There is an almost continual determination of genes and
their artifacts in the identification of specific cancer related presence. We
have examined many of these before and each time there appears another target
it is essential to ask; why this new target is chose and what does it do that
results in a metastatic condition? The current analysis focuses on a small
class of proteins which facilitate endocytosis, the bringing in of materials
from the cell’s surface.
In a recent paper the authors have identified the small 200
nucleotide protein Ras7 as a significant factor in the metastatic behavior of
melanoma. We first examine that express consideration, then examine the details
behind it and consider some observations.
As Alonso-Curbelo et al state:
Although common cancer hallmarks are well established,
lineage-restricted oncogenes remain less understood.
Here, we report an inherent dependency of melanoma cells
on the small GTPase RAB7, identified within a lysosomal gene cluster that
distinguishes this malignancy from over 35 tumor types.
Analyses in human cells, clinical specimens, and mouse
models demonstrated that RAB7 is an early-induced melanoma driver whose levels
can be tuned to favor tumor invasion, ultimately defining metastatic risk.
Importantly, RAB7 levels and function were independent of
MITF, the best-characterized melanocyte lineage-specific transcription factor.
Instead, we describe the neuroectodermal master modulator
SOX10 and the oncogene MYC as RAB7 regulators. These results reveal a unique
wiring of the lysosomal pathway that melanomas exploit to foster tumor
progression.
The above observation contains an interesting connection
between several elements:
1. Extracellular Matrix elements (ECM) which we have
examined before as factors in driving melanoma metastasis.
2. Lysosome activity which is the counter to exosomes, which
we have examined as markers for melanoma.
3. Cell surface receptors and ligands which reflect cell
cycle activation and control.
4. Transcription factor management through MITF and MYC
which result in loss of cell cycle homeostasis.
Thus to a degree in this paper we seem to be seeing a
multifaceted response modulated by the RAB gene products. Thus it is worth a
brief review of endosomes and RAB and then a re-examination of the
transcription factors in the context of cell cycle control.
The authors especially note the following:
· Melanoma-restricted lysosomal gene cluster uncovers
tumor-type-specific roles of RAB7: RAB7 is one of several RAB genes and it
has a specific functionality which at face value does not relate directly to
the management of transcription factors. Thus it is necessary to examine that
in some detail.
·
RAB7-controlled pathways
selectively modulate melanoma cell phenotypes: One does not look at all the
RAB7 pathways and what controls them. We often think of MITF, SOX10 and MYC as
having other control elements.
·
RAB7 is an early-induced
melanoma driver that defines patient prognosis: RAB7 modification by some
form of upregulation which in turn is driven by oncogenes is a significant
factor in metastatic behavior. The details will be examined somewhat herein.
·
MYC and SOX10 regulate
RAB7 in an oncogene- and lineage-dependent manner, respectively: The issue
is that it is MYC and SOX10 that modulate RAB7 upwards and thus use it as a
means to bring more elements into the cell which are then broken down via a
lysosome and use by the cell in its proliferation. The question which seems
unanswered is; if MYC and COX10 are transcription factors and also oncogenes,
then how do they regulate RAB7 and if they up-regulate RAB7 what does that
result in since RAB7 drives endosomic activity?
Now there is a commentary which adds to what is presented
above. It states[1]:
The results of the study could help to determine the
development of metastasis in patients suffering from the disease….
What is the function of these genes? Strangely, the
factors that are increased in melanoma share a common mechanism: the formation
of vesicles called endosomes.
Endosomes are machinery that tumour cells, via a process
called endocytosis, can use to incorporate components into their environment
and obtain energy by degrading them via autodigestion or autophagy. Autophagy
is also used for self-cleaning to eliminate other proteins as well as damaged
or unneeded cellular components.
Thus at the heart of this theory is that RAB7 is activated
and results in increased processing of endosomes taking proteins from the
surface of the cell and breaking them down and allowing the cells to
proliferate more aggressively. We will try to demonstrate some of this activity
but many of the details are still wanting. They continue:
Among all the genes that control endocytosis, the authors
of the study focused specifically on one, called RAB7; this gene is highly
expressed in melanoma cells. After more than six years of research, the
research team …showed that RAB7 acts as an orchestra director, determining the
fate of melanoma cells: at high concentrations of RAB7, cellular autodigestion
is very active, and this allows tumour cells to obtain energy, prevent the
accumulation of toxic components and thus divide and proliferate; when RAB7 is
reduced, cells use endosomes to recycle metastatic proteins, favouring their
dispersal throughout the body.
To some degree this is a growth and proliferation issue.
What specifically is driving RAB7 is open for debate and it does not appear to
be any mutation in the gene or its product. The emphasis on RAB7 as the key
factor may be begging the question in light of the many other factors known to
take a role in melanoma development.
