Effect of a p38 MAPK inhibitor on FFA-induced hepatic insulin resistance in vivo.

The mechanisms whereby prolonged plasma free fatty acids elevation, as found in obesity, causes hepatic insulin resistance are not fully clarified. We herein investigated whether inhibition of p38 mitogen-activated protein kinase (MAPK) prevented hepatic insulin resistance following prolonged lipid infusion. Chronically cannulated rats were subdivided into one of four intravenous (i.v.) treatments that lasted 48 h: Saline (5.5 µl min(-1)), Intralipid plus heparin (IH, 20% Intralipid+20 U ml(-1) heparin; 5.5 µl min(-1)), IH+p38 MAPK inhibitor (SB239063) and SB239063 alone. During the last 2 h of treatment, a hyperinsulinemic (5 mU kg(-1) min(-1)) euglycemic clamp together with [3-(3)H] glucose methodology was carried out to distinguish hepatic from peripheral insulin sensitivity. We found that SB239063 prevented IH-induced hepatic insulin resistance, but not peripheral insulin resistance. SB239063 also prevented IH-induced phosphorylation of activating transcription factor 2 (ATF2), a marker of p38 MAPK activity, in the liver. Moreover, in another lipid infusion model in mice, SB239063 prevented hepatic but not peripheral insulin resistance caused by 48 h combined ethyloleate plus ethylpalmitate infusion. Our results suggest that inhibition of p38 MAPK may be a useful strategy in alleviating hepatic insulin resistance in obesity-associated disorders.

NADPH oxidase inhibition prevents beta cell dysfunction induced by prolonged elevation of oleate in rodents.

AIMS/HYPOTHESIS: The activation of NADPH oxidase has been implicated in NEFA-induced beta cell dysfunction. However, the causal role of this activation in vivo remains unclear. Here, using rodents, we investigated whether pharmacological or genetic inhibition of NADPH oxidase could prevent NEFA-induced beta cell dysfunction in vivo. METHODS: Normal rats were infused for 48 h with saline or oleate with or without the NADPH oxidase inhibitor apocynin. In addition, NADPH oxidase subunit p47(phox)-null mice and wild-type littermate controls were infused with saline or oleate for 48 h. This was followed by measurement of NADPH oxidase activity, reactive oxygen species (ROS) and superoxide imaging and assessment of beta cell function in isolated islets and hyperglycaemic clamps. RESULTS: Oleate infusion in rats increased NADPH oxidase activity, consistent with increased total but not mitochondrial superoxide in islets and impaired beta cell function in isolated islets and during hyperglycaemic clamps. Co-infusion of apocynin with oleate normalised NADPH oxidase activity and total superoxide levels and prevented beta cell dysfunction. Similarly, 48 h NEFA elevation in wild-type mice increased total but not mitochondrial superoxide and impaired beta cell function in isolated islets. p47(phox)-null mice were protected against these effects when subjected to 48 h oleate infusion. Finally, oleate increased the levels of total ROS, in both models, whereas inhibition of NADPH oxidase prevented this increase, suggesting that NADPH oxidase is the main source of ROS in this model. CONCLUSIONS/INTERPRETATION: These data show that NADPH-oxidase-derived cytosolic superoxide is increased in islets upon oleate infusion in vivo; and whole-body NADPH-oxidase inhibition decreases superoxide in concert with restoration of islet function.

Deletion of p47phox attenuates the progression of diabetic nephropathy and reduces the severity of diabetes in the Akita mouse.

