Effects of triiodothyronine, dexamethasone and estradiol-17 beta on GH mRNA in rat pituitary cells in culture as revealed by in situ hybridization.

The GH lines of rat pituitary tumour cells have been largely used to study the regulation of GH mRNA. In order to investigate the role of T3, dexamethasone and estradiol-17 beta on GH expression in non-tumoural pituitary cells, we have used in situ hybridization techniques performed on rat anterior pituitary cells in monolayer culture. The amounts of mRNA encoding for GH, as evaluated by counting the number of grains per somatotrope, were markedly reduced after 4 days of culture in a steroid-free medium supplemented with an hypothyroid calf serum. Addition of T3 or dexamethasone for 3 days increased GH mRNA levels. The concomitant administration of the two hormones produced a synergistic effect on GH mRNA levels which became higher than those observed after T3 or dexamethasone administration alone. However, this effect did not restore GH mRNA levels to those measured in monolayer pituitary cells grown in medium containing 10% fetal calf serum. Moreover GH mRNA levels appeared higher in male than in female pituitary cells. The administration of E2 to pituitary cell cultures from both male and female rats produced an increase by 15, and 12.8% in GH mRNA levels in male and female, respectively. This stimulatory effect of E2 in cell culture was competitively blocked by simultaneous incubation with the antiestrogen LY156758 (Keoxifene). These results demonstrate that T3, dexamethasone as well as E2 act directly on somatotropic cells to regulate GH gene expression.