Defining "the key to the fate of the tumour
cell", …is just one of many new aspects of melanoma uncovered by this
study. "Finding which mechanisms determine why melanoma is so aggressive
is very complex because more than 80,000 mutations have been described for this
tumour", …
This study is also relevant for clinical work. One
application is the prognosis of the melanoma: the authors show in tumour
biopsies that the amount of RAB7 in a cutaneous tumour defines the risk of
developing metastasis. "This study opens avenues for the potential use of
proteins that control vesicles and regulate autophagy as novel markers of
patient survival", …
Furthermore, these results help to understand the
mechanism of action of a compound that, as the group discovered in 2009, is
lethal in melanoma cells as well as in other tumour cells. This RNA-based
nanoparticle compound kills the cells by acting on the formation of vesicles.
The above seems to imply that if one can stop the formation
of such vesicles that one then possibly “starves” the specific cells. The issue
is targeting just these vesicles and not all vesicles. This observations is
noted as follows:
"We knew how our nanoparticles act inside tumour
cells, but not how they selectively incorporate inside the cells", …The
size of these molecules requires cells to form endosomes in order to be able to
trap the compound. This study demonstrates that this endosome formation (via
RAB7) is very active in tumour cells but not in normal cells. Normal cells,
therefore, do not incorporate RNA nanoparticles, reducing the risk of toxic
effects.
However RAB7 proteins do function in all cells, taking in
items and participating in the “digestion” of many such items, many of which if
left undigested could be harmful to the cell. Thus it could be problematic to
try such a direct approach. To some degree it may be akin to DNMT interference
drugs used to block hypermethylation.
The work … could lead to the development of novel drugs
that selectively target the mechanism of cell autodigestion as a potential
therapeutic strategy.
The target of a specific drug then must block either the
generation of Rab7 or the Rab7 directly. The sequellae of such blockage may be
significant.
We have previously discussed
exosomes and cancer prognosis. Exosomes are what cells eject. Endosomes are
what cells ingest and process. They do this in a complex manner and utilize
lysosomes which have digestive capabilities which allow for the extraction of
cell nutrients.
Endocytosis has several key
functions for a cell[2]:
- It internalizes nutrients found outside the cell.
- It facilitates and regulates the expression of cell surface proteins so that cells can control the up-take of certain ligands.
- It facilitates the uptake of extracellular debris as well as other ECM items.
- Recovery of membrane structure.
The endosomes are the transport vehicles and they function
in concert with lysosomes which are enzymatically charged digestive vacuoles.
Together they take in and process what is on the cell’s surface and without the
cell.
We depict this process generally below:
Now we can examine this process alongside several others as
we show below.
Note above we show both Rab7 and Rab5 actions.
As Tabata et al note:
Endocytosis involves the intracellular transport of
extracellular and plasma membrane substrates to endosomes/lysosomes and is
involved in many physiological processes. Autophagy is also a process in which
cytoplasmic constituents, including organelles, are transported within double- membraned
autophagosomes to lysosomes for degradation.
Autophagy has divergent physiological roles in cancer,
infection, immunity, and other processes. Many reports suggest there is a
common element between the endosomal and autophagic pathways, but these
commonalities have not been fully elucidated….
In this study, we described a novel RH protein domain
that associates directly with Rab7. Rubicon and PLEKHM1 negatively regulate
endosomal transport by binding to Rab7 via their RH domains.
Furthermore, this study provides novel insight into the
function of PLEKHM1. These two RH domain–containing proteins also have several
significant differences. Rubicon must bind to the Beclin 1–PI3-kinase complex in
addition to Rab7 for its function, whereas PLEKHM1 only requires Rab7 binding.
Rubicon, unlike PLEKHM1, is involved in autophagosome maturation. Both of these
proteins localize to endosomes but do not show complete colocalization.
Therefore, these two RH domain proteins seem to function through
different mechanisms. PLEKHM1 bound not only to wild-type Rab7 and the Rab7QL
mutant but also to the Rab7TN mutant in a yeast two-hybrid assay and
immunoprecipitation analyses in mammalian cells, suggesting that PLEKHM1 may
bind to both the GTP-bound and GDP-bound forms of Rab7. However, the TN mutant
is predicted to have reduced affinity for both GDP and GTP and may behave as a
nucleotide-free Rab7 depending on the assay conditions.
Hence, in order to determine the precise nucleotide
dependence, we performed an in vitro GST pulldown assay and found that recombinant
PLEKHM1 strongly interacted with GTP
S-loaded Rab7 but only minimally interacted with GDP-loaded Rab7. The data convincingly confirm that PLEKHM1 preferentially binds to the GTP-bound form of Rab7, corroborating our hypothesis that PLEKHM1, like Rubicon, is a Rab7 effector.