AIMS/HYPOTHESIS: Reactive oxygen species (ROS) contribute to diabetes-induced glomerular injury and endoplasmic reticulum (ER) stress-induced beta cell dysfunction, but the source of ROS has not been fully elucidated. Our aim was to determine whether p47(phox)-dependent activation of NADPH oxidase is responsible for hyperglycaemia-induced glomerular injury in the Akita mouse, a model of type 1 diabetes mellitus resulting from ER stress-induced beta cell dysfunction. METHODS: We examined the effect of deleting p47 (phox) (also known as Ncf1), the gene for the NADPH oxidase subunit, on diabetic nephropathy in the Akita mouse (Ins2 (WT/C96Y)) by studying four groups of mice: (1) non-diabetic mice (Ins2 (WT/WT)/p47 (phox+/+)); (2) non-diabetic p47 (phox)-null mice (Ins2 (WT/WT)/p47 (phox-/-)); (3) diabetic mice: (Ins2 (WT/C96Y)/p47 (phox+/+)); and (4) diabetic p47 (phox)-null mice (Ins2 (WT/C96Y)/p47 (phox-/-)). We measured the urinary albumin excretion rate, oxidative stress, mesangial matrix expansion, and plasma and pancreatic insulin concentrations in 16-week-old mice; we also measured glucose tolerance and insulin sensitivity, islet and glomerular NADPH oxidase activity and subunit expression, and pro-fibrotic gene expression in 8-week-old mice. In addition, we measured NADPH oxidase activity, subunit expression and pro-fibrotic gene expression in high glucose-treated murine mesangial cells. RESULTS: Deletion of p47 (phox) reduced kidney hypertrophy, oxidative stress and mesangial matrix expansion, and also reduced hyperglycaemia by increasing pancreatic and circulating insulin concentrations. p47 (phox-/-) mice exhibited improved glucose tolerance, but modestly decreased insulin sensitivity. Deletion of p47 (phox) attenuated high glucose-induced activation of NADPH oxidase and pro-fibrotic gene expression in glomeruli and mesangial cells. CONCLUSIONS/INTERPRETATION: Deletion of p47 (phox) attenuates diabetes-induced glomerular injury and beta cell dysfunction in the Akita mouse.

Glucose-induced beta cell dysfunction in vivo in rats: link between oxidative stress and endoplasmic reticulum stress.

AIMS/HYPOTHESIS: Endoplasmic reticulum (ER) stress has been implicated in glucose-induced beta cell dysfunction. However, its causal role has not been established in vivo. Our objective was to determine the causal role of ER stress and its link to oxidative stress in glucose-induced beta cell dysfunction in vivo. METHODS: Healthy Wistar rats were infused i.v. with glucose for 48 h to achieve 20 mmol/l hyperglycaemia with or without the co-infusion of the superoxide dismutase mimetic tempol (TPO), or the chemical chaperones 4-phenylbutyrate (PBA) or tauroursodeoxycholic acid (TUDCA). This was followed by assessment of beta cell function and measurement of ER stress markers and superoxide in islets. RESULTS: Glucose infusion for 48 h increased mitochondrial superoxide and ER stress markers and impaired beta cell function. Co-infusion of TPO, which we previously found to reduce mitochondrial superoxide and prevent glucose-induced beta cell dysfunction, reduced ER stress markers. Similar to findings with TPO, co-infusion of PBA, which decreases mitochondrial superoxide, prevented glucose-induced beta cell dysfunction in isolated islets. TUDCA was also effective. Also similar to findings with TPO, PBA prevented beta cell dysfunction during hyperglycaemic clamps in vivo and after hyperglycaemia (15 mmol/l) for 96 h. CONCLUSIONS/INTERPRETATION: Here, we causally implicate ER stress in hyperglycaemia-induced beta cell dysfunction in vivo. We show that: (1) there is a positive feedback cycle between oxidative stress and ER stress in glucose-induced beta cell dysfunction, which involves mitochondrial superoxide; and (2) this cycle can be interrupted by superoxide dismutase mimetics as well as chemical chaperones, which are of potential interest to preserve beta cell function in type 2 diabetes.

Type 1 diabetes mellitus and major depressive disorder: evidence for a biological link.

AIMS/HYPOTHESIS: A growing body of research suggests that the prevalence of major depressive disorder (MDD) in children and youth with type 1 diabetes mellitus is significantly higher than that of youth without type 1 diabetes and is associated with increased illness severity. The objective of this article is to review the current literature on the pathophysiology of these two common diseases with respect to potential areas of overlapping biological dysfunction. METHODS: A search of English language articles published between 1966 and 2010 was conducted and augmented with manual review of reference lists from the identified publications. RESULTS: The evidence suggests plausible mechanisms whereby a biological relationship between type 1 diabetes and MDD may exist. These include the effects of circulating cytokines associated with autoimmune diabetes, the direct impact of insulin deficiency on neurogenesis/neurotransmitter metabolism, the effects of the chronic hyperglycaemic state, occurrence of iatrogenic hypoglycaemia and the impact of basal hyperactivity of the hypothalamic-pituitary-adrenal axis. CONCLUSIONS/INTERPRETATION: Shared biological vulnerabilities may be implicated in the comorbidity of type 1 diabetes and MDD. Further research is warranted to determine the magnitude of associations and confirm their observation in clinical populations.

