INHIBITION OF PROTEIN-KINASE-C PREVENTS ASBESTOS-INDUCED C-FOS AND C-JUN PROTOONCOGENE EXPRESSION IN MESOTHELIAL CELLS

Asbestos and the phorbol ester tumor promoter, 12-O-tetradecanoylphorb ol-13-acetate (TPA), increase c-fos and c-jun mRNA levels and AP-1 DNA binding activity in rat pleural mesothelial (RPM) cells, a target cel l of asbestos-induced mesotheliomas (N. H. Heintz et al., Proc. Natl. Acad, Sci, USA, 90: 3299-3303, 1993), Because protein kinase C (PKC) i s the intracellular receptor of phorbol ester tumor promoters and asbe stos is a putative tumor promoter in the respiratory tract, we hypothe sized that PKC might play a critical role in asbestos-induced cell sig naling pathways associated with regulation of proto-oncogenes. Using a panel of PKC antibodies, we identified PKC alpha as the major PKC iso zyme in RPM cells, We then pretreated cells with phorbol ester dibutyr ate to down-modulate PKC or with calphostin C, a specific PKC inhibito r, to determine if depletion of PKC alpha could block asbestos-induced c-fos/c-jun expression, Quantitation of Northern blots showed that fi ber-associated c-fos/c-jun mRNA levels were significantly lower either after PKC alpha down-modulation or pretreatment with calphostin C. In addition, to determine whether tyrosine kinases also were involved in proto-oncogene activation by asbestos, tyrphostin AG82 or herbimycin A was added to RPM cells before exposure to asbestos, These inhibitors decreased crocidolite-induced c-fos but not c-jun levels, suggesting that tyrosine kinases have different regulatory roles in asbestos-indu ced c-fos versus c-jun signaling pathways, The ability to block induct ion of asbestos-induced proto-oncogene expression using pharmacologica l intervention may be important in prevention and treatment of asbesto s-induced proliferative diseases including lung cancers, mesothelioma, and pulmonary fibrosis.