Chronic leptin treatment enhances insulin-stimulated glucose disposal in skeletal muscle of high-fat fed rodents

Obesity and metabolic disease: is adipose tissue the culprit?

Obesity is a risk factor for the development of type 2 diabetes and CVD. Is adipose tissue the culprit in the relationship between obesity and metabolic disease? It is certainly possible to argue that adipose tissue function is disturbed in obesity in such a way that adverse consequences may follow. For instance, lipolysis is down regulated, the sensitivity of lipolysis to insulin is reduced and there are disturbances in the regulation of adipose tissue blood flow. However, when examined critically these changes can be seen as adaptations to the increased adipose tissue mass, making the situation better rather than worse. In terms of the many peptide and other factors now known to be secreted from adipose tissue, it is easier to argue that adipose tissue is the culprit. However, for no single 'adipokine' is there as yet unequivocal evidence of a link between adipose tissue secretion and adverse metabolic events in other tissues. The best documented of these adipokines in relation to insulin resistance is adiponectin. Here, unusually, adiponectin confers insulin sensitivity, and its secretion is down regulated in obesity. It could be again that adipose tissue has down regulated its function in an attempt to compensate for its increased mass, although certainly that down-regulation is too extreme. On balance, it is clear that adipose tissue is a link in the chain of events leading to metabolic disease, but in many respects it is an innocent intermediary trying to deal with the consequences of positive energy balance, the real culprit.

Hyperglycemia, impaired glucose tolerance and elevated glycated hemoglobin levels in a long-lived mouse stock

We have previously demonstrated that two wild-derived stocks of mice, Idaho and Majuro, are significantly longer-lived than mice of a control stock (DC) generated as a four-way cross of commonly used laboratory strains of mice. This study provides independent confirmation of this earlier finding, as well as examining serum glucose, insulin, leptin, glycated hemoglobin (GHb), cataract severity, and glucose tolerance levels in each of the stocks. Both the mean (+20%) and maximum (+13%) life span of the Idaho mice were significantly increased relative to the DC stock, while in the Majuro mice only maximum (+15%) life span was significantly increased. In addition, Majuro mice were hyperglycemic in both the fed and fasted states compared both to laboratory-derived and Idaho stocks, had significantly elevated GHb levels and cataract scores, and were glucose intolerant although serum insulin levels did not differ between stocks. Body weight and body mass index (BMI)-corrected leptin levels were also dramatically (1.5-3-fold) higher in the Majuro mice. The longevity of Id mice was not accompanied by changes in serum glucose and insulin levels, or glucose tolerance compared to DC controls, although GHb levels were significantly lower in the Idaho mice. Taken together, these findings suggest that neither a reduction of blood glucose levels nor an increase in glucose tolerance is necessary for life span extension in mice.

Insulin-stimulated plasma membrane association and activation of Akt2, aPKC zeta and aPKC lambda in high fat fed rodent skeletal muscle

Several recent reports using cell lines have suggested that both Akt and atypical protein kinase C (aPKC) zeta/lambda are translocated to the plasma membrane (PM) in response to insulin. However, it has yet to be determined in skeletal muscle whether: (1) insulin increases PM-associated Akt2, aPKC zeta and/or lambda protein concentration, (2) the activity of these kinases is altered by insulin at the PM, and (3) high fat feeding alters the insulin-stimulated PM concentration and/or activity of Akt2 and aPKC zeta/lambda. Sprague-Dawley rats were randomly assigned to either normal (n=16) or high fat (n=16) dietary groups. Following a 12 week dietary period, animals were subjected to hind limb perfusions in the presence (n=8 per group) or absence (n=8 per group) of insulin. In normal skeletal muscle, total PI3-kinase, Akt2 and aPKC zeta/lambda activities were increased by insulin. PM-associated aPKC zeta and lambda, and aPKC zeta/lambda activity, but not Akt2 or Akt2 activity, were increased by insulin in normal muscle. High fat feeding did not alter total skeletal muscle Akt2, aPKC zeta or aPKC lambda protein concentration. Insulin-stimulated total PI3-kinase, Akt2 and aPKC zeta/lambda activities were reduced in the high fat fed animals. Insulin-stimulated PM aPKC zeta, aPKC lambda, aPKC zeta/lambda activity and GLUT4 protein concentration were also reduced in high fat fed animals. These findings suggest that in skeletal muscle, insulin stimulates translocation of aPKC zeta and lambda, but not Akt2, to the PM. In addition, high fat feeding impairs insulin-stimulated activation of total aPKC zeta/lambda and Akt2, as well as PM association and activation of aPKC zeta and lambda.