Insulin-receptor interaction in isolated fat cells

Insulin-independent diabetes: a defect in the activity of lipoprotein lipase in adipose tissue

The activity of lipoprotein lipase (E.C.3.1.1.3.) has been measured in adipose tissue from insulin-independent diabetics with hypertriglyceridaemia, non-diabetics with hypertriglyceridaemia and control patients, all of whom were obese. Although all groups showed an increase of plasma insulin after oral glucose, both the diabetic and nondiabetic hypertriglyceridaemics had impaired activities of lipoprotein lipase in adipose tissue compared to the obese normals (p less than 0.02, p less than 0.03, respectively). A course of insulin therapy (20 u.o.d.) for one week increased the activity of lipoprotein lipase extracted from adipose tissue, lowered plasma triglycerides and improved triglyceride clearance from plasma in a group of diabetics with hypertriglyceridaemia (mean plasma triglyceride 8.7 mmol/l). Our results suggest that a feature in the development of insulin resistance in adult diabetics may be a failure of maintenance of key intracellular enzyme activities involved in lipid metabolism.

Insulin binding by a cell line (HT 29) derived from human colonic cancer

Insulin binding was demonstrated in cultured HT 29 cells originating from a human colon carcinoma. At 37 degrees and in complete medium, the binding of [125I]insulin (1-4x10-10M) reaches a maximum in 40 min and the cell associated radioactivity remains constant for at least 4 h. No degradation of the hormone is observed under these conditions. The binding is proportional to the number of cells and its pH optimum is 7.8. In the presence of excess insulin 50% of the [125I]insulin is dissociated from the complex after 10 min. At equilibrium, insulin binding is specific: proinsulin is 25 times less potent than native insulin in competing with [125I]insulin and related polypeptide hormones are inactive. Scatchard analysis indicates two classes of binding sites (1400 sites/cell of "high affinity" e.g. 4.7 x 108 M-1, and 20 000 sites of "low affinity" e.g. 4 x 107 M-1). The binding of insulin to this non-target cell shows the same kinetic characteristics and specificity as found for insulin in its target cells, except that HT 29 cells do not degrade the hormone. The problem of the correlation between insulin binding and a biological effect in these cells remains to be elucidated.

Interrelationship and control of glucose metabolism and lipogenesis in isolated fat-cells. Effect of the amount of glucose uptake on the rates of the pentose phosphate cycle and of fatty acid synthesis

In order to study the quantitative relationship between fatty acid synthesis and pentose phosphate-cycle activity under different hormonal and dietary conditions affecting the extent of glucose uptake, cells isolated from rat epididymal adipose tissue were incubated in bicarbonate buffer containing [U-(14)C]-, [1-(14)C]- or [6-(14)C]-glucose. From the amount of glucose taken up, the production of lactate and pyruvate, and the incorporation of (14)C from differently labelled [(14)C]glucose into CO(2), fatty acids and glyceride glycerol, the rates of glucose metabolism via different pathways and the extent of lipogenesis under various experimental conditions were determined. The contribution of the pentose phosphate-cycle to glucose metabolism under normal conditions was calculated to be 8%. Starvation and re-feeding, and the presence of insulin, caused an enhancement of glucose uptake, pentose phosphate-cycle activity and fatty acid synthesis. Plots of both pentose phosphate-cycle activity and fatty acid synthesis versus glucose uptake revealed that the extent of glucose uptake, over a wide range, determines the rates of fatty acid synthesis and glucose metabolism via the pentose phosphate cycle. A balance of formation and production of nicotinamide nucleotides in the cytoplasm was established. The total amount of cytoplasmic NADH and NADPH formed was only in slight excess over the hydrogen equivalents required for the synthesis of fatty acids, glyceride glycerol and lactate. Except in cells from starved animals, the pentose phosphate cycle was found to provide only about 60% of the NADPH required for fatty acid synthesis. The results are discussed with respect to an overall control of the different metabolic and biosynthetic reactions in the fat-cells by the amount of glucose transported into the cell.