The results in this report establish that adipocytes from the visceral fat depot express lower levels of apoE compared to those from the subcutaneous abdominal fat depot in humans, mice, rats, and baboons. Depot differences are significant in obese humans but are somewhat smaller than in the animal experimental models, perhaps related to less marked genetic heterogeneity present in the animal models. In rats and mice, we demonstrate that depot-specific differences are maintained in obesity. In baboons, we demonstrate that compared to subcutaneous abdominal fat, there is no difference in apoE expression in subcutaneous gluteal fat, but that apoE expression in pericardial fat is lower (similar to visceral fat) but higher in epicardial fat. Provocatively, results in freshly isolated mature adipocytes compared to those in adipocytes differentiated from pre-adipocytes in vitro, indicate that precursor cells are already committed to depot-specific differences in apoE expression.
We have previously shown that reactive oxygen species or TNFα from macrophages in the adipose tissue stromovascular fraction can significantly reduce adipocyte apoE expression [22
]. It seems reasonable, therefore, to assume that the lower levels of adipocyte apoE measured in visceral and pericardial adipose tissue depots could result from the increased macrophage infiltration (as marked by increased CD68 expression) and an increased inflammatory milieu (as marked by increased TNFα expression) we detect in these depots. However, our results also demonstrate that depot-specific differences in adipocyte apoE expression are maintained in precursor cells that have divided, undergone differentiation, and been maintained in culture for 14 days. This observation suggests that adipocyte precursor cells in unique adipose tissue depots are already committed to a specific level of apoE expression. This observation is consistent with those of others showing that depot-specific differences in adipocyte gene expression are also maintained in longterm culture for other genes including adiponectin and genes involved in insulin signaling [24
]. In fact, pre-adipocyte strains derived from single pre-adipocytes have been shown to maintain depot-specific cell characteristics after 40 divisions in culture [25
]. In the case of apoE expression, the long-term exposure to differing adipose tissue inflammatory milieu () could play a role in imprinting preadipocyte precursors.
Our previous work has established that endogenous adipocyte apoE expression has important implications for the acquisition of substrate from triglyceride-rich lipoprotein particles like VLDL [14
]. ApoE knockout adipocytes are lipid-poor and triglyceride synthesis is lower compared to WT adipocytes in the absence and presence of extracellular VLDL. We have also previously shown that the difference in triglyceride synthesis between EKO and WT adipocytes can be corrected by adenoviral expression of apoE in EKO adipocytes [13
]. Based on the results of these prior studies, adipocytes derived from depots with lower expression of apoE would be predicted to manifest lower levels of triglyceride synthesis in the absence and presence of VLDL (); and increasing the expression of apoE in adipocytes harvested from depots with low apoE expression should increase acquisition of substrate and TG mass (). Although our study does not address depot-specific differences in apoE expression as they relate to gender, it is interesting to speculate that a gender effect could contribute to the well-established differences in adipose tissue distribution between human males and females.
Many examples of differential gene expression in cells derived from subcutaneous versus visceral fat have been documented, along with important systemic implications deriving from this differential expression. Depot-specific effects for gene expression in pericardial and epicardial depots have more recently come to attention [10
]. Cross-sectional associations between altered gene expression in pericardial and epicardial fat, and the presence of human disease (i.e. coronary atherosclerosis or obesity) have also recently been reported [10
]. Our results establish a depot-specific influence on apoE expression level in pericardial and epicardial adipose tissue depots. ApoE expression in the pericardial depot is lower than that in subcutaneous fat, similar to that observed in visceral fat. However, apoE expression in epicardial adipose tissue is higher than that observed in any depot examined. An interesting potential implication of apoE gene expression in epicardial fat relates to the absence of a fascia between the epicardial fat pad and the epicardial coronary arteries; the site of atherothrombotic disease that gives rise to clinically important myocardial ischemia. There are several well-established mechanisms by which apoE secreted from the epicardial fat could favorably impact the evolution of atherothrombotic disease in the coronary arteries [27
In summary, our results demonstrate significant heterogeneity in the expression of adipocyte apoE across adipose tissue depots in several species. These differences in adipocyte apoE expression have implications for adipocyte triglyceride and VLDL metabolism, as predicted by the results of prior in vitro and in vivo mechanistic studies. Some differences in adipocyte apoE expression across adipose tissue depots could be related to depot-specific differences in macrophage infiltration and inflammatory microenvironment as we have already demonstrated that TNFα and reactive oxygen species produced by adipose tissue macrophages can significantly suppress adipocyte apoE expression. Because of the important role that has been established for endogenous adipocyte apoE expression in overall adipose tissue substrate flux, the depot-specific changes we currently demonstrate in adipocyte apoE expression could have important implications for modulating the accumulation of triglyceride in these depots.