Endocrine Abstracts

ACTH is the product of proteolytic processing of its precursor, proopiomelanocortin (POMC) and, in turn, is subject to proteolytic degradation. Regulation of proteolysis is crucial to several intracellular processes, including protein activation and inactivation, and thus contributes to active peptide concentrations. The proteosome 26S/ubiquitin system (UPS) is the main, non-lisosomial intracellular proteolytic pathway and alterations to this system have been observed in both neoplastic and degenerative disorders.

Aim of the present study was to evaluate the role of UPS in ACTH turnover regulation within pituitary corticotropes.

Methods: Rat anterior pituitary primary cell cultures were incubated with MG132, an UPS inhibitor, at increasing concentrations (0.001–100 nM) for up to 48 h. Control incubations were performed with plain medium only. Medium and cell content was collected after 4, 24 and 48 h and ACTH quantified by IRMA and western blotting.

Results: Incubation with MG132 was associated with an increase in ACTH secretion at 4 h (0.01 nM: 177±27.5% control and 1 nM: 129.8±18.9% control, both P<0.05 vs control wells) and 24 h (0.01 nM: 131.6±8.1% control, P<0.05 vs control wells). Likewise, an increase in ACTH content was observed after 24 h incubation with MG132 (0.01 nM: 148±33% control, 1 nM: 187±33% control, 100 nM: 137.1±11.3% control, all P<0.05 vs control wells). Western blotting confirmed quantitative data. No significant changes in either ACTH secretion or cell content were observed after 48 h incubation with MG132.

Conclusions: This study demonstrates that the proteosome/ubiquitin system modulates ACTH concentrations within anterior pituitary cells. Further, these results indicate that regulation of ACTH proteolytic degradation contributes to corticotrope secretory activity.