Objective: Macrophages are the main drivers of obesity-induced adipose tissue inflammation that is related to hypertrophy-induced adipocyte death and leads to major health impairments due to induction of insulin resistance. Upon entry into a tissue, monocytes differentiate to macrophages and polarize into various phenotypes depending on the stimuli in their environment. In adipose tissue of lean mice, anti-inflammatory M2 macrophages dominate, while in the obese state the phenotype of macrophages changes into a pro-inflammatory M1 polarization type by yet unknown mechanisms. In human adipose tissue, a peculiar macrophage phenotype with mixed polarization characteristics has been described.
Osteopontin (OPN) is a cytokine shown to be highly expressed in adipose tissue in obesity and to contribute to obesity-induced adipose tissue inflammation and insulin resistance. I hypothesized an impact of presence of OPN on the survival of monocytes and the polarization of macrophages.
Methods: I phenotyped adipose tissue macrophages (ATMs) from obese OPN-deficient (Spp1-/-) and wild type C57BL/6 (WT) mice. I further investigated human monocytes, bone marrow derived macrophages (BMDMs) from Spp1-/- and WT mice as well as human monocyte-derived macrophages (MDM), which were in vitro polarized in presence of OPN and thrombin cleaved OPN. I assessed survival, apoptosis, polarization markers on gene expression and on protein level and phagocytic activity.
Results: Although OPN-deficient ATMs surprisingly expressed more M1 surface marker CD11c they did not up-regulate inflammatory cytokine gene expression. Conversely, M2 marker mannose receptor (CD206) was less expressed. Polarization of murine BMDMs was not affected by genetic OPN-deficiency or presence of exogenous OPN. OPN promoted survival and prevented apoptosis of human monocytes and this effect was weakened by thrombin cleavage. In contrast to murine BMDM, OPN induced skewing of M1/M2-related gene expression in human MDMs and diminished inflammatory cytokine production, while surface markers indicated a mixed phenotype. Thrombin cleavage of OPN, which is known to enhance OPN actions mediated by integrin-binding, abrogated these effects. Strikingly, phagocytic activity was elevated by presence of OPN during polarization in both human MDMs and in murine BMDMs.
Conclusions: My data indicate that OPN may be a macrophage survival signal in obese adipose tissue and promotes macrophages to take up, e.g., liberated lipids and cell debris as required due to hypertrophy-induced adipocyte death. This function, however, is not clearly related to a prototypic M1 or M2 phenotype.