Diabetes mellitus is a metabolic disease characterized by chronic hyperglycemia and insulin dysfunction, which often results in secondary complications associated with blood flow. Four major types of diabetes are defined by the American Diabetes Association: type 1 diabetes (T1D, insulin insufficiency), type 2 diabetes (T2D, insulin resistance), gestational diabetes (occurring during pregnancy), and other types, which includes cystic fibrosis related diabetes (CFRD). Although each of these types of diabetes is associated with insulin dysfunction, it is important to note that the treatment of oral medications or exogenous insulin, although sufficient to effectively manage blood glucose levels, is not sufficient to completely cure diabetes. Secondary complications resulting from diabetes include retinopathy, nephropathy and neuropathy, along with dysfunctions in immune response. In recent years, a surge in the research of C-peptide, the 31 amino acid peptide co-secreted with insulin, has revealed that C-peptide may actually help to ameliorate some of these secondary complications, including neuropathy and immune response. Although initial studies revealed promising results, a phase 2b clinical trial was shut down in 2014 due to indistinguishable results in placebo and treatment groups in T1D patients. To enhance our knowledge of these downstream problems, we here report the effect of C-peptide/zinc/albumin on RBCs and peripheral neutrophils (an immune cell) that mimic those from CF/CFRD patients using a chemical inhibitor (CFTRinh-172). C-peptide and zinc binding to RBCs confirms previously published data, and binding to CFTR-inhibited RBCs is increased from the control, however, the molar binding ratio of C-peptide to zinc remains 1:1. ATP release is increased in control and CFTR-inhibited cells only in the presence of the ensemble of C-peptide/zinc/albumin, but is abolished when any one of these components is missing. Similarly, in neutrophils, we observe a 1:1 molar binding ratio of C-peptide to zinc when albumin is present in both control and CFTR-inhibited cells. We also observed a significant increase in intracellular calcium, intracellular NO, extracellular NO, glucose uptake, and rate of chemotaxis in neutrophils that had been treated with the ensemble, but not when any one component was missing. In both RBCs and neutrophils, we observed an increase in membrane expression of glucose transporter 1 (GLUT1) only when C-peptide, zinc and albumin were present. Insulin did not have this effect. The results reported here are the first to indicate that C-peptide may be acting on cells that contain primarily GLUT1, and that the mechanism of action may be similar to the way that insulin acts in muscle and fat cells, by translocating GLUT4 to the plasma membrane. These results are also the first to suggest that C-peptide may directly be affecting peripheral neutrophils, which could have significant implications in the treatment of immune complications in all forms of diabetes. Additionally, these results are the first to examine the effect this peptide and its metal may have on patients who have CF, and may point to its use a potential therapy in both T2D and CFRD.
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