SUPPRESSION OF PHOSPHOLIPASE-C BETA-FAMILY, GAMMA-FAMILY, AND DELTA-FAMILY ALTERS CELL-GROWTH AND PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE LEVELS

Phosphatidylinositol-specific phospholipase C (PLC) activity reflects a summation of the activities of three families, beta, gamma, and delt a, each of which is regulated differently. In order to understand the contribution of each family to cell proliferation signaling, expressio n of each family was suppressed by use of an inducible expression vect or for antisense PLC sequences in a single cell line, FTO-2B rat hepat ocytes. Activation of second messengers of PLC [diacylglycerol (DAG) a nd inositol 1,4,5-tris-(phosphate) (IP3)] was dramatically reduced, pr oviding a strategy for probing the consequences of PLC deficiency on c ell function. Importantly, while one PLC family was suppressed, the ot her PLCs actively responded to specific stimuli, suggesting parallel a nd independent signaling pathways for each PLC family in FTO-2B cells. Selective suppression of each PLC family altered cell growth markedly and differentially. The rank order for suppression of cell growth by loss of a PLC family was gamma > delta > beta. Exploration of down-str eam growth regulators revealed that loss of beta and gamma, but not de lta, families was associated with markedly reduced basal ms and protei n kinase C activity. Moreover, suppression of each of the three PLC fa milies caused remarkably reduced basal and stimulated MAP kinase activ ities. Interestingly, cellular levels of PIP2 were increased and drama tically correlated with growth inhibition rate in the clones with supp ressed PLC activity, suggesting that PIP2 itself can serve as a second messenger of cell growth regulation.