Abstract

We have previously reported that omental (OM) preadipocytes respond less well to the prodifferentiating effects of thiazolidinediones than do preadipocytes from subcutaneous (SC) depots. This finding is consistent with in vivo alterations in fat distribution that occur in humans treated with thiazolidinediones. To explore these site-related differences further, we used real-time RT-PCR to quantify the specific mRNAs encoding peroxisome proliferator-activated receptor (PPAR) γ1 and γ2 and found that both isoforms were more highly expressed in SC than in OM preadipocytes. After 10 days of thiazolidinedione treatment, preadipocytes from both depots showed a small and comparable increase in expression of PPARγ1 mRNA (1.7 ± 0.2-fold [P = 0.007]) and 1.3 ± 0.1-fold [P = 0.008] increase for SC and OM, respectively). There was a much larger increase in PPARγ2 expression, which was significantly greater in SC compared with OM preadipocytes (11.1 ± 2.8-fold [P = 0.0003] and 5.5 ± 1.7-fold [P = 0.0003], respectively; P = 0.014 for SC versus OM). To establish whether the refractoriness of OM preadipocytes to differentiation was unique to activators of the PPARγ pathway, we examined the effects of the retinoid X receptor (RXR) ligand LG100268. As assessed by glycerol-3-phosphate dehydrogenase activity, LG100268 had a greater effect on the differentiation of SC compared with OM preadipocytes when examined alone (SC = 5.7 ± 1.7-fold vs. OM = 1.9 ± 0.6-fold; P < 0.05) or in combination with rosiglitazone (SC = 27.0 ± 7.5 vs. OM = 10.6 ± 3.6-fold; P < 0.05). Consistent with this, RXRα mRNA levels were also higher in SC than in OM preadipocytes. In summary, the previously reported insensitivity of OM preadipocytes to the differentiating effects of thiazolidinediones may relate to their lower basal levels of PPARγ1 and γ2 mRNA and their diminished capacity to upregulate PPARγ2 expression in response to ligand. That omentally derived cells also show reduced responsiveness to the prodifferentiating actions of an RXR ligand and a lower expression of RXRα in the undifferentiated state suggests that they may have a more generalized resistance to differentiation.