To further explore the nature of the mitochondrial dysfunction and insulin resistance that occur in the muscle of young, lean, normoglycemic, insulin-resistant offspring of parents with type 2 diabetes (IR offspring), we measured mitochondrial content by electron microscopy and insulin signaling in muscle biopsy samples obtained from these individuals before and during a hyperinsulinemic-euglycemic clamp. The rate of insulin-stimulated muscle glucose uptake was approximately 60% lower in the IR offspring than the control subjects and was associated with an approximately 60% increase in the intramyocellular lipid content as assessed by 1 H magnetic resonance spectroscopy. Muscle mitochondrial density was 38% lower in the IR offspring. These changes were associated with a 50% increase in IRS-1 Ser312 and IRS-1 Ser636 phosphorylation and an approximately 60% reduction in insulin-stimulated Akt activation in the IR offspring. These data provide new insights into the earliest defects that may be responsible for the development of type 2 diabetes and support the hypothesis that reductions in mitochondrial content result in decreased mitochondrial function, which predisposes IR offspring to intramyocellular lipid accumulation, which in turn activates a serine kinase cascade that leads to defects in insulin signaling and action in muscle. IntroductionRecent magnetic resonance spectroscopy (MRS) studies have revealed increased intramyocellular lipid content associated with reduced mitochondrial phosphorylation activity in the muscle of young, lean, normoglycemic, insulin-resistant offspring of parents with type 2 diabetes (IR offspring) (1). These data suggest a potential role of mitochondrial dysfunction in the pathogenesis of insulin resistance and type 2 diabetes; however, the underlying mechanism responsible for this reduced mitochondrial activity remains unknown.Increases in the intramyocellular concentration of fatty acid metabolites have been postulated to activate a serine kinase cascade, causing increased phosphorylation of IRS-1 on critical serine sites, which blocks insulin receptor phosphorylation of IRS-1 on tyrosine sites. This results in reduced insulin-stimulated IRS-1-associated PI3K activity (2-5), decreased insulin-stimulated glucose transport activity (3), and reduced muscle glycogen synthesis (6, 7). However, there is currently little evidence that serine phosphorylation of IRS-1 is a key molecular event for this process in humans or whether or not there are associated alterations in insu-

Recent studies using magnetic resonance spectroscopy have shown that decreased insulin-stimulated muscle glycogen synthesis due to a defect in insulin-stimulated glucose transport activity is a major factor in the pathogenesis of type 2 diabetes. The molecular mechanism underlying defective insulin-stimulated glucose transport activity can be attributed to increases in intramyocellular lipid metabolites such as fatty acyl CoAs and diacylglycerol, which in turn activate a serine/threonine kinase cascade, thus leading to defects in insulin signaling through Ser/Thr phosphorylation of insulin receptor substrate (IRS)-1. A similar mechanism is also observed in hepatic insulin resistance associated with nonalcoholic fatty liver, which is a common feature of type 2 diabetes, where increases in hepatocellular diacylglycerol content activate protein kinase C-, leading to reduced insulin-stimulated tyrosine phosphorylation of IRS-2. More recently, magnetic resonance spectroscopy studies in healthy lean elderly subjects and healthy lean insulin-resistant offspring of parents with type 2 diabetes have demonstrated that reduced mitochondrial function may predispose these individuals to intramyocellular lipid accumulation and insulin resistance. Further analysis has found that the reduction in mitochondrial function in the insulin-resistant offspring can be mostly attributed to reductions in mitochondrial density. By elucidating the cellular and molecular mechanisms responsible for insulin resistance, these studies provide potential new targets for the treatment and prevention of type 2 diabetes. Diabetes 55 (Suppl. 2):S9 -S15, 2006 T ype 2 diabetes is rapidly emerging as one of the greatest global health challenges of the 21st century. The World Health Organization estimates that by the year 2030, ϳ366 million people will be afflicted with diabetes (1). This looming epidemic is also expected to trigger a steep rise in the complications associated with diabetes, such as ischemic heart disease, stroke, neuropathy, retinopathy, and nephropathy.Developing better treatments and novel prevention strategies for type 2 diabetes is therefore a matter of great urgency. To accomplish this goal, it is necessary to better understand the pathogenesis of type 2 diabetes. Although the underlying cause remains unknown, recent studies have clearly demonstrated that insulin resistance plays a critical role in the development of type 2 diabetes (2-5). This brief review will focus on recent magnetic resonance spectroscopy (MRS) studies that have shed new light on the cellular and molecular mechanisms of insulin resistance in humans. EARLY DEFECTS IN THE PATHOGENESIS OF TYPE 2 DIABETESOver the past 2 decades, our group has extensively used in vivo MRS to noninvasively probe the cellular and molecular mechanisms of insulin resistance in humans. This technique is ideal for human studies, since it is capable of monitoring particular intracellular metabolite concentrations and kinetics noninvasively and in real time (6 -22). Furthermore, in contrast...

SummaryRecent studies have demonstrated a strong relationship between aging-associated reductions in mitochondrial function, dysregulated intracellular lipid metabolism, and insulin resistance. Given the important role of the AMP-activated protein kinase (AMPK) in the regulation of fat oxidation and mitochondrial biogenesis, we examined AMPK activity in young and old rats and found that acute stimulation of AMPK-α2 activity by 5′-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) and exercise was blunted in skeletal muscle of old rats. Furthermore, mitochondrial biogenesis in response to chronic activation of AMPK with β-guanidinopropionic acid (β-GPA) feeding was also diminished in old rats. These results suggest that aging-associated reductions in AMPK activity may be an important contributing factor in the reduced mitochondrial function and dysregulated intracellular lipid metabolism associated with aging.