Promyelocytic leukaemia zinc finger protein (PLZF) is a glucocorticoid- and progesterone-induced transcription factor in human endometrial stromal cells and myometrial smooth muscle cells.
We have previously characterized PRL production in human myometrial tissue maintained in explant culture. Here we describe PRL gene expression and its regulation in smooth muscle cells isolated from normal human myometrium. Onset of PRL secretion occurred spontaneously after several days in culture and increased over time without exogenous stimulation. PRL secretion could be further simulated by the addition of PGE(2) or relaxin, both of which were also shown to increase cAMP formation in smooth muscle cells. Likewise, treatment with 8-Br-cAMP led to an elevation of PRL secretion. By reverse transcription/polymerase chain reaction, we demonstrate that smooth muscle cells transcribe the PRL gene from the alternative decidual-type dPRL promoter, located upstream of the pituitary promoter. Treatment with PGE(2), relaxin, and 8-Br-cAMP resulted in an increase in dPRL transcript abundancy. The effect of cAMP was transcriptional as shown by the induction of transfected dPRL promoter/reporter gene fusion constructs. A fragment of 332 bp flanking the dPRL transcription start site was sufficient to mediate cAMP inducibility. In parallel with the increase in PRL secretion, we detected an increase in cAMP formation and PGE(2) secretion in cultured smooth muscle cells. We propose the presence of a paracrine positive feedback mechanism that may reflect the physiological situation in vivo where an increase in myometrial adenylate cyclase activity throughout pregnancy has been reported.