RESUMEN
Extracellular matrix and growth factors cooperate to regulate signaling pathways and gene transcription in adherent cells. However, the mechanism of extracellular matrix signaling is poorly defined. In mammary gland, the expression of milk protein genes is controlled by cross-talk between signals derived from the basement membrane protein, laminin, and the lactogenic hormone, prolactin. Signals from basement membrane are transduced by beta1 integrins and are required for prolactin to activate DNA binding of the milk protein gene transcription factor, Stat5. Here we show that basement membrane is necessary for tyrosine phosphorylation of the prolactin receptor and thus directly affects cytokine signaling and differentiation at the level of the plasma membrane. Prolactin does not induce tyrosine phosphorylation of its receptor, Jak2, or Stat5 in nondifferentiated breast epithelia cultured on collagen I, and we show that this is due to a vanadate-sensitive activity that inhibits the prolactin pathway. We suggest that protein-tyrosine phosphatases are novel targets for regulation by extracellular matrix and in mammary cells represent an additional control to the requirement of integrins for milk protein production.