The above analysis does reflect on a different binding
domain and process from what was introduced earlier. We shall come back to this
issue later.
We will now examine the RAB proteins in some detail. Vesicle
targeting and fusion with acceptors on the membrane uses the collection of
proteins at the vesicle membrane. These are managed by the Rab family of proteins
which are small G proteins[3]. G
proteins and Rab in particular are classified as amplifier, rectifier and
organizing proteins. They can use GTP to act as a switch and these proteins are
used by cells to make movement, transformations and activations. The Rab
subclass of the Ras proteins plays a key role in the endocytosis and
integration with lysosomes….
The following describes several of the Rab proteins and
their functions[4]:
RAB Type
|
Function
|
Rab1 and Rab2
|
Control vesicular traffic
from the endoplastic reticulum to the Golgi apparatus.
|
Rab6
|
Controls inter Golgi
traffic.
|
Rab8
|
Controls transport from the
Golgi apparatus to the plasma membrane.
|
Rab4, Rab5, Rab9
|
Regulate endocytosis.
|
Rab7
|
Specialized for recycling
of down-modulated membrane receptors.
|
RAB7A 3q21.3
RAB7B 1q32
|
Specific sub-types of Rab7
|
From NCBI we have[5]:
RAB family members are small, RAS-related GTP-binding
proteins that are important regulators of vesicular transport. Each RAB protein
targets multiple proteins that act in exocytic / endocytic pathways. This gene
encodes a RAB family member that regulates vesicle traffic in the late
endosomes and also from late endosomes to lysosomes. This encoded protein is
also involved in the cellular vacuolation of the VacA cytotoxin of Helicobacter
pylori. Mutations at highly conserved amino acid residues in this gene have
caused some forms of Charcot-Marie-Tooth (CMT) type 2 neuropathies.
As we had discussed above the Rab 5 and 7 facilitate the
endocytic paths for bringing in materials from without the cell. Thus they are
simply transport facilitators. It is alleged that they can be easily targeted
for suppression but then again they appear to be active elements in many cells
and the blockage of them in a systemic manner may potentially have significant
negative effects.
Let us go back to a statement made by the authors:
Importantly, RAB7
levels and function were independent of MITF, the best-characterized melanocyte
lineage-specific transcription factor. Instead, we describe the neuroectodermal
master modulator SOX10 and the oncogene MYC as RAB7 regulators. These results
reveal a unique wiring of the lysosomal pathway that melanomas exploit to
foster tumor progression.
MITF has been examined before in the case of melanoma. MYC
and SOX10 are transcription factors and MYC is a well-known oncogene. How they
then regulate RAB7 is a key question. Are they transcription factors that
up-regulate the gene? If so, then what has activated MYC and is this the better
target. Is RAB7 then just an artifact of such up-regulation. The same can be
asked about SOX10.
As regards to cancers, Rab7 has been studied extensively. Zhang
et al have presented results where they state:
Aberrant endocytosis and altered lysosomal function
result in defective growth-factor transport and unbalanced levels of surface proteins,
such as integrins and E-cadherin, leading to tumorigenesis and cancer
metastasis. Rab GTPases, as master regulators in membrane traffic, are proved
to be involved in cancer development. Rab25 is a well-established tumorigenesis
associated Rab and is highly homologous to Rab11, and endogenously overexpressed
in most ovarian and breast cancer samples in a constitutively active form,
which is unique among Rab proteins. …provided data indicating that
overexpression of Rab25 promotes cell transformation, inhibits apoptosis and induces
tumour progression, probably through the PI3K/AKT signalling pathway. Rab25 may
also be related to other cancer such as OC/PPC (ovarian/primary peritoneal
serous carcinoma) and prostate cancer.
Zhang et al continue as follows:
The results …showed that thyroid hormone production was
regulated by Rab5a and Rab7. cAMP stimulation elevated the expression of Rab5a
and Rab7 in adenomas, linking Rab7 to the formation of benign thyroid
autonomous adenomas …also found Rab7 is overexpressed in DMPM (diffuse
peritoneal malignant mesothelioma).
In addition, v-Src induces activation of Rab7, which may
be related to epithelial-to-mesenchymal transition during tumour progression …
indicate that Rab7 is involved in a cell survival pathway. Upon growth-factor
depletion, Rab7 down-regulates surface nutrient transporters through endocytic
degradation, preventing growth-factor-independent survival, but inhibition of
Rab7 sustains surface nutrient transporters, thus promoting long-term cell survival,
which is dependent on the AKT survival signalling pathway. Furthermore, …
inhibition of Rab7 co-operated with the adenoviral E1A protein to promote
transformation of p53−/− MEFs (mouse embryonic fibroblasts), thus Rab7 was
proposed to act as a potential tumour suppressor.
however, there is insufficient evidence to conclude that
Rab7 functions as a tumour suppressor. As mentioned above, Rab7 is actually
overexpressed in some cancer cells or tissues, as described previously, and the
transformation effects of dominant-negative Rab7 required the crucial help of
the E1A protein and the absence of p53 … and these studies were carried out
under nutrient starvation condition which may differ slightly from the
environmental conditions for tumorigenesis that are usually rich in growth
factors.
