• adipogenesis
• adiponectin
• adipose tissue
• haptoglobin
• hepatosteatosis
• high fat diet
• inflammation
• macrophage
• obesity
• size

INTRODUCTION— Haptoglobin (Hp) is an inflammatory and adiposity marker, its expression during obesity being specifically induced in the white adipose tissue (WAT). The low chronic inflammatory state, caused by massive adipose tissue macrophage (ATM) infiltration found in obesity, and low adiponectin have been implicated in the development of insulin resistance and hepatosteatosis. The aim of this project was to get further insights into Hp function in WAT and to investigate whether and how Hp interferes with the onset of obesity-associated complications. To this end, we metabolically profiled and analyzed the histology and the gene expression of the WAT in the Hp deficient model both in standard and obesity conditions. To get further insights into Hp function in the adipocyte, the adipogenic potential of Hp−/− mouse embryonic fibroblasts (MEFs) was also evaluated.

MATERIALS AND METHODS— Hp null (Hp−/−) and wild type (WT) mice were metabolically profiled under chow food diet (CFD) and high fat diet (HFD) feeding, by assessing physical parameters, glucose tolerance, insulin sensitivity, insulin response to glucose load, liver triglyceride content, plasma levels of leptin, insulin, glucose and adiponectin. ATM content and adipocyte size were evaluated by using immunohistochemistry. Adiponectin expression was measured in Hp-treated, cultured 3T3-L1 and human adipocytes. Primary MEFs were isolated from embryos of WT and Hp−/− embryos at 15 days post coitum and induced to progress to adipose conversion using an adipogenic hormonal cocktail.

RESULTS— Metabolic profiling did not reveal any genotype-related difference in CFD animals. HFD Hp−/− mice showed significantly higher glucose tolerance, insulin sensitivity, glucose-stimulated insulin secretion, adiponectin expression and reduced hepatomegaly / steatosis compared with HFD WT mice. The WAT of HFD Hp−/− mice showed higher activation of insulin signaling cascade, lower ATM and higher adiponectin expression. Hp was able to inhibit adiponectin expression in cultured adipocytes. Average size and percentage of very large adipocytes were respectively smaller and reduced in HFD Hp−/− mice as compared to HFD WT. Lean adult Hp−/− showed significantly larger adipocytes and lower subcutaneous WAT expression of aP2 and LPL with respect to WT. Morphometric analysis of adipocytes in lean and obese mice revealed an attenuation of obesity-related changes in Hp−/− mice compared to WT. MEFs from Hp−/− mice were less capable to accumulate triglycerides and exhibited lower expression of PPARgamma, aP2, FAS, LPL, Leptin.

CONCLUSIONS— We demonstrated that, in the absence of Hp, obesity-associated insulin resistance and hepatosteatosis are attenuated, this being consistent with reduced ATM content, increased plasma adiponectin and higher WAT insulin sensitivity. Moreover, Hp deficiency tends to blunt the diet effect on the size of adipocytes, which show less susceptibility to develop hypertrophy upon obesity; in addition Hp deficiency impacts negatively on adipogenesis.