Antibodies directed to the insulin receptor. Clinical aspects and applications to the study of insulin action

Insulin receptor antibodies inhibit insulin uptake by the liver: in vivo 123I-insulin scintigraphic scanning and in vitro characterization in autoimmune hypoglycemia

BACKGROUND

Insulin receptor antibodies can induce severe hypoglycemia or insulin resistance in rare autoimmune syndromes. In vitro properties of these antibodies occasionally explain the clinical features of the syndrome, but direct evidence of their in vivo activity is poor. We studied a 58-year-old male with rheumatoid arthritis who presented with hypoglycemic coma.

METHODS AND RESULTS

Antibodies were detected by inhibition of 125I-insulin binding to human insulin receptor-3T3 cells by the patient's serum. By immunofluorescence, they were immunoglobulin G of all four subclasses, immunoprecipitated insulin receptors from biotin-labeled cells, and triggered phosphorylation of the beta subunit of the insulin receptor. Insulin binding on the patient's red blood cells was markedly reduced. A biodistribution study after intravenous 123I-Tyr A14 insulin showed a marked inhibition of tracer uptake by the liver, reaching 10% of the injected dose (controls, mean +/- SD, 21.1 +/- 1.7%; n = 10). Time activity curves generated on the liver and on the heart were parallel, with a T1/2 of 11.5 minutes for both, suggesting that no specific uptake occurred in the liver, where tracer activity represented only the blood pool. Clearance of insulin from the blood was indeed slower than in controls and mainly occurred through the kidneys. Analysis of plasma 123I-insulin immunoreactivity and trichloroacetic acid precipitate showed that insulin degradation did not occur as in normal controls.

CONCLUSIONS

In this patient with hypoglycemic syndrome, insulin receptor antibodies with in vitro insulin-like activity are capable of blocking in vivo the access of insulin to the liver receptor compartment, as directly demonstrated by the markedly altered biodistribution of intravenously injected 123I-insulin.

Hypoglycaemia in the adult

Hypoglycaemia is a relatively common cause for referral of patients to the accident and emergency departments of hospitals but most of it is iatrogenic. Occasionally, however, hypoglycaemia is due to any one of up to a hundred different disorders. In some, hypoglycaemia is the cause of intermittent neuroglycopenic symptoms that lead to their referral to medical outpatients for investigation. Only the most important are discussed here. Hyperinsulinism due to abnormal beta-cell function is an uncommon but important cause of spontaneous hypoglycaemia. The diagnosis is suspected from the history of episodes of altered consciousness confirmed by demonstrating raised plasma insulin, C-peptide and proinsulin levels in peripheral blood in the presence of hypoglycaemia. Differentiation of the various causes of endogenous hyperinsulinism before surgery is difficult if not impossible and the low predictive value of most of the localizing techniques that are available makes them an additional and unnecessary cost, producing little clinical benefit. Hypoglycaemia caused by non-islet cell tumours (NICTH) is seemingly rarer than hyperinsulinism from insulinoma and tends to occur in older patients. The clinical features are similar to those of hyperinsulinism but laboratory investigation reveals appropriately depressed plasma insulin, C-peptide and proinsulin levels in the presence of hypoglycaemia. The plasma IGF-II:IGF-I ratio is characteristically high and the concentration of the E-domain of proIGF-II is raised. Autoimmune hypoglycaemia is more common in some countries than others and is most often due to autoantibodies to insulin (AIS). It may also be caused by autoantibodies to the insulin receptor and possibly to autoantibodies that are stimulatory to pancreatic beta-cells. Contrary to popular belief, idiopathic reactive hypoglycaemia is rare and only one of the possible causes of the postprandial syndrome. It is characterized by a low blood glucose concentration in blood collected during a spontaneous symptomatic episode but not at other times. Its cause is unknown. Other causes of hypoglycaemia include endocrinopathies of various kinds; sepsis including malaria; congestive cardiac failure; hepatic and renal insufficiencies; diverse inborn errors of metabolism; and exogenous toxins, of which alcohol is probably the commonest.