Resistance to subcutaneous and intramuscular insulin associated with deficiency of insulin-like growth factor (IGF) 2

A Possible Novel Effect for Dapagliflozin in the Management of Subcutaneous Insulin Resistance Syndrome: A Report of Two Cases

INTRODUCTION

Subcutaneous insulin resistance syndrome (SIRS) is a rare condition in which patients poorly respond to subcutaneous (SC) insulin but maintain a normal response to intravenous (IV) insulin. The underlying pathophysiology remains elusive. Several treatment regimens have been tested for the management of SIRS, none of which included a sodium-glucose cotransporter-2 inhibitor (SGLT-2).

CASE PRESENTATION

Two cases of type 1 diabetes initially achieved adequate glycemic control with subcutaneous insulin. Both cases later progressed into recurrent diabetic ketoacidosis that would resolve following IV insulin administration. Further investigation revealed unresponsiveness to SC, but not IV, insulin and the clinical diagnosis of SIRS was established accordingly. HbA1c values for cases 1 and 2 were 11% on 400 units/day of SC insulin, and 12% on 350 - 400 units/day of SC insulin, respectively. The patients required very high doses of intramuscular (IM) insulin. Subsequently, dapagliflozin as adjunct therapy significantly reduced the patients' IM insulin requirements beyond the anticipated dose reduction. Ultimately, case 1 achieved an HbA1c of 7 - 8% on 90 units/day of IM insulin and 10 mg/day of dapagliflozin, and case 2 achieved an HbA1c of 7 - 8% on 120 units/day of IM insulin and 10 mg/day of dapagliflozin.

CONCLUSIONS

These are the first reported cases of SIRS in which dapagliflozin, an SGLT-2 inhibitor, was used. The substantial reduction in the IM insulin dose following the addition of dapagliflozin in our reported cases of SIRS suggests a possible novel mechanism for dapagliflozin beyond its glucosuric effects. In this report, we present a hypothetical basis for this possible novel mechanism.

Degradation of porcine relaxin by glutathione-insulin transhydrogenase and a neutral peptidase

The susceptibility of porcine relaxin and 125I-polytyrosyl-porcine relaxin to degradation by 3 purified enzymes involved in the degradation of insulin and proinsulin was examined. Rat liver glutathione-insulin transhydrogenase (GIT), which cleaves disulfide bonds in insulin, catalyzed a time- and concentration-dependent increase in trichloroacetic acid (TCA)-soluble radioactivity of relaxin. The Sephadex G-50 profile of the reaction products revealed conversion to the A- and B-chains. Relaxin competitively inhibited the degradation of insulin by GIT; however, kinetic analysis revealed insulin to be preferred over relaxin as a substrate. Rat liver cytosol neutral thiol peptidase (NTP) catalyzed a time- and concentration-dependent increase in the TCA solubility of relaxin and a shift in the Sephadex G-50 radioactivity profile to low molecular weight products. Kinetic analysis revealed that insulin and B-chain are preferred over relaxin as substrates for NTP. A third enzyme, rat kidney neutral metalloendopeptidase, which degrades proinsulin and insulin C-peptide but not insulin, also did not degrade porcine relaxin.

Insulin resistance in total lipodystrophy: evidence for a pre-receptor defect in insulin action

The cause of insulin resistance in lipodystrophic diabetes is unknown but has generally been ascribed to dysfunction at either the receptor or post receptor level. In a 14 year-old girl with total acquired lipodystrophy, subcutaneous and intravenous insulin requirements approximated 600 units daily. However, circulating total and free insulin levels were not increased, and during testing by the euglycemic clamp method, the glucose response to increasing free insulin concentrations was within the range found in eight subjects with insulin-dependent diabetes. Insulin clearance during the euglycemic clamp was 43, 98, 115, and 116 mL/kg/min at each of four insulin infusion rates compared to means of 13, 13, 12, and 11 in the control subjects with diabetes. No detectable degrading activity was present in serum, and serum inhibited insulin degradation normally. Binding of insulin to IgG, IgM, and IgE was not increased, insulin binding to monocytes and erythrocytes was not sufficiently abnormal to account for the the insulin resistance, and insulin receptor increased insulin clearance or accelerated degradation of insulin by tissues.

The renal metabolism of insulin

The kidney plays a pivotal role in the clearance and degradation of circulating insulin and is also an important site of insulin action. The kidney clears insulin via two distinct routes. The first route entails glomerular filtration and subsequent luminal reabsorption of insulin by proximal tubular cells by means of endocytosis. The second involves diffusion of insulin from peritubular capillaries and subsequent binding of insulin to the contraluminal membranes of tubular cells, especially those lining the distal half of the nephron. Insulin delivered to the latter sites stimulates several important processes, including reabsorption of sodium, phosphate, and glucose. In contrast, insulin delivered to proximal tubular cells is degraded to oligopeptides and amino-acids by one of two poorly delineated enzymatic pathways. One pathway probably involves the sequential action of insulin protease and either GIT or non-specific proteases; the other probably involves the sequential action of GIT and lysosomal proteases. The products of insulin degradation are reabsorbed into the peritubular capillaries, apparently via simple diffusion. Impairment of the renal clearance of insulin prolongs the half-life of circulating insulin by a number of mechanisms and often results in a decrease in the insulin requirement of diabetic patients. Much needs to be learned about these metabolic events at the subcellular level and how they are affected by disease states. Owing to the heterogeneity of cell types within the kidney and to their anatomical and functional polarity, investigation of these areas will be challenging indeed.