The emerging role of complement c3 as a biomarker of insulin resistance and cardiometabolic diseases: Preclinical and clinical evidence

Background: An intricate network of reciprocal interactions between adipose tissue and immune system have been largely demonstrated, leading to the well-accepted concept of insulin resistance as a low grade inflammatory state and, conversely, chronic high-grade inflammation as a dysmetabolic condition. Immune homeostasis is regulated by several players including the complement system, a complex protein network at the crossroad between the innate and adaptive arms of the human defences against pathogens. Objective: Complement C3 represents the nodal point of the complement cascade independently of the pathway recruited. Aim of this review is to collect the evidence supporting the role of complement C3 as a candidate biomarker of insulin resistance and other metabolic disorders. Method: We reviewed the available evidence pointing to a role of complement system, and in particular complement C3, in insulin resistance and other cardiometabolic diseases including diabetes (T2D), hypertension, non-alcoholic fatty liver disease (NAFLD) and atherosclerosis. Results: Compelling preclinical evidence demonstrated a role of adipose-tissue C3 and its cleavage products C3a and acylation stimulating protein (ASP) in adipose tissue inflammation and insulin resistance. To further support this hypothesis, several clinical studies, both cross-sectional and longitudinal, confirmed this association in independent cohorts. Moreover, preliminary evidence support a role of complement C3 in other cardiometabolic diseases such as hypertension, NAFLD and atherosclerosis. Conclusion: Future studies are needed to fully confirm the usefulness of C3 as a clinical biomarker and to establish accurate cut-off values. Moreover, the therapeutic potential of C3 modulation in either cardiometabolic and inflammatory diseases need to be investigated.