Phospholipase C (EC 3.1.4.11): a malignancy linked signal transduction enzyme.

AIM

To elucidate the relationship between phospholipase C, phosphatidylinositol-4,5-bisphosphate (PIP2) phosphodiesterase, EC 3.1.4.11 (PLC) and transformation, progression and proliferation, steady-state activity was determined in a spectrum of transplantable solid rat hepatomas of different growth rates, and in a variety of normal rat organs.

METHODS

To measure PLC activity in the plasma membrane fraction a standard isotopic method was developed using exogenous PIP2 as substrate. PLC activity was linear with time for 2.5 min and proportional with protein concentrations over a range of 5 to 29 micrograms per 50 microliters reaction mixture.

RESULTS

The apparent K(m) for PIP2 was 0.3 mM in both normal liver and rapidly growing hepatoma 3924A. PLC activity in adult rat liver was 747 and 986 nmol/h/mg protein; it increased 1.7- to 2.1-fold in hepatomas of slow and intermediate growth rates and 3.7-fold in rapidly growing hepatoma 3924A. By contrast, no significant difference was observed between 24 hours regenerating and sham-operated livers. PLC activity was high in thymus, spleen, testis, bone marrow, lung, brain, heart and renal cortex, but in the same range in skeletal muscle and liver. PLC has a relatively long half-life as there were no significant changes in PLC specific activity in bone marrow during the 9 hour period after intraperitoneal injection of the protein biosynthetic inhibitor, cycloheximide. PLC was subject to nutritional regulation because at 3 days' starvation the specific activity in rat bone marrow decreased to 78% of control values and recovered after 1-day refeeding.

CONCLUSION

The results indicate that PLC is a transformation- and progression-linked signal transduction enzyme. This work and recent studies showing increased 1-phosphatidylinositol 4-kinase (EC 2.7.1.67) and 1-phosphatidylinositol 4-phosphate-5-kinase (EC 2. 7. 1. 68) provide evidence of a gain in function for a markedly elevated capacity of signal transduction sequence in transplantable rat hepatomas of different growth rates. Inhibition of the activities of kinases and PLC may provide sensitive targets for selective cancer therapy by down-regulating the up-regulated signal transduction cascade.