Cecilia Hofmann

New oral thiazolidinedione antidiabetic agents act as insulin sensitizers

O f the 12 million people in the U.S. afflicted with NIDDM (1), nearly 40% are currently being treated with oral hypoglycemic agents (2); it is clearly imperative to seek optimal treatment for these patients. Development of treatment agents has followed principally from empirical observations of potent hypoglycemic actions. Intense recent research studies have produced an enhanced understanding of cellular derangements underlying the NIDDM condition. The identification of specific cellular deficits in NIDDM presents opportunities for development of targeted treatments. NIDDM is now known to involve several anomalies. These include I) aberrant pancreatic insulin secretion and 2) insulin resistance in peripheral tissues, with reduced glucose uptake particularly in muscle, and hepatic glucose overproduction (3-8). Although the order of appearance of these derangements during development of diabetes remains unknown, the overt diabetic condition is characterized by the coexistence of defective insulin secretion with insulin resistance. When fasting hyperglycemia becomes severe, i.e., >12 mM, the ability of insulin to suppress hepatic glucose production becomes impaired (7), and the insulin-resistant liver may thus be involved in maintaining the diabetic state. Candidate hypoglycemic agents could conceivably have corrective actions at any of these sites (Fig. 1). Sulfonylurea agents now used as oral therapy for NIDDM appear to act generally as secretagogues to increase the availability of insulin (1,9). Support for this finding comes from the observation that sulfonylurea drugs are not effective hypoglycemic agents in either insulin-dependent diabetic patients or in diabetic animal models lacking functional (3cells due to chemical destruction or surgical removal of the pancreas (1). An alternate NIDDM treatment strategy would be to use agents that act principally on target tissues as insulin sensitizers, thus overcoming insulin resistance. Compounds known as thiazolidinediones have been reported to have such actions. These agents are currently undergoing clinical trials for efficacy in treating NIDDM, and are the subject of this commentary.

Altered glucose transporter mRNA abundance in a rat model of endotoxic shock.

To better understand molecular mechanisms of glucose transport in shock, we studied glucose transporter isoform mRNA abundance after injection of S. enteritidis endotoxin (40 mg/kg) or saline. Six to 8 hours after injection, endotoxin-treated animals compared to controls became hypoglycemic (44 +/- 6 vs. 111 +/- 4 mg/dl) and lactacidemic (5.9 +/- 0.5 vs. 1.3 +/- 0.1). At such times, tissue RNA was isolated and hybridized to Riboprobes for GLUT1 (erythrocyte), GLUT2 (liver), and GLUT4 (muscle/fat) glucose transporter isoforms and expressed as percent of control. GLUT1 mRNA abundance was increased in fat (660%, p less than .05), soleus muscle (314%, p less than .05), and liver (871%, p less than .001) of endotoxin-treated rats. Soleus muscle GLUT4 mRNA levels were increased (+33%, p less than .02), while liver GLUT2 mRNA levels were markedly decreased (-58%, p less than .01). The overall increase in GLUT1 mRNA abundance accompanied by lowered liver GLUT2 mRNA levels may either cause or reflect profoundly altered glucose transport.

Monoclonal antibodies to the human insulin receptor mimic a spectrum of biological effects in transfected 3T3/HIR fibroblasts without activating receptor kinase.

The effects of four monoclonal antibodies to the alpha subunit of the human insulin receptor were studied in transfected mouse 3T3 fibroblasts expressing human insulin receptors (3T3/HIR). Three antibodies, MA-5, MA-20, and MA-51, mimicked insulin stimulation of the uptake of both 2-deoxy-D-glucose and alpha-aminoisobutyrio acid, and S6 kinase activity. Antibody MA-5 also mimicked insulin stimulation of [3H]thymidine incorporation and cell growth. Although these antibodies mimicked insulin stimulation of biological effects, they failed to significantly activate insulin receptor tyrosine kinase activity. These studies suggest, therefore, that the insulin receptor can signal a variety of cellular functions without stimulation of receptor kinase activity.

Down-regulation of tissue specific tumor necrosis factor-alpha in the liver and lung after burn injury and endotoxemia.