Oral taurine but not N-acetylcysteine ameliorates NEFA-induced impairment in insulin sensitivity and beta cell function in obese and overweight, non-diabetic men.

AIMS/HYPOTHESIS: Antioxidants have been shown to ameliorate lipid-induced impairment of insulin action and beta cell function, both in vitro and in animal studies. The aim of the present study was to examine the effects of two orally administered antioxidants, N-acetylcysteine (NAC) and taurine (TAU), on lipotoxicity in humans. METHODS: Nine non-diabetic men, who were either overweight or obese, underwent three studies each, 4-6 weeks apart, in random order: (1) i.v. infusion of saline for 48 h (SAL); (2) i.v. infusion of Intralipid and heparin for 48 h to mimic chronic elevation of plasma NEFA (IH); and (3) IH infusion for 48 h with concurrent oral NAC (IH+NAC). Six men underwent similar studies except for study 3, where instead of NAC they received a 2 week pretreatment with oral TAU (IH+TAU). RESULTS: For both the NAC and TAU studies, a 48 h IH infusion alone without antioxidant impaired insulin sensitivity (S(I), 63% and 62% of SAL in NAC and TAU studies, respectively) and beta cell function, as evidenced by a reduction in disposition index (DI, 55% and 54% of SAL in NAC and TAU studies, respectively). NAC failed to prevent the lipid-induced increase in levels of the plasma oxidative stress marker malondialdehyde and did not prevent the lipid-induced reduction in S(I) or DI, whereas TAU completely prevented the rise in malondialdehyde and decreased 4-hydroxynonenal, and significantly improved S(I) (91% of SAL) and DI (81% of SAL). CONCLUSIONS/INTERPRETATION: Oral TAU ameliorates lipid-induced functional beta cell decompensation and insulin resistance in humans, possibly by reducing oxidative stress.

Differential effects of monounsaturated, polyunsaturated and saturated fat ingestion on glucose-stimulated insulin secretion, sensitivity and clearance in overweight and obese, non-diabetic humans.

AIMS/HYPOTHESIS: Prolonged elevation of plasma specific fatty acids may exert differential effects on glucose-stimulated insulin secretion (GSIS), insulin sensitivity and clearance. SUBJECTS AND METHODS: We examined the effect of oral ingestion, at regular intervals for 24 h, of an emulsion containing either predominantly monounsaturated (MUFA), polyunsaturated (PUFA) or saturated (SFA) fat or water (control) on GSIS, insulin sensitivity and insulin clearance in seven overweight or obese, non-diabetic humans. Four studies were conducted in each individual in random order, 4-6 weeks apart. Twenty-four hours after initiation of oral ingestion, subjects underwent a 2 h, 20 mmol/l hyperglycaemic clamp to assess GSIS, insulin sensitivity and insulin clearance. RESULTS: Following oral ingestion of any of the three fat emulsions over 24 h, plasma NEFAs were elevated by approximately 1.5- to 2-fold over the basal level. Ingestion of any of the three fat emulsions resulted in reduction in insulin clearance, and SFA ingestion reduced insulin sensitivity. PUFA ingestion was associated with an absolute reduction in GSIS, whereas insulin secretion failed to compensate for insulin resistance in subjects who ingested SFA. CONCLUSIONS/INTERPRETATION: Oral ingestion of fats with differing degrees of saturation resulted in different effects on insulin secretion and action. PUFA ingestion resulted in an absolute reduction in insulin secretion and SFA ingestion induced insulin resistance. Failure of insulin secretion to compensate for insulin resistance implies impaired beta cell function in the SFA study.

A pilot randomised controlled trial to reduce colorectal cancer risk markers associated with B-vitamin deficiency, insulin resistance and colonic inflammation.

