Accumulating evidence indicated that PGC-1β may play a role in energy homeostasis through its effects on substrate metabolism and mitochondrial activity [4
]. However, the function and relevance of PGC-1β in the control of whole-organism energy metabolism is not well defined. To address this question, we generated and phenotyped a PGC1βKO mouse model. The PGC1βKO mouse is viable and apparently healthy. However, using a phenotyping strategy that combines physiological stress challenges guided by information obtained from a systems biology approach, we found that PGC1βKO mice have a general defect in mitochondrial function, a defect that is partly compensated for by mechanisms that are usually involved in adaptation to increased energy demands, such as up-regulation of PGC-1α.
The effect of PGC-1β controlling mitochondrial activity was investigated at the mRNA level using spotted array technology and bioinformatics pathway analysis. This approach showed that deletion of PGC-1β results in a significant mitochondrial phenotype. Multi-tissue comparisons demonstrated that PGC-1β controls the level of expression of ETC and OxPhos genes across all the organs studied and that this defect can only be partially compensated for in BAT and WAT by up-regulation of PGC-1α. However, the decrease in mRNA expression of ETC components is not necessarily seen at the protein level. These data suggest that although PGC-1β may be a controller of mitochondrial gene expression, in the absence of PGC-1β, additional factors such as protein degradation may be counter-regulated in specific tissues in an attempt to normalise levels of mitochondrial activity.
Our initial hypothesis was that dysregulation of ETC gene expression would increase the likelihood that the PGC1βKO mouse would become obese. However, despite its mitochondrial phenotype, the PGC1βKO mouse showed elevated resting metabolic rate and lower body weight compared to wild type littermates under ambient room temperature conditions. This phenotype is possibly due to the compensatory increase in expression of PGC-1α and its target genes in BAT. The increase in BAT PGC-1α expression may cause the higher degree of energy expenditure and relatively conserved mitochondria volume fraction despite reductions in ETC gene and protein expression. PGC-1α was also up-regulated in PGC-1β–deficient WAT, but its induction was not robust enough to produce a BAT-like histology or induction of typical BAT genes. Instead, within the reduced WAT content of the PGC1βKO mice, there was a greater proportion of larger, hypertrophic adipocytes. This result suggests that PGC-1β may play an as yet unknown role in white adipocyte biology, and the nature of this role requires further investigation.
Of interest, despite marked elevation of PGC-1α levels in PGC1βKO BAT, we did not observe a full restoration of the expression of mitochondrial genes back to WT levels. However, unlike the heart or skeletal muscle, organs where PGC-1α was not up-regulated, the mitochondrial fraction of BAT tissue was preserved in PGC1βKO mice. Thus, there may be partial functional overlap between PGC-1α and PGC-1β, concerning mitochondrial structure and function, which allows the development of an appropriate mitochondrial BAT content but fails to correct deficiencies in ETC composition. BAT was the only tissue out of three metabolically relevant tissues examined using electron microscopy that displayed relatively normal mitochondrial fraction, suggesting that further tissue function–specific mechanisms may be able to correct for PGC-1β ablation. However, the BAT gene expression pattern in PGC1βKO mice also showed altered expression of genes involved in intermediary metabolism, a pattern of gene expression that may also be primarily the result of PGC-1α up-regulation. Our conclusion from these gene expression analyses is that whereas PGC-1β may be required to set the basal level of mitochondrial ETC gene expression, it is not essential for normal expression of metabolic pathways such as glycolysis, the citric acid cycle, and fatty acid oxidation.
We hypothesized that the elevated resting metabolic rate seen in ambient-temperature housed PGC1βKO mice may be reversed under conditions of thermoneutrality, particularly if it was caused by compensatory up-regulation of PGC-1α. Under thermoneutral conditions, levels of PGC-1α gene expression are suppressed in BAT in response to the lower thermogenic and therefore oxidative demands of life at 30 °C. At thermoneutral conditions, PGC-1α mRNA expression was equivalent in WT and PGC1βKO BAT. Interestingly, under thermoneutral conditions, the PGC1βKO mouse exhibited reduced basal metabolic rate compared to the WT mouse. The energy expenditure measurements with mice acclimatised to 4 °C and 30 °C were performed at 33 °C. This measurement at thermoneutrality gives an indication of the basal metabolic rate. Under these experimental conditions, we can conclude that the PGC1βKO mouse has a reduced basal metabolic rate. The data from mice housed in ambient (22 °C) conditions were also generated at ambient conditions (). Therefore, this measurement represents energy expenditure in “normal” environmental conditions, when the body weights of the PGC1βKO mice are lower than WT. Under these experimental conditions, this measurement is composed of both basal metabolic rate and any additional metabolic effort required to maintain body temperature in this suboptimal thermal environment. Given that basal metabolic rate is reduced in 30 °C– and 4 °C–acclimatised PGC1βKO mice, it could be assumed that at ambient temperatures, basal metabolic rate is also reduced, but that the metabolic adaptation required for body temperature defense is actually greater in the PGC1βKO mouse. This suggests that PGC-1β may contribute to BAT thermogenesis in states of low metabolic demand (as is the case for BAT at 30 °C), whereas induction of PGC-1α regulates BAT thermogenesis as environmental temperatures fall.