…another view on the function of Rab7 in apoptosis.
Inhibiting the upstream regulator RabGGT prominently induces apoptosis of germ
cells in Caenorhabditis elegans and mammalian cancer cells. … examined the
effects of knockdown of Rab5, Rab7 and components of the HOPS complex by RNA interference
in C. elegans, and found that knockdown of both Rab proteins promoted germ
cells apoptosis…
Taken together, the underlying mechanism for cancer, cell
survival and apoptosis regulated by Rab7 is still not yet understood. Rab7 is
also emerging as a regulator for the autophagic pathway, another mechanism for
cell death and survival, which is related to many diseases, such as cancer and
heart failure.
The autophagic process is initiated by engulfment of
cytoplasmic materials into a unique membrane (phagophore) to form an autophagosome;
the autophagosome then undergoes maturation through fusion with endosomal
vesicles and lysosomes to form a lysoautophagosome, in which materials are
degraded to provide nutrients and energy for cell survival under nutrient
depletion.
It is thus fair to say that Rab7 functioning is still a work
in progress.
Now it is worth a brief discussion of MITF which has been
noted as an element in melanoma metastasis. As noted by Carreira et al[6]:
It is widely held that cells with metastatic properties
such as invasiveness and expression of matrix metalloproteinases arise through
the stepwise accumulation of genetic lesions arising from genetic instability
and “clonal evolution.”
By contrast, we show here that in melanomas invasiveness
can be regulated epigenetically by the microphthalmia-associated transcription
factor, Mitf, via regulation of the DIAPH1 gene encoding the
diaphanous-related formin Dia1 that promotes actin polymerization and
coordinates the actin cytoskeleton and microtubule networks at the cell
periphery.
Low Mitf levels lead to down-regulation of Dia1,
reorganization of the actin cytoskeleton, and increased ROCK-dependent
invasiveness, whereas increased Mitf expression leads to decreased
invasiveness.
Significantly the regulation of Dia1 by Mitf also
controls p27Kip1-degradation such
that reduced Mitf levels lead to a p27Kip1-dependent
G1 arrest. Thus Mitf, via regulation of Dia1, can both inhibit invasiveness and
promote proliferation.
The results imply variations in the repertoire of
environmental cues that determine Mitf activity will dictate the
differentiation, proliferative, and invasive/migratory potential of melanoma
cells through a dynamic epigenetic mechanism.
Note that the discussion above shows that overexpression of
MITF leads to less invasiveness. We show the details of the pathway dynamics
below.
Thus the MITF function, in their view, is critical. The
recent work demonstrates that we can through an understanding of the pathways
then target specific pathway control proteins by understanding their structure.
We can already control B RAF in certain circumstances by targeting its
specificity and that controlling the path but allowing MITF control in a broad
sense may actually be much more powerful if the results hold for clinical
applications.
The ability to find, characterize, and design binding site
specific blocking agents is an essential step in a broader control of multiple
cancers. From Eichoff we have:
In various cell types the MITF gene is transcribed from
different promoters to generate cell type-specific isoforms. In the
melanocytic-lineage, Wnt signaling is required for expression of the M-Mitf
isoform essential for melanocyte development. M-Mitf has a role in the
differentiation of neural crest cells to melanoblasts and melanocytes, as well
as in their survival and maintenance, and is identified as the master regulator
of melanocyte development . M-Mitf
regulates the expression of melanocyte lineage-specific pigment-producing
factors such as DCT and tyrosinase (TYR). Both the proliferation of melanocytes
and M-Mitf expression and melanocyte proliferation is Wnt signal driven and
inhibited by the Wnt signal inhibitor dickkopf-related protein 1 (Dkk1). Dkk1
antagonises Wnt signaling by binding to the Wnt receptor complex and inducing
its internalisation. On palmoplantar skin, fibroblast release of Dkk1 is an
important regulator of the hypopigmentation which is characteristic of these
tissues.
The importance of Wnt signalling is also a critical factor
here. We have examined this in detail elsewhere.
