and DNA double-strand breaks (DSBs)
[13, 14]. These studies suggest that B-
Raf or N-Ras activation initiates S-phase
entry, causing stress at the DNA replica-
tion fork which culminates in DNA DSBs,
activation of a cell response mechanism,
cell cycle arrest, and OIS.
Bypassing OIS
OIS has been extensively studied and fo-
cus has been given to the role of INK4a,
ARF, and p53 in this process. Restoration
of p16 expression in INK4a/ARF-/- mu-
rine melanocytes was demonstrated to
rescue the senescent phenotype, while
reexpression of ARF actually induced cell
death [15]. Contrasting these findings,
establishment of ARF expression in a
transgenic mouse model appeared es-
sential to induce senescence; further-
more, when combined with activation of
N-Ras and INK4a, loss of ARF trans-
formed murine melanocytes, in a man-
ner independent of p53 [16]. While seem-
ingly diverse, the collective data strongly
implicate the CDKN2a locus for its roles
in preventing tumourigenesis through
the induction of OIS.
Much evidence of the cooperation of
MAPK activation and senescence bypass
in melanocytic transformation has come
from studies using a murine model of
constitutive activation of H-Ras, where
individual loss of INK4a, ARF, or p53
leads to melanomagenesis [17-19]. Other
work has shown that short term expres-
sion of B-Raf V600E in human melano-
cytes leads to a proliferation burst, but
long term expression results in cell cycle
arrest associated with p16 upregulation
and SA-ßGal activity [9, 10]. No evidence
of p53 or p21 activity in this process was
observed. Ironically, cell cycle arrest was
observed even in p16-deficient fibro-
blasts, suggesting that other genes may
be involved in the initiation of OIS. Con-
trasting these results, Patton et al. have
shown that zebrafish solely expressing
B-Raf V600E form non-malignant nevi,
but when combined with p53 deficiency,
these lesions develop into invasive mela-
nomas [20]. p16 inactivation, however, is
not sufficient for melanocyte immortali-
sation in vitro, possibly implicating other
immortalizing factors, such as hTERT
[21]. In fact, hTERT contributes to the
malignant transformation of human me-
lanocytes when combined with activated
N-Ras or PI3K and disruption of both the
Rb and p53 pathways [22]. B-Raf activa-
tion, however, did not result in a similar
phenotype.
Most recently, a genome-wide RNAi
approach identified 17 genes that block
the proliferative arrest imposed by B-Raf
activation [23]. IGFBP7, an insulin-like
growth factor binding protein, is upregu-
lated after B-Raf activation in melano-
cytes and induces senescence in an auto-
crine/paracrine manner. The mechanism
involving this secreted factor seems to be
independent of both p16 and p53, an ob-
servation that is interesting, particularly
considering that p53 was another gene
identified that synergises with V600E to
bypass OIS in the same study.
Conclusion
While the data do not always correlate
well, it appears that potentiation of onco-
genic activity combined with an escape
from tumour suppressor mechanisms is
necessary to transform melanocytes into
melanoma cells. The evidence discussed
here underscores the mechanisms em-
ployed by cells to repress tumour forma-
tion. The participation of well-known tu-
mour suppressor genes p16, ARF, and
p53 are prominent in this response, par-
ticularly in the realm of OIS – to what ex-
tent each of these genes contributes to
evasion of transformation is still not en-
tirely clear. The most recent data open
the possibility of other mechanisms, such
as DDRR and secreted factors (i.e.,
IGFBP7), might play a role in melano-
cytic transformation.
References
[1] Davies, H., et al., Nature 417, 949-954 (2002)
[2] Pollock, P.M., et al., Nat. Genet. 33, 19-20
(2003)
[3] Curtin et al., N Engl. J.Med. 353, 2135-2147
(2005)
[4] Curtin et al., J Clin Oncol 24, 4340-4346
(2006)
[5] Polsky, D., et al., Cancer Res. 61, 7642-6
(2001)
[6] Kraehn, G.M., et al., Br J Cancer 84, 72-79
(2001)
[7] Hayflick, L., Exp. Cell Res 37, 614-36 (1965)
[8] Clark, W.J. Jr., et al., J Natl Cancer Inst 81,
1893-904 (1989)
[9] Michaloglou, C., et al., Nature 436, 720-4
(2005)
[10] Gray-Schopfer, V.C., et al., Br. J Cancer 95,
496-505 (2006)
Further references are available from
the author
www.eMagazineBIOforum.com
CONTACT
Meenhard Herlyn DVM, PhD
The Wistar Institute
Philadelphia, PA, USA
Tel.: +1 215 898 3950
Fax: +1 215 898 0980
herlynm@wistar.org
Fig. 1: Progression of melanocytic transformation. Normal melanocytes acquire oncogenic mutations that initiate a transient proliferative burst followed by
entrance into a senescent state (nevi); it is currently unknown whether all oncogene-affected melanocytes enter senescence or a small percentage acquire
additional genetic abnormalities (i.e., mutations, deletions/amplifications) that enable bypass of oncogene-induced senescence. In either event, the acquisi-
tion of those genetic anomalies appears to transform benign nevi into malignant melanoma.
C A N C E R R E S E A R C H
BIOforum Europe 6/2008 • 3
Anexo 1 - Revisão publicada