The effect of acute and chronic insulin administration on insulin-like growth factor-I expression in the pituitary-intact and hypophysectomised rat

Obesity-related promotion of aberrant crypt foci in DMH-treated obese Zucker rats correlates with dyslipidemia rather than hyperinsulinemia

BACKGROUND

Obesity and energy restriction modulate the development of precancerous aberrant crypt foci (ACF) in animal models of colon cancer.

AIM

Investigation of the major obesity-associated determinants for ACF-development and underlying mechanisms leading to ACF-modulation, such as changes in DNA damage or colonocytes hyperproliferation.

METHODS

Lean and obese Zucker rats fed ad libitum (a.l.) or obese pair fed (p.f.) were induced with 1,2-dimethylhydrazine (DMH) for colon cancer. Multiple regression analyses were performed to identify major metabolic factors correlated with ACF number and size (aberrant crypts/ACF). DNA damage is analyzed by the comet-assay, epithelial proliferation by immunohistochemistry.

RESULTS

Aberrant crypt foci number was significantly elevated in Zucker obese a.l. (205.7+/-65.4 vs. lean 9.5+/-6.3, P<0.05) and is reduced by pair feeding in Zucker obese rats (81.4+/-28.5 vs. obese a.l., P<0.05). Compared to lean the ACF size was higher in Zucker obese a.l. (2.1+/-0.3 vs. lean 1.3+/-0.2., P<0.05) but is not reduced by pair feeding (1.7+/-0.2; P>0.05). While ACF number and size were modulated by genotype and/or pair feeding the DMH-induced DNA damage and hyperproliferation in colonocytes did not differ significantly between groups. Regression analysis showed that plasma parameters associated with lipid-metabolism (triglycerides, cholesterol, malondialdehyde) significantly correlated with the ACF number and size while parameters linked to carbohydrate-metabolism (glucose, insulin) were weaker determinants.

CONCLUSION

Obesity or pair feeding-associated modulation of ACF correlate with parameters related to lipid-metabolism but is not accompanied by changes in DNA damage and proliferation.

Possible relationship between insulin resistance and remnant-like lipoprotein particles in coronary endothelial dysfunction

BACKGROUND

High insulin resistance and elevated remnant lipoprotein levels both correlate with impaired coronary vascular endothelial function. Hyperinsulinemia induces abnormalities of lipid metabolism. However, the correlation among insulin resistance, remnant lipoproteins, and endothelial function has not been clinically elucidated.

HYPOTHESIS

This study was designed to elucidate the correlation among insulin resistance, remnant lipoproteins, and acetylcholine (ACh)-induced coronary artery response.

METHODS

Forty-nine patients suspected of having ischemic heart disease, but without angiographically significant atherosclerotic coronary artery disease, underwent an ACh provocation test. Fasting venous blood was taken early in the morning on the day coronary angiography was performed. The insulin resistance index (IR) was determined from fasting plasma glucose and insulin concentrations, using the homeostasis model assessment (HOMA). Serum levels of remnant-like lipoprotein particle cholesterol (RLP-C) were measured.

RESULTS

Homeostasis model assessment IR was significantly higher (3.65 +/- 1.38 vs. 0.75 +/- 0.14, p < 0.05) and log-transformed HOMA (Log HOMA) was even more significantly higher (0.20 +/- 0.12 vs. -0.29 +/- 0.08, p < 0.001) in the ACh-positive group (n = 23) than in the ACh-negative group (n = 26). The serum RLP-C level was also higher in the ACh-positive group than in the ACh-negative group (4.37 +/- 0.63 vs. 2.52 +/- 0.18 mg/dl, p < 0.01). Log HOMA and RLP-C levels correlated with each other (R = 0.54, p < 0.001). Multiple regression analysis indicated that only the RLP-C level was a dependent predictor of Log HOMA in various lipid profiles.

CONCLUSIONS

Both high insulin resistance and elevated remnant lipoprotein levels correlated and might have a crucial role in the impairment of coronary vascular endothelial function, even in patients without angiographically significant coronary artery disease.

Hyperinsulinemia, but not other factors associated with insulin resistance, acutely enhances colorectal epithelial proliferation in vivo

The similarity in risk factors for insulin resistance and colorectal cancer (CRC) led to the hypothesis that markers of insulin resistance, such as elevated circulating levels of insulin, glucose, fatty acids, and triglycerides, are energy sources and growth factors in the development of CRC. The objective was thus to examine the individual and combined effects of these circulating factors on colorectal epithelial proliferation in vivo. Rats were fasted overnight, randomized to six groups, infused iv with insulin, glucose, and/or Intralipid for 10 h, and assessed for 5-bromo-2-deoxyuridine labeling of replicating DNA in colorectal epithelial cells. Intravenous infusion of insulin, during a 10-h euglycemic clamp, increased colorectal epithelial proliferation in a dose-dependent manner. The addition of hyperglycemia to hyperinsulinemia did not further increase proliferation. Intralipid infusion alone did not affect proliferation; however, the combination of insulin, glucose, and Intralipid infusion resulted in greater hyperinsulinemia than the infusion of insulin alone and further increased proliferation. Insulin infusion during a 10-h euglycemic clamp decreased total IGF-I levels and did not affect insulin sensitivity. These results provide evidence for an acute role of insulin, at levels observed in insulin resistance, in the proliferation of colorectal epithelial cells in vivo.

