Effects of cytokines on preadipocyte differentiation and Wnt signalling

Sammanfattning: The relationship between amount of adipose tissue, the Metabolic Syndrome and Type 2 diabetes has been recognized for several years. However, the mechanisms for this remain to be clarified. Not all obese individuals develop insulin resistance or Type 2 diabetes and these perturbations are also seen in non-obese individuals. Adipose cell enlargement is a marker of obesity, but adipocytes have a limited capacity for lipid storage. Adipose cell growth and differentiation are important steps for normal lipid storage. Interestingly, obesity and insulin resistance are associated with an inflamed adipose tissue and infiltration of macrophages. Thus, the cytokine levels are expected to be increased in the adipose tissue under those conditions. This thesis is focused on elucidating the consequences of increased adipose tissue cytokine levels on preadipocyte development and function.We first measured interstitial IL-6 concentrations (Paper I) and found that local IL-6 concentrations in the adipose tissue were markedly higher than in plasma. Interestingly, the interstitial IL-6 concentrations were positively correlated with adipose cell size. These high concentrations suggest that IL-6 might act as an autocrine/paracrine regulator in the adipose tissue. Adipose tissue biopsies were also incubated in vitro to examine the effects of IL-6. Several important adipose tissue differentiation markers such as adiponectin, aP2 and PPARgamma2 were decreased. Thus, high local IL-6 concentrations, as seen in obesity, can impair the differentiation of the adipose cells.In Paper II, we characterized the effects of PPARgamma and C/EBPalpha of adiponectin and aP2 expression. We differentiated C/EBPalpha-/- fibroblasts and found that PPARgamma2 was necessary for adiponectin expression but C/EBPalpha was required for full gene activation. We also found that the PPARgamma ligand, pioglitazone, increased adiponectin expression in the absence of C/EBPalpha suggesting that the adiponectin promoter contains functional PPRE elements. Both IL-6 and TNFalpha reduced the expression of the differentiation markers adiponectin and aP2. However, over-expressing C/EBPalpha prevented this effect of IL-6.In Paper III, we found that IL-6 impaired the terminal differentiation of preadipocytes to adipose cells and reduced the lipid accumulation. However, TNFalpha completely prevented differentiation. IL-6, like TNFalpha, reduced the expression of most genes related to normal adipocyte function, insulin signalling and action. Remarkably, the canonical Wnt signalling pathway remained activated in the presence of either IL-6 or TNFalpha. This was associated with low axin and high beta-catenin, thereby keeping the cells in a proliferative mode and preventing the terminal differentiation. Instead, both IL-6 and TNFalpha promoted an inflammatory phenotype of the (pre)adipocytes.CONCLUSIONS: Interstitial IL-6 levels in the adipose tissue were markedly higher than circulating IL-6 concentrations and correlated positively with adipose cell size. The inhibitory effect of IL-6 on adiponectin mRNA levels was prevented by over-expressing C/EBPalpha. Both IL-6 and TNFalpha impaired, or prevented, the normal preadipocyte differentiation to mature adipose cells and, instead, promoted an inflammatory phenotype. This was associated with a maintained Wnt signalling, thus preventing the normal differentiation. Together, these findings provide an explanation to why obesity is associated with insulin resistance, inflammation and ectopic lipid accumulation in other tissues.