MYC is a well know transcription factor whose effects are
often strongly seen in many cancers. It is an oncogene. As Nillson and
Cleveland note regarding Myc:
At first glance, the selection for Myc activation in
cancer seemed obvious. First, it was quickly established that enforced Myc
expression was sufficient to provoke the entry and continuous,
mitogen-independent, proliferation of cells and that it effectively blocked
terminal cell differentiation.
Subsequently, Myc was shown to be necessary for traverse
into S phase of the cell cycle, a finding recently underscored by the
conditional knockout of c-Myc. Thus, not surprisingly, both c-Myc and N-Myc are
essential for vertebrate development.
In addition, numerous studies showed that Myc activation
was sufficient to provoke diverse cancers and, more recently, that Myc is
continuously required to maintain the transformed state.
Finally, to round out the story was the revelation that
c-Myc functioned as an angiogenic switch, and that its expression was in fact
essential for proper and coordinate regulation of angiogenic and
anti-angiogenic factors in cancer and development. This was satisfying – now we
know why Myc activation was so pervasive in cancer.
As Yamamura et al state:
Rho GTPases are small G proteins that regulate various
cellular processes, including cytoskeletal dynamics, migration, vesicle
trafficking, cell proliferation, apoptosis and transcription. Rho GTPases,
their regulators and their effectors have been suggested to control tumor
formation and progression. RhoA has been found to control cancer metastasis and
progression. Recently, the c-Myc–Skp2–Miz1 complex was shown to activate the
RhoA gene.
The Ras superfamily incorporates the Ras, Rho, Rab Art and
Ran families[7].
Now Rab7 is in the Rho GTPase class it may be considered controlled via Myc in
a similar manner and mentioned above.
As stated in NCBI for Skp2[8]:
This gene encodes a member of the F-box protein family
which is characterized by an approximately 40 amino acid motif, the F-box. The
F-box proteins constitute one of the four subunits of ubiquitin protein ligase
complex called SCFs (SKP1-cullin-F-box), which function in
phosphorylation-dependent ubiquitination. The F-box proteins are divided into 3
classes: Fbws containing WD-40 domains, Fbls containing leucine-rich repeats,
and Fbxs containing either different protein-protein interaction modules or no
recognizable motifs. The protein encoded by this gene belongs to the Fbls
class; in addition to an F-box, this protein contains 10 tandem leucine-rich repeats.
This protein is an essential element of the cyclin A-CDK2
S-phase kinase. It specifically recognizes phosphorylated cyclin-dependent
kinase inhibitor 1B (CDKN1B, also referred to as p27 or KIP1) predominantly in
S phase and interacts with S-phase kinase-associated protein 1 (SKP1 or p19).
In addition, this gene is established as a protooncogene
causally involved in the pathogenesis of lymphomas. Alternative splicing of
this gene generates three transcript variants encoding different isoforms.
And similarly for Miz1[9]:
This gene encodes a member of the protein inhibitor of
activated STAT (PIAS) family. PIAS proteins function as SUMO E3 ligases and
play important roles in many cellular processes by mediating the sumoylation of
target proteins. Alternatively spliced transcript variants encoding multiple
isoforms have been observed for this gene.
Isoforms of the encoded protein enhance the sumoylation
of specific target proteins including the p53 tumor suppressor protein, c-Jun,
and the androgen receptor. A pseudogene of this gene is located on the short
arm of chromosome 4. The symbol MIZ1 has also been associated with ZBTB17 which
is a different gene located on chromosome 1.
Also note that Miz1 is also called PIAS2 protein inhibitor
of activated STAT, 2.
As Qu et al state:
In addition to the PI3K/Akt pathway, c-Myc also plays a
role during TGF-b-induced EMT. It has been demonstrated that high TGF-b levels
are often associated with melanoma progression, and so does the Akt1, c-Myc,
and SKP2 (S-phase kinase-associated protein 2) levels. However, it is not clear
how these signals are interacted and integrated in melanoma metastasis.
The above observation specifically details c-Myc and Skp2
but also indicates the lack of specificity and clarity as to the roles of each,
no less the specifics on the role of these in Rab7. Likewise Qu et al continue:
SKP2 is the substrate recognition subunit of SCF (SKP1-
CUL1-F-box protein) ubiquitin ligase complex. Aberrant SKP2 expression plays an
active role in tumorigenesis owing to its central role in degradation of a
number of cyclin-dependent kinase inhibitors including p27kip1, p21cip1, and
p57. SKP2 was overexpressed in melanoma and its levels were correlated with
metastasis. SKP2 regulates c-Myc protein stability and activity at both
transcriptional and post-translational levels. Whether and how SKP2 is
regulated during TGF-b-induced EMT remains to be elucidated.
Again we have a lingering level of uncertainty.
SOX10 is a transcription factor and has been identified with
melanoma by many authors[10].
