Activation of the Integrated Stress Response and Metabolic Dysfunction in a Murine Model of Sleep Apnea
Intermittent hypoxia (IH) induces activation of the integrated stress response (ISR), but its role in IH-induced visceral white adipose tissue (vWAT) insulin resistance is unknown. CHOP is activated by chronic ISR, whereas GADD34 dephosphorylates the subunit of translation initiation factor 2 (eIF2α), leading to termination of the ISR. We hypothesized that CHOP/Gadd34 null mice would not manifest evidence of insulin resistance after IH exposures. Eight-week-old CHOP/GADD34−/− (double mutant [DM]) and wild-type (WT) littermates were randomly assigned to IH or room air (RA) exposures for 6 weeks. Glucose and insulin tolerance tests were performed, and regulatory T cells (Tregs) and macrophages in vWAT were assessed. Phosphorylated eIF2α:total eIF2α, ATF4, XBP1 expression, and insulin-induced pAKT/AKT expression changes were examined in vWATs. Single GADD34−/− and PERK+/− mice were also evaluated. Body weight and vWAT mass were reduced in DM and WT mice after IH. M1/M2 macrophages and inflammatory macrophages (Ly-6chigh) were significantly increased in WT vWAT but remained unchanged in DM mice. Tregs were significantly decreased in WT vWAT but not in DM mice. Systemic insulin and glucose tolerance tests revealed insulin resistance in IH-WT but not in IH-DM mice. Similarly, decreased pAKT/AKT responses to exogenous insulin emerged in IH-WT compared with RA-WT mice, whereas no significant differences emerged in IH-DM compared with DM-RA. Chronic ISR activation appears to contribute to the insulin resistance and vWAT inflammation that characteristically emerge after long-term IH exposures in a murine model of obstructive sleep apnea.