Relationship between insulin receptor tyrosine kinase activity and internalization in monocytes of non-insulin-dependent diabetes mellitus patients

Insulin receptor tyrosine-kinase activity is altered in both muscle and adipose tissue from non-obese normoglycaemic insulin-resistant subjects

We performed i.v. insulin tolerance test in 30 non-obese (BMI < 30 male and < 28 female) non-diabetic (by oral glucose tolerance test) subjects and subdivided them into three groups of 10 subjects each, according to their insulin sensitivity (K(itt) values). Then we compared the tyrosine-kinase activity of immunopurified insulin receptors (using 32P-ATP and poly-glu-tyr (4:1) from both muscle and adipose tissue in 7 of the most insulin-sensitive and 7 of the most insulin-resistant subjects. No difference was observed between the two groups in the basal (no insulin) receptor tyrosine-kinase activity from both tissues. In contrast, tyrosine-kinase activity response to insulin was significantly higher (p < 0.05 by 2-way ANOVA test) in receptors from both tissues of insulin-sensitive subjects. In addition, a decreased tyrosine-kinase sensitivity to insulin was observed in muscle, but not adipose, tissue of insulin-resistant subjects (insulin ED50 being 0.87 +/- 0.05 nmol/l vs 2.03 +/- 0.07, p < 0.05 in insulin-sensitive and -resistant subjects). Insulin ED50 of muscle receptor tyrosine-kinase significantly (p = 0.001) correlated to both K(itt) values (r = -0.79) and plasma insulin values at 120 min during OGTT (r = + 0.80). Insulin receptor content, as assessed by radioimmunoassay, was similar in both muscle (7.9 +/- 1.3 and 9.2 +/- 1.9 ng/mg protein) and adipose tissue (8.2 +/- 1.3 and 7.5 +/- 1.4) of insulin-sensitive and -resistant subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin receptor tyrosine kinase activity is reduced in monocytes from non-obese normoglycaemic insulin-resistant subjects

Insulin sensitivity has been quantified by i.v. insulin tolerance test (0.1 U/kg of body weight) in 18 (11 male/7 female) non-obese (body mass index range 19-25 kg/m2) normoglycaemic subjects. We then compared the tyrosine kinase activity and internalization of insulin receptor in monocytes from the six most insulin-sensitive (group 1) and the six most insulin-resistant (group 3) subjects. Tyrosine kinase activity was measured on immunopurified receptors using 32P-ATP and poly-glutamic acid 4: tyrosine 1, sodium salt (poly-glu-tyr 4:1). Insulin internalization was studied by incubating cells with 1 nmol/l 125I-insulin and measuring total cell-bound and intracellular 125I-insulin by an acid dissociation procedure. Basal (in the absence of insulin) receptor kinase activity was similar in both groups. Maximal (in the presence of 100 nmol/l insulin) kinase activity was 41% lower in group 3 (13.8 +/- 3.6 fmoles 32P-ATP incorporated vs 23.3 +/- 4.0, p = 0.1). Delta increment of receptor kinase activity after insulin stimulation (calculated by subtracting basal from maximal activity) was significantly (p < 0.05) reduced in group 3 (21.3 +/- 3.8 vs 11.1 +/- 2.1) and significantly (p < 0.05) correlated to the in vivo insulin sensitivity. Both total cell-bound (0.70 +/- 0.09% of total radioactivity added vs 0.83 +/- 0.15) and intracellular (0.39 +/- 0.05 vs 0.44 +/- 0.09) 125I-insulin were similar in the two groups. These data suggest that in non-obese, normoglycaemic subjects a defective insulin receptor tyrosine kinase activity may contribute to the development of insulin resistance.(ABSTRACT TRUNCATED AT 250 WORDS)