Effects on synthesis of milk proteins

Among its effects on the epithelial cells of the mammary alveoli prolactin stimulates the production of the major milk proteins casein and lactalbumin [64,66-69,74-76]. Mouse mammary explants incubated in vitro with insulin and a corticosteroid produce very little of these proteins, but addition of prolactin to the culture medium induces their synthesis 10-20-fold within about 24 h. As has already been discussed, there is some uncertainty about the importance of glucocor- 

Induction of milk proteins appears to involve increased transcription of the appropriate genes. Levels of mRNA for casein increase substantially [66,69,74-77] (Fig. 6), though to a lower extent than the increase in casein production, suggesting that an increased rate of translation of mRNA (or possibly decreased protein degradation) may also be important. The increased levels of mRNA may be partly due to decreased degradation as well as increased transcription [74]. 

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In addition to its effects on synthesis of milk proteins themselves, prolactin also induces the production in the mammary gland of other proteins/enzymes involved in production of milk components. Lactose synthetase and enzymes of lipid metabolism, including acetyl-CoA carboxylase and lipoprotein lipase, are among these inducible enzymes [75]. 

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