Regulation of c-fos expression by EGF and GnRH in specific anterior pituitary cells from proestrous female rats

C-fos is an early expression oncogene that can be stimulated by a variety of regulators. It is expressed by subsets of all pituitary cells, with increased expression seen in proestrous rats. However, in freshly dispersed pituitary cells studied during different stages of the cycle, there is limited expression of fos by luteinizing hormone (LH) cells and little basal expression by cells with follicle-stimulating hormone (FSH) antigens. Proestrus is a time during which pituitary gonadotropes express peak levels of receptors for gonadotropin-releasing hormone (GnRH) and epidermal growth factor (EGF). We hypothesized that if GnRH or EGF stimulated fos activity in gonadotropes they would be most effective during the peak expression of their receptors. Anterior pituitaries were removed, cut into small pieces, and stimulated for 30 min. Total RNA was then collected and analyzed by Northern analysis. Both EGF and GnRH caused an increase in c-fos mRNA levels in the anterior pituitary gland compared with unstimulated pituitary glands assayed immediately after removal from the pituitary. However, the stimulatory effects were no greater than those seen with medium alone. This suggested that fos expression could be stimulated by local factors either in the pituitary or the medium itself. The second phase of the study focused on pituitary cells plated for 1 hr and then stimulated with EGF and GnRH for 15 min. Dual immunocytochemistry was done to learn which cell types expressed the fos proteins. After 15 min, EGF and GnRH both increased the percentages of fos-bearing cells above levels seen in medium alone. EGF stimulated fos proteins in subsets of FSH, adrenocorticotropin (ACTH), and growth hormone (GH) cells. GnRH increased fos proteins in subsets of ACTH and GH cells. These results suggest that EGF and GnRH may regulate fos expression, but not necessarily in gonadotropes. They also highlight the need for carefully timed experiments because endogenous factors in the pituitary itself may stimulate immediate early gene expression.