Biochemical basis of hyperinsulinemia in SCHAD deficiency

Abstract

ABSTRACT: Glucose-stimulated insulin secretion (GSIS) by the pancreatic beta cell involves the Ca2+ influx/KATP-dependent pathway and the amplification/KATP-independent pathway(s), stimulated by hitherto unidentified signalling molecules. We have suggested that lipid mediators play a key role in GSIS, though their biochemical nature is not clear. Patients homozygous for a mutation in the beta-oxidation enzyme short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) (catalyzing the penultimate reaction of the beta-oxidation of fatty acid of 8-4 C chain length) are hyperinsulinemic. This suggests that SCHAD plays an important role in the control of insulin secretion (IS), but the molecular mechanism responsible for this increased IS is not known. We hypothesized that SCHAD deficiency could lead to altered cellular levels of short-chain fatty acid (FA) oxidation intermediates or their derivatives (CoA, carnitine) and these could regulate IS. RNAi-knockdown of SCHAD expression (by ~80%) by shRNA and siRNA (as assessed by Western blot) in the rat insulinoma cell line INS832/13 led to an increase in both fuel- (glucose, gln+leu) and non-fuel- (high KCl) induced IS in the absence of FA, but not in their presence. The latter finding is consistent with the view that some FA intermediates play a key role in the regulation of IS, even in the absence of exogenous FA. Under these conditions, as expected, [1-14C]-palmitate oxidation was not affected, because SCHAD is involved in the beta-oxidation of short-chain FA. However, the oxidation of [1-14C]-octanoate was surprisingly increased (measured as 14CO2 release), possibly because 6-4 carbon FA intermediates are released faster from the multienzyme complex, catalyzing the oxidation of short-chain FA, allowing more molecules of octanoate to interact with the complex per unit of time. Since hydroxybutyryl-carnitine accumulates in SCHAD-deficient patients and short-chain FA or their derivatives likely accumulate in SCHAD deficient cells, we monitored the effect of these intermediates on GSIS with or without SCHAD knockdown. Hydroxybutyrate and octanoate potentiated GSIS, and, thus, these or their derivatives could contribute to the increased IS seen in absence of SCHAD. Collectively, these results are consistent with the view that hyperinsulinemia in SCHAD deficiency is due at least in part to an intrinsic beta-cell defect and suggest that some short-chain FA intermediates or their derivatives (to be determined) play a role in the regulation of IS in response to both fuel and non-fuel stimuli.