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Aging, like obesity, is associated with metabolic and inflammatory alterations within adipose tissue in older individuals. Younger females are protected from adipose inflammation, but older post-menopausal females exhibit exaggerated visceral adiposity correlated with increased disease risk. Obesity accelerates the onset and progression of age-associated diseases, but it is unclear if aging and obesity drive adipose tissue dysfunction in a sexually dimorphic fashion. We investigated adipose tissue metabolism and inflammation in a diet-induced obesity model in young and old mice. We identified age related sex differences in adipose tissue macrophages (ATMs), fibrosis and lipid metabolism in male and female visceral fat depot (GWAT). Although aging normalized body weights between the sexes, females remained protected from proinflammatory ATMs and stimulated lipolysis failed to adversely affect the inflammatory state even with obesity. Older obese males had augmented CD11c+ ATMs and higher insulin levels, while females showed increased visceral adiposity and exaggerated Pparγ, and Pgc1α expression. Obesity in aging demonstrated similar expression of GWAT p53, p16, p21, Timp1 and Tgfβ1 in both sexes. Our studies suggest that even with aging, female GWAT shows an attenuated inflammatory response compared to males due to an efficient oxidative metabolism combined with an active tissue remodeling state.

Obesity-induced chronic inflammation is associated with metabolic disease. Results from mouse models utilizing a high-fat diet (HFD) have indicated that an increase in activated macrophages, including CD11c+ adipose tissue macrophages (ATMs), contributes to insulin resistance. Obesity primes myeloid cell production from hematopoietic stem cells (HSCs) and Toll-like receptor 4 (TLR4), and the downstream TIR domain-containing adapter protein-inducing interferon-β (TRIF)- and MyD88-mediated pathways regulate production of similar myeloid cells after lipopolysaccharide stimulation. However, the role of these pathways in HFD-induced myelopoiesis is unknown. We hypothesized that saturated fatty acids and HFD alter myelopoiesis by activating TLR4 pathways in HSCs, differentially producing pro-inflammatory CD11c+ myeloid cells that contribute to obesity-induced metabolic disease. Results from reciprocal bone marrow transplants (BMTs) with Tlr4-/- and WT mice indicated that TLR4 is required for HFD-induced myelopoiesis and production of CD11c+ ATMs. Experiments with homozygous knockouts of Irakm (encoding a suppressor of MyD88 inactivation) and Trif in competitive BMTs revealed that MyD88 is required for HFD expansion of granulocyte macrophage progenitors and that Trif is required for pregranulocyte macrophage progenitor expansion. A comparison of WT, Tlr4-/-, Myd88-/-, and Trif-/- mice on HFD demonstrated that TLR4 plays a role in the production of CD11c+ ATMs, and both Myd88-/- and Trif-/- mice produced fewer ATMs than WT mice. Moreover, HFD-induced TLR4 activation inhibited macrophage proliferation, leading to greater accumulation of recruited CD11c+ ATMs. Our results indicate that HFD potentiates TLR4 and both its MyD88- and TRIF-mediated downstream pathways within progenitors and adipose tissue and leads to macrophage polarization.

Men have a statistically higher risk of metabolic and cardiovascular disease than premenopausal women, but the mechanisms mediating these differences are elusive. Chronic inflammation during obesity contributes to disease risk and is significantly more robust in males. Prior work demonstrated that compared with obese males, obese females have reduced proinflammatory adipose tissue macrophages (ATMs). Given the paucity of data on how sex hormones contribute to macrophage responses in obesity, we sought to understand the role of sex hormones in promoting obesity-induced myeloid inflammation. We used gonadectomy, estrogen receptor–deficient alpha chimeras, and androgen-insensitive mice to model sex hormone deficiency. These models were evaluated in diet-induced obesity conditions (high-fat diet [HFD]) and in vitro myeloid assays. We found that ovariectomy increased weight gain and adiposity. Ovariectomized females had increased ATMs and bone marrow myeloid colonies compared with sham-gonadectomized females. In addition, castrated males exposed to HFD had improved glucose tolerance, insulin sensitivity, and adiposity with fewer Ly6chi monocytes and bone marrow myeloid colonies compared with sham-gonadectomized males, although local adipose inflammation was enhanced. Similar findings were observed in androgen-insensitive mice; however, these mice had fewer CD11c+ ATMs, implying a developmental role for androgens in myelopoiesis and adipose inflammation. We concluded that gonadectomy results in convergence of metabolic and inflammatory responses to HFD between the sexes, and that myeloid estrogen receptor alpha contributes minimally to diet-induced inflammatory responses, whereas loss of androgen-receptor signaling improves metabolic and inflammatory outcomes. These studies demonstrate that sex hormones play a critical role in sex differences in obesity, metabolic dysfunction, and myeloid inflammation.

It is well known in clinical and animal studies that women and men have different disease risk as well as different disease physiology. Women of reproductive age are protected from metabolic and cardiovascular disease compared with postmenopausal women and men. Most murine studies are skewed toward the use of male mice to study obesity-induced metabolic dysfunction because of similar protection in female mice. We have investigated dietary obesity in a mouse model and have directly compared inflammatory responses in males and females. In this review we will summarize what is known about sex differences in diet-induced inflammation and will summarize our data on this topic. It is clear that sex differences in high-fat diet-induced inflammatory activation are due to cell intrinsic differences in hematopoietic responses to obesogenic cues, but further research is needed to understand what leads to sexually dimorphic responses.

Males are known to have profound adipose tissue macrophage (ATM) accumulation in gonadal white adipose tissue (GWAT) during obesity, whereas females are protected from such an inflammatory response even with increased adiposity. The inflammatory tone in males is linked to insulin resistance and might be the underlying cause for sex differences in metabolic disease. Factors regulating the meta-inflammatory response remain unclear but enhanced lipid storage in females may explain the reduced inflammatory response to high-fat diets. In this study, we evaluated lean and obese females with stimulated lipolysis to understand whether a stress release of free fatty acids (FFAs) could induce female ATMs. We demonstrate that in both lean and obese females, GWAT CD11c(−) resident ATMs accumulate with β-3 adrenergic receptor–stimulated lipolysis. Lipolysis elevated serum FFA, triglyceride, and IL-6 levels in females that corresponded to significant phosphorylated hormone-sensitive lipase and adipose triglyceride lipase protein expression in obese female GWAT compared with males. Increased lipolytic response in obese females was associated with crown-like structures and induced Il6, Mcp1, Arg1, and Mgl1 expression in obese female GWAT, suggesting an environment of lipid clearance and adipose remodeling. With this finding we next investigated whether lipid storage and lipolytic mediators differed by sex. Diacylglycerol, ceramides, phospholipids, and certain fatty acid species associated with inflammation were elevated in male GWAT compared with obese female GWAT. Overall, our data demonstrate a role for GWAT lipid storage and lipolytic metabolites to induce inflammation in males and induce remodeling in females that might explain sex differences in overall metabolic health.

Obesity-induced chronic inflammation is associated with metabolic disease. Results from mouse models utilizing a high-fat diet (HFD) have indicated that an increase in activated macrophages, including CD11c(+) adipose tissue macrophages (ATMs), contributes to insulin resistance. Obesity primes myeloid cell production from hematopoietic stem cells (HSCs) and Toll-like receptor 4 (TLR4), and the downstream TIR domain–containing adapter protein–inducing interferon-β (TRIF)- and MyD88-mediated pathways regulate production of similar myeloid cells after lipopolysaccharide stimulation. However, the role of these pathways in HFD-induced myelopoiesis is unknown. We hypothesized that saturated fatty acids and HFD alter myelopoiesis by activating TLR4 pathways in HSCs, differentially producing pro-inflammatory CD11c(+) myeloid cells that contribute to obesity-induced metabolic disease. Results from reciprocal bone marrow transplants (BMTs) with Tlr4(−/−) and WT mice indicated that TLR4 is required for HFD-induced myelopoiesis and production of CD11c(+) ATMs. Experiments with homozygous knockouts of Irakm (encoding a suppressor of MyD88 inactivation) and Trif in competitive BMTs revealed that MyD88 is required for HFD expansion of granulocyte macrophage progenitors and that Trif is required for pregranulocyte macrophage progenitor expansion. A comparison of WT, Tlr4(−/−), Myd88(−/−), and Trif(−/−) mice on HFD demonstrated that TLR4 plays a role in the production of CD11c(+) ATMs, and both Myd88(−/−) and Trif(−/−) mice produced fewer ATMs than WT mice. Moreover, HFD-induced TLR4 activation inhibited macrophage proliferation, leading to greater accumulation of recruited CD11c(+) ATMs. Our results indicate that HFD potentiates TLR4 and both its MyD88- and TRIF-mediated downstream pathways within progenitors and adipose tissue and leads to macrophage polarization.