K L Luskey

Amylin secretion from the rat pancreas and its selective loss after streptozotocin treatment.

Amylin, a peptide copackaged with insulin in beta-cell granules, was measured in the effluent of the perfused rat pancreases by means of a newly developed specific radioimmunoassay. Its secretion parallels that of insulin in response to 20 mM glucose, 10 mM arginine, or the combination thereof. The relative molar amount of secreted amylin was estimated to be 25-37% that of insulin. Treatment with a borderline diabetogenic dose of streptozotocin reduced amylin response without significantly changing the insulin response. A severely diabetogenic dose of streptozotocin totally abolished amylin release and markedly reduced insulin release. The selective impairment of amylin secretion in streptozotocin-treated rats could represent an early manifestation of beta-cell depletion or injury.

Coordinate regulation of amylin and insulin expression in response to hypoglycemia and fasting

Amylin is a 37–amino acid peptide synthesized in the pancreatic β-cell and cosecreted with insulin. In situ hybridization of nondiabetic rat pancreas shows that insulin and amylin RNA are both localized within the islet of Langerhans in a similar distribution. After 12 days of insulin-induced hypoglycemia (mean blood glucose 3.0 ± 0.4 mM [54 ± 8 mg/dl]), both insulin and amylin RNA fell > 95%. However, maintenance of euglycemia by simultaneous infusion of glucose with insulin did not suppress insulin or amylin RNA. Fasting suppressed amylin and insulin secretion from the isolated, perfused pancreas 70 and 58%, respectively, and with refeeding, secretion rates recovered to fed levels. Despite these changes in the rates of secretion, the relative ratio of amylin to insulin was not significantly different in fed, fasted, or refed rats. The molar ratio of insulin to amylin was estimated to be 100:2.3–2.6. Both insulin and amylin RNA was suppressed ∼ 50% in response to fasting. Thus, although the absolute amounts of insulin and amylin change substantially under the conditions tested, the relative amounts of these peptides do not change.

Regulation of Cholesterol Synthesis: Mechanism for Control of HMG CoA Reductase

Publisher Summary Essentially all mammalian cells need cholesterol. Sterols are utilized for the synthesis of plasma membranes in all cells and for the production of steroid hormones, lipoproteins, and bile acids in appropriate cell types. Cells possess an enzymatic pathway to synthesize the cholesterol they need. This involves a series of over 20 reactions in which the simple building block—acetyl coenzyme A (CoA)—is ultimately assembled into a sterol molecule. However, this is not the only way by which cells can get cholesterol; they also possess a system to internalize lipoproteins from the external environment. To regulate intracellular synthesis, cholesterol or a closely related sterol metabolite acts via negative feedback mechanisms to suppress the activities of several key enzymes involved in the early steps of this pathway. These enzymes catalyze the synthesis of acetoacetyl CoA, the synthesis of 3-hydroxy-3-methylglutaryl (HMG) CoA, the reduction of HMG CoA to form mevalonate, and the phosphorylation of mevalonate. When cells are grown in the absence of cholesterol, all of these enzymes are induced, and their activities decline after the addition of cholesterol. Of these reactions, the production of mevalonate from HMG CoA is the most critical regulated step. This reaction is catalyzed by the enzyme HMG CoA reductase. This chapter discusses what has been learned recently about the molecular mechanisms that are responsible for regulating the expression of HMG CoA reductase at the level of messenger RNA synthesis and protein degradation. It further reviews the structural characteristics of HMG CoA reductase, regulation of HMG CoA reductase degradation, structure of the HMG CoA reductase mRNA and gene, and transcriptional regulation of HMG CoA reductase.