Stimulation of Combinatorial Expression of Prolactin and Glycoprotein Hormone α-Subunit Genes by Gonadotropin-Releasing Hormone and Estradiol-17β in Single Rat Pituitary Cells during Aggregate Cell Culture

Previously we showed the existence of rat and mouse anterior pituitary cells coexpressing mRNA from two or more hormone genes in which production and/or storage of the corresponding hormones were not detectable. To substantiate a putative function for these cells, we investigated whether these phenotypes were retained during long-term reaggregate cell culture and whether protagonist regulatory factors could expand cell populations expressing particular hormone mRNA combinations. After 4-wk culture and treatments, aggregates were trypsinized and single cells collected by means of a fluo-rescence-activated cell sorter. Hormone mRNAs were detected by single-cell RT-PCR. Combinatorial hormone mRNA expression was retained in culture. Both estradiol (E2) and GnRH (1 nm) markedly augmented the proportion of cells expressing prolactin (PRL) mRNA together with other hormone mRNAs and cells expressing glycoprotein subunit (GSU)-α mRNA together with other hormone mRNAs. GnRH strongly increased the proportion of cells containing αGSU mRNA alone, but E2 did not. GnRH and (E2) affected the expansion of a population (∼20% of all cells) coexpressing PRL and αGSU mRNA without βGSUs. Immunostaining of stored hormone on tissue sections revealed colocalization of PRL and αGSU in the E2- but not in the GnRH-treated cells. The present findings suggest that cells coexpressing different pituitary hormone mRNAs form a distinct population that survives without extrapituitary factors. Their occurrence can be markedly modified by regulatory factors. Certain hormone regimens favor unique coexpressions distinctly at mRNA and protein level. These peculiar characteristics support the notion that combinatorial expression of hormone genes in the pituitary serves a biological role.