Phorbol ester stimulates prolactin release but reduces prolactin mRNA in the human B-lymphoblastoid cell line IM-9-P3

The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) stimulated prolactin (PRL) release from the PRL producing human B-lymphoblastoid cell line IM-9-P3 within 30 min with an EC50 of 5 x 10(-9) M. Increased release was entirely attributable to a loss from intracellular PRL pools. No change in hPRL mRNA was observed during 8 h of exposure to TPA. Prolonged exposure of the cells to 2 x 10(-7) M TPA, however, led to a maximal reduction of hPRL mRNA levels after 24 h and a subsequent recovery by 72 h. Secretory rates followed a corresponding kinetic. The relative abundance of c-myc mRNA was not affected, although a persistent inhibition of cellular proliferation occurred upon chronic exposure to TPA. The addition of dibutyryl cAMP caused a minor transient increase in hPRL secretion by 35% after 1 h.