Colorectal cancer risk is associated with biochemical markers for B-vitamin deficiency, insulin resistance and colonic inflammation, suggesting that these three conditions are each involved in colon carcinogenesis. We expected that dietary supplements of folic acid, n-3 fatty acids and calcium would reduce the markers and thus possibly cancer risk. We therefore randomised 98 participants, with previous colonic polyps or intramucosal carcinomas, to a combined treatment of supplementary folic acid, fish oil and calcium carbonate, or placebos for 28 days. Blood and faecal samples were obtained prior to and at the conclusion of the intervention and analysed for plasma folate, homocysteine, insulin, free fatty acids, triglycerides and faecal calprotectin. In addition, plasma vitamin B12, thiamin, glucose and C-reactive protein were assessed. Our supplemental strategy modestly affected some of the biomarkers associated with folate metabolism and insulin resistance, but had no effect on those associated with colonic inflammation. This pilot study demonstrates the feasibility and practicality of clinical trials aimed at reducing diet-related biochemical risk markers for colon cancer. We suggest that long-term intervention studies with tumour-related end points should be undertaken when the intervention agents used are found effective in short-term biochemical risk marker trials.

Prolonged increase of plasma non-esterified fatty acids fully abolishes the stimulatory effect of 24 hours of moderate hyperglycaemia on insulin sensitivity and pancreatic beta-cell function in obese men.

AIMS/HYPOTHESIS: A prolonged increase of plasma NEFA impairs acute glucose-stimulated insulin secretion (GSIS) in vitro and in vivo. Our study therefore examined the combined effect of increased plasma NEFA and glucose on GSIS in humans. METHODS: We examined GSIS on four occasions in eight obese men during a 10 mmol/l hyperglycaemic clamp and after a 24-h infusion of (i) normal saline, (ii) intralipid and heparin to raise plasma NEFA about two-fold above basal, (iii) 20% dextrose to raise plasma glucose to about 7.5 mmol/l and (iv) intralipid and heparin combined with 20% dextrose to raise plasma NEFA and glucose. RESULTS: In study (iii) insulin sensitivity was about 20% greater than in study (i) and the disposition index was about 50% higher. Insulin sensitivity tended to be lower in study (ii) whereas the disposition index was lower than in study (i), confirming previous observations. The combination of increased plasma NEFA and glucose (study iv) reduced insulin sensitivity in comparison with study (i) and completely abolished the increase in insulin sensitivity and disposition index seen in study (iii), but did not reduce the latter to a lower value than that in the saline control study (study i). CONCLUSIONS/INTERPRETATION: We showed that a prolonged increase of plasma NEFA completely abolishes the stimulatory effect of moderate hyperglycaemia on insulin sensitivity and beta-cell function in obese humans. This suggests that previous observations, showing that a prolonged increase of plasma NEFA impairs pancreatic beta-cell function, also apply to the hyperglycaemic state.

Effect of increased plasma non-esterified fatty acids (NEFAs) on arginine-stimulated insulin secretion in obese humans.

AIMS/HYPOTHESIS: We have shown previously that the increase of plasma non-esterified fatty acids for 48 h results in decreased glucose-stimulated insulin secretion in lean and non-diabetic obese subjects. It is currently not known if a prolonged increase in non-esterified fatty acids also impairs the insulin secretory response to non-glucose secretagogues. METHODS: Heparin and intralipid (to increase plasma non-esterified fatty acid concentrations by about two- to fourfold) or normal saline was infused intravenously for 48 h in 14 non-diabetic obese subjects. On the third day in both studies, insulin, C-peptide, proinsulin, and insulin secretion rate were assessed in response to an intravenous arginine infusion at fasting glucose concentration and a second arginine infusion after a 60-min 11 mmol/l hyperglycaemic clamp. RESULTS: There were no significant differences detected in acute (5 min) or total (90 min) arginine-stimulated C-peptide or insulin secretion response in the heparin-intralipid study compared with the control group at fasting glucose or during hyperglycaemia. CONCLUSION/INTERPRETATION: We have shown that a prolonged increase in plasma NEFA does not blunt arginine-stimulated insulin secretion or plasma insulin concentrations in non-diabetic obese subjects. These findings suggest that the previously demonstrated NEFA-induced impairment in insulin secretory response to glucose cannot be generalized for non-glucose secretagogues.