Insulin administration suppresses an increase in insulin-like growth factor binding protein-2 gene expression stimulated by fasting in the chicken

1. We examined the changes in plasma IGF-I concentration and tissue IGFBP-2 gene expression of young fasted chickens refed a commercial diet or administered bovine insulin intravenously. 2. Plasma IGF-I concentration was decreased by fasting for 2 d. Although plasma IGF-I concentration was increased by refeeding, it didn't recover to the level of chickens fed a commercial diet ad libitum. 3. Insulin administration lowered plasma IGF-I concentration compared to other groups. 4. Hepatic IGFBP-2 mRNA was increased by fasting for 2 d and decreased by refeeding for 6 h. Insulin administration also decreased hepatic IGFBP-2 gene expression stimulated by fasting to the level of refed chickens. 5. IGFBP-2 mRNA in the gizzard was increased by fasting for 2 d and tended to decrease after refeeding for 6 h. Insulin administration decreased gizzard IGFBP-2 gene expression to less than that in refed chickens. 6. There was no between-treatment difference in IGFBP-2 mRNA in the brain and kidney. 7. These results suggest that the changes in IGFBP-2 gene expression in the liver and gizzard by fasting and refeeding might be partly regulated by the alteration in plasma insulin concentration.

Serum N-acetyl-beta-D-gulucosaminidase activity increases in association with insulin resistance in patients with coronary artery disease

N-acetyl-beta-D-glucosaminidase (NAG) is released from lysosomes, but the clinical significance of its serum activity in the pathogenesis of coronary artery disease has not been well understood. We measured serum NAG activity by a colorimetric method in consecutive 168 patients suspected of having coronary artery disease who underwent diagnostic coronary angiography. In addition, we evaluated the relationship between the activity and severity of coronary artery disease, as well as various coronary risk factors. Serum NAG activity was higher in the multi-vessel disease group than in the no stenotic lesion group (9.2+/-2.3 vs. 7.8+/-1.8 U/l, P<0.01) and in the single-vessel disease group (vs. 8.2+/-2.2 U/l, P<0.05). In all patients, Gensini score was closely correlated with the serum NAG activity (r = 0.39, P<0.001). Multiple regression analysis showed that serum NAG activity was correlated with plasma insulin level (r = 0.49, P<0.01), but not correlated with other coronary risk factors. In 126 patients without apparent diabetes mellitus, serum NAG was also correlated with plasma insulin level (r = 0.37, P<0.01) and additionally with insulin resistanc determined by homeostasis model assessment (r = 0.47, P<0.01). Our results suggested that serum NAG activity correlates with the severity of coronary artery disease in relation to plasma insulin level and insulin resistance, and thus can be an indicator of coronary artery disease based upon abnormalities of glucose metabolism.

Insulin resistance in systemic hypertension: pharmacotherapeutic implications

Systemic hypertension, a vascular disease with multiple origins, now is being linked to subtle abnormalities in glucose metabolism, which include insulin resistance and hyperinsulinemia. These conditions often occur together in patients with obesity, noninsulin-dependent diabetes, or both. Hyperinsulinemia and insulin resistance may cause systemic hypertension through multiple mechanisms. Insulin has a salt-retaining effect on the kidney. Also, insulin can augment catecholamine release, increase vascular sensitivity to vasoconstrictor substances, and decrease vascular sensitivity to vasodilator substances. In addition, insulin can increase production of tissue growth factors and help retain sodium and calcium in cells. Insulin resistance in patients can be treated with regular aerobic exercise, weight reduction, and a high-fiber diet. Pharmacologic approaches include hypoglycemic drugs, weight-reducing agents, and certain antihypertensive drugs that may have a favorable impact on both blood pressure and insulin resistance.

Abnormal arachidonate distribution in low-density lipoprotein and thoracic aorta in hyperinsulinemia

The mechanism by which hyperinsulinemia promotes atherogenesis is unknown. The effects of hyperinsulinemia on risk factors for atherosclerosis were investigated by subcutaneously injecting rats daily with an insulin-zinc suspension (20 U/kg) for 12 weeks. After this period, body mass and food consumption did not differ significantly between control and insulin-treated animals. Daily insulin injection significantly increased urinary excretion of epinephrine and decreased urinary excretion of norepinephrine and dopamine, but had no significant effect on blood pressure or heart rate. Although insulin decreased plasma triglyceride concentration by 44% (P < .01), the triglyceride to protein ratio in plasma low-density lipoprotein (LDL) was increased by 34% (P < .05) in insulin-treated rats; the cholesterol to protein and triglyceride to protein ratios remained unaffected, indicating a change in the quality of the LDL particle. Insulin also increased the percentage of arachidonic acid (20:4) in LDL triglycerides by 37% (P < .05). In contrast, cholesteryl esters and triglycerides in the thoracic aorta were significantly increased (49% and 91%, respectively) by insulin treatment. Insulin increased the percentage of monounsaturated fatty acids and decreased the percentage of n-6 fatty acids, including arachidonate, in aortic triglycerides. Insulin also increased the percentage of palmitoleic acid (16:1) and decreased the percentages of saturated fatty acids and n-6 fatty acids in aortic cholesteryl esters. These results indicate that insulin induced deposition of cholesteryl esters and triglycerides, especially those containing monounsaturated fatty acids, and abnormal arachidonate distribution in LDL and tissues. The data further suggest that the development of atherosclerosis in response to hyperinsulinemia may be associated with arachidonate-rich triglycerides in LDL.

