Induction of casein synthesis by prolactin and inhibition by progesterone in the pseudopregnant rabbit treated by colchicine without any simultaneous variations of casein mRNA concentration.

Prolactin injected into pseudopregnant rabbits induced casein synthesis and it provoked an accumulation of casein mRNA and of ribosomal RNA. Colchicine, which has been shown to block the prolactin signal, totally prevented the accumulation of beta-casein mRNA, when injected with the hormone. However, the drug did not hamper the initiation of casein synthesis and the accumulation of ribosomal RNA. The effect of prolactin injected with colchicine on casein synthesis was totally abrogated by progesterone administered simultaneously and it was essentially unmodified by glucocorticoids. These results suggest that prolactin controls casein gene expression by supporting the accumulation of casein mRNAs and by stimulating the translation of these mRNAs, through independent mechanisms. Progesterone which is known to prevent the accumulation of casein mRNAs is also a potent inhibitor of casein mRNA translation whereas glucocorticoids exert their effect essentially by favouring the accumulation of casein mRNA but not by modifying their translation efficiency.