Calcium regulation of prolactin gene expression: opposing effects of extracellular CaCl2 and Ca2+ ionophores.

Previous studies have demonstrated that the high basal level of transcription of the rat PRL gene in pituitary tumor GH3 cells is dependent on [CA2+]e. In the present study, we have extended these findings by examining the effects of the Ca2+ ionophores, A23187 and ionomycin, on [Ca2+]i, and on PRL mRNA levels and glucose-regulated protein (GRP) mRNA levels in GH3 cells cultured in a low Ca2+, serum-free medium (SFM). Using digital imaging microscopy of individual Fura 2-loaded GH3 cells in SFM plus 0.4 mM CaCl2, extranuclear and nuclear [Ca2+] were both about 70 nM. Addition of 600 nM ionomycin increased these levels by 10-fold within minutes, and by about 45-fold after 120 min. As previously published, addition of 0.4 mM CaCl2 to GH3 cells cultured in SFM significantly increased PRL mRNA, and had little or no effect on GRP78 and GRP94 mRNA after 16 h. Addition of 0.4 mM CaCl2 plus 100 nM A23187 significantly increased GRP78 and GRP94 mRNA. Surprisingly, the Ca2+ ionophore significantly inhibited PRL gene expression below that obtained in 0.4 mM CaCl2 without A23187. This same pattern of stimulation of GRP78 gene expression, but inhibition of PRL gene expression, was observed with 125 and 600 nM ionomycin. Both Ca2+ ionophores had no effect on histone 3 mRNA, and A23187 depressed PRL gene expression at a concentration (50 nM) that did not affect protein synthesis. Although A23187 reproducibly lowered PRL mRNA levels, it slightly inhibited its degradation in cells in which RNA synthesis was blocked by actinomycin D.(ABSTRACT TRUNCATED AT 250 WORDS)