Disordered fat storage and mobilization in the pathogenesis of insulin resistance and type 2 diabetes

The primary genetic, environmental, and metabolic factors responsible for causing insulin resistance and pancreatic beta-cell failure and the precise sequence of events leading to the development of type 2 diabetes are not yet fully understood. Abnormalities of triglyceride storage and lipolysis in insulin-sensitive tissues are an early manifestation of conditions characterized by insulin resistance and are detectable before the development of postprandial or fasting hyperglycemia. Increased free fatty acid (FFA) flux from adipose tissue to nonadipose tissue, resulting from abnormalities of fat metabolism, participates in and amplifies many of the fundamental metabolic derangements that are characteristic of the insulin resistance syndrome and type 2 diabetes. It is also likely to play an important role in the progression from normal glucose tolerance to fasting hyperglycemia and conversion to frank type 2 diabetes in insulin resistant individuals. Adverse metabolic consequences of increased FFA flux, to be discussed in this review, are extremely wide ranging and include, but are not limited to: 1) dyslipidemia and hepatic steatosis, 2) impaired glucose metabolism and insulin sensitivity in muscle and liver, 3) diminished insulin clearance, aggravating peripheral tissue hyperinsulinemia, and 4) impaired pancreatic beta-cell function. The precise biochemical mechanisms whereby fatty acids and cytosolic triglycerides exert their effects remain poorly understood. Recent studies, however, suggest that the sequence of events may be the following: in states of positive net energy balance, triglyceride accumulation in "fat-buffering" adipose tissue is limited by the development of adipose tissue insulin resistance. This results in diversion of energy substrates to nonadipose tissue, which in turn leads to a complex array of metabolic abnormalities characteristic of insulin-resistant states and type 2 diabetes. Recent evidence suggests that some of the biochemical mechanisms whereby glucose and fat exert adverse effects in insulin-sensitive and insulin-producing tissues are shared, thus implicating a diabetogenic role for energy excess as a whole. Although there is now evidence that weight loss through reduction of caloric intake and increase in physical activity can prevent the development of diabetes, it remains an open question as to whether specific modulation of fat metabolism will result in improvement in some or all of the above metabolic derangements or will prevent progression from insulin resistance syndrome to type 2 diabetes.     

Intracranial venous sinus thrombosis complicating AIDS-associated nephropathy

An alert and oriented 27-year-old African American woman with AIDS presented with a 10-day history of fever, cough productive of yellow sputum, nausea, and vomiting and a 1-day history of excruciating headache and photophobia. Her condition rapidly deteriorated into a coma with decorticate and then decerebrate posture, and she died 3 weeks later. There was evidence of extensive intracranial venous sinus thrombosis (ICVST), renal vein thrombosis (RVT), and multiple cerebral hemorrhagic infarcts due to a hypercoagulable state complicating AIDS-associated nephrotic syndrome. This is the first reported case of fatal ICVST and RVT with extensive cerebral hemorrhagic infarcts complicating nephrotic syndrome in a patient with AIDS.     

Measurement of intracellular vitamin C levels in human lymphocytes by reverse phase high performance liquid chromatography (HPLC)

OBJECTIVES: Vitamin C plays an active role in many important metabolic processes, such as collagen formation and the prevention of bleeding. Although overt scurvy is now rare, there is evidence that subclinical vitamin C deficiency is still quite common. Serum and plasma vitamin C measurements do not correlate well with tissue levels while lymphocyte vitamin C levels provide the most accurate assessment of the true status of vitamin C stores and are not affected acutely by circadian rhythm or dietary changes. We report a specific and reproducible reverse phase high performance liquid chromatographic method (HPLC) for the quantification of vitamin C in lymphocytes. METHODS: Reverse phase HPLC with a UV detection system was used. National Committee for Clinical Laboratory Standards (NCCLS) guidelines were followed for evaluation. Sample stability testing for lymphocyte vitamin C was performed for a period of 24 h at room temperature and 4 degrees C. Lymphocyte vitamin C levels were measured in 51 children. RESULTS: Lymphocyte vitamin C measurement with HPLC revealed very good analytical sensitivity with a 1.42 microg/10(8) lymphocyte lower limit of detection on repeated testing. An external standard curve was used for quantification, which showed a linear range of 1.25-100 microg/10(8) lymphocyte with a correlation coefficient of 0.989. Precision studies showed an inter-assay repeatability coefficient of variance (CV) between 0.25-9.98% and a within-assay coefficient of variance between 1.2-12.49%. The inter-assay CV for a period of 20 days was less than 10% for concentrations equal to or less than 1.42 microg/10(8) lymphocytes and less than 5.5% for concentrations between 5-100 microg/10(8) lymphocytes. Vitamin C was most stable at 4 degrees C, with a 0.31% decrease after 3 h and 2.35% after 4 h. At room temperature, vitamin C loss was more significant, with losses of 8.44% and 15.6% at 3 and 4 h, respectively, at a concentration of 29.9 microg/10(8) lymphocytes. CONCLUSIONS: The proposed HPLC method offers a reliable and reproducible technique for the quantification of intracellular vitamin C. Lymphocyte samples can be rapidly prepared and represent a more homogeneous tissue sample source for intracellular vitamin C measurement as compared to serum. To ensure stability, lymphocyte lysates should be prepared and stored at or below -20 degrees C within 2 h of blood collection.     