Growth hormone (GH) and insulin-like growth factor-I (IGF-I) treatment of the GH-deficient dwarf rat: differential effects on IGF-I transcription start site expression in hepatic and extrahepatic tissues and lack of effect on type I IGF receptor mRNA expression

The rat IGF-I gene consists of six exons, with exons 3 and 4 forming a 'core' mature IGF-I coding region to which alternate 5' and 3' regions are spliced. Transcription occurs from four dispersed start sites (ss) approximately 382 (ss 1), approximately 343 (ss 2), approximately 245 (ss 3) and approximately 30-40 (ss 4) basepairs (bp) from the 3' end of exon 1, and from a region 50-70 bp from the 3' end of exon 2. The expression of ss mRNAs displays tissue-specific and ontogenic regulation. Alternate splicing of exon 5 produces E-peptide coding domain variants (Ea and Eb mRNAs), with the Eb form found predominantly in the liver. The regulation of IGF-I mRNA expression by GH and IGF-I in the GH-deficient dwarf (dw/dw) rat was investigated using antisense RNA probes in a solution hybridization RNase protection assay to detect leader exon and E domain variant mRNAs. GH treatment of dw/dw and normal Lewis rats increased the expression of all liver leader exon ss and E domain variants coordinately (1.6-1.9-fold increase, p < 0.01), although the increase observed in Eb transcripts was significantly higher in the dw/dw compared to the normal rat (p < 0.05). In kidney, GH treatment significantly increased exon 1 ss 3 and ss 4 transcripts by approximately 40% (p < 0.05). The expression of the other start sites was not affected by GH, suggesting that transcription factors may regulate start site usage independently. GH treatment was associated with a significant increase in IGF-I mRNA expression in skeletal muscle (p < 0.05) but not cardiac muscle or spleen. IGF-I treatment was associated with minor (approximately 20%) but significant (p < 0.05) reductions in IGF-I mRNA expression in the liver and kidney of dw/dw rats, suggesting that IGF-I can suppress IGF-I mRNA expression. IGF-I treatment did not affect IGF-I mRNA expression in cardiac and skeletal muscle of dw/dw rats. IGF-I receptor mRNA was detected in extrahepatic tissues only, and was not affected by either GH or IGF-I treatment. In summary, start site-specific regulation by GH was observed in kidney. GH increased IGF-I mRNA expression in muscle, kidney and liver, but had no effect in heart or spleen in the dw/dw rat. Our data suggest that systemic IGF-I can feedback on hepatic and renal IGF-I mRNA expression in the GH-deficient state.

Distribution of IGF-I mRNA and IGF-I binding sites in the rat kidney

In the present study we have investigated the distribution of IGF-I mRNA and IGF-I binding sites in the rat kidney. The distribution of IGF-I mRNA was investigated using a simple and sensitive non-radioactive in situ hybridisation technique based on probe labelling with digoxigenin labelled-UTP followed by detection with conventional immunocytochemical techniques. IGF-I mRNA was found predominantly in medullary collecting ducts and sparsely in cortical collecting duct cells. In addition IGF-I mRNA was expressed in scattered proximal tubular cells in the cortex and in cells confined to the glomerular tuft. IGF-I binding sites were studied using radiolabelled IGF-I and conventional autoradiographical techniques on tissue sections. It was found that IGF-I binding sites were widely distributed throughout the entire kidney and that the specific binding was highest in the inner medulla. These findings add further complexity to the understanding of IGF-I production and action on renal structures.

Insulin regulates insulin-like growth factor I mRNA in rat hepatocytes

To evaluate the regulatory role of growth hormone (GH) and insulin on insulin-like growth factor I (IGF-I) mRNA levels, we employed primary rat hepatocytes. Cells were incubated for 16 h with 10 nM insulin, 10 nM GH, or a combination thereof, and IGF-I mRNA levels were analyzed by Northern blotting. Insulin results in 2.5-fold and GH in 3.8-fold higher IGF-I mRNA levels than hormone-free controls, and a combination of insulin and GH had an additive effect (6.7-fold). The effect of 10 nM insulin was constant at variable GH concentrations. Therefore, GH and insulin affect IGF-I mRNA levels independently of each other. The half-maximal effective dose of insulin was 4.7 X 10(-10) M, and, in kinetic experiments, insulin was effective within 2 h. These findings demonstrate that insulin modulates hepatic IGF-I production by a direct regulation of the transcript levels of IGF-I.