The effect of systemic versus portal insulin delivery in pancreas transplantation on insulin action and VLDL metabolism.

Combined kidney-pancreas transplantation (KPT) with anastomosis of the pancreatic vein to the systemic circulation (KPT-S) or to the portal circulation (KPT-P) provides a human model in which the chronic effects of portal versus systemic insulin delivery on glucose and VLDL metabolism can be examined. Despite similar plasma glucose and C-peptide levels, KPT-S (n = 9) had an approximate twofold elevation of fasting and intravenous glucose-stimulated plasma insulin levels compared with both KPT-P (n = 7) and healthy control subjects (n = 15). The plasma free fatty acid (FFA) levels were elevated in both transplant groups versus control subjects, but the plasma insulin elevation necessary to lower plasma FFA by 50% was approximately two times higher in KPT-S versus KPT-P and control subjects. Endogenous glucose production was similar in KPT-S and KPT-P, despite approximately 35% higher hepatic insulin levels in the latter, and was suppressed to a greater extent during a euglycemic-hyperinsulinemic clamp in KPT-S versus KPT-P. Total-body glucose utilization during the euglycemic-hyperinsulinemic clamp was approximately 40% lower in KPT-S versus KPT-P, indicating peripheral tissue but not hepatic insulin resistance in KPT-S versus KPT-P. Both transplant groups had an approximate twofold elevation of triglyceride (TG)-rich lipoprotein apolipoprotein B (apoB) and lipids versus control subjects. Elevation of VLDL-apoB and VLDL-TG in both transplant groups was entirely explained by an approximately 50% reduction in clearance of VLDL compared with healthy control subjects. In the presence of increased FFA load but in the absence of hepatic overinsulinization and marked hepatic insulin resistance, there was no elevation of VLDL secretion in KPT-S versus KPT-P and control subjects. These findings suggest that chronic systemic hyperinsulinemia and peripheral tissue insulin resistance with the consequent elevation of plasma FFA flux are insufficient per se to cause VLDL overproduction and that additional factors, such as hepatic hyperinsulinemia and/or gross insulin resistance, may be an essential prerequisite in the pathogenesis of VLDL overproduction in the common form of the insulin resistance syndrome.

Low-dose IGF-I has no selective advantage over insulin in regulating glucose metabolism in hyperglycemic depancreatized dogs.

At supraphysiological levels, IGF-I bypasses some forms of insulin resistance and has been proposed as a therapeutic agent in the treatment of diabetes. Unfortunately, side effects of high-dose IGF-I (100-250 microg/kg) have precluded its clinical use. Low-dose IGF-I (40-80 microg/kg), however, shows minimal side effects but has not been systematically evaluated. In our previous study under conditions of declining glucose, low-dose IGF-I infusion was more effective in stimulating glucose utilization, but less effective in suppressing glucose production and lipolysis than low-dose insulin. However, under conditions of hyperglycemia, we could not observe any differential effects between high-dose infusions of IGF-I and insulin. To determine whether the differential effects of IGF-I and insulin are dose-related or related to the prevailing glucose level, 3 h glucose clamps were performed in the same animal model as in the previous studies, i.e. the moderately hyperglycemic (175 mg/dl) insulin-infused depancreatized dog, with additional infusions of low-dose IGF-I (67.8 microg/kg, i.e. 29.1 microg/kg bolus plus 0.215 microg/kg( )per min infusion; n=5) or insulin 49.5 mU/kg (9 mU/kg bolus plus 0.45 mU/kg per min; n=7). As in the previous study under conditions of declining glucose, low-dose IGF-I had significant metabolic effects in vivo, in our model of complete absence of endogenous insulin secretion. Glucose production was similarly suppressed with both IGF-I and insulin, by 54+/-3 and 56+/-2% s.e. (P=NS) respectively. Glucose utilization was stimulated to the same extent (IGF-I 5.2+/-0.2, insulin 5.5+/-0.3 mg/kg per min, P=NS). Glucagon, free fatty acid, glycerol, alanine and beta-hydroxybutyrate, were suppressed, while lactate and pyruvate levels were raised, similarly with IGF-I and insulin. We conclude that: (i) differential effects of IGF-I and insulin may be masked under hyperglycemic conditions, independent of the hormone dose; (ii) low-dose IGF-I has no selective advantage over additional insulin in suppressing glucose production and lipolysis, nor in stimulating glucose utilization during hyperglycemia and subbasal insulin infusion when insulin secretion is absent, as in type 1 diabetes mellitus.

