Growth factors and melanomas.

Understanding the growth constraints imposed on normal human melanocytes may help to elucidate the processes conferring growth advantage to melanoma cells. Several synergistic growth factors have been identified for normal human melanocytes. They include fibroblast growth factors (FGF), hepatocyte growth factor/scatter factor, mast/stem cell growth factor, and the neuropeptides endothelin-1, 2 and 3 (ET-1, ET-2, ET-3). From this group of peptides, only basic FGF (bFGF/FGF2) appears, so far, to play a role in autonomous growth of melanoma cells. Aberrant expression of FGF2 is due to activation of an otherwise repressed gene by a mechanism that may involve the transcriptional activity of wild-type p53. The growth factors and activated receptors aberrantly expressed in melanoma cells act in concert with molecules that control cell cycle progression. These proteins bind to, and regulate cyclin-dependent kinase (CDK), such as CDK4, responsible for phosphorylation of retinoblastoma (RB) and dissociation of RB-E2F1 inhibitory complexes, thereby allowing progression through the cell cycle. Constitutive CDK4 activity in melanomas may be the results of inactivation of the negative regulators known as CDK inhibitor p16INK4, and/or p21; and/or overexpression of cyclin D, the positive CDK4 regulator. This complex set of changes in melanoma cells can lift growth constraints by inducing unregulated expression of genes promoting transition from GI to S phase of the cell cycle.