The role of insulin in regulating protein catabolism in mammalian cells

Zinc and insulin metabolism

A review of experimental studies of the effect of zinc nutrition on insulin metabolism is presented. In addition to a short introduction to the synthesis, secretion, and action of insulin, the effects of zinc deficiency-specifically on glucose tolerance, insulin secretion, insulin synthesis and storage, and on total insulin-like activity-are dealt with. The concentrations of zinc and chromium in serum, pancreas, and liver are compared to those of zinc-deficient animals and pair-fed controls.In contrast to pair-fed controls, zinc-deficient rats had unaltered proinsulin contents after glucose stimulation, but they showed a diminished glucose tolerance, lowered serum insulin content, and an elevated total insulin-like activity. The serum zinc concentration of the deficient animals was greatly reduced and did not change during glucose stimulation, whereas it rose in the case of the pair-fed controls. The serum chromium concentration increased in both groups in response to glucose stimulation. In the pancreas of the deficient animals, the zinc concentration was reduced 60% and it increased during the glucose tolerance test. In the liver there were no significant differences. The chromium concentrations were elevated in both the pancreas and liver of the zinc-deficient rats by 60 and 100%, respectively, and were not influenced by glucose injection.These studies show clearly that nutritional zinc deficiency influences insulin metabolism and action.

Effects of insulin on cultured rat brain cells: stimulation of ornithine decarboxylase activity

Growth-promoting peptide hormones, including growth hormone and insulin, stimulate rat brain ornithine decarboxylase (ODC; EC 4.1.1.17) activity in vivo (Roger et al., 1974; Roger and Fellows, 1980). To determine if this is a result of a direct action on brain, we have investigated the effect of peptide hormones in primary cell cultures of brain from fetal rats of 20 days gestational age. Significant stimulation of ODC activity was observed 4 h after administration of porcine insulin and bovine growth hormone. On a molar basis, growth hormone was less potent than insulin. By contrast, glucagon, enkephalin, and angiotensin II did not stimulate ODC in this system. At 25 ng/ml, insulin stimulated ODC activity approximately threefold, with maximum stimulation of five- to sevenfold reached at 1 microgram/ml. After a 1-h lag, insulin-stimulated ODC activity increased to a maximum between 5 h and 8 h and returned to basal levels by 24 h. The apparent Km of ODC, 5.66 +/- 1.16 microM, was not significantly altered by insulin treatment, nor was any enzyme activator found in mediating insulin actions. Additional evidence suggests that insulin stimulation of ODC activity involves both de novo synthesis of the enzyme and a prolongation of ODC half-life by 50%. These findings, implicating insulin as a regulator of ODC activity in brain cells, suggest the possible involvement of insulin or an insulin-like peptide in the control of growth and development of the CNS.

The influence of exercise on muscle lysosomal enzymes

Representative lysosomal enzyme activities were measured in muscles taken 0 h and 24 h after an acute exercise run that was completed without any overt signs of fatigue. The animals had progressed 2 and 4 weeks into a standard exercise program which typically produces adaptive changes in the working muscles. There was an increase in acetylglucosaminidase activity (12%) in the fast-twitch red muscle section of all animals that participated in the training program. This small increase may be representative of a delayed response found after more exhausting exercise. The single exercise bout, however, did not cause any acute change in lysosomal activity nor alter the partition of lysosomal enzymes between the "free" and particulate fractions. Thus, altered lysosomal enzyme activity does not appear to be a contributing influence that challenges muscle fiber homeostasis during moderately intense running.

Organ culture of adult mouse intestine IV. Stimulation of glucose-6-phosphatase in vitro

Explants of adult mouse jejunum have been maintained in organ culture with or without fructose added to the medium in order to stimulate the intestinal glucose-6-phosphatase (G-6-Pase). When the fructose is added, at the beginning of the culture, a three-fold increase of G-6-Pase in measured during the first 24 h. If the fructose is added after 24 h of culture, no significant increase of the G-6-Pase is registered in comparison with the controls. Proteins, DNA content and dissacharidase activities are not modified during the culture. Alkaline phosphatase activity presents a twofold increase in the controls and stimulated explants. The ultrastructural localization of the G-6-Pase is not altered during the culture.

Regulation of cardiac protein balance by hydrocortisone: interaction with insulin

In fetal mouse hearts in organ culture the rate of protein synthesis was substantially reduced and the rate of protein degradation slightly increased by hydrocortisone in the absence of insulin, but in the presence of insulin the steroid caused a small increase in protein synthesis and a significant reduction in protein degradation. Hydrocortisone promoted the net uptake (or reduced the net release) of branched-chain amino acids independent of insulin and independent of simultaneous changes in protein balance. The specific activities of the lysosomal enzymes cathepsin D and glucosaminidase were reduced by hydrocortisone in all media, whereas the specific activity of creatine kinase increased when the medium contained insulin but decreased in the absence of insulin. It is concluded that hydrocortisone regulates cardiac protein balance via alterations both in synthesis and in degradation. Some of the hormone's myocardial effects are influenced by insulin so that hydrocortisone is anabolic in its presence but catabolic in its absence.