Two protein kinase-inhibitors, 6-dimethyl amino purine and 2-amino purine inhibited induction of β-casein synthesis by prolactin when added to the culture medium of rabbit mammary explant and cells. The accumulation of the mRNA for αs1- and β-caseins and for whey acidic protein did not take place in the presence of the inhibitors whereas β-actin mRNA concentration was not altered. In the same experimental conditions, H7, an inhibitor of protein kinase C and, to a lower extent, of protein kinase A did not prevent prolactin from acting. These data suggest for the first time that specific protein kinases are involved in the transduction of the prolactin signal to milk protein genes.

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