Targeting Circulating Adipocyte Protein 2 in the Context of Type 1 Diabetes Incidence and Pathogenesis

Abstract

Type 1 diabetes (T1D) remains a growing problem worldwide, with a rise in incidence among very young children (9 months to young adolescent), as well as an emerging population of adults developing T1D pathogenesis and disease. Over a 100 years since its discovery, insulin replacement remains the main form of treatment for T1D-related hyperglycemia. However, exogenous insulin replacement will never be comparable to residual insulin production by endogenous pancreatic beta cells. This fact is driven home by the risk of the secondary complications that plague T1D-insulin dependent patients—a 40% increased risk of retinopathy, neuropathy, nephropathy, and vascular disease, as a result of injectable, exogenous insulin failing to maintain adequate glycemic control. Maintaining residual beta cell function, significantly reduces the risk of these secondary complications. Therefore, due to the frequency of secondary complications, and the failure of injectable, exogenous insulin to maintain residual beta cell function, other therapies are needed to maintain residual beta cell mass in order to improve glycemic control. These drugs would be drugs that would ideally be used in concert with immune-mediated therapies to reduce immune infiltration and prevent beta cell dysfunction. Based on the stipulations that a new T1D drug would improve glycemia and ameliorate beta cell function, we became interested in the role that a fatty acid carrier, adipocyte protein 2 (aP2) may play in T1D incidence and pathogenesis. Previous work by our lab and others has shown that this intracellular fatty acid carrier aP2 mediates cytokine secretion and inflammatory signaling; glycemic control; hepatic glucose production; and partial restoration of insulin secretion, in models of type 2 diabetes. A recent development of a monoclonal antibody to target the circulating form of aP2, secreted from adipocytes, had shown an attenuation in type 2 diabetes-related mechanisms. We hypothesized that these improvements to glycemic control upon targeting aP2 action may be relevant in the context of T1D. We began by investigating the role of genetic deficiency of aP2 in mediating T1D incidence and pathogenesis. Genetic deficiency of aP2 in the spontaneously developing, non-obese diabetic (NOD) model yielded promising results, showing a significant reduction in T1D incidence. Treatment of the fast-onset, viral, RIP-LCMV-glycoprotein (GP) model with either genetic or therapeutic deficiency of aP2 led to protection against T1D incidence and hyperglycemia. Genetic and therapeutic deficiency of aP2 yielded reduced inflammation and immune infiltration to the pancreas. Based on these results, we hypothesized that aP2-deficiency may be mediating incidence through reduced immune infiltration to the islets. Surprisingly, comparing aP2-antibody and vehicle-treated controls, we found that there were no quantitative differences in immune composition of the pancreas. There were no differences in the percentage of B cells, dendritic cells, CD4-T cells, CD8-T-cells, and FOXP3 regulatory T-cells in whole pancreas, draining lymph nodes and spleen. Based on the lack of an immune phenotype in aP2-deficient mice versus controls, we hypothesized that aP2 may be mediating the metabolic arm of T1D pathogenesis. We found that aP2-antibody treated mice had significantly reduced hepatic glucose production and improved glucose tolerance as compared to vehicle controls. These improvements in glucose tolerance were not accompanied by changes in peripheral insulin sensitivity. If there had been changes in insulin sensitivity this would have pointed to improved peripheral glucose uptake, and reduced burden of insulin secretion on the beta cells. No differences in peripheral insulin sensitivity directed us to an islet-inherent phenotype. AP2-antibody treated mice, as compared to vehicle controls, had increased insulin secretion both in max peak and duration. This led us to investigate the role of aP2-deficiency in intrinsic beta cell function. We found that aP2-antibody treated mice had increased beta cell number, specifically of small islets. These islets were highly functional and secretory by nature, and secrete more insulin in response to hyperglycemia or incretin stimulation (GLP-1) than control islets. Ex vivo and in vitro analysis showed that aP2-deficiency seems to be protecting these small islets from pancreatic cell death as there were not significant indications pointing to a proliferative phenotype of these small pancreatic beta cells. Instead, aP2 deficiency seemed to protect the islets from beta cell apoptosis, as shown by reduced TUNEL or cell death analysis in pancreatic sections from RIP-LCMV-GP mice. Additionally, exposure to recombinant aP2 treatment appeared to mediate beta cell dysfunction and loss of insulin secretion in response to hyperglycemia. This beta cell dysfunction ultimately culminates in beta cell death, which can be mimicked by 24- and 40-hour recombinant aP2 treatment under fasting conditions. Therefore, our hypothesis is that aP2-deficiency protects small islets by preventing beta cell death and increasing the relative percentage of small, highly functional islets. Investigations into the translational relevance of this work have also yielded promising results. Examination of serum aP2 levels in normoglycemic, antibody-negative patients or positive non-diabetic subjects as compared to antibody positive new-onset T1-diabetic patients, showed significantly (≈2-fold) increased serum aP2 levels. This result reveals that serum aP2 levels may be relevant to T1D incidence and pathogenesis in humans. Ongoing work will seek to demonstrate the relationship of serum aP2 levels with other anthropometric indicators of T1D, including beta cell function (i.e. C-peptide levels). Overall this work demonstrates a novel role for the fatty acid binding protein, aP2, in mediating T1D incidence and pathogenesis, and supports the possibility that therapeutically targeting of this molecule may restore glycemic control, improve beta cell function, and preserve beta cell mass, as shown in two mouse models of T1D.