Pancreatic β-Cell research associated with metabolic syndrome and type 2 diabetes

Metabolic syndrome (MetS) encompasses multiple risk determinants including obesity, hypertension, lipid abnormalities, and insulin resistance.¹ Currently, there is no clear answer as to which factor, if any, is the original cause of MetS and which are consequential to the syndrome.² Even with this uncertainty, these risk determinants play an important role in type 2 diabetes, as evidenced by the fact that 65% to 85% of individuals with type 2 diabetes also have metabolic syndrome.^3,4,5

One area of research that overlaps MetS and type 2 diabetes focuses on how the pancreatic b-cell combats hyperglycemia caused by insulin resistant peripheral tissue. For MetS, pancreatic b-cells secrete more insulin to compensate for insulin resistance. Similarly, the impaired glucose tolerance observed in patients with pre-type 2 diabetes is also compensated for by increased secretion of insulin by pancreatic b-cells.⁶ As type 2 diabetes progresses, however, the ability of pancreatic b-cells to secrete sufficient levels of insulin to stimulate glucose uptake by the insulin resistant peripheral tissue diminishes due to decreased pancreatic b-cell mass/function, and, as a result, blood glucose levels rise.⁷

The above described alterations in b-cell secretion are not limited to just insulin and C-peptide. Greater secretion of proinsulin is also attributed to MetS- and type 2 diabetes-associated b-cell dysfunction.^8,9 For example, as glucose intolerance progresses to type 2 diabetes, the release of proinsulin can surpass that of insulin as a result of incomplete or no post-translational processing of proinsulin,¹⁰ and can be observed as an increase in the proinsulin:insulin ratio.¹¹

Much of the above discussion is based upon research that utilized the enzyme-linked immunosorbent assay (ELISA) format. Commercially available ELISA kits for the quantification of human proinsulin-derived proteins—proinsulin, insulin, and C-peptide—have been available for many years. Until recently, commercially available ELISA kits for the quantification of rodent proinsulin-derived proteins were only capable of accurately measuring insulin. This was due to the fact that rodents possess two proinsulin genes and the gene products (isoforms) for mouse and rat insulin are highly conserved; approximately 4% of the amino acid positions vary between rodent insulin isoforms. Such conservation does not exist between the mouse and rat C-peptide isoforms—approximately 16% of the amino acid positions vary between the gene I and gene II C-peptide isoforms. This made commercial rodent insulin ELISA kit development fairly straightforward compared to that for rodent C-peptide and proinsulin. Now, however, there is no impediment to measuring C-peptide and proinsulin isoforms in mouse or rat samples.

During the 2010 American Diabetes Association Scientific Sessions, the utility of measuring proinsulin-derived gene I and gene II isoforms in a single ELISA kit was demonstrated for each of mouse insulin, C-peptide, and proinsulin (ALPCO Diagnostics, Salem, NH).¹² Male db/db mice were fed standard chow and treated with rosiglitazone (TZD) or vehicle for 29 days. If just insulin levels were measured, an investigator would only have known that secretion of insulin increases with TZD treatment (Figure 1). With the use of mouse C-peptide and proinsulin ELISA kits, it is evident that C-peptide and proinsulin levels also increase with TZD treatment. Without these additional kits, a complete assessment of how proinsulin processing in pancreatic b-cells is affected by TZD would not be as easily determined.

Figure 1. Serum insulin, C-peptide, and proinsulin levels from male db/db mice (Jackson Labs, Bar Harbor, ME) fed standard chow (Harland Teklab, Madison, WI) and treated with vehicle (n=6) or rosiglitazone (TZD; 10 mg/kg; n=7) for 29 days (serum samples from lean littermates (n=8) were assessed as controls; p<0.05 between TZD treated groups and all other groups by one-way ANOVA/Tukey's).

Whether or not scientific research is associated with pancreatic b-cell activity as it relates to MetS or type 2 diabetes, consistent performance characteristics are crucial for any assay format used in the laboratory. Significant variability with assay characteristics such as sensitivity, cross-reactivity, intra- and inter-assay precision, linearity of dilution, and spike recovery can lead to inconsistent, and sometimes useless, data. Currently, there are a number of assays available, including multiplex platforms, which have demonstrated variability-associated limitations in performance.^13,14 Therefore, thorough due diligence should be conducted in selecting how to measure an analyte of interest before any samples are tested.

Leon F. Hebert Jr., PhD, is Product Manager at ALPCO Diagnostics and has more than 20 years experience working in the academic and biotechnology sectors.

References

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