Muscle Arnt/Hif1β Is Dispensable in Myofiber Type Determination, Vascularization and Insulin Sensitivity

Abstract

Aryl Hydrocarbon Receptor Nuclear Translocator/ hypoxia-inducible factor 1 beta (ARNT/ HIF1β), a member of bHLH-PAS family of transcriptional factors, plays a critical role in metabolic homeostasis, insulin resistance and glucose intolerance. The contributions of ARNT in pancreas, liver and adipose tissue to energy balance through gene regulation have been described. Surprisingly, the impact of ARNT signaling in the skeletal muscles, one of the major organs involved in glucose disposal, has not been investigated, especially in type II diabetes. Here we report that ARNT is expressed in the skeletal muscles, particularly in the energy-efficient oxidative slow-twitch myofibers, which are characterized by increased oxidative capacity, mitochondrial content, vascular supply and insulin sensitivity. However, muscle-specific deletion of ARNT did not change myofiber type distribution, oxidative capacity, mitochondrial content, capillarity, or the expression of genes associated with these features. Consequently, the lack of ARNT in the skeletal muscle did not affect weight gain, lean/fat mass, insulin sensitivity and glucose tolerance in lean mice, nor did it impact insulin resistance and glucose intolerance in high fat diet-induced obesity. Therefore, skeletal muscle ARNT is dispensable for controlling muscle fiber type and metabolic regulation, as well as diet-induced weight control, insulin sensitivity and glucose tolerance.

Loss of ARNT in skeletal muscle limits muscle regeneration in aging

“…Following activation, there were no differences noted in mouse weight or hind limb muscle mass between the ARNT mKO mice and littermate controls (Supporting Figure S2). The ARNT mKO mice exhibited 90% reduction in the skeletal muscle ARNT expression (Supporting Figure S2), similar to previous reports 13 . ARNT deletion in young mice hindered skeletal muscle regeneration in comparison to similarly treated littermate controls, as indicated by smaller regenerating muscle fibers on DPI 5 ( P < .001) and DPI 10 ( P < .001) in ARNT mKO mice (Figure 5A‐C).…”

“…These findings suggest that the impaired muscle regeneration seen in the ARNT mKO mice was not likely dependent on vascular or perfusion changes within the muscle. Others have similarly demonstrated that constitutive skeletal muscle‐specific knockout of ARNT inhibits hypoxia signaling without limiting vasculogenesis 13 …”

“…The ARNT mKO mice exhibited 90% reduction in the skeletal muscle ARNT expression (Supporting Figure S2), similar to previous reports. 13 ARNT deletion in young mice hindered skeletal muscle regeneration in comparison to similarly treated littermate controls, as indicated by smaller regenerating muscle fibers on DPI 5 (P < .001) and DPI 10 (P < .001) in ARNT mKO mice ( Figure 5A-C). Importantly, muscle-specific deletion of ARNT did not result in any decline in the proportion of SMPs within the muscle, or myogenic differentiation potential or fusion index following culture, thereby demonstrating the absence of intrinsic SMP defects in the ANRT mKO model ( Figure 5D-H).…”

“…Within the nucleus, the heterodimer of ARNT and HIF‐1α, or ARNT and HIF‐2α, binds to the hypoxia response element and induces transcriptional activity 10,11 . Initially, ARNT was thought to be constitutively expressed, 12 but more recent studies have shown in multiple tissue types that ARNT is dynamically regulated and responds to stimuli 13 . Despite its critical role in hypoxia signaling, ARNT regulation during aging or muscle regeneration has not been previously characterized.…”

Cryptochromes Suppress HIF1α in Muscles

“…However, the ability of circadian proteins and HIF1α to form functional heterodimers in a physiological context has been controversial ( Cowden and Simon, 2002 ). Although HIF1α/BMAL1 heterodimers are unable to support normal vascularization in HIF1β-deficient mouse embryos ( Cowden and Simon, 2002 ), it was recently demonstrated that muscle-specific deletion of HIF1β has no impact whatsoever on muscle vascularization ( Badin et al., 2016 ). This finding suggests that an alternate partner, perhaps BMAL1, may cooperate with HIF1α to support muscle vascular remodeling.…”