Endocrine Abstracts

The transcriptional activator Bmal1, reaching acrophase in the morning and bathyphase in the evening, is the main regulator of circadian clock. A mutual interaction between circadian clock and glucocorticoids (GCs) has been described as well as their implication in muscle metabolism. The current in vitro study aims at investigating Bmal1 involvement in muscle catabolism induced by GCs circadian exposure. To this aim, mouse myocytes C2C12 were serum-shocked to synchronize clock genes oscillation. Using acrophase and bathyphase of Bmal1, hydrocortisone (HC) administrations were aligned to daytime of human cortisol peaks and nadir. During Bmal1 acrophase, myocytes were exposed to physiological concentration of 450nM and to non-physiological concentrations of 650nM and 750nM HC. During Bmal1 bathyphase, myocytes were exposed to physiological concentration of 150nM and to non-physiological concentrations of 177nM and 300nM HC. Atrogenes circadian expression was measured by RT-qPCR, while mTOR and KLF15 expression by WB. To address Bmal1 involvement in GC-induced muscle catabolism, myocytes Bmal1-knockdown (Bmal1-KD) were performed using siRNA technology and mTOR and KLF15 expression after HC exposures were compared to negative control (NC). PI3K (P100α)-GR/IRS-1 interactions in Bmal1 bathyphase and acrophase as well as in Bmal1-KD and NC myocytes exposed to 300nM HC were monitored by co-immunoprecipitation (Co-IP). Localization of structural protein myosin and of glucocorticoid receptor (GR) were detected by immunofluorescence (IF). At Bmal1 acrophase and bathyphase, no significant change in atrogenes levels was revealed. Conversely, only at Bmal1 bathyphase, 300nM HC significantly inhibited mTORC1 (61.03%; P=0.005 vs 150nM) and pp70S6K (49.80%; P=0.002 vs 150nM) and significantly stimulated KLF15 expression (53.34%; P=0.02 vs 150nM). In Bmal1-KD myocytes, displaying lower Bmal1 expression than NC (65%; P=0.02), 300nM HC significantly decreased mTORC1 (32.62%; P=0.02 vs NC) and significantly increased KLF15 expression (54.20%; P=0.02 vs NC). Co-IP demonstrated that 300nM HC, both in bathyphase and Bmal1-KD myocytes, facilitated PI3K (P110α)-GR interaction (93.88% and 97.36% vs acrophase and NC respectively), simultaneously reducing PI3K (P110α)-IRS-1 (31.01% and 27.58% vs acrophase and NC respectively) interaction. IF for heavy chain myosin revealed that compared to NC, myocytes Bmal1-KD plus 150nM HC showed reduced myotubes formation and nuclear GR expression, whereas Bmal1-KD plus 300nM HC showed reduced myotubes formation, increased nuclear GR expression and myosin co-localization. These data demonstrated that Bmal1 down-regulation contributes to muscle catabolism induced by GCs triggering PI3K (P110α)-GR interaction and inhibiting myotube formation.