Growth Factors and Tyrosine Protein Kinases in Normal and Malignant Melanocytes

Melanomas are highly variable with respect to aberrant gene expression and chromosomal lesions but share a common characteristic of an acquired independence from environmental growth factors that are needed for proliferation of normal melanocytes. Receptors with tyrosine kinase activity play a critical role in normal melanocyte proliferation and in the uncontrolled growth of melanomas. Normal human melanocytes depend on exogenous peptide growth factors such as basic fibroblast growth factor (bFGF), hepatocyte growth factor (HGF), or mast cell growth factor (MGF), all of which stimulate receptors with tyrosine kinase activity. In contrast, human melanoma cells from primary nodular and metastatic lesions grow autonomously partially because of inappropriate production of bFGF and continuous activation of the bFGF-receptor kinase. Animal models also provide evidence for the importance of receptor-tyrosine kinases in normal melanocyte proliferation and in malignant transformation. In the mouse, genes residing in three loci in which inactivation mutations lead to piebaldism, the dominant spotting (W), patch (Ph), and Sl encode, respectively, the receptor-kinases c-kit and platelet derived growth factor receptor, and the ligand for c-kit: MGF. In vivo transformation of mouse melanocytes to melanoma, due to constitutive expression of a transmembrane tyrosine kinase, the oncogene ret, was recently demonstrated in transgenic mice. Studies on a fish model, Xiphophorus, in which melanoma is inherited, showed that the dominant tumor inducing gene, Tu, encodes an EGF-receptor related tyrosine kinase which is expressed only in melanomas and not in normal tissues. Taken together, the results suggest that the uncontrolled growth of melanomas is due, in large part, to constitutive activation of receptors with tyrosine kinase activity.