Macrophages inhibit adipogenesis of mesenchymal precursor cells via wnt-5a in humans

Introduction: Several clinical and molecular studies suggest that inflammatory mechanisms are important in the pathogenesis of metabolic diseases. In this respect it is of interest, that macrophages found in adipose tissue of human subjects with insulin resistance and type 2 diabetes inhibit adipogenic differentiation of mesenchymal precursor cells. While TNF-α was shown to be important in this process in mice, anti-TNF-α therapy in humans did not exhibit a major benefit on insulin sensitivity. Therefore the aim of the present study was to identify alternative signalling mechanisms in the interaction of macrophages and adipogenic precursor cells in humans.

Methods: Human peripheral monocytes were isolated from blood samples from healthy volunteers and induced to transform into macrophages under cell culture conditions using macrophage colony stimulating factor (M-CSF). Expression of wnt-5a was investigated in these cells on RNA level by real-time RT-PCR. To examine wnt-5a expression in macrophages on protein level in vivo, immunohistochemistry was performed of subcutaneous adipose tissue biopsies from human subjects who underwent elective abdominal surgery. Using neutralizing anti-wnt-5a antibodies in the differentiation medium we examined whether wnt-5a influences adipogenesis of mesenchymal precursor cells. Intracellular signalling of wnt-5a was examined by western blotting experiments, promoter reporter gene analysis and electromobility gel shift assay.

Results: Peripheral human monocytes, human macrophages under cell culture conditions and macrophages within human adipose tissue biopsies in vivo were found to express wnt-5a. When adipogenesis was induced in mesenchymal precursor cells in the presence of neutralizing anti-wnt-5a antibodies, differentiation was enhance compared to control cells, as shown by increased expression of the molecular marker PPAR-γ and accelerated Oil-red-O lipid staining. These findings suggest that macrophages inhibit preadipocyte differentiation via wnt-5a in humans. On a cellular level, wnt-5a was found to activate the c-Jun N-terminal kinase (JNK) dependent non canonical pathway in human mesenchymal precursor cells.

Conclusions: It has been shown in previous reports that wnt-signalling is involved in the communication between cells on various stages of the preadipocyte differentiation process. The data obtained in the present study demonstrate that it is also important in the interaction of macrophages and adipogenic precursor cells suggesting that wnt signalling molecules are essential in integrating different biological systems, e.g. the metabolic and the immune system.