Estrogen enhances browning in adipose tissue by M2 macrophage polarization via heme oxygenase-1.

Loss of ovarian function results in increased fat mass, leading to the accumulation of adipose tissue macrophages that participate in chronic inflammation. We hypothesized that ovariectomy (OVX)-induced increases in body weight and fat mass are associated with decreased adipose tissue (AT) browning due to estrogen (E2 ) deficiency. In mice, OVX decreased AT browning along with increased body weight, fat mass, and size of lipid droplets 12 weeks after surgery. Exogenous E2 recovered the OVX-induced changes. AT browning was enhanced by M2 macrophages induced by exogenous E2. E2 -induced M2 polarization occurred due to the increased expression of heme oxygenase-1 (HO-1) in macrophages, leading to decreased reactive oxygen species levels. Collectively, we demonstrated that E2 enhances AT browning via M2 polarization mediated by HO-1.

Carbon monoxide reverses adipose tissue inflammation and insulin resistance upon loss of ovarian function.

We hypothesized that carbon monoxide (CO) might suppress chronic inflammation, which led to metabolic disturbances. Ovariectomy (OVX) was performed in mice to mimic chronic inflammation secondary to loss of ovarian function. OVX increased fat mass and the infiltration of highly inflammatory CD11c cells into adipose tissue (AT), resulting in a disturbance of glucose metabolism. Treatment of CO attenuated these; CO decreased recruitment of CD11c-expressing cells in AT and reduced expression of CD11c in bone marrow-derived macrophages, protecting them from M1 polarization. Upregulated cGMP and decreased reactive oxygen species were responsible for the inhibitory activity of CO on CD11c expression; knockdown of soluble guanylate cyclase or heme oxygenase-1 using small interfering RNAs reduced this inhibition substantially. Improved OVX-induced insulin resistance (IR) by CO was highly associated with its activity to attenuate AT inflammation. Our results suggest a therapeutic value of CO to treat postmenopausal IR by reducing AT inflammation.

MicroRNA-29b Enhances Osteoclast Survival by Targeting BCL-2-Modifying Factor after Lipopolysaccharide Stimulation.

Recent findings suggest that microRNAs (miRs) play a critical role in osteoclastogenesis, which regulates bone loss. We hypothesized that inflammation induces miR-29b, which increases the survival rate in osteoclasts (OCs), leading to bone loss. The expression level of miR-29b increased in OC stimulated by lipopolysaccharide (LPS) in an in vitro system which correlated with its increase in tibiae from mice that received LPS injections compared with those that received vehicle treatment. An miR-29b mimic increased OC survival rate without any change in OC differentiation, and furthermore, the inhibition of endogenous miR-29b induced by LPS decreased OC survival rate. Increased OC survival rate after overexpression of miR-29b was associated with antiapoptotic activity, as shown by staining annexin V-positive cells. We found that a target gene of miR-29b is BCL-2-modifying factor (Bmf), which acts as a proapoptotic factor, and that miR-29b binds to the 3'-UTR of Bmf. Our data demonstrate that LPS-induced miR-29b increases the number of OC by enhancing OC survival through decreased BMF.

MCP-1 deficiency enhances browning of adipose tissue via increased M2 polarization.

Obesity is strongly associated with chronic inflammation for which adipose tissue macrophages play a critical role. The objective of this study is to identify monocyte chemoattractant protein-1 (MCP-1, CCL2) as a key player governing M1-M2 macrophage polarization and energy balance. We evaluated body weight, fat mass, adipocyte size and energy expenditure as well as core body temperature of Ccl2 knockout mice compared with wild-type mice. Adipose tissues, differentiated adipocyte and bone marrow-derived macrophages were assessed by qPCR, Western blot analysis and histochemistry. MCP-1 deficiency augmented energy expenditure by promoting browning in white adipose tissue and brown adipose tissue activity via increasing the expressions of Ucp1, Prdm16, Tnfrsf9, Ppargc1a, Nrf1 and Th and mitochondrial DNA copy number. MCP-1 abrogation promoted M2 polarization which is characterized by increased expression of Arg1, Chil3, Il10 and Klf4 whereas it decreased M1 polarization by decreased p65 nuclear translocation and attenuated expression of Itgax, Tnf and Nos2, leading to increased browning of adipocytes. Enhanced M2 polarization and attenuated M1 polarization in the absence of MCP-1 are independent. Collectively, our results suggest that the action of MCP-1 in macrophages modulates energy expenditure by impairing browning in adipose tissue.

Impaired insulin signaling upon loss of ovarian function is associated with a reduction of tristetraprolin and an increased stabilization of chemokine in adipose tissue.

Loss of ovarian function can activate inflammation and lead to insulin resistance (IR). IR is also a core feature of obesity and obesity-associated metabolic dysfunction. Tristetraprolin/zinc finger protein 36 (TTP) interferes with TNF-α production by destabilizing TNF-α mRNA, and mice deficient in TTP develop a complex syndrome of inflammatory disease (Carballo et al., 1998; Taylor et al., 1999). We hypothesized that ovariectomy (OVX) might also prime inflammation by reducing tristetraprolin/zinc finger protein 36 (TTP) levels. We used a mouse OVX model to study impaired insulin signaling due to loss of ovarian function by evaluating Akt activity upon insulin stimulus. Impaired insulin signaling was initially detected in adipose tissue (AT) at 4 weeks after OVX, and then spread to liver and muscle, finally resulting in systemic IR at 12 weeks after OVX. OVX decreased TTP protein levels and increased adipocyte size, oxidative stress, chemokine expression and fat mass in AT by 4 weeks after surgery. TTP deficiency due to TTP gene deletion induced aberrant insulin signaling and increased chemokine expression and macrophage numbers in AT but did not increase adipocyte size, oxidative stress, or fat mass, suggesting that it promotes insulin signaling by decreasing AT inflammation independent of oxidative stress and adiposity. OVX, like TTP deficiency, increased the stability of chemokine transcripts as assessed from their half-lives. Our data indicate that the impaired insulin signaling resulting from OVX is due to an OVX-induced reduction of TTP and the resulting stabilization of inflammatory chemokines.

MicroRNA-155 induces autophagy in osteoclasts by targeting transforming growth factor ß-activated kinase 1-binding protein 2 upon lipopolysaccharide stimulation.

The autophagy pathway has been suggested to influence skeletal structure by modulating bone metabolism. Recent findings suggest that microRNAs (miR) play a critical role in autophagy. We hypothesized that inflammation induces miR-155, which enhances autophagy in osteoclasts (OC), leading to inflammatory bone loss. The expression of miR-155 was elevated in tibiae from LPS-injected mice and in OC stimulated by lipopolysaccharide (LPS) compared with vehicle treatment. Overexpression of miR-155 enhanced autophagy as well as differentiation in OC, whereas inhibition of endogenous miR-155 decreased both. Transforming growth factor ß-activated kinase 1-binding protein 2 (TAB2) was identified as a target gene of miR-155 via binding to the 3'-UTR of TAB2, which directly interacts with BECLIN1. BECLIN1 was dissociated from TAB2, which started to associate with TAK1 when autophagy was induced. Our data demonstrate that LPS-induced miR-155 promoted autophagy to increase OC formation via decreased TAB2.

MicroRNA-183 increases osteoclastogenesis by repressing heme oxygenase-1.

Emerging evidence suggests that microRNAs (miRs) influence skeletal structure by modulating osteoclastogenesