THE CO-MITOGENIC COMBINATION OF TRANSFORMING GROWTH-FACTOR BETA(1) AND BOMBESIN PROTECTS PROTEIN-KINASE C-DELTA FROM LATE-PHASE DOWN-REGULATION, DESPITE SYNERGY IN DIACYLGLYCEROL ACCUMULATION

Bombesin induces the down-regulation of protein kinase C-delta (PKC-de lta) and PKC-epsilon in Swiss 3T3 cells. Simultaneous addition of tran sforming growth factor beta(1) (TGF beta(1)) selectively blocks PKC-de lta down-regulation at mid-S-phase, whereas PKC-epsilon levels continu e to decline. Northern blot analysis shows that PKC-epsilon levels cou ld be controlled in part at the level of transcription; PKC-delta mRNA levels remained constant at these later times. Bombesin induces a sus tained elevation of some species of diacylglycerol (DAG), consistent w ith the observed loss of PKC-delta and PKC-epsilon. Interestingly, the combination of bombesin and TGF-beta(1) produces an even greater DAG response. Flow cytometric analysis demonstrates that bombesin induces only 15% of the cells to enter the cell cycle, in contrast to the comb ination of TGF beta(1) plus bombesin which induces 75-80% of the cells to progress through the cycle. The protection of PKC-delta from down- regulation under conditions of sustained DAG elevation correlates with the mitogenic response and implies that the down-regulation process i tself is regulated. Consistent with this, it is demonstrated that bomb esin plus TGF beta(1) protects PKC-delta from phorbol ester-induced do wn-regulation.