[Phosphorylation of adrenal cortex proteins by protein kinase C induced by prolactin]

The in vitro effect of prolactin (PRL) on protein phosphorylation in male guinea pig adrenal cortex has been studied. It was found that 60 min after addition of PRL (10 micrograms/ml) to slices of adrenocortical tissue the rate of protein phosphorylation increased considerably. Autoradiographic studies revealed that at least part of the proteins are substrates for protein kinase C, since their phosphorylation increased in the presence of specific activators of protein kinase C (Ca2+, diacylglycerol, phosphatidyl serine) in the incubation medium. The molecular masses of these proteins were 95, 93, 90, 12 and 10 kDa. In extracts of PRL-treated adrenocortical tissue phosphorylation of these proteins was especially well-pronounced. Addition of staurosporine (10 nM) during incubation of the slices abolished the PRL effect on protein phosphorylation without any effect on the rate of unstimulated phosphorylation. Taking into account the ability of isolated nuclei to respond to PRL by activation of protein kinase C, the time course of PRL stimulation of protein phosphorylation was studied using preparations of isolated nuclei. The maximal increase of 32P incorporation into the proteins from [gamma-32P]ATP was observed after 60 min. Staurosporine only slightly attenuated the PRL effect.