[Comparison of the mechanism of action of insulin and prolactin].

One of the major actions of prolactin in the mammary cell is to activate the expression of casein genes by enhancing the transcription rate of the genes. Anti-prolactin receptor antibodies can mimic prolactin action when added to mammary cells, suggesting that a relay is formed at the membrane level and transferred to the target genes. None of the classical hormone intracellular relays can account for the transfer of the prolactin message. The incubation of membranes from various tissues containing prolactin receptor with prolactin provokes the release of a factor which specifically accelerates the transcription of the beta-casein gene in isolated mammary nuclei. The factor is thermostable, inactivated by trypsin and is eluted from G-10 Sephadex as a molecule smaller than 1000 daltons. The action of the factor is prevented by phosphatase inhibitors. These results are strikingly reminiscent of those obtained with insulin which activates enzymes via a factor released from the membranes and which dephosphorylates the enzymes. The analogy between the action mechanism of prolactin and insulin is discussed.