They are characterized by the Sry-box binding site. Sry-box is a variant of the
HMG domain. Sox10 has been identified with the performance of many functions.
It is involved in maintaining pluripotency of neural crest cells, melanocytes
are derived from neural crests, it remotes survival and proliferation, and in
terminal differentiation. It requires other cell pathways to affect its
results. In mice it has been shown that deletion of Sox10 results in the elimination
of all melanocytes.
Sox10 also works directly with MITF and it is through this
joint action that it supports proliferation and survival. Sox10 influences the
M promoter as well as MITF.Sox10 expression precedes MITF expression.
From Eichoff we have:
In adult human skin, stem cells are found in the hair
follicle where lineage-specific differentiation of neural crest cells to
melanoblasts and melanocytes occurs in response to changes in signaling. Among
the genes involved several are transcription factors that include the Wnt
pathway target gene Microphthalmia-associated transcription factor (MITF),
paired domain-and homeodomain-containing transcription factor 3 (PAX3), and
Sry-related transcription factor 10 (SOX10).
Importantly, it has been shown that the loss of any of
these factors results in the failure of melanoblasts to develop (White &
Zon, 2008). The final fate determination
for melanocytes occurs when migrating melanoblasts come into contact with
epidermal keratinocytes, which regulate their rate of replication to establish
a stable keratinocyte/melanocyte ratio (Fukunaga-Kalabis et al, 2006;
Valyi-Nagy et al, 1993).
Differentiated human melanocytes remain strictly
localized at the basement membrane and cannot survive within the upper epidermal
layers unless transformed into nevi or melanoma cells.
The observations of melanocyte survival in the basement
membrane are key. For example in melanoma in situ, as compared to superficial
spreading melanoma, the melanocyte breaks loose from the basement membrane and
wanders upward. The presence of melanocytes in the upper layers is often
pathognomonic for MIS. However as we have discussed elsewhere the movement is
also driven be E cadherin changes and β catenin expression.
From Shakhova et al we have:
Giant congenital naevi are pigmented childhood lesions
that frequently lead to melanoma, the most aggressive skin cancer. The
mechanisms underlying this malignancy are largely unknown, and there are no
effective therapies. Here we describe a mouse model for giant congenital naevi
and show that naevi and melanoma prominently express Sox10, a transcription
factor crucial for the formation of melanocytes from the neural crest.
Strikingly, Sox10 haploinsufficiency counteracts NrasQ61K-driven congenital
naevus and melanoma formation without affecting the physiological functions of
neural crest derivatives in the skin.
Moreover, Sox10 is also crucial for the maintenance of
neoplastic cells in vivo. In human patients, virtually all congenital naevi and
melanomas are SOX10 positive. Furthermore, SOX10 silencing in human melanoma
cells suppresses neural crest stem cell properties, counteracts proliferation
and cell survival, and completely abolishes in vivo tumour formation. Thus,
SOX10 represents a promising target for the treatment of congenital naevi and
melanoma in human patients.
The above observation regarding Sox10 for survival begs the
question; why? Silencing Sox10 may silence the melanocyte.
As Hoek et al state:
Upregulation of SOX10 by Mitf-transfection is an
interesting finding as SOX10 has long been held to be a regulator of MITF …indicating
the possibility that these transcription factors regulate each other’s
expression. It may be that the myelinating cell genes mentioned here are
detected because they are directly regulated by SOX10 … while its gene is being
regulated by MITF, rather than being directly regulated by MITF itself. This,
nevertheless, suggests that MITF may have a role alongside SOX10 in regulating
the processes of myelination.
The role of MITF and SOX10 are known to be aligned, and one
preceding the other. Yet the complete details of the interactions appears to be
yet determined.
As Saskia et al state[11]:
The transcription factor SOX10 (SRY (sex determining
region Y)-box 10) has a key role in the embryonic development of melanocytes.
Recently, it has been suggested that SOX10 is highly relevant for melanoma
development and survival. However, the distinct functions and downstream
targets of SOX10 in melanoma remain widely unknown. In this study, we inhibited
SOX10 via RNA interference in different human melanoma cell lines and found a
significantly reduced invasion capacity in vitro and in the chick embryo model.
This recent paper still reflects the uncertainty for the
downstream control of SOX10. Just what it does as regards to RAB7 is yet to be
determined.
At later time points, SOX10 inhibition reduced
proliferation and induced cell death. We identified melanoma inhibitory
activity (MIA) as a direct target gene of SOX10, which is an essential protein
for melanoma cell migration and invasion. Expression levels of SOX10 and MIA
strictly correlated in melanoma cell lines, and SOX10 inhibition reduced MIA
expression and promoter activity. Direct binding of SOX10 to the MIA promoter
was demonstrated by electrophoretic mobility shift assay and chromatin
immunoprecipitation.
