Transforming growth factor-beta (TGF beta) is important in the maturation and function of the mammary gland and is present in milk. We have examined whether, in addition to inhibiting lactogenesis, TGF beta exerts acute regulatory effects on lactating mammary cells. The isoform TGF beta 1 at 5 and 50 ng/ml suppressed the onset of lactation and the subsequent production of beta-casein by differentiating mouse mammary explants from pregnant mice. By contrast, it did not inhibit protein synthesis or secretion from acini isolated from lactating-mouse mammary gland or protein secretion from explants from lactating mice. These data indicate that TGF beta inhibits the onset of casein secretion, but is not an acute regulator of casein synthesis or secretion from differentiated lactating mammary cells.

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