Trayhurn and Wood (30
) first hypothesized that hypoxia of AT might play a role in insulin resistance. This hypothesis was based on experiments demonstrating that surrogate markers of hypoxia were increased in the AT of obese animals (30
). We found that reduced AT pO2
in overweight/obese subjects and AT pO2
strongly correlates with percent body fat. We also found a significant reduction in AT temperature in overweight/obese subjects and a strong inverse correlation with percent total fat. This is in accordance with previous findings (31
) showing a lower skin temperature in obesity. The presence of capillary rarefaction suggests that decreased AT perfusion might also play a role. Indeed, there is evidence to suggest that obese subjects have lower blood flow in abdominal adipose subcutaneous tissue (32
). Further work using direct measures of both AT pO2
and direct measures of AT blood flow such as xenon washout are needed to formally address this hypothesis. There is additional evidence that obese mice have decreased oxygenation in epididymal and retroperitoneal AT (6
) and that weight loss increases oxygenation (6
). Choban et al. (33
) observed an increased incidence of postsurgical wound infections in obese patients. This same group, measuring AT pO2
in the upper arm, subsequently showed that decreased oxygenation of AT contributes to the increased risk of infection (34
). AT mass and adipocyte hypertrophy are closely related to the metabolic complications of obesity (35
). In vivo data suggested a role for hypoxia in insulin resistance even though AT pO2
has never been measured in human subjects (7
). We were unable to demonstrate a correlation between insulin sensitivity and AT pO2
. A lack of correlation of AT pO2
and insulin sensitivity may be due to lack of power because this AT pO2
and inflammation are just two of many factors that induce insulin resistance or perhaps because there is simply no meaningful biological relationship. The correlation between macrophage content and insulin resistance is modest (36
). One limitation of our study is that only one region of the body was measured and only at one depth.
Most hypoxic tissues develop strong transcriptional, metabolic, and secretory responses to reduced oxygenation in order to increase capillary density and to correct the hypoxia. Hypoxia turns on genes that act to increase oxygen availability by decreasing oxygen consumption (switching “on” anaerobic glycolysis) and stimulating angiogenesis. Hypoxia target genes expressed in AT include PDK1 and VEGF (37
). Consistent with this data, VEGF and PDK1 are upregulated in adipocytes cultured in 1% oxygen (a hypoxic environment) (6
). In contrast to our expectations, we found that lower AT pO2
in overweight/obese subjects did not induce an increase in hypoxia targets (PDK1, VEGF). Also, capillary density and VEGF were decreased in overweight/obese AT along with a lower AT pO2
. This suggests that the transcriptional counterregulatory system was not activated. One should note, however, that the lowest value of AT pO2
in the overweight/obese group was 29 mmHg; this corresponds to 3.8% oxygen compared with the 1% oxygen used in the cell culture experiments. This suggests that overweight/obese subjects have low AT pO2
but not low enough to mount a counterregulatory response driven during a response to hypoxia. Consistent to our results, Lijnen et al. (5
) found that obese mice have both lower VEGF and lower vascular density in AT. In addition, it is known that spleen, thymus, and retina have low pO2
in normal rats (38
), suggesting that angiogenesis is activated at different levels of oxygenation for different types of tissue or that oxygenation could influence angiogenesis. One limitation of our study is that we did not measure VEGF protein, and this should be addressed in future experiments.
Recent data suggests that PPARγ1 might be required for angiogenesis in AT (39
). PPARγ drives VEGF (and angiogenesis) (26
). We found a strong positive correlation between PPARγ1 and VEGF and between VEGF and AT pO2
. One way to interpret this data is that PPARγ1 drives angiogenesis in human AT and therefore is a key controller of AT pO2
. More work is needed to test this hypothesis.
AT expansion (adipogenesis) during development is preceded by a wave of neovascularization (40
). Vascular plasticity may play a role in the ability of AT to increase or decrease in size (29
). Our data suggests that reduced capillary density might restrict the growth of AT. Our finding of reduced capillary density in subcutaneous AT in obesity suggests the hypothesis that the failure of the vasculature to expand with increasing subcutaneous obesity might limit adipogenesis in subcutaneous depot. If visceral AT were not similarly restricted, this might allow for the growth of visceral AT. Further work measuring AT pO2
and capillary density in omental and mesenteric AT is needed to test this hypothesis.
Previous studies have shown that COL6 is abundantly expressed by adipocytes (42
), and obese mice have increased COL6 expression in the extracellular matrix (43
). We found that overweight/obese subjects with low AT pO2
have greater expression of COL6, and COL6 expression increased with increased body fat and fat-cell size. Scherer et al. (44
) suggests that proteolytic fragments of COL6 promote tumor growth through prosurvival and proliferation signaling pathways such as Akt and β-catenin. This suggests that new blood vessel formation is restricted by increased extracellular matrix or that a reduction in angiogenesis leads to increases in the formation of the extracellular matrix as exemplified by COL6. Further work is needed to separate these two possibilities.
AT inflammation has received much attention as an important factor in insulin resistance and type 2 diabetes (6
). Previous in vitro and in vivo preclinical studies showed that hypoxia induces inflammation that might contribute to insulin resistance (6
). We found that in humans, AT pO2
correlates with macrophage markers (CD68 and MAC2/CD163). In addition, AT secretion of MIP1α, a potent macrophage chemokine (46
), increased as AT pO2
decreased. This is consistent with recent data showing upregulation of MIP1α in obesity (48
). This is supportive of the hypothesis that lower oxygenation drives inflammation by upregulating adipocyte chemokine secretion but, as discussed previously, not by activating the classic hypoxia pathway and VEGF. However, it is possible that inflammation could drive hypoxia. Given that MIP1α has been implicated in angiogenesis, it is unclear why MIP1α is up when capillary density is down. Further work is needed to understand the factors regulating angiogenesis in human AT.
In summary, we provide direct evidence of lower AT pO2 in overweight/obese subjects and that the most likely causes are decreased capillary density and reduced expression of the angiogenic factors like VEGF and PPARγ1 and increased expression of COL6. This suggests that low AT pO2 in overweight/obese subjects does not result in a complete counterregulatory response to reduced AT pO2 and that neovascularization is not activated. Decreased AT pO2 was paralleled by an increase in the expression and secretion of the chemokine and markers of macrophage infiltration. These data suggests that proangiogenic therapies might reduce AT inflammation, improve insulin action, and reduce cardiovascular disease risk in obesity and type 2 diabetes.