Phosphoinositide 3-kinase in rat liver nuclei.
Biochemical and immunochemical data from the present investigation reveal the existence of a p85/p110 phosphoinositide 3-kinase (PI 3-kinase) in rat liver nuclei. 32P-Labeling of membrane phosphoinositides by incubating intact nuclei with [gamma-32P]ATP results in the formation of [32P]phosphatidyl-inositol 3,4, 5-trisphosphate [PtdIns(3,4,5)P3], accompanied by small quantities of [32P]phosphatidylinositol 3-phosphate [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-metabolizing enzymes, including phospholipase C (PLC), phosphoinositide phosphatase, and diacylglycerol (DAG) kinase. As a result, exposure of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] to the nuclear extract in the presence of [gamma-32P]ATP generates a series of 32P-labeled D-3 phosphoinositides and phosphatidic acid (PA) in an interdependent manner. On the basis of the immunological reactivity and kinetic behavior, 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 the cell. Moreover, we test the premise that nuclear PI 3-kinase may, in part, be regulated through the control of substrate availability by PtdIns(4,5)P2-binding proteins. Effect of CapG, a nuclear actin-regulatory protein, on PI 3-kinase activity is examined in view of its unique Ca2+-dependent PtdIns(4, 5)P2-binding capability. In vitro data show that the CapG-mediated inhibition of nuclear PI 3-kinase is prompted by PKC phosphorylation of CapG and elevated [Ca2+]. This CapG-dependent regulation provides a plausible link between nuclear PLC and PI 3-kinase pathways for cross-communications. Taken together, these findings 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 transducing signals from the plasma membrane to the nucleus in response to diverse physiological stimuli.
Lu
P J
PJ
Division of Medicinal Chemistry and Pharmaceutics, College of Pharmacy, University of Kentucky, Lexington 40536, USA.
Hsu
A L
AL
Wang
D S
DS
Yan
H Y
HY
Yin
H L
HL
Chen
C S
CS
D000818:Animals; D001692:Biological Transport; D002467:Cell Nucleus / Q000201:enzymology* / Q000276:immunology / Q000648:ultrastructure; D015151:Immunoblotting; D007425:Intracellular Membranes / Q000201:enzymology; D008099:Liver / Q000201:enzymology* / Q000276:immunology / Q000648:ultrastructure; D016253:Microscopy, Immunoelectron; D009687:Nuclear Proteins / Q000276:immunology / Q000378:metabolism* / Q000648:ultrastructure; D019869:Phosphatidylinositol 3-Kinases / Q000276:immunology / Q000378:metabolism* / Q000648:ultrastructure; D010716:Phosphatidylinositols / Q000378:metabolism; D010766:Phosphorylation; D051381:Rats
D009687:Nuclear Proteins; D010716:Phosphatidylinositols; D019869:Phosphatidylinositol 3-Kinases