Obesity is associated with a low-grade systemic chronic inflammatory state characterized by abnormal production of pro- and anti-inflammatory adipocytokines leading to immune dysfunction and contributing to increased disease risk. Precise triggers for obesity-induced inflammation are not yet fully understood. Activation of PBMCs is an important initial step in the cascades of events leading to many inflammatory diseases including insulin resistance. Since expression of receptors on cells is a key element in the regulation of proinflammatory cytokines, we determined whether TLRs (TLR2 and TLR4) and inflammatory cytokines expression was simultaneously modulated on PBMCs in obesity.
Our data demonstrated alterations in TLR2 and TLR4 expressions in PBMCs and adipose tissues from obese subjects. In parallel, abnormalities in cytokines expression were found in PBMCs from obese individuals. Moreover, we found that there was a strong association between TLRs’ expression and cytokines (IL-6 and TNF-) measured simultaneously in PBMCs. Recent findings indicate that the TLRs which are up-regulated in the affected tissue of most inflammatory disorders can mediate crosstalk between the immune systems and body metabolism . Increased TLRs activity was reported in patients with metabolic syndrome . Elevated TLR2 and TLR4 expression was assessed in artherosclerotic lesions . Higher expression of TLRs 3 and 4 in other conditions, such as an early stage of RA suggests that modulation in TLR expression does result during inflammatory states . We found that markedly increased TLR2 and TLR4 levels in adipose tissue from obese subjects related with their increased expression on PBMCs. This could be the result of migration of inflammatory monocytes/macrophages from the peripheral compartment to the adipose tissue. Shi et al. (2006) argued that the increased expression of TLR4 mRNA in total adipose tissue extracts in two models of obesity could be due, in part, to increased numbers of macrophages known to reside in fat tissue of obese animals. Since a preferential macrophage infiltration into obese adipose tissue was demonstrated , it was suggested that the toll-like receptors’ expression in adipose tissue was mainly due to macrophages .
The enhanced expression of cytokines we observed in obese subjects could be explained by an elevated expression of TLRs on PBMCs. Other factors, such as increased free fatty acids, can influence TLRs responsiveness and explain the increased response observed in obese individuals. Saturated free fatty acids which are elevated in case of obesity are able to augment TLR induced cytokine production [29, 30]. This could be a reason, in part, that obese individuals are more prone to developing insulin resistance. There is a clear link between TLR activation and insulin resistance. Free fatty acids cause insulin resistance in TLRs dependent manner. Recently pro-inflammatory effects of resistin were seen through its activation of TLR4  and other endogenous ligands such as HMGB1 or hyaluronan fragments or HSPs also served as TLR4 or TLR2 activators . Mice lacking TLR2 are substantially protected from diet-induced adiposity, insulin resistance, hypercholesterolemia, and hepatic steatosis and TLR2 deletion was associated with attenuation of adipocyte hypertrophy as well as diminished macrophage infiltration and inflammatory cytokine expression . It was reported that the absence of TLR2 attenuated local inflammatory cytokine expression and related signaling and increased insulin action specifically in the liver . Notably, only a few studies so far have described the responsiveness of TLRs to free fatty acid in insulin resistance and cytokine production. Interestingly, TLR expression modulation on PBMCs and adipose tissues in obese individuals remains poorly defined. The present data show over-expression of TLR2 and TLR4 on both PBMCs and adipose tissues which may explain their increased response to endogenous TLR ligands. Our findings suggest that TLRs modulation is linked with expression of proinflammatory cytokines in obese individuals.
The prior data demonstrate a correlation between TLR2/4 expression and BMI in subjects with type 2 diabetes ; however, the exact mechanism by which these two clinical predictors are interlinked remains undefined. There is lacking information about the relation of TLRs modulation and BMI in obese individuals. Expression of TLR2 and TLR4 on PBMCs varied widely among the individuals with different BMI that we observed. The extent of the obesity-induced up-regulation of TLR2/TLR4 genes and related proinflammatory cytokines cascades related to the BMI values. We found that TLR2 and TLR4 and inflammatory cytokines were overexpressed in PBMCs from obese subjects and this TLRs/cytokines overexpression correlated with the BMI. Increased expression of TLRs was correlated with FBG and HbAIc. More importantly, our data show a remarkable difference in the level of expression of toll-like receptors between obese individuals with and without diabetes. Overall, these results suggest that the overexpression of TLR2 and TLR4 on both PBMCs and adipose tissues together with the enhanced production of proinflammatory cytokines may pave way for the development of insulin resistance in obese individuals, leading to type 2 diabetes.
In conclusion, we found that TLR2 and TLR4 were overexpressed on PBMCs/adipose tissues from obese subjects which correlated with the increased expression of proinflammatory cytokines. This association may explain a potential pathophysiological link between obesity and inflammation, to subsequently result in development of insulin resistance and type 2 diabetes.