Ectopic expression of MIA in SOX10-inhibited melanoma
cells restored the invasion capacity, supporting the hypothesis that MIA is
responsible for SOX10-mediated melanoma cell invasion. Our data provide
evidence for a critical role of SOX10 in melanoma cell invasion through the
regulation of MIA and highlight its role as a therapeutic target in melanoma.
The last observation was also present in the comments by Wegner.
The interrelationship between SOX10 and MIT and MITF and MIA are somewhat
understood. The details again need clarification.
This analysis of the Rab7 protein as a driver of melanoma
metastatic dissemination appears to be predicated on the observation that
endosomes can transport into the cell items which foster growth and
proliferation. Further it is alleged that Sox10 and Myc are regulators of Rab7,
and unlike their normal roles as oncogene transcription factors that in this
case they appear to up-regulate Rab7 which in turn provides the cell with added
nutrients that enable growth and proliferation. We have tried to piece together
the details of this assertion and there appears to be multiple missing steps.
1. Pathway Control by Myc and Sox10: What are the pathway
control elements related to the RAB7 controls? We have a general understanding
but many details are yet to be fully elucidated.
2. Actions precipitated by Rab7 that promote Growth and
Proliferation: What does RAB7 do to drive proliferation? Admittedly it can
bring in nutrients but specifically what and why?
3. Mechanisms for the Control of Rab7: Just how do various
sets of enzymes/proteins control RAB7? Is it just transcription control, or are
there epigenetic factors as well?
4. Control of Myc and Sox10 versus Control of Rab7: What
controls the two transcription factors which in turn activate RAB7?
5. It is known that there exists a melanoma stem cell. What
is the relationship between the up-regulated RAB7 and the stem cell
characteristics. In Girouard and Murphy we have an excellent overview of the
melanoma stem cell. Thus if we accept the stem cell model is the RAB behavior
an identifies of such a cell or is it a just an artifact?
Now from KEGG we have the following generic progression of
melanoma and identified gene and pathway changes[12]:
Note that we do not see the targets discussed herein. In
fact there are many alternative targets that have been discussed at length
elsewhere.
References:
1. Albinet, V., et al, Dual role of sphingosine kinase-1 in
promoting the differentiation of dermal fibroblasts and the dissemination of
melanoma cells, Oncogene (2014) 33, 3364–3373.
2. Alonso-Curbelo, D., et al, RAB7 Controls Melanoma Progression by
Exploiting a Lineage-Specific Wiring of the Endolysosomal Pathway, http://www.cell.com/cancer-cell/abstract/S1535-6108%2814%2900218-9
, DOI: http://dx.doi.org/10.1016/j.ccr.2014.04.030
3. Barre, B., N. Perkins, The Skp2 Promoter Integrates Signaling
through the NF-kB, p53, and Akt/GSK3b Pathways to Regulate Autophagy and
Apoptosis, Molecular Cell 38, 524–538, May 28, 2010.
4. Bunz, F., Principles of Cancer Genetics, Springer (New York)
2008.
5. Cassimimeris, L., et al, Cells, Bartlett (Boston) 2011.
6. Cremona, C., A Behrens, ATM
signalling and cancer, Oncogene (2014) 33, 3351–3360.
7. Eichoff, O., Signaling pathways in melanoma, PhD Thesis, ETH
ZURICH, 2010.
8. Gellmann, E., et al, Molecular Oncology, Cambridge (New York)
2014.
9. Girouard, S., G. Murphy, Melanoma stem cells: not rare, but well
done, Laboratory Investigation (2011) 91, 647–664
10. Gutierrez, M., et al, Rab7
is required for the normal progression of the autophagic pathway in mammalian
cells, Journal of Cell Science 117, 2687-2697 Published by The Company of
Biologists 2004
11. Hearing V., S. Leong, From Melanocytes to Melanoma, Humana
(Totowa, NJ) 2006.
12. Hoek, K., et al, Novel MITF targets identified using a two-step
DNA microarray strategy, Pigment Cell Melanoma Res. 21; 665–676
13. Karamchandani, J., et al., Sox10 and S100 in the Diagnosis of
Soft-tissue Neoplasms, Appl Immunohistochem Mol Morphol, Volume 00, Number 00,
2012
14. McGarty, T., Melanoma Genomics (DRAFT #2) 2013. http://www.telmarc.com/Documents/Books/Melanoma%20Genomics%2007.pdf
15. Mendelsohn, et al, The Molecular Basis of Cancer, Elsevier (New
York) 2014.
16. Ni, J., et al, Cancer Stem Cells in Prostate Cancer
Chemoresistance, Current Cancer Drug Targets, 2014, 14, 225-240.
