B CELL ACTIVATING FACTOR AND MATRIX METALLOPROTEINASE-2 AS NOVEL MODULATORS OF INSULIN RESISTANCE AND GLUCOSE UTILIZATION

Abstract

Obesity and insulin resistance are accompanied by chronic low-grade inflammation within the adipose tissue mediated by the infiltration of immune cells. In recent years, many studies have highlighted the importance of understanding the interplay between the body's metabolic state and the activation of the immune system in type 2 diabetes. Here, we identify novel roles for B cell-activating factor (BAFF) and matrix metalloproteinase 2 (MMP2) in the progression of insulin resistance and glucose utilization, respectively. BAFF is a critical TNF-family cytokine that regulates homeostasis and peripheral tolerance of B2 cells. BAFF overproduction promotes autoantibody generation and autoimmune diseases. During obesity, BAFF is predominantly produced by white adipose tissue (WAT), and IgG autoantibodies against adipocytes are identified in the WAT of obese humans. However, it remains to be determined if the autoantibodies formed during obesity affect WAT remodeling and systemic insulin resistance. Here, we show that IgG autoantibodies are generated in high-fat diet (HFD)-induced obese mice that bind to apoptotic adipocytes and promote their phagocytosis by macrophages. Using murine models of obesity in which the gonadal WAT undergoes remodeling, we found that BAFF neutralization depleted IgG autoantibodies, increased the number of dead adipocytes, and exacerbated WAT inflammation and insulin resistance. RNA sequencing of the stromal vascular fraction from the WAT of BAFF-neutralized mice revealed decreased expression of immunoglobulin light-chain and heavy-chain variable genes and impaired B cell activation and phagocytosis pathways. In vitro, plasma IgG fractions from BAFF-neutralized mice reduced the phagocytic clearance of apoptotic adipocytes. Altogether, our study suggests that IgG autoantibodies developed during obesity dampen WAT inflammation and systemic insulin resistance. Glucose transporter 4 (GLUT4) expression on white adipocytes is critical for absorbing excess blood glucose, failure of which promotes hyperglycemia. MMPs play a crucial role in remodeling the white adipose tissue (WAT) during obesity. MMPs have multiple protein substrates, and surprisingly, it is unknown if they can directly target GLUT4 on the adipocyte surface and impair glucose absorption. We identified MMP2 as the highly active gelatinase, a class of MMP, in the gonadal WAT of HFD-induced obese mice. In vitro, metabolic studies in 3T3-L1 adipocytes revealed MMP2 attenuated glucose absorption and glycolysis, which were recovered by an MMP2 inhibitor. In silico structural analysis using AlphaFold identified a putative MMP2 cleavage site on the extracellular domain of GLUT4. Further, in a substrate competition assay, a peptide mimicking the MMP2 cleavage site on GLUT4 attenuated the cleavage of an MMP substrate by MMP2. Altogether, our results suggest a novel mechanism of impaired glucose absorption by adipocytes, which may contribute to hyperglycemia during obesity.