The ability of adipocytes to oxidize FAs and uncouple mitochondrial respiration in response to βAR activation is best understood in brown adipocytes, where the high density of mitochondria and the presence of the unique protein UCP1 allow for robust OCR and energy expenditure. Obviously, white adipocytes have less oxidative capacity than brown adipocytes, and their ability to oxidize FA is much lower (24
). Nevertheless, a case can be made for assessing the relevance of WAT as a target for increasing energy expenditure based on the sheer amount of the tissue as a percentage of body mass. In fact, evidence exists in rodent adipocytes for a moderate increase in FA oxidation in response to βAR activation (12
), and various futile cycles have been examined in adipocytes (32
Several years ago, a collection of unrelated reports suggested that epinephrine could increase OCR in rat epididymal fat pads ([33
] and references therein) or primary cultures of rat adipocytes (14
). βAR-stimulated lipolysis in these adipocytes was also associated with both FA-dependent IMM depolarization (15
) and a decrease in ATP levels (14
). A more direct link between lipolysis and ATP levels was shown in 3T3-L1 cells in which the generic lipase inhibitor orlistat suppressed FSK-stimulated lipolysis as well as the drop in ATP content (36
). Because a high OCR together with mitochondrial depolarization and ATP reduction are common characteristics of mitochondrial uncoupling, all together these studies suggest there is mitochondrial uncoupling in response to βAR activation. However, no single study has presented cohesive evidence for βAR-induced uncoupling in white adipocytes, especially in human adipocytes, and a molecular mechanism was not identified.
Here, we find an acute increase in the cellular energetics of human white adipocytes in response to βAR activation or cAMP-elevating agents that is dependent on the FAs liberated by lipolysis. In following cAMP-induced changes in cellular respiration over time, the increases in both aerobic (measured as OCR) and anaerobic (measured as ECAR) respiration were immediate and transient. The ECAR response was faster than OCR, implying that glucose oxidation preceded FA oxidation. Importantly, a major portion of cAMP-induced OCR was Oligo-insensitive, implying that some respiration was independent of ATP synthase activity. These results, together with mitochondrial depolarization, represent mitochondrial uncoupling.
The physiological significance of mitochondrial uncoupling in white fat is not fully understood and is debated (13
) (rev. in 38
). One consequence of mitochondrial uncoupling in WAT is an increase of the AMP/ATP ratio and activation of AMP kinase to 1
) promote ATP-generating processes such as FA and glucose oxidation ([38
] and references therein) and 2
) inhibit ATP-consuming pathways such as lipogenesis, triglyceride synthesis and to some extent lipolysis (36
). Another outcome of uncoupling in WAT could be protection against mitochondrial-derived reactive oxygen species and oxidative stress directly within the adipocyte, which can lead to cellular damage (12
). Interestingly, we observed that adipocytes derived from obese donors had a significantly lower OCR response to βAR activation. In adipose tissue from obese individuals, there is evidence for an impaired lipolytic response in vivo and a reduced mitochondrial mass (10
), with a variety of mechanisms proposed (rev. in 26
). Although we did not observe a difference in lipolysis, our results suggest that there is a difference in the ability of the mitochondria from obese versus lean subjects to increase respiratory activity in response to βAR stimulation, mirroring notions that an impaired response of WAT to catecholamines at some level could be significant in the development or maintenance of obesity (27
The ability of FAs to uncouple mitochondrial respiration has been extensively investigated, particularly in tissues such as BAT, muscle, and liver because of their high aerobic capacity. The molecular basis mediating FA-induced uncoupling has been vigorously discussed, and a number of mitochondrial carriers have been suggested (2
). In BAT, the general consensus is that FAs allosterically activate the “carrier” or “channel” properties of UCP1 (46
) (mechanistically still debated in some circles). By comparison, in WAT UCP1 levels are fairly negligible whereas the UCP1 homologue, UCP2, is abundant (47
). UCP2 has periodically been proposed to mediate proton leak or mitochondrial uncoupling ([1
] and references therein), but this point is not fully agreed upon (48
). However, regarding UCP2 there was no difference in the OCR response to ISO in WAT of both wild-type and Ucp2
-null mice. Moreover, the contribution of ANT2, ANT3, PiC, and VDAC1 was excluded. Instead, a significant portion of βAR and cAMP-induced OCR was inhibited by the specific PTP inhibitor, CSA, as well as by suppressing BAX expression—altogether suggesting that the opening of the mitochondrial PTP by the regulatory protein BAX is important for the observed mitochondrial uncoupling in WAT. Given that the amplitude of the OCR and uncoupling response in the obese subjects was reduced, we measured whether there was a change in expression of BAX and the PTP in obese WAT. In our sample set, no differences were detected in the expression levels of BAX (or VDAC1 or ANTs) between lean and obese WAT. However, proteins of the BCL-2 family are known to be post-translationally modified, which controls their activity and regulates PTP opening (30
). Such modifications in WAT mitochondria could reasonably be a basis for the observed differences in mitochondrial function between lean and obese, but will require further study and must be extended to a larger sample set.
Because white adipocytes comprise the bulk of adipose tissue in the body, even a moderate, sustained ability of adipocytes to oxidize FAs could have several positive consequences. First, limiting the amount of FAs released into the circulation (13
) would be protective against lipotoxicity in other tissues, and second, it might contribute to whole body energy expenditure. The concept of mitochondrial uncoupling as a strategy to reduce body weight has been clearly demonstrated with the past clinical use of such strong global uncouplers as 2,4-dinitrophenol, but that enterprise was not without major side effects, including fatalities (rev. in 48
). Therefore, greater understanding of the mechanism(s) responsible for this cAMP-dependent uncoupling in WAT and the role of the PTP is warranted. The potential to identify novel agents and/or processes that could be harnessed for selectively stimulating moderate uncoupling are needed to combat the metabolic disease epidemic.