Changes in polyphosphoinositide levels in rat liver nuclei in response to prolactin, a known hepatic mitogen

The effect of prolactin action on nuclear polyphosphoinositide synthesis was investigated in isolated rat liver nuclei. An increased uptake of phosphate from [γ32P]adenosinetrisphosphate was observed in both phosphatidylinositol 4-phosphatet and phosphatidylinositol 4,5-biphosphate with a maximum response at 10−12 M concentration of hormone. Pulse-chase experiments in isolated nuclei following prolactin treatment indicate that the observed increase in accumulation of phosphatidyloinositol 4-phosphate and phosphatidyl-inositol 4,5-biphosphate is mainly due to a decrease in their rate of turnover possibly induced by a change in activity of polyphosphoinositide-specific monesterases. In vitro prolactin also reduces the activity of nuclear phospholipase C specific for phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-biphosphate. Moreover, this feature is stronly supported by the concomitant decrease in nuclear diacylglycerol mass. Thus these data suggest that once prolactin reaches the nucleus an intranuclear signalling is evoked through inositol lipid metabolism.