Presence and activation of nuclear phosphoinositide 3-kinase C2 beta during compensatory liver growth

Highly purified liver nuclei incorporated radiolabeled phosphate into phosp hatidylinositol 4-phosphate (PtdIns(4)P), PtdIns(4,5)P-2 and PtdIns(3,4,5)P -3. When nuclei were depleted of their membrane, no radiolabeling of PtdIns (3,4,5)P-3 could be detected showing that within the intranuclear region th ere are no class I phosphoinositide 3-kinases (PI3K)s. In membrane-depleted nuclei harvested 20 h after partial hepatectomy, the incorporation of radi olabel into PtdIns(3)P was observed together with an increase in immunoprec ipitable PI3K-C2 beta activity, which is sensitive to wortmannin (10 nM) an d shows strong preference for PtdIns over PtdIns(4)P as a substrate. On Wes tern blots PI3K-C2 beta revealed a single immunoreactive band of 180 kDa, w hereas 20 h after partial hepatectomy gel shift of 18 kDa was noticed, sugg esting that observed activation of enzyme is achieved by proteolysis. When intact membrane-depleted nuclei were subjected to short term (20 min) expos ure to mu -calpain, similar gel shift together with an increase in PI3K-C2 beta activity was observed, when compared with the nuclei harvested 20 h af ter partial hepatectomy, Moreover, the above-mentioned gel shift and increa se in PI3K-C2 beta activity could be prevented by the calpain inhibitor cal peptin. The data presented in this report show that, in the membrane-deplet ed nuclei during the compensatory liver growth, there is an increase in Ptd Ins(3)P formation as a result of PI3K-C2 beta activation, which may be a ca lpain-mediated event.