Insulin Action in Isolated Rat Thymocytes

Serum modulation of insulin action: effects on rat gingival fibroblast metabolism

Serum is reported to reduce the sensitivity of cells in culture to insulin. The effect of serum concentration in the growth medium on the responsiveness of control (C) and streptozotocin diabetic (D) rat gingival fibroblasts to insulin was measured by monitoring cellular DNA, RNA, total protein and medium hydroxyproline (collagen) levels, as well as the cellular uptake of C14-alpha-NH2-isobutyrate (alpha-AIB) and H3-2-deoxyglucose (2DG). The cells were grown in alpha-MEM at 5, 10, 15 or 20% FCS with 0, 10(-12), 10(-10), 10(-8) and 10(-6) M insulin used at each serum level. Insulin effects in the absence of serum were not assessed. For both the C and D rat cells, the DNA increased proportionately with increasing serum and insulin levels. In contrast, RNA and total cell protein increased with increase in insulin and decrease in serum, the magnitude of the effect being greater in C than in D cells. The insulin stimulation of both 2DG and alpha-AIB uptake and of collagen secretion varied inversely with serum concentrations. The magnitude of the insulin-serum interaction on metabolite uptake was greater for the D rat cells. These data indicate that serum significantly reduced the cell response to insulin stimulated metabolite uptake and collagen secretion, but was without apparent effect on the intracellular insulin responsive parameters. They suggest that serum factor(s) interfere with the availability of insulin to the cell and that the D rat cells are most affected.

Insulin-cholera toxin binding unit conjugate: a hybrid molecule with insulin biological activity and cholera toxin binding specificity

The polypeptide hormone insulin and the binding unit of cholera toxin (CTB) were coupled via a disulfide bond. This hybrid molecule had 1/30 the ability of native insulin to bind to the insulin receptor and 1/30 the biological activity of native insulin in H35 rat hepatoma cells and rat adipocytes. Thus, in these two cell types that are very sensitive to insulin, the biological activity of the hybrid molecule was as predicted on the basis of the ability of the molecule to interact with the insulin receptor. In contrast, in HTC rat hepatoma cells and rat thymocytes, two poorly responsive cell types, the insulin-CTB conjugate had 1/3 the biological activity of native insulin, a value 10 times greater than its insulin receptor binding potency. This increased activity of the conjugate did not appear to be due to cholera toxin in the preparation, since a control of uncoupled CTB had no biological activity. Furthermore, native cholera toxin increased intracellular levels of cAMP by 20-fold, whereas the conjugate had no effect on cAMP levels. The CTB moiety did, however, contribute to the biological activity of the conjugate, since the activity of the hybrid molecule, like cholera toxin, was inhibited by gangliosides, whereas the activity of native insulin was not. Finally, the binding to thymocytes of insulin-CTB conjugate, but not insulin, was inhibited by gangliosides. Thus, a hybrid hormone molecule has been constructed which has insulin-like biological activity with the receptor specificity of cholera toxin in poorly responsive cells.

Effect of insulin on glucose oxidation and amino isobutyric acid transport and binding of insulin in chicken thymocytes

In chicken thymocytes isolated from 15--40 day-old chickens, after a 2 h incubation at 37 degrees C, insulin stimulated amino isobutyric acid uptake (maximal response: 40--50% of increase at 1 microgram insulin/ml and half maximal response at 60 ng/ml) by specifically stimulating the influx without altering the efflux. Insulin also stimulated glucose oxidation (maximal response: 11% of increase at 1 microgram insulin/ml). Binding of 125I-labelled chicken insulin to thymocytes was rapid and higher at 15 degrees C than at 37 degrees C. At steady state, (90 min at 15 degrees C), chicken, porcine and goose insulins were equipotent in inhibiting the binding of 125I-labelled chicken insulin. Maximal binding capacity was estimated at 1250 pg insulin/10(8) cells, i.e., 1250 binding sites/cell with an apparent dissociation constant of 200 ng insulin/ml at 15 degrees C. Degradation of 125I-labelled chicken insulin in the incubation medium was negligible at 15 degrees C but very noticeable at 37 degrees C. Therefore, the low level of insulin binding at 15 degrees C reflects a true scarcity of insulin receptors in chicken thymocytes as compared to rat thymocytes.

Serum-dependent regulation of alpha-aminoisobutyric acid uptake in bovine granulosa cells

The removal of serum from the medium of ovarian granulosa cells in exponential or confluent stages of growth results in a rapid and pronounced decrease in the rate of transport of the non-metabolizable amino acid, alpha-aminoisobutyric acid. This decrease is rapidly and completely reversed by the addition of serum. The decrease and its reversal are insensitive to inhibitors of RNA and protein syntheisis and are unaffected by a number of other metabolic inhibitors. The serum requirement cannot be replaced by peptide hormones known to stimulate cell division and secretion by these cells. These data are consistent with a model of post-translational control of AIB transport by a high-molecular-weight component of serum.