Mammary explants from 14-15-d-pregnant mice synthesize and secrete milk proteins in culture in response to insulin, hydrocortisone, and prolactin. Here we demonstrate that transforming growth factor beta (TGF beta) treatment suppresses, in a dose dependent and reversible manner, the ability of explants to synthesize and secrete milk caseins. TGF beta does not affect the level of casein mRNA within explants but inhibits casein synthesis posttranscriptionally. We also show increased expression of TGF beta 2 and TGF beta 3 in intact mammary gland as pregnancy progresses, with reduced expression of all three TGF betas at the onset of lactation. These findings suggest that endogenously produced TGF beta may limit the accumulation of milk caseins that are produced in the mammary gland during pregnancy.

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