Effects of Glucocorticoids on Casein Gene Expression in the Rabbit

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Abstract
Milk protein synthesis is initiated by prolactin and a glucocorticoid. In the rabbit, prolactin alone is sufficient. Glucocorticoids potentiate the action of prolactin. The stimulatory effect of glucocorticoids was evaluated after injections of hydrocortisone acetate alone or associated with prolactin by measurements of the total RNA and DNA content of mammary glands, the lactose synthetase [EC 2.4.1.22] activity, casein synthesis and the concentration of casein mRNA in total cellular RNA and in polysomal RNA by hybridization with its cDNA [cytoplasmic DNA]. The glucocorticoid, totally inactive alone, had a stimulatory effect proportional to the dose injected when prolactin was present. This effect was more evident with low doses of prolactin. Glucocorticoids proceeded by amplifying the capacity of prolactin to enhance the concentration of casein mRNA available for translation. A parallel effect of glucocorticoids on translation of casein mRNA was suspected. Glucocorticoids injected with low doses of prolactin were unable to mimic all the effects of high doses of prolactin alone.