In obesity, white adipose tissue (WAT) inflammation is linked to insulin resistance. Increased adipocyte chemokine (C-C motif) ligand 2 (CCL2) secretion may initiate adipose inflammation by attracting the migration of inflammatory cells into the tissue. Using an unbiased approach, we identified adipose microRNAs (miRNAs) that are dysregulated in human obesity and assessed their possible role in controlling CCL2 production. In subcutaneous WAT obtained from 56 subjects, 11 miRNAs were present in all subjects and downregulated in obesity. Of these, 10 affected adipocyte CCL2 secretion in vitro and for 2 miRNAs (miR-126 and miR-193b), regulatory circuits were defined. While miR-126 bound directly to the 3′-untranslated region of CCL2 mRNA, miR-193b regulated CCL2 production indirectly through a network of transcription factors, many of which have been identified in other inflammatory conditions. In addition, overexpression of miR-193b and miR-126 in a human monocyte/macrophage cell line attenuated CCL2 production. The levels of the two miRNAs in subcutaneous WAT were significantly associated with CCL2 secretion (miR-193b) and expression of integrin, α-X, an inflammatory macrophage marker (miR-193b and miR-126). Taken together, our data suggest that miRNAs may be important regulators of adipose inflammation through their effects on CCL2 release from human adipocytes and macrophages.

Wnt signaling regulates adipogenesis and adipocyte function. Secreted frizzled-related proteins (SFRPs) are a family of secreted proteins (SFRP1-5) that bind and inhibit Wnts. Several members, including SFRP5, have recently been implicated in adipocyte dysfunction in obesity. Our objective was to characterize the expression, secretion, and function of the SFRP family in human white adipose tissue (WAT) and fat cells. SFRP1-5 mRNA expression was measured in human sc and visceral WAT from lean and obese individuals and correlated to insulin sensitivity. SFRP secretion from WAT explants was assessed by ELISA. Gene expression of SFRPs in cultured adipocytes during and after differentiation was determined. Functional analyses were done by gene silencing or incubations with recombinant SFRPs. SFRP1-4, but not SFRP5, mRNA levels were altered in obesity. However, although SFRP1 was down-regulated and correlated positively with insulin sensitivity, SFRP2-4 were up-regulated, particularly in visceral WAT, and associated with insulin resistance. Only SFRP1, SFRP2, and SFRP4 were secreted from WAT, thereby constituting adipokines. Individual knockdowns of SFRP1, SFRP2, or SFRP4 during adipogenesis did not affect terminal differentiation. Incubations with SFRP1 reduced the secretion of the proinflammatory cytokines IL-6 and monocyte chemotactic protein-1 (MCP1) and increased the release of adiponectin. SFRP1, SFRP2, and SFRP4 are adipokines, the expression of which correlates with insulin sensitivity. For SFRP1, this may be related to effects on the secretion of IL-6, MCP1, and adiponectin. In contrast to recent murine findings implicating SFRP5 in metabolic dysfunction, this SFRP is neither regulated by obesity nor actively secreted from human WAT.

Hypoxia modulates the production of key inflammation-related adipokines and may underlie adipose tissue dysfunction in obesity. Here we have examined the effects of hypoxia on glucose transport by human adipocytes. Exposure of adipocytes to hypoxia (1% O(2)) for up to 24 h resulted in increases in GLUT-1 (9.2-fold), GLUT-3 (9.6-fold peak at 8 h), and GLUT-5 (8.9-fold) mRNA level compared to adipocytes in normoxia (21% O(2)). In contrast, there was no change in GLUT-4, GLUT-10 or GLUT-12 expression. The rise in GLUT-1 mRNA was accompanied by a substantial increase in GLUT-1 protein (10-fold), but there was no change in GLUT-5; GLUT-3 protein was not detected. Functional studies with [(3)H]2-deoxy-D-glucose showed that hypoxia led to a stimulation of glucose transport (4.4-fold) which was blocked by cytochalasin B. These results indicate that hypoxia increases monosaccharide uptake capacity in human adipocytes; this may contribute to adipose tissue dysregulation in obesity.

Obesity is a metabolic condition usually accompanied by insulin resistance (IR), type 2 diabetes (T2D), and dyslipidaemia, which is characterised by excessive fat accumulation and related to white adipose tissue (WAT) dysfunction. Enlargement of WAT is associated with a transcriptional alteration of coding and non-coding RNAs (ncRNAs). For many years, big efforts have focused on understanding protein-coding RNAs and their involvement in the regulation of adipocyte physiology and subsequent role in obesity. However, diverse findings have suggested that a dysfunctional adipocyte phenotype in obesity might be also dependent on specific alterations in the expression pattern of ncRNAs, such as miRNAs. The aim of this review is to update current knowledge on the physiological roles of miRNAs and other ncRNAs in adipose tissue function and their potential impact on obesity. Therefore, we examined their regulatory role on specific WAT features: adipogenesis, adipokine secretion, inflammation, glucose metabolism, lipolysis, lipogenesis, hypoxia and WAT browning. MiRNAs can be released to body fluids and can be transported (free or inside microvesicles) to other organs, where they might trigger metabolic effects in distant tissues, thus opening new possibilities to a potential use of miRNAs as biomarkers for diagnosis, prognosis, and personalisation of obesity treatment. Understanding the role of miRNAs also opens the possibility of using these molecules on individualised dietary strategies for precision weight management. MiRNAs should be envisaged as a future therapeutic approach given that miRNA levels could be modulated by synthetic molecules (f.i. miRNA mimics and inhibitors) and/or specific nutrients or bioactive compounds.