Modulation of Basal and Corticotropin-Releasing Factor- Stimulated Proopiomelanocortin Gene Expression by Vasopressin in Rat Anterior Pituitary*

Abstract

In several anterior pituitary hormone systems, factors that stimulate hormone release also stimulate hormone biosynthesis. In the corticotropes of the anterior pituitary, CRF stimulates ACTH release as well as cAMP accumulation and transcription of the POMC gene. Arginine vasopressin (AVP) is a well documented coregulator of ACTH release and potentiates CRF-stimulated cAMP accumulation. The present studies were undertaken to determine if AVP also potentiates the early effects of CRF on POMC gene expression in rat anterior pituitary primary cultures. We have measured the levels of the POMC primary transcript, processing intermediate, and mature mRNA in nuclear RNA samples and POMC mRNA in cytoplasmic RNA samples as well as ACTH release after treatment with CRF and/or AVP. After a 30-min treatment with 0.5 nM CRF, POMC primary transcript levels were increased by 200-400%. Thirty-minute or 1-h treatments with AVP alone (10 or 100 nM) did not affect primary transcript levels, but increased the amount of the processing intermediate, while a 2-h incubation with 100 nM AVP significantly decreased POMC primary transcript levels. The effects of CRF on the POMC primary transcript and nuclear processing intermediate were not potentiated by cotreatment with AVP, although a potentiation of CRF-stimulated ACTH release was observed. POMC nuclear and cytoplasmic mRNA levels were not affected by these 2-h or less treatments, while an 18-h incubation with 0.5 nM CRF alone or in combination with 100 nM AVP increased POMC cytoplasmic mRNA levels to about 140% of vehicle-treated control values. When a 1-h AVP treatment (100 nM) preceded presentation of CRF, we observed an attenuation of the stimulation of POMC primary transcript and processing intermediate levels by the 1-h CRF treatment. The effects of CRF on POMC primary transcript and processing intermediate levels in these experiments are consistent with a CRF-induced increase in POMC gene transcription. Our findings that AVP decreased primary transcript levels and attenuated the effects of CRF on this end point suggest that: 1) AVP clearly does not act in a synergistic manner with CRF to stimulate POMC gene expression; and 2) AVP possibly affects RNA stability and/or processing.