Glucocorticoid hormones (cortisol in man, corticosterone in rodent; CORT) are vital for maintaining normal homeostasis in multiple systems ((e.g., the cardiovascular, metabolic, and immune systems), and for an optimal response to acute and chronic stress. Plasma CORT is released from the adrenal zona fasciculata (ZF) in response to circulating levels of pituitary-derived adrenocorticotropin hormone (ACTH). ACTH binds to the MC2-receptor on the membrane of ZF cells and rapidly stimulates de novo biosynthesis of CORT by activating cAMP/PKA/CREB-mediated transcription of steroidogenic proteins and in turn CORT synthesis. The release of ACTH and CORT is governed by an ultradian rhythm (in the rat: hourly pulses of hormone over a 24hr-cycle). Our current in vivo studies indicate that phosphorylation and synthesis of the steroidogenic proteins in the rat adrenal are also subject to an ultradian rhythm.
To help understand how the body translates the ultradian rhythm of ACTH stimulation to the adrenal gland, we have been studying steroidogenic pathways in the recently developed (and long-awaited) mouse ZF-ATC7 cell line. We have found that hourly pulses of 10 nM ACTH are able to stimulate CREB-phosphorylation (pCREB) and steroidogenic gene transcription in ATC7 cells in the same dynamic manner to that observed in vivo. In contrast, when the same dose of ACTH is applied as a constant stimulus, the pCREB and steroidogenic gene transcriptional response is prolonged and exaggerated. In addition, when a large stress-dose of ACTH (100 nM) is applied after these treatment regimes, a significant increase in pCREB is only achieved in cells that have been exposed to pulsatile, rather than constant, ACTH. Together this supports our in vivo observations that pulsatile ACTH is important for the optimal responsiveness of the adrenal, and that ATC7 cells are a suitable tool to investigate the regulation of CORT ultradian rhythm in vitro.