Chronic intraperitoneal insulin delivery, as compared with subcutaneous delivery, improves hepatic glucose metabolism in streptozotocin diabetic rats.

We have previously shown that chronic insulin treatment by the intraperitoneal route normalizes the elevated glucose production (GP) in streptozotocin (STZ) diabetic rats, while insulin delivered by the subcutaneous route only partially normalizes GP. To investigate the biochemical mechanism of the effect of chronic insulin delivery by either route on hepatic glucose metabolism, we measured the hepatic activity of glucose 6-phosphatase (G6Pase) and glucokinase (GK). Four groups of rats were used: (1) nondiabetic rats (N, n = 7), (2) untreated STZ diabetic rats (D, n = 8), (3) diabetic rats treated intraperitoneally (IP, n = 6), or (4) subcutaneously (SC, n = 8) (both 3 U of insulin/d). Glucose levels, higher in D, were normalized by insulin treatment regardless of route. Peripheral insulin levels were lowest in D and highest in SC as expected (N, 162 +/- 18 pmol/L; D, 66 +/- 12; IP, 360 +/- 96; SC, 798 +/- 198). STZ diabetes resulted in a 10-fold decrease in GK (P < .001), and a 2-fold increase in G6Pase activity (P < .01). Both intraperitoneal and subcutaneous treatments normalized G6Pase activity. In contrast, with subcutaneous but not intraperitoneal treatment, GK activity was still 35% less than normal (SC v N, P < .05). Glucose 6-phosphate (G6P) levels did not differ among the groups. In summary: (1) the increase in GP in D reflected increased activity of G6Pase and reduced activity of GK, (2) the partial suppression of GP with subcutaneous insulin treatment reflected correction of increased G6Pase activity, but only partial correction of low GK activity, and (3) the normalization of GP with intraperitoneal insulin treatment reflected correction of both increased G6Pase activity and low GK activity. Our current studies indicate that chronic intraperitoneal insulin treatment is superior to subcutaneous treatment with regard to hepatic glucose metabolism.

Increased responses of glucagon and glucose production to hypoglycemia with intraperitoneal versus subcutaneous insulin treatment.

The study aim was to investigate the effect of the route of insulin treatment on the glucagon and glucose production (GP) responses to hypoglycemia in the diabetic rat. Experiments were performed in 4 groups of rats: (1) streptozotocin (STZ)-induced diabetic, untreated (D, n = 7), (2) diabetic treated with subcutaneous insulin (DSC, n = 8), (3) diabetic treated with intraperitoneal insulin (DIP, n = 6), and (4) normal control (N, n = 10). Slow-release insulin implants were used in DSC and DIP rats for 10 to 14 days (3 U/d). A hyperinsulinemic (120 pmol x kg(-1) x min(-1) insulin)-hypoglycemic (glycemia = 2.5 +/- 0.1 mmol/L) clamp following an isoglycemic basal period was performed in 5-hour fasted rats. Basal plasma glucose was normalized in both DSC and DIP rats; however, in DSC but not DIP rats, glucose normalization required peripheral hyperinsulinemia. Tracer-determined GP, which was elevated in D rats, was completely normalized in DIP but only partially corrected in DSC rats. Basal glucagon levels were similar in all groups. During hypoglycemia, GP was suppressed in D rats (delta, -28.9 +/- 5.0 micromol x kg(-1) x min(-1), moderately increased in DSC rats (delta, 6.1 +/- 5.6, P < .01 v D), but markedly increased in DIP and N rats (delta, 34.5 +/- 4.5 for DIP and 16.8 +/- 2.8 for N; P < .01 vD, P < .05 for DIP v DSC or N). Plasma glucagon increased 6-fold in N (945 +/- 129 pg/mL), only doubled in D (424 +/- 54), and tripled in DSC (588 +/- 83), but increased 5-fold in DIP rats (1,031 +/- 75, P < .05 v D and DSC). We conclude that in STZ-diabetic rats, (1) intraperitoneal but not subcutaneous insulin treatment normalizes basal GP, and (2) intraperitoneal insulin treatment as compared with subcutaneous treatment alleviates peripheral hyperinsulinemia and results in increased glucagon and GP responses to hypoglycemia.