Rosiglitazone improves intestinal lipoprotein overproduction in the fat-fed Syrian Golden hamster, an animal model of nutritionally-induced insulin resistance

We have recently shown that the fructose-fed Syrian Golden hamster, a non-diabetic animal model of nutritionally-induced insulin resistance and hyperlipidemia, is characterized by intestinal lipoprotein overproduction. In order to determine whether intestinal lipoprotein overproduction is specific to fructose feeding or applies generally to other models of insulin resistance, we studied intestinal lipoprotein production and the response to insulin sensitization in the high fat-fed Syrian Golden hamster. Syrian Golden Hamsters were fed either (1). chow (CHOW), (2). 60% fat (FAT) or (3). 60% fat with rosiglitazone, 20 micromol/kg per day (FAT + RSG) for 5 weeks. Euglycemic hyperinsulinemic clamp studies confirmed that FAT is a good model of insulin resistance and rosiglitazone treatment resulted in a significant improvement in insulin sensitivity. In addition, there was a significant approx. two- to four-fold increase in intestinal apoB48 particle production in FAT. Rosiglitazone treatment resulted in partial normalization of apoB48-containing intestinal lipoprotein secretion. In summary: (1). the fat-fed Syrian Golden Hamster is a good model of nutritionally-induced insulin resistance, (2). intestinal overproduction of lipoproteins appear to contribute to the hypertriglyceridemia of insulin resistance in this animal model and (3). insulin sensitization with rosiglitazone ameliorates intestinal apoB48 particle overproduction in the fat-fed Syrian Golden Hamster. These data further support the link between insulin resistance and intestinal lipoprotein overproduction.     

Glial fibrillary acidic protein and vimentin expression is regulated by glucocorticoids and neurotrophic factors in primary rat astroglial cultures

The neurotrophic factors epidermal growth factor (EGF), basic fibroblast growth factor, (bFGF), insulin-like growth factor I (IGF-I) and insulin (INS) regulate neural and astroglial cell functions. Glucocorticoids may influence the metabolism of astroglial compartment and are key hormones in neurodegenerative events. This study was designed to assess the interactions between growth factors and dexamethasone (DEX) on cytoskeletal proteins (GFAP and vimentin) expression in 25 days in vitro (DIV) astrocyte cultures. An increase in GFAP and vimentin expression was observed after 12 h pretreatment with bFGF and subsequent treatment for 60 h with DEX. GFAP immunoreactivity was decreased after 24 h progression growth factors (EGF, IGF-I and INS) addition, when compared to control 36 h DEX and bFGF-pretreated cultures for the last 12 h. Vimentin immunoreactivity was decreased after 12 h bFGF pretreatment and subsequent 60 h DEX addition in astrocyte cultures compared to 12 h bFGF-pretreated ones. Pretreatment for 36 h with DEX plus bFGF in the last 12 h and subsequent treatment for 24 h with DMEM (Dulbecco's modified Eagle medium; DMEM) + BSA (bovine serum albumine) (harvesting), or with progression growth factors (EGF, IGF-I or INS) alone or two of them together, stimulated GFAP expression, compared to untreated controls. Immunochemical analysis of the mitogen-activated protein kinase ERK2 suggests an involvement of this enzyme in the control of GFAP expression. The above findings support the view of an interactive and complex dialogue between growth factors and glucocorticoids during astroglial cell proliferation and maturation in culture. This may have implications in therapeutic approach of neurologic disorders associated with astrogliosis, including cerebrovascular disease.     

Hepatic Regulation of Apolipoprotein B

Reviews in Endocrine and Metabolic Disorders -