Phosphorylation of nuclear phospholipase C beta 1 by extracellular signal-regulated kinase mediates the mitogenic action of insulin-like growth factor I

It is well established that a phosphoinositide (PI) cycle which is operatio nally distinct from the classical plasma membrane PI cycle exists within th e nucleus, where it is involved in both cell proliferation and differentiat ion. However, little is known about the regulation of the nuclear PI cycle. Here, we report that nucleus-localized phospholipase C (PLC) beta1, the ke y enzyme for the initiation of this cycle, is a physiological target of ext racellular signal-regulated kinase (ERK). Stimulation of Swiss 3T3 cells wi th insulin-like growth factor I (IGF-I) caused rapid nuclear translocation of activated ERK and concurrently induced phosphorylation of nuclear PLC be ta1, which was completely blocked by the MEK inhibitor PD 98059, Coimmunopr ecipitation detected a specific association between the activated ERK and P LC beta1 within the nucleus. In vitro studies revealed that recombinant PLC beta1 could be efficiently phosphorylated by activated mitogen-activated p rotein kinase but not by PKA, The ERK phosphorylation site was mapped to se rine 982, which lies within a PSSP moth located in the characteristic carbo xy-terminal tail of PLC beta1. In cells overexpressing a PLC beta1 mutant i n which serine 982 is replaced by glycine (S982G), IGF-I failed to activate the nuclear PI cycle, and its mitogenic effect was also markedly attenuate d. Expression of S982G was found to inhibit ERK-mediated phosphorylation of endogenous PLC beta1. This result suggests that ERK-evoked phosphorylation of PLC beta1 at serine 982 plays a critical role in the activation of the nuclear PI cycle and is also crucial to the mitogenic action of IGF-I.