Role of cell-cell adhesion in the regulation of prolactin gene expression by extracellular CaCl(2)

We have investigated a role for calcium-dependent cell-cell adhesion in the regulation of prolactin gene expression in rat pituitary GH(3) cells. Cells cultured in a calcium-free, serum-free medium (SFM) express low levels of prolactin and growth hormone mRNA. As expected, addition of 0.5 mM CaCl(2) to GH(3) cells in SFM produced a specific, severalfold increase in prolactin mRNA levels. CaCl(2) also promoted intercellular adhesion, during which cells assembled end-to-end in to cords. Prolactin mRNA increased after a delay of several hours. This latency period ranged from 4-12 h among different experiments, but always occurred after the onset of cell-cell adhesion. The voltage-sensitive calcium channel (VSCC) blocker, nitrendipine, inhibited the CaCl(2)-induced increase in prolactin mRNA without affecting cord formation. However, the VSCC agonist, BAY K-8644, was unable to induce prolactin gene expression prior to the onset of intercellular adhesion at 8 h, even though it produced a cellular response (tyrosine phosphorylation of a ca. 130-kDa protein) within 30 min. Blocking cell-cell adhesion inhibited the calcium-dependent induction of prolactin gene expression. Low levels (0.0025-0.02%) of trypsin blocked cell-cell adhesion and the prolactin mRNA induction by CaCl(2) without affecting the levels of other mRNAs or cell-matrix adhesion. Heparin also specifically blocked the induction of both cell-cell adhesion and prolactin gene expression. Based on these data, we propose a role for both VSCCs and calcium-dependent cell-cell adhesion in the induction of prolactin gene expression by extracellular CaCl(2).