A.D.A. is in the Department of Biochemistry and C.M.K. is in the Department of Biostatistics and Medical Informatics at the University of Wisconsin-Madison, Madison, Wisconsin 53706, USA. attie@biochem.wisc.edu or kendzior@biostat.wisc.edu
Muscle mitochondrial respiratory metabolism is reduced in aging and diabetes. Now, coordinated changes in expression of genes involved in oxidative phosphorylation have been found in individuals with type 2 diabetes mellitus (DM2). Peroxisome proliferator activator protein- co-activator-1 (PGC-1) seems to be in charge of this orchestrated change in gene expression.
Glucose oxidation and glucose production must be finely tuned to maintain blood glucose within a narrow range. Glucose sensing by pancreatic -cells stimulates insulin secretion. Insulin promotes glucose clearance in skeletal muscle and adipose tissue and attenuates glucose production in the liver. In DM2, the insulin signal is impaired by a combination of dampened responsiveness to insulin (insulin resistance) and an inability to compensate for the poor response with an adequate supply of insulin.
Skeletal muscle is responsible for most insulin-mediated glucose oxidation. There is considerable plasticity in skeletal muscle oxidative capacity. Exercise improves muscle respiratory capacity1 by inducing mitochondrial proliferation2, whereas in insulin resistance and DM2, there is a reduction in muscle mitochondria and whole-body oxygen consumption3. This correlates with a reduction in slow-twitch oxidative muscle fiber and an increase in fast-twitch glycolytic muscle fiber4. PGC-1 is a transcriptional co-activator that is essential for mitochondrial biogenesis. It mediates muscle fiber type switching and is responsive to exercise5. Vamsi Mootha and colleagues6 have now identified coordinated changes in genes involved in oxidative phosphorylation in human DM2—and PGC-1 seems to regulate these changes.
Probing subtlety In contrast to rodents, it has been difficult to find consistent changes in gene expression associated with diabetes in human subjects; genetic variability can mask true differences in expression or cause spurious identification of differentially expressed genes. Rather than analyze mRNAs individually, Mootha et al.6 surveyed compilations of mRNAs (gene sets) based on function, chromosomal location or other factors that tie them together. The Gene Set Enrichment Approach (GSEA), introduced by the authors, uses the Kolmogorov−Smirnov test statistic (similar to a Wilcoxon statistic) to identify those sets that contain a large proportion of genes that are differentially regulated between two samples. Of 149 gene sets, one set, containing genes involved in oxidative phosphorylation (OXPHOS), showed consistent changes in expression in individuals with DM2; 94 of 106 transcripts in the gene set were less abundant in these individuals. These changes were too small to be detected when the data were analyzed gene by gene. The GSEA proved useful here and should be considered in similar studies as a method for identifying groups of coordinately regulated genes.
Expression of PGC-1 was modestly (20%) reduced in muscle samples from individuals with DM2. The authors hypothesized that PGC-1 might mediate the changes in OXPHOS gene expression. Strong evidence of this connection emerged from an in vitro study in which ectopic expression of PGC-1 upregulated most of the genes in the OXPHOS collection. Though subtle, these changes in gene expression correlated with important physiological outcomes; OXPHOS gene expression accounted for 24% of the variability in oxygen consumption in the experimental subjects.
Cause or effect? Is the defect in OXPHOS gene expression a consequence or a cause of impaired glucose tolerance? Cells sense 'energy charge' by responding to the concentration of ATP versus ADP and AMP (Fig. 1). During muscle contraction ATP levels drop, leading to a relative increase in ADP and AMP. AMP activates the AMP-activated protein kinase (AMPK; ref. 7). In turn, AMPK mediates contraction-stimulated glucose transport in muscle. This occurs through an insulin-independent pathway that involves translocation of the GLUT4 glucose transporter to the plasma membrane8. AMPK induces expression of PGC-15, but the dominant effect of exercise on PGC-1 is through calcium/calmodulin-dependent protein kinase IV (refs. 9,10).
