GROWTH REGULATION OF HUMAN MELANOMA: FACTORS INVOLVED IN THE EXPRESSION OF THE TRANSFORMED PHENOTYPE (SOFT AGAR, GROWTH FACTORS, PLATELETS, ENDOTHELIAL CELLS, PARACRINE).

Abstract

Cellular transformation is accomplished in vitro through the concerted action of growth factors and oncogenes. This association has demonstrated that malignant growth results from aberrations in pathways that normally operate to control proliferation. Activation of genes that code for growth factors, their receptors, and/or molecules essential in the transduction of signals from the cell surface to the nucleus are all potential mechanisms by which tumor cells could establish a selective growth advantage over normal cells. This dissertation addresses the question of what oncogenic mechanisms are important in the development and progression of human melanoma. These studies show that melanoma growth is regulated by endogenous substances produced by the melanoma cells themselves (autocrine stimulation), as well as by exogenous substances supplied by neighboring cells and platelets (paracrine stimulation). These factors work to drive the expression of the transformed phenotype for melanoma as evidenced by induction of serum-free soft agar growth. Human platelets were found to the the richest source of paracrine growth promoters. The factor from human platelets was characterized and partially purified. Melanoma cells respond to this 60,000 molecular weight, disulfide-bond-containing protein in colony formation assays. In addition, the protein has endothelial cell growth factor activity. Purified fractions which promoted optimal colony formation for human melanoma cells also maximally stimulated monolayer growth of bovine aortic endothelial cells, while melanocytes were nonresponsive. This implies that melanoma cells are expressing receptors for a protein which plays no known or apparent role in the normal growth of melanocytes. Melanoma cells are sensitive to growth regulatory molecules of autocrine and paracrine nature. This dissertation provides clues to the genetic lesions which have occurred in these melanoma cells to influence their proliferation. The aberrations appear to reside in those genes important in growth factor pathways at the level of endogenous production and misguided response to exogenous factors through receptor expression. We can not hope to fully inhibit the proliferation of tumor cells until we identify and understand those forces which drive their growth. These studies have increased our knowledge of those signals which stimulate melanoma cellular proliferation, and thus provide insight into important therapeutic targets.