Burn injury and endotoxin lead to the development of a systemic inflammatory response. Because tumor necrosis factor-alpha (TNF-alpha) is a component of the proinflammatory response, we have determined the effect of burn injury and endotoxin in a murine model of thermal on tissue specific TNF-alpha levels in the liver and lung. Male mice were divided into four groups and injected with endotoxin (ETX) (2.5 mg/kg intraperitoneally) or saline (CNTL) or subjected to a 16% full-thickness scald burn (B), or ETX administration 72 hours after burn injury (B+ETX). Animals were killed at 0 to 24 hours after ETX or CNTL, 0 to 72 hours after B, and 72 to 96 hours after B+ETX (ETX administration 72 hours after B). TNF-alpha mRNA by Northern blot and protein analysis by enzyme-linked immunosorbent assay were determined and protein expressed as nanogram per gram of tissue. Statistical analysis was performed using analysis of variance with significance at p < 0.05. Burn injury did not result in detectable levels of liver or lung TNF protein or mRNA. Endotoxin administration resulted in a near six-fold rise in liver TNF protein compared with controls at 1, 2, and 6 hours after ETX (p < 0.05 to p < 0.001). Liver mRNA remained elevated from 20 minutes to 24 hours after ETX versus CNTL (p < 0.05). Endotoxin injection produced a persistent lung TNF protein elevation reaching significance at 1 and 2 hours (p < 0.001) and a rise in mRNA at 40 minutes to 6 hours (p < 0.05) versus CNTL. The liver showed a trend of reduced mRNA after B+ETX versus ETX (p = NS), whereas protein levels were reduced by 50 to 60% at 1 and 2 hours (p < 0.01). Lung mRNA values after B+ETX were only 40% compared with ETX at nearly all time points (p < 0.001) but were 15 times above CNTL values at 2 hours (p < 0.05). Based on these results, we conclude that burn injury did not cause an increase in liver or lung tissue specific TNF-alpha. However, the presence of a preexisting burn injury dramatically altered the response to endotoxin and the primary point of regulation appears to be at the posttranscriptional level.

Long-term effect of radiation on thyroid function and tumor formation.

Abstract Cellular injury coupled with hypersecretion of thyroid stimulating hormone (TSH) is considered the mechanism for radiation-induced thyroid tumors. This hypothesis was tested by administering 40 μCi of 131 I to male and female rats. Serum thyroxine (T 4 ) decreased significantly overtime in male and female control and radiated animals. T 4 was essentially the same in 6- and 12-month-old radiated and control animals, but a significant decrease in T 4 was noted in radiated females at 21 months. TSH decreased overtime in control animals, but a significant increase in TSH was required in radiated animals to maintain T 4 levels. Thyrocalcitonin (TC) was significantly greater in 21-month-old control females than in males or radiated females. A significant increase in follicular cell thyroid neoplasms in radiated animals was associated with the elevated TSH levels. The importance of sex in thyroid tumor formation is illustrated by the fact that males had a significantly higher frequency of thyroid neoplasms than did females. C-cell hyperplasia and medullary thyroid carcinoma only occurred in control animals and accounted for their higher TC levels. This study confirms that administration of low doses of 131 I leads to an increased frequency of thyroid tumor formation in rats and that direct cellular injury plus hypersecretion of TSH seem responsible for these radiation-induced tumors.

Insulin-ricin B hybrid molecules: Receptor binding and biological activity in a minimal-deviation hepatoma cell line

Hybrid molecules were produced by covalently coupling the hormone insulin to the binding chain B of the plant toxin ricin. Binding of the insulin-ricin B hybrid to minimal-deviation hepatoma cells occurred primarily through ricin-specified cell-surface carbohydrates (galactose, N-acetylgalactosamine) since 125I-insulin-ricin B binding to cells could be 90% displaced by 50 mM lactose. [14C]Glucose incorporation into glycogen was maximally stimulated approximately 80% by insulin, whereas maximum stimulation by insulin-ricin B hybrid was greater than 100%. Ricin B chain alone was non-stimulating at concentrations tested (10(-9)-10(-7) M). Furthermore, the stimulation of [14C]glycogen labeling mediated by the hybrid was markedly inhibited by 1 mM lactose, while this sugar had no effect on the stimulation mediated by native insulin. Additionally, a preparation of ricin B shown to actively displace up to 80% of the binding of 125I-hybrid to cells also inhibited hybrid-mediated [14C]glycogen production. These results indicate that insulin-ricin B hybrid molecules possess toxin-specified binding abilities while evoking the insulin-associated cellular response of stimulated incorporation of [14C]glucose into glycogen. Such results thus suggest the possibility that alternate cell-surface receptors may play a role in conveying insulins intracellular metabolic-control signals.

ENDOTOXIN ALTERS DEXTROSE INSULIN SECRETION IN SUCKLING RATS

To better understand the molecular mechanisms of decreased insulin secretion during endotoxic shock, isolated pancreatic islets from 10 day old rats were studied Pancreatic islets were isolated A hours after an ip injection of saline or endotoxin (LPS: 0.1mg/kg: LD90 at 24 hours). Glucose transporter GLUTI and 2 mRNA abundance in isolated islets were determined by Northern blots, and morphologic changes of islets were observed. Five islets were incubated for 60 minutes in 10 ml of RPMI media with dextrose (500mg/ml) to determine insulin release (pM). Abundance of glucose transporter isotorm in LPS treated group was expressed as percent of saline treated controla.LPS did not alter islet anatomy. Insulin response to dextrose was decreased in the LPS treated group. The abundance of mRNA for GLUT1 was increased and GLUT2, decreased, thus showing divergent regulation of the two transporter isoforms. Altered glucose transporter gone expression in pancreatic islets may help explain decreased insulin release in the young rat with endotoxic shock.