MEMBRANE SIGNAL TRANSDUCTION IN TUMOR PROMOTION

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The goal of these studies is to determine the required biochemical events that occur between tumor promoter-receptor interaction and the activation of effectors of neoplastic transformation. Candidate second messengers include protein phosphorylation, reactive oxygen generation, and calcium mobilization. Both activation of protein kinase C (PKC) and the subsequent loss of PKC activity may be on the signal transduction pathway for 12-0- tretradecanoylphorbol-13-acetate (TPA)-promoted transformation. A C-kinase substrate of 80 kDa has been found to be differentially phosphorylated in P-, P+, and neoplastically transformed JB6 cells, with little or no phosphorylated 80-kDa phosphoprotein (pp80) seen in transformed cells. This pp80 is postulated to be a tumor suppressor. Pharmacological analogs of calcium, the lanthanides, promote neoplastic transformation in JB6 cells by a PKC-independent pathway. The lanthanides, like phorbol esters, induce transformation in (activated) pro-1- or pro- 2-transfected P- cells. This indicates that tumor promoters can collaborate with activated pro genes to bring about neoplastic transformation by either PKC-dependent or PKC-independent pathways. The synthesis of nuclear proteins of 15 and 16 kDa is TPA inducible in P+, but no in P- cells, an event that may account, in part, for the promotion sensitivity of P+ cells. Finally, P+ and P- cells differ in a transient, TPA-stimulated focus-associated expression of cellular P21 H-ras and an irreversible change in actin configuration, suggesting a possible collaboration of cytoskeletal, cytoplasmic and nuclear proteins with activated pro genes to bring about transformation.