Mechanisms linking diet and colorectal cancer: the possible role of insulin resistance.

Diet is clearly implicated in the origin of colorectal cancer, with risk factors for the disease including reduced consumption of vegetables, fiber, and starch and increased consumption of red meat and animal fat. Several hypotheses have been developed to explain these associations. Most recently, McKeown-Eyssen and Giovannucci noted the similarity of the risk factors for colorectal cancer and those for insulin resistance and suggested that insulin resistance leads to colorectal cancer through the growth-promoting effect of elevated levels of insulin, glucose, or triglycerides. We briefly review the evidence from observational, epidemiological, and experimental animal studies linking diet with insulin resistance and colorectal cancer. The evidence suggests that diets high in energy and saturated fat and with high glycemic index carbohydrate and low levels of fiber and n-3 fatty acids lead to insulin resistance with hyperinsulinemia, hyperglycemia, and hypertriglyceridemia. We then consider how insulin, the related insulin-like growth factors, triglycerides, and nonesterified fatty acids could lead to increased growth of colon cancer precursor lesions and the development of colorectal cancer. Finally, we consider the implications of this scheme on possible future research directions, including studies of satiety and clinical tests of the importance of insulin resistance in the colon carcinogenesis process.

Effect of JTT-501 on net hepatic glucose balance and peripheral glucose uptake in alloxan-induced diabetic dogs.

JTT-501, a new insulin sensitizer, improves peripheral glucose uptake in insulin-resistant animals such as KK-Ay mice and Zucker fatty rats. However, the effect of JTT-501 on hepatic glucose metabolism has not been addressed. To investigate this effect, experiments were performed on 6 alloxan-diabetic dogs. Three experiments were conducted for each dog: the treatment experiment, which followed a 10-day oral treatment with JTT-501 30 mg x kg(-1) x d(-1), and 2 control experiments 2 weeks before and 2 weeks after the treatment experiment. A hyperinsulinemic-hyperglycemic clamp was performed with the tracer dilution method (intraportal insulin infusion rate, 18 pmol x kg(-1) x min(-1)). Arterial hyperglycemia (approximately 10 mmol/L) was maintained by adjusting the peripheral glucose infusion rate. After a 45-minute basal period (period I), portal glucose infusion (22.2 micromol x kg(-1)min(-1)) was administered for 120 minutes (period II). This was followed by a 90-minutes recovery period (period III). JTT-501 increased insulin-stimulated glucose utilization (P < .05) and enhanced insulin-mediated suppression of glucose production (P < .05) in periods I and III. Net hepatic glucose balance (NHGB) determined by the arterial-venous (A-V) difference method was increased by JTT-501 in period II (P < .01). We conclude that JTT-501 enhances both hepatic and peripheral insulin sensitivity and therefore may have important therapeutic effects in type 2 diabetes.

Prolonged elevation of plasma free fatty acids impairs pancreatic beta-cell function in obese nondiabetic humans but not in individuals with type 2 diabetes.

