As eluded above, PGC-1α has now emerged as a master regulator of several major metabolic pathways, including adaptive thermogenesis, glucose and fatty acid metabolism, oxidative phosphorylation, mitochondrial biogenesis, fiber type switching in skeletal muscle, and heart development [21–26]. Numerous studies also indicate that PGC-1α is a key factor involved in the regulation of insulin sensitivity [30–32, 35, 40] and adaptive responses to regular endurance exercise, leading to enhanced oxidative capacity of the skeletal muscle, and consequently, increased capacity for both lipid and carbohydrate utilization [36, 38, 39, 45]. However, less is known about the potential beneficial actions of PGC-1α as a mediator of exercise-induced improvement in hepatic insulin sensitivity and how exercise impacts PGC-1α expression in the liver. Thus, the current study was initiated with an overall goal to assess the relative effects of exercise on the PGC-1α protein expression in two representatives of slow oxidative (soleus) and high-oxidative fast (plantaris) muscle types as well as in the liver and its relevance to whole body insulin sensitivity and circulating insulin and fatty acid levels. The rationale for the present study was based on our previous findings that exercise training enhances insulin sensitivity in rats and that this effect is accompanied by a reduction in circulating insulin concentration [41, 46, 47]. This phenomenon has been observed in normal rats [41, 46, 47] as well as rats fed diets enriched with glucose, fructose or sucrose . The results of the current experiments not only confirm our previous observations that exercise training attenuates the plasma levels of insulin, but also lead to a significant reduction in the circulating levels of free fatty acids. More importantly, in the present study, we have demonstrated that exercise training selectively upregulates the PGC-1α expression in high-oxidative fast plantaris muscle, but have no effect on its expression in either slow oxidative soleus muscle or the liver.
It is clear from the data presented that plasma FFA levels were lower in exercise trained chow fed normal rats. However, we cannot tell if lower FFA levels noted in exercise trained rats were due to increased fatty acid oxidation by liver and/or skeletal muscle or simply the negative impact of running on the hormone-sensitive lipase (HSL) and adipose triacylglycerol lipase (ATGL), the two key enzymes involved in lipid hydrolysis and release of FFA from adipose tissue depots. The latter possibility is unlikely given that optimal functioning of both HSL and ATGL is necessary in maintaining adequate supply of FFA to sustain whole body substrate metabolism in response to exercise [49, 50]. A more likely possibility that needs to be considered is that exercise training may lower protein expression of liver MTP, a molecule that is essential for VLDL assembly and secretion , and in turn causes increased channeling of FFA for their disposal through oxidation. Indeed, it has been shown that exercise training reduces VLDL synthesis and/or secretion at least in high-fat fed rats most likely through modulation of MTP . Whether or not exercise-induced negative regulation of MTP-VLDL ultimately turns out to be responsible for the attenuation of dyslipidemia will depend on the results of future studies, but the observed reduction in circulating FFA levels supports the view that this possibility is worthy of continued consideration.
Exercise is known to cause an acute and transient increase in skeletal muscle PGC-1α gene expression [36, 37, 53, 54]. Conversely, cessation of physical activity reduces the skeletal muscle PGC-1α mRNA expression . It is suggested that exercise-increased levels of skeletal muscle PGC-1α leads to enhanced mitochondrial electron transport activities that enables cells to meet rising energy demand. We show here that spontaneous wheel running up regulates PGC-1α protein expression only in plantaris muscle, but not in soleus muscle. These results are in agreement with a previous study showing that voluntary wheel running of female ICR mice was associated with a selective increase in PGC-1α protein expression in plantaris muscle . However, we cannot tell if the enhanced PGC-1α protein expression in plantaris muscle noted in the running rats was due to the effect of exercise training or simply the reflection of plantaris muscle hypertrophy . Moreover, it is also unclear whether steady state increases in PGC-1α expression results from increased de novo synthesis, decreased degradation or both. Clearly, further studies will be necessary to investigate these possibilities.
The observation that exercise selectively up regulates the expression of PGC-1α only in the plantaris muscle points to a potential dissociation between exercise-mediated modulation of PGC-1α and improvements in skeletal muscle oxidative metabolism. Most of the previous studies have shown a positive correlation between exercise-induced changes in total skeletal PGC-1α mRNA and mitochondrial electron transport activities [36, 37, 53, 54, 57, 58]. Our results raise the possibility that contrary to the previous findings, PGC-1α may not function as a global regulator of oxidative metabolism in response to exercise. Instead, we suggest that exercise-mediated improvement in skeletal muscle oxidative metabolism may involve improved PGC-1α action in specific muscle types (e.g., oxidative muscle such as plantaris muscle) or exercise-induced recruitment other biologically active compounds leading to increased functional efficacy of PGC-1α [59–65]. In addition, exercise-induced potential alterations in acetylation, phosphorylation, and/or methylation status of PGC-1α could also positively impact its function (65). Furthermore, since we have used 5 weeks of voluntary wheel running exercise regimen, there remains the possibility that we may have missed any potential transient increases in PGC-1α in soleus muscle studied here. Indeed, exercise is known to cause an acute and transient increase in PGC-1α gene expression in various skeletal muscle types [36, 37, 53, 54, 66] The follow-up studies are underway to assess the impact of short- and long-term exercise training on the expression of PGC-1α not only in these two muscle types, but also in a variety of other skeletal muscle types with differing fiber composition . Also, experiments are in progress to determine effects of short-term and long-term exercise treatment regimen on the acetylation, phosphorylation, and/or methylation status of PGC-1α in specific skeletal muscle types as well as in liver.
Previous studies have shown that expression in the liver is robustly induced in response to acute food deprivation, insulin resistance or diabetes [25–28], but reduced in obesity . Both loss-of-function and gain-of-function studies have shown that PGC-1α over expression results in hepatic insulin resistance, while PGC-1α deficiency is accompanied by alterations in hepatic energy and lipid metabolism, lipid accumulation (steatosis), insulin signaling and diminished hepatic gluconeogenesis [33–35]. These observations have led to a general thinking that hepatic PGC-1α may function as a negative modulator of insulin sensitivity, and lipid and glucose metabolism. Despite our expectations that exercise-mediated improvement in hepatic oxidative metabolism [68–70] should accompany a significant decline in PGC-1α expression, we found no change in its expression at the end of 5-weeks of exercise training of rats. It remains possible that at some point beyond 5-weeks of training, PGC-1α expression may be significantly altered or that exercise may indirectly influence PGC-1α through modulation of SIRT1 which serves as a regulator of PPARα-PGC-1α directed gene expression in liver . Likewise, it is possible that we may have missed a transient change (decrease) in PGC-1α expression during the early phase of the exercise treatment regimen. Moreover, we tested the effect of exercise only in male Wistar rats, in which animals were subjected to voluntary wheel running starting at 5 weeks of age. Thus, our findings do not preclude the possibility that other important factors such as strain, sex, and age at the start of the exercise regimen may render animals more responsive to exercise-induced changes in PGC-1α expression in the liver.
In conclusion, results of the present study indicate that 5 weeks of voluntary wheel training exercise results in a significant reduction in circulating levels of insulin and FFA. Western blot analysis indicated that exercise selectively up regulated the PGC-1α protein expression, a major mediator of exercise-induced improvements in skeletal muscle oxidative metabolism and insulin sensitivity, in the high-oxidative plantaris muscle. In contrast, exercise training had no effect on the PGC-1a expression in either the slow-oxidative soleus muscle or liver. Based on these findings, we conclude that PGC-1α plays a minor role in exercise-mediated improvements in insulin resistance (sensitivity) and lowering of circulating FFA levels.