Human Adipose Tissue
3.4 Discussion
The premise of this chapter was to understand the relevance of BAT1 in human adipose tissue and understand whether its perceived roles in other tissues and cell line studies may be influential in human adipocytes. To examine this, a series of molecular techniques were utilized. Initially studies investigated the presence of BAT1 in lean and obese Abdominal Sc and Om AT through microarray analysis. These studies revealed that BAT1 was expressed in human Abd Sc and Om AT altered by adiposity and T2DM status.
Although no functional studies explored the role of BAT1 in this chapter following microarray analysis further studies addressed whether increasing adiposity and/or diabetic status may reduce the expression of BAT1 within the AT. Interestingly initial microarray studies identified that BAT1 expression was reduced with obesity in either Abd Sc or Abd Om AT. Subsequent analysis by RT-PCR confirmed this observation with BAT1 mRNA expression reduced with increasing adiposity in both fat depots and a further reduction in subjects with T2DM. However it should be noted that microarray data has to be interpreted cautiously as although an extremely sensitive technique, it can lead to false positives and therefore as in this case the data should always been supported with specifically designed probe sequences that can ensure the data is accurate. To affirm the findings from the real time data subsequent studies examined protein expression of BAT1 and confirmed the real time PCR data. Additionally, whilst it was established that BAT1 was expressed at the gene and protein level in human adipose tissue further studies determine the expression of BAT1 in isolated human adipocytes, as assessed by undifferentiated and differentiating human pre-
adipocyte Chub 7 cells. These findings established that pure cultured human adipocytes express BAT1.
Although obesity reduced expression of BAT1 in either Abd Sc or Abd Om AT it was interesting to note that BAT1 expression was observed to be much higher in lean AbdSc AT than in either omental AT groups (lean or obese) or in either Abd Sc or Om depot once subjects became diabetic. Such an observation may be due to the knowledge that Abd Sc appears to represent the least insulin resistant tissue compared with Abd Om AT or an epicardial AT. However Abd Sc AT importance should not be underestimated in the pathogenesis of T2DM and may represent the last adipose tissue which tries to buffer ectopic fat from other tissues reducing inflammatory responses (Carobbio et al. 2011). With BAT1 mRNA expression reduced in obese and T2DM states this may incapacitate BAT1 functionality to reprise the production of inflammatory cytokines and co-current insulin resistance. The BAT 1 expression would also tend to fit with the subjects at most risk of insulin resistance, with men tending to show a lower BAT 1 expression compared with women.
Taken these finding together, indicate that if BAT1 expression is suppressed with increasing adiposity and remain suppressed, through to the development of T2DM and thereafter this may reduce the capacity to response to the inflammatory insults. Such BAT 1 suppression in the adipose tissue of obese subjects could be associated with unhindered local release of inflammatory cytokines e.g. IL-6, TNF-that generate a local pro-inflammatory state initially with later systemic effects. Interestingly, within the subgroup of the obese subjects (with or without diabetes) BAT1 was significantly more suppressed in the Abd Sc adipose tissue compared with Om AT. This may not be what
many studies would anticipate as omental AT is often viewed as the critical site in the generation of the metabolic disturbances (e.g. Isomaa et al. 2001; Kahn, Flier 2000; Kissebah & Krakower 1994; Larsson et al. 1984). This finding however, could imply a more important role of peripheral Abd Sc AT in the generation of inflammation in obesity than other studies might consider currently (Carobbioet al. 2011). Although often noted in Abd Sc AT is to have comparable or significant adipokine protein expression and release compared with Om AT (McGee et al. 2011; Harte et al. 2005; Fisheret al. 2005; McTernan et al. 2003; McTernanet al. 2002). Besides AT depot being important for BAT 1 expression these current studies also examined predictors of BAT1 expression such as BMI, age, gender and T2DM status. The statistic analysis revealed that increased BMI, male gender and presence of diabetes were all associated with suppressed BAT1 expression.
In conclusion this chapter has identified that BAT1 is clearly expressed in human adipose tissue and isolated human adipocytes, further that fat depot, adiposity, diabetic status and gender can impact on the expression of BAT 1. However, as the human adipose tissue contains several cell types except of adipocytes, including fibroblasts, macrophages, lymphocytes and endothelial cells, and some particular cell types increase with increasing adiposity for e.g. macrophages (Weisberg et al. 2003) and lymphocytes (Kintscher et al. 2008; Wu et al. 2007) it will be important to establish if BAT1 expression is regulated in human adipocytes. Further to understand whether local release of inflammatory cytokines influence BAT 1 expression and as such suggest how obesity and diabetic status may affect BAT1 activity.