Figure 1. PGC-1 induces expression of mitochondrial respiratory genes and of GLUT4.
Reduced expression of PGC-1 and inefficient glucose oxidation might lead to spillover of glucose into the glucosamine pathway. Glucosamine can also downregulate mitochondrial respiratory genes. Exercise induces PGC-1 through calcium/calmodulin-dependent protein kinase IV and, secondarily, through AMPK. As fatty acid oxidation is entirely dependent on mitochondrial respiration and glucose oxidation can occur anaerobically, attenuated mitochondrial respiration will have a greater effect on fatty acid oxidation. Accumulation of fatty acyl-CoA causes insulin resistance.
Under conditions of chronically low energy charge as might occur when mitochondrial respiration is compromised, insulin-independent glucose transport systems might be upregulated. The greater glucose load, along with reduced aerobic glucose oxidation, might siphon excess glucose to the glucosamine pathway, leading to insulin resistance. Indeed, infusion of glucosamine under hyperinsulinemic conditions leads to downregulation of OXPHOS gene expression and lower O2 consumption11. Similar changes in OXPHOS gene expression occur with overfeeding11. Conversely, caloric restriction and exercise are associated with lower glucosamine production and higher glucose tolerance. Caloric restriction leads to changes in gene expression mediated by histone deacetylation, in which NAD+ has an essential role12. Under these conditions, the mitochondrial redox NAD+/NADH ratio might favor histone deacetylation
This model, however, leaves many unanswered questions. Does the glucosamine pathway mediate the downregulation of PGC-1 in people with impaired glucose tolerance and DM2? Does the level of coupling of respiration to ATP production have a role in mitigating potentially detrimental effects of PGC-1 downregulation? What is the range of gene variation contributing to the physiological variation in respiratory rate? Do changes in respiration rate affect gene expression through changes in NAD+ metabolism (through histone deacetylation)? Are changes in OXPHOS gene expression restricted to skeletal muscle or do they also occur in liver, adipose tissue and pancreatic -cells?
A thrifty genotype in an abundant environment The thrifty genotype hypothesis states that past periods of famine selected for genotypes favoring efficient energy storage13. With overnutrition, these genotypes are pro-diabetic. A thrifty genotype might result in reduced muscle oxidative capacity. As fatty acid oxidation is entirely mitochondrial and glucose oxidation can be aerobic or anaerobic, this would be consistent with an observed decrease in fatty acid oxidation relative to glucose oxidation (a decreased respiratory quotient). The resulting increase in myocellular fatty acyl-CoA and triglyceride promote insulin resistance.
The high incidence of diabetes among Mexican-Americans has been attributed to a high prevalence of the thrifty genotype. Patti et al. evaluated gene expression patterns in diabetic Mexican-Americans14. Like Mootha et al.6, they detected reduced expression of mitochondrial respiratory genes and PGC-1. Moreover, non-diabetic subjects with a family history of diabetes had a significant 34% reduction in PGC-1 expression compared with those individuals with no family history of diabetes. This raises the possibility that reduced PGC-1 expression levels might be a marker of a pre-diabetic condition.
There are numerous potential bottlenecks in respiratory metabolism, and so it is not surprising that various types of mitochondrial inherited or acquired mutations can cause diabetes and might well be responsible for the mitochondrial dysfunction that accompanies aging15. The new insights into OXPHOS-regulating genes and DM2 suggest that clinical tests to assess respiratory capacity might help to identify people at highest risk for developing diabetes.
at the crossroads of type 2 diabetes
co-activator-1
(PGC-1
-cells stimulates insulin secretion. Insulin promotes glucose clearance in skeletal muscle and adipose tissue and attenuates glucose production in the liver. In DM2, the insulin signal is impaired by a combination of dampened responsiveness to insulin (insulin resistance) and an inability to compensate for the poor response with an adequate supply of insulin.
is a transcriptional co-activator that is essential for mitochondrial biogenesis. It mediates muscle fiber type switching and is responsive to exercise