Our recent in vivo observations in healthy nonobese humans have demonstrated that prolonged elevation of plasma free fatty acids (FFAs) results in diminished glucose-stimulated insulin secretion (GSIS) when the FFA-mediated decrease in insulin sensitivity is taken into account. In the present study, we investigated whether obese individuals and patients with type 2 diabetes are more sensitive than healthy control subjects to the inhibitory effect of prolonged elevation of plasma FFAs on GSIS. In seven patients with type 2 diabetes and seven healthy nondiabetic obese individuals, we assessed GSIS with a programmed graded intravenous glucose infusion on two occasions, 6-8 weeks apart, with and without a prior 48-h infusion of heparin and Intralipid, which was designed to raise plasma FFA concentration approximately twofold over basal. The nondiabetic obese subjects had a significant 21% decrease in GSIS (P = 0.0008) with the heparin and Intralipid infusion, associated with a decrease in whole body insulin clearance. The impairment in GSIS was evident at low (<11 mmol/l) but not at higher plasma glucose concentrations. In contrast, the patients with type 2 diabetes had a slight increase in GSIS (P = 0.027) and no change in insulin clearance, although there was marked interindividual variability in response. Plasma proinsulin concentrations measured in a subset of subjects were not altered in either group by the infusion of heparin and Intralipid. In summary, 1) obese nondiabetic individuals are susceptible to a desensitization of GSIS with heparin and Intralipid infusion, and 2) patients with type 2 diabetes do not demonstrate such susceptibility when FFAs are elevated approximately twofold above basal with heparin and Intralipid. Our results suggest that FFAs could play an important role in the development of beta-cell failure in obese individuals who are at risk for developing type 2 diabetes. They do not, however, seem to further deteriorate the beta-cell function of patients who already have established type 2 diabetes and may even result in a slight increase in GSIS in this latter group.

Possible mechanisms relating diet and risk of colon cancer.

Two recent developments in cancer epidemiology and experimental carcinogenesis provide the basis for two possible mechanisms relating diet and colon cancer risk. The first development is the accumulating epidemiological evidence for an association between insulin resistance and colonic adenomas and cancers. This evidence suggests the following mechanism: the consumption of excess dietary energy results in the development of insulin resistance with increased circulating levels of insulin, triglycerides, and non-esterified fatty acids. These circulating factors subject colonic epithelial cells to a proliferative stimulus and also expose them to reactive oxygen intermediates. These long-term exposures result in the promotion of colon cancer. The second development is the continuing identification of agents that significantly inhibit experimental colon carcinogenesis. These observations suggest the following mechanism: focal loss of epithelial barrier function resulting from a failure of terminal differentiation results in the "leak" of a presently undefined toxin and a focal inflammatory response characterized by evidence of the activation of the COX-2 enzyme and an oxidative stress with the release of reactive oxygen intermediates. The resulting focal proliferation and mutagenesis give rise to aberrant crypt foci and adenomas. The process is inhibited by: (a) demulcents confined to the colonic lumen that "repair" the surface; (b) anti-inflammatory agents; or (c) antioxidants. The two mechanisms, i.e., insulin resistance acting throughout the body and focal epithelial barrier failure acting locally, can describe most of the known relationships between diet and colon cancer risk.

Acute enhancement of insulin secretion by FFA in humans is lost with prolonged FFA elevation.

The in vivo effect of elevated free fatty acids (FFA) on beta-cell function in humans remains extremely controversial. We examined, in healthy young men, the acute (90 min) and chronic (48 h) effects of an approximately twofold elevation of plasma FFA vs. control on glucose-stimulated insulin secretion (GSIS). GSIS was studied in response to a graded intravenous glucose infusion (peak plasma glucose, approximately 10 mmol/l, n = 8) and a two-step hyperglycemic clamp (10 and 20 mmol/l, n = 8). In the acute studies, GSIS was significantly higher, insulin sensitivity index (SI) was lower, and disposition index (DI = insulin sensitivity x insulin secretion) was unchanged with elevated FFA vs. control [2-step clamp: DI = 8.9 +/- 1.4 x 10(-3) l2. kg-1. min-2 in control vs. 10.0 +/- 1.9 x 10(-3) l2. kg-1. min-2 with high FFA, P = nonsignificant (NS)]. In the chronic studies, there was no difference in absolute GSIS between control and high FFA studies, but there was a reduction in SI and a loss of the expected compensatory increase in insulin secretion as assessed by the DI (2-step clamp: DI = 10.0 +/- 1.2 x 10(-3) l2. kg-1. min-2 in control vs. 6.1 +/- 0.7 x 10(-3) l2. kg-1. min-2 with high FFA, P = 0.01). In summary, 1) acute and chronic FFA elevation induces insulin resistance; 2) with acute FFA elevation, this insulin resistance is precisely countered by an FFA-induced increase in insulin secretion, such that DI does not change; and 3) chronic FFA elevation disables this beta-cell compensation.