17. Nillson, J., J. Cleveland, Myc pathways provoking cell suicide
and cancer, Oncogene (2003) 22, 9007–9021
18. Qu et al, A Signal Transduction Pathway from TGF-b1 to SKP2 via
Akt1 and c-Myc and its Correlation with Progression in Human Melanoma, Journal
of Investigative Dermatology (2014) 134, 159–167.
19. Saskia, A. et al, SOX10 Promotes Melanoma Cell Invasion by
Regulating Melanoma Inhibitory Activity, Nature Cell Biology,14, 882–890, (2012),
doi:10.1038/ncb2535,
20. Shakova, O., et al, Sox10 promotes the formation and maintenance
of giant congenital naevi and melanoma, http://www.nature.com/ncb/journal/v14/n8/full/ncb2535.html
21. Straume, O., et al, Loss of Nuclear p16 Protein Expression Correlates
with Increased Tumor Cell Proliferation (Ki-67) and Poor Prognosis in Patients with
Vertical Growth Phase Melanoma, Vol. 6, 1845–1853, May 2000 Clinical Cancer
Research.
22. Tabata, K., et al, Rubicon and PLEKHM1 Negatively Regulate the Endocytic/Autophagic
Pathway via a Novel Rab7-binding Domain, Molecular Biology of the Cell, Vol.
21, 4162–4172, December 1, 2010
23. Utical, Y., et al, Diagnostic and Prognostic Biomarkers in
Melanoma: Current State of Play, Current Clinical Pathology, DOI
10.1007/978-1-60761-433-3_2, 2012.
24. Yamamura et al, MicroRNA-34a suppresses malignant transformation
by targeting c-Myc transcriptional complexes in human renal cell carcinoma, Carcinogenesis
vol.33 no.2 pp.294–300, 2012.
25. Zhang, M., et al, Rab7: roles in membrane trafficking and
disease, Biosci. Rep. (2009) / 29 / 193–209
26. Zhong, W., et al, SOXs in human prostate cancer: implication as progression
and prognosis factors, BMC Cancer 2012, 12:248.
White Papers:
The following are various White Papers we have authored
which are referred to herein.
No. 114 NOTCH, miR-146a and Melanoma, June 2014
No. 112 Prostate Cancer: miR-34, p53, MET and Methylation,
May 2014
No. 111 CRISPR and Cancer, April 2014
No. 110 ERG and Prostate Cancer, January 2014
No. 108 Cancer Cell Dynamics, January 2014.
No. 107 Prostate
Cancer Genetic Metrics, January 2014
No. 106 Divergent Transcription, December 2013
No. 104 Prostate Cancer and Blood Borne Markers, December
2013
No. 103 Prostate Cancer Indolence, December 2013
No. 102 MDS and Methylation, August 2013
No. 101 Exosomes and Cancer, August 2013
No. 100 lncRNA and Prostate Cancer, August 2013
No. 99 SNPs and Cancer Prognostics, July 2013
No. 98 CCP and Prostate Cancer, July 2013
No. 97 ATF2 and Melanoma, July 2013
No. 96 PD-1 and
Melanoma Therapeutics, June 2013)
No. 95 MER Tyrosine
Kinase Receptors and Inhibition, June 2013
No. 94 Melanoma Therapeutics, May 2013
No. 93 Cancer Cell Dynamics April 2013
No. 91 Methylation and Cancer, March 2013
No. 90 Telomeres and Melanoma, February 2013
No. 89 miRNA and Melanoma, January 2013
No. 88 Extracellular Matrix vs. Intracellular Pathways
No. 87 Prostate Cancer Prognostic Markers
No. 86 Cancer Models for Understanding, Prediction, and
Control
No. 85 Prostate Cancer Stem Cells
No. 84 Epistemology of Cancer Genomics
No. 83 Prostatic Intraepithelial Neoplasia
No 82 Prostate Cancer: Metastatic Pathway Identification
No 81 Backscatter Radiation and Cancer, November 2010
No 80 PSA Evaluation Methodologies, November 2010
No 79 The PSA Controversy, November 2010.
[1] http://www.news-medical.net/news/20140628/CNIO-researchers-identify-over-40-genes-that-predict-aggressiveness-of-melanoma.aspx
[2]
See p 351 Cassimeris et al.
[3]
See Marks pp 32-37. “small G” proteins are monomers about 200 amino acids in
length. They are in the Ras superfamily and thus Rab is also in that family.
[4]
See Marks et al, p 383.
[6]
Carreira, S., et al, Mitf regulation of Dia1 controls melanoma proliferation
and invasiveness, Genes Dev. 2006 20: 3426-3439.
[7]
See Marks et al p 354.
[10]
See Wegner pp 71-80 in Hearing and Leong.
Posts