It has been suggested that PGC-1β plays a role in BAT accumulation and function during cold exposure [6
]. PGC1βKO mice tolerated cold acclimatisation, despite the fact that under these conditions, PGC1βKO mice had decreased ETC gene expression and had decreased maximal thermogenic capacity in BAT, as shown by the lower oxygen consumption rate in response to adrenergic stimulation. Both WT and PGC1βKO mice up-regulated PGC-1α to a similar extent under conditions of cold exposure, suggesting that there is a maximal capacity of PGC-1α up-regulation in BAT. Alternatively, survival in cold may also be facilitated by the ability of the mice to maintain some fraction of increased metabolic activity through continued shivering [22
]. Therefore, although the total metabolic capacity provided by PGC-1α in cold-exposed PGC1βKO mice was sufficient for their survival, our results indicate that PGC-1β regulates a large fraction of the thermogenic capacity of the BAT tissue.
Studies focused in other metabolically relevant organs where PGC-1β is well expressed also indicated that ablation of PGC-1β resulted in impaired metabolic capacity. For example, mitochondrial volume fraction in soleus of PGC1βKO mice was decreased, and permeabilised soleus muscle fibres from PGC1βKO mice had reduced oxygen consumption and ATP synthesis compared to WT-derived fibres. Similar experiments using isolated soleus mitochondria from PGC1βKO mice failed to show any abnormalities in metabolic performance, indicating that metabolic defects in PGC1βKO soleus tissue relate to lower mitochondrial density relative to WT, although individual mitochondria retain similar metabolic properties.
The heart is the third oxidative tissue that is a major site of PGC-1β expression. Absence of PGC-1β in heart also led to a reduction of mitochondrial fraction, consistent with reduced cardiac expression of ETC genes. However, despite these potential bioenergetic defects, PGC1βKO mice have normal basal heart rates and ventricular contractility. Left ventricular function was also maintained after dobutamine treatment, suggesting that despite their mitochondrial defect, the PGC-1β–deficient hearts are able to adapt normally to imposed hemodynamic loads. The only difference in adrenergic stress that we observed between the PGC1βKO mice and WT was a blunting of the expected increase in heart rate in the PGC1βKO mice. A similar defect in heart rate regulation was also found in the PGC-1α knockout mice [14
], suggesting that both PGC-1α and PGC-1β may have a direct effect in controlling the activity of heart pacemakers. Overall, our results indicate that ablation of PGC-1β impairs heart mitochondrial function but that this defect is not severe enough to induce heart failure. However, a more chronic intervention study would be required to establish the relative importance of these PGC isoforms in cardiac energy supply and control.
To this point, we have discussed exclusively a role for PGC-1β in oxidative metabolism. Nonetheless, PGC-1β may also play an important role in regulating hepatic lipid production [11
]. Contrary to previous reports, we did not observe either PGC-1β induction after 24-h HFD in the liver of our control mice or differences in FAS
gene expression between controls and PGC1βKO mice. However, we did observe a reduction in circulating total cholesterol and alterations in the lipoprotein-associated cholesterol pattern in chow-fed conditions. Feeding for 24 h with a saturated fat–enriched HFD was associated with severe hepatic lipid accumulation and decreased total triglycerides, cholesterol, and VLDL and LDL-cholesterol plasma levels in PGC1βKO mice compared to WT. These results agree with a recent report suggesting that PGC-1β is a likely mediator of VLDL secretion by acting via interactions with Foxa2 to alter microsomal transfer protein expression [17
]. However, it is also possible that PGC-1β is necessary for additional aspects of hepatic lipid handling, including modification of lipid storage pathways and for controlling the balance between fatty acid synthesis and oxidation. Indeed, our pathway analysis suggests that the ETC in liver is expressed at a lower level and this may contribute to the steatosis after acute HFD treatment.
Our results indicate that PGC-1β has a well-defined role in controlling mitochondrial gene expression and function in many different organs. Despite this, PGC-1β can be ablated without overt metabolic failure, at least in unstressed conditions. This may be due in part to robust compensatory mechanisms, as demonstrated by the up-regulation of PGC-1α expression in WAT and BAT. Interestingly, other organs such as liver, muscle, or heart did not show up-regulation of PGC-1α under the conditions investigated. Following from these differences in PGC-1α expression, it is likely that the lean phenotype of this mouse model at ambient temperature should be considered the result of overcompensation mediated by up-regulation of PGC-1α, at least in BAT and WAT. Conversely, defects observed in skeletal muscle, heart, and liver are more likely to be the result of the absence of PGC-1β given the lack of PGC-1α induction in those tissues. When considering the roles played by PGC-1α and PGC-1β, our results show that there are specific effects of PGC-1β that cannot be compensated for by PGC-1α. Taken altogether, our results indicate that PGC-1β seems to cover basal bioenergetic needs whereas PGC-1α provides the extra bioenergetic support required under conditions of increased energy demand.