The serine/threonine kinase TPL2 integrates inflammatory inputs from the MAP kinase and IKK/NF-κB signaling pathways (24
) and is upregulated in the inflamed adipose tissue of obese mice and humans (31
). These observations suggest that TPL2 actions promote the inflammatory and metabolic complications of obesity. The current study used a diet-induced obesity paradigm with TPL2KO and WT mice to provide the first demonstration that TPL2 deletion reduces peripheral inflammation, hepatic steatosis, and improves whole-body insulin resistance in the obese state. A key finding of our study was that whole-body insulin sensitivity was improved in obese TPL2KO mice compared with obese WT controls. The observed improvement in glucose-insulin homeostasis was due to increased glucose uptake in skeletal muscle and also increased suppression of hepatic glucose output.
Importantly, the improvement in whole body insulin sensitivity in obese TPL2KO mice occurred in the absence of any reductions in adipose depot mass, suggesting that an alternative mechanism promotes enhanced insulin sensitivity in TPL2KO mice. Specifically, our results suggest that the improved metabolic profile of TPL2KO mice reflects the salutary effects of TPL2 gene ablation on inflammation.
Two key components of the TPL2-mediated signaling axis in nonhematopoetic and hematopoietic cells and tissues are the MAP kinases ERK and JNK (26
), both of which are implicated in obesity-associated insulin resistance. JNK and ERK activity are both increased in adipocytes from obese patients with diabetes, and this MAPK activation has been linked to the development of insulin resistance, either directly or indirectly, by increased production of downstream inflammatory mediators such as TNF-α and IL-6 (8
). Phosphorylation of both JNK and ERK was increased in the adipose tissue of WT mice in response to diet-induced obesity, and this increase was attenuated by ≥50% in TPL2KO mice.
Adipose tissue inflammation is largely due to the proinflammatory actions of recruited BMD immune cells (4
). In both rodents and humans (43
), the metabolic complications of obesity are associated with the accumulation of proinflammatory macrophages (F4/80+
cells) that express the dendritic cell marker CD11c in adipose tissue (13
). These macrophages selectively localize to CLS surrounding dead and dying adipocytes (6
). Genetic ablation of CD11c+
cells results in significant reductions in CLS and improves metabolic profiles in obese mice (17
). Our data suggest that TPL2 is a key regulator of CD11c+
macrophage infiltration and inflammation in obese adipose tissue. Gene expression of F4/80
were dramatically decreased in the adipose tissue of obese TPL2KO mice, suggesting reduced infiltration or retention of CD11c+
macrophages. Consistent with this observation, the frequency of CLS and mRNA levels of MCP-1
were both significantly reduced. Our data do not rule out the possibility that reduced adipose tissue inflammation in TPL2KO mice reflects altered inflammatory responses of T-cells (47
) or other adipose tissue immune cells.
TPL2 is a master regulator of ERK-dependent gene expression downstream of TLRs in hematopoietic cells, including macrophages (25
). Thus, in addition to its effects on adipose tissue macrophage recruitment, TPL2 ablation is likely to have direct effects on macrophage inflammatory gene expression in obesity. We therefore determined the effect of TPL2 ablation on lipopolysaccharide-induced IL-6
, and TNF-α
expression in BMDM from WT and TPL2KO mice. We observed reduced secretion of TNFα, MCP-1, and IL-6 from TPL2KO BMDM. These data are in line with the gene expression data from the adipose tissue or liver, or both, of obese WT and TPL2KO mice and suggest TPL2 signaling in immune cells is an important factor in the initiation and maintenance of obesity-induced peripheral inflammation.
In addition to reducing adipose tissue immune cell infiltration and inflammation, ablation of TPL2 signaling in obese mice reduced hepatic lipid accumulation and inflammation. Decreased hepatic inflammation and lipid accumulation are both associated with improved insulin action in the liver. Hepatic inflammation is linked to enhanced triglyceride deposition in the liver; therefore, TPL2 ablation may reduce hepatic steatosis by directly reducing hepatic inflammation, for example, in Kupffer cells.
Alternatively our observations are consistent with studies reporting positive associations between visceral adipose tissue inflammation and hepatic inflammation in obese humans (49
) and mice (13
). Future studies using tissue-specific TPL2 ablation will be required to determine if the observed reduction in hepatic inflammation is a result of reduced adipose tissue inflammation or is caused by a lack of TPL2 signaling in the liver.
Accumulation of triglyceride and active lipid species (i.e., diacylglycerols) has been implicated in the pathogenesis of insulin resistance (40
) and could be a common denominator in the improved metabolic profile observed in liver and muscle. Triglyceride accumulation was reduced in the liver and skeletal muscle in obese TPL2KO mice compared with obese WT mice. Consistent with reduced ectopic triglyceride accumulation, the clamp studies demonstrated increased sensitivity to insulin’s actions in liver and skeletal muscle. Insulin-stimulated Akt phosphorylation was enhanced in skeletal muscle but not in the liver of obese TPL2KO mice. This discrepancy between clamp and Akt data presumably reflects differential responses to an acute insulin bolus versus the integrated physiologic response to a steady-state insulin infusion during the clamp. It is intriguing that reduced ectopic triglyceride was observed in obese TPL2KO mice without a relative reduction in basal or clamped plasma free fatty acid concentrations. Future studies will address the mechanism(s) by which TPL2 reduces ectopic triglyceride accumulation in the liver and skeletal muscle and its role in improving glucose-insulin homeostasis.
In summary, the current study demonstrates a significant role for TPL2 in obesity-induced inflammation and metabolic dysfunction. TPL2 deletion in the obese state prevents MAPK activation, proinflammatory cytokine expression, and the accumulation of immune cells in peripheral tissues, resulting in reduced hepatic steatosis and improved insulin sensitivity. The recent report of increased TPL2 activity in human obesity (31
) and the results of the current study suggest that TPL2 is a promising therapeutic target for the inflammatory and metabolic complications of obesity.