PHOSPHOINOSITIDE 3-KINASE IN RAT-LIVER NUCLEI

Biochemical and immunochemical data from the present investigation rev eal the existence of a p85/p110 phosphoinositide 3-kinase (PI 3-kinase ) in rat liver nuclei. P-32-Labeling of membrane phosphoinositides by incubating intact nuclei with [gamma-P-32]ATP results in the formation of [P-32]phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P-3], accompanied by small quantities of [P-32]phosphatidylinositol 3-phosp hate [PtdIns(3)P]. Studies with subnuclear fractions indicate that the PI 3-kinase is not confined to nuclear membranes. The nuclear soluble fraction also contains PI 3-kinase and an array of inositide-metaboli zing enzymes, including phospholipase C (PLC), phosphoinositide phosph atase, and diacylglycerol (DAG) kinase. As a result, exposure of phosp hatidylinositol 4,5-bisphosphate [PtdIns(4,5)P-2] to the nuclear extra ct in the presence of [gamma-P-32]ATP generates a series of P-32-label ed D-3 phosphoinositides and phosphatidic acid (PA) in an interdepende nt manner. On the basis of the immunological reactivity and kinetic be havior, the nuclear PI 3-kinase is analogous, if not identical, to PI 3-kinase alpha, and constitutes about 5% of the total PI 3-kinase in t he cell. Moreover, we test the premise that nuclear PI 3-kinase may, i n part, be regulated through the control of substrate availability by PtdIns(4,5)P-2-binding proteins. Effect of CapG, a nuclear actin-regul atory protein, on PI 3-kinase activity is examined in view of its uniq ue Ca2+-dependent PtdIns(4,5)P-2-binding capability. In vitro data sho w that the CapG-mediated inhibition of nuclear PI 3-kinase is prompted by PKC phosphorylation of CapG and elevated [Ca2+]. This CapG-depende nt regulation provides a plausible link between nuclear PLC and PI 3-k inase pathways for cross-communications. Taken together, these finding s provide definite data concerning the presence of an autonomous PI 3- kinase cycle in rat liver nuclei. The nuclear location of PI 3-kinase may lead to a better understanding regarding its functional role in tr ansducing signals from the plasma membrane to the nucleus in response to diverse physiological stimuli.