Friend or foe: the paradoxical roles of MG53 in diabetes mellitus.

MG53 is predominantly expressed in striated muscles. The role of MG53 in diabetes mellitus has been gradually elucidated but is still full of controversy. Some reports have indicated that MG53 is upregulated in animal models with metabolic disorders, and that muscle-specific MG53 upregulation is sufficient to induce whole-body insulin resistance and metabolic syndrome through targeting both the insulin receptor (IR) and IR substrate-1 (IRS-1) for ubiquitin-dependent degradation. Additionally, MG53 has been identified as a myokine/cardiokine that is secreted from striated muscles into the bloodstream and circulating MG53 has further been shown to trigger insulin resistance by binding to the extracellular domain of the IR, thereby allosterically inhibiting insulin signaling. Conversely, other studies have reported findings that contradict these results. Specifically, no significant change in MG53 expression in striated muscles or serum has been observed in diabetic models, and the MG53-mediated degradation of IRS-1 may be insufficient to induce insulin resistance due to the compensatory roles of other IRS subtypes. Furthermore, sustained elevation of MG53 levels in serum or systemic administration of recombinant human MG53 (rhMG53) has shown no impact on metabolic function. In this review, we will fully characterize these two disparate views, strive to provide critical insights into their contrasts and propose several specific experimental approaches that may yield additional evidence. Our goal is to encourage the scientific community to elucidate the effects of MG53 on metabolic diseases and the molecular mechanisms involved, thereby providing the theoretical basis for the treatment of metabolic diseases and the applications of rhMG53.