Endocrine Abstracts

Intra-abdominal adiposity is associated with insulin resistance and increased cardiovascular morbidity and mortality. Obesity occurs as a consequence of increased adipocyte size (hypertrophy) and number (differentiation or hyperplasia). Whilst differences in gene expression between omental (om) and subcutaneous (sc) adipose tissue have been described, the molecular mechanisms that underpin the differences in adipose tissue biology and the depot specific metabolic risks that they convey have not been explained. Consequently, there is a need to identify factors that are involved in adipogenesis and adipose tissue distribution. Acetyl CoA carboxylase (ACC) is the rate limiting step in fatty acid synthesis and is regulated at the level of mRNA expression, protein phosphorylation and proteasomal degradation. These mechanisms have not been fully explored in human adipose tissue. We have hypothesized that the E3 ligase, Smurf (isoforms 1 and 2), which is believed to target ACC for ubiquitination, may be an important regulator of ACC in human adipose tissue.

Paired om and sc adipose tissue biopsies were obtained from patients undergoing elective abdominal surgery. Expression of key components of the lipogenic pathways were examined by real-time PCR and western blotting. In addition, observations were endorsed in primary cultures of differentiating adipocytes and in a novel human sc cell line (chub-s7).

Across adipocyte differentiation (sc and om primary cultures and chub-s7 cells), fatty acid binding protein 4 (FABP4), fatty acid synthase (FAS) and ACC expression increased (e.g. om FABP4: 26.9-fold, P<0.05; FAS: 5.3-fold, P<0.05; ACC: 50-fold, P<0.005). ACC was more highly expressed in differentiated sc adipocytes (om: 1 vs sc: 3.7-fold P<0.05). Smurf 1 and 2 mRNA expression decreased across differentiation (Smurf1: 1 vs 0.24(sc), 0.3(om); Smurf2: 1 vs 0.45(sc), 0.4(om), P<0.05). In addition, in chub-s7 cells there were parallel changes in protein expression (Smurf: 1 vs 0.43-fold, P<0.05; ACC 1 vs 7.7-fold P<0.05). Consistent with our hypothesis, absolute levels were significantly lower in sc compared to om cells (Smurf2: 1(om) vs 0.7(sc), P<0.05).

We have described depot specific differences in lipogenic gene expression across adipocyte differentiation. Enhanced lipid storage in sc cells (decreasing free fatty acid availability) may be important in explaining depot specific metabolic risk. In addition, our data endorse the novel hypothesis that Smurf proteins regulate ACC and as such may represent a potential therapeutic target.