Objective: Recent literature has reported preserved residual beta-cell function (C-peptide "microsecretion") in many individuals with long-standing type 1 diabetes (T1D). However, the concentrations of detectable insulin/C-peptide in the serum are usually very low, and beta-cell mass is typically negligible. Proinsulin is measurable in the early years after diagnosis, consistent with the presence of residual functioning beta cells. However, individuals are not expected to secrete significant amounts of proinsulin beyond the early years after diagnosis. Our primary objective was to measure the prohormone, proinsulin, in a heterogeneous cohort of individuals with long-standing T1D. We also sought to assess whether proinsulin secretion might occur in certain individuals despite the absence of measurable C-peptide.
Methods: Random postmeal proinsulin concentrations were measured in 97 subjects with T1D (disease duration >3 years) recruited from within the T1D Exchange Clinic Network participants who took part in the Residual C-peptide Study.
Results: Forty-nine of these subjects had undetectable baseline and stimulated C-peptide (C-peptide [-]), and 48 of them had detectable C-peptide concentrations (C-peptide [+]). All the C-peptide (+) subjects had detectable serum proinsulin. Eight (16%) of the C-peptide (-) subjects had detectable serum proinsulin.
Conclusion: We report the observation that proinsulin secretion persists in a proportion of individuals with long-standing T1D, even in the absence of measurable C-peptide. It is not yet clear why certain patients with T1D retain the ability to secrete proinsulin many years after diagnosis.
Abbreviations: CP = C-peptide CV = coefficient of variation ELISA = enzyme-linked immunosorbent assay IQR = inter-quartile range MMTT = mixed-meal tolerance test NIBSC = National Institute for Biological Standards and Control PI = proinsulin T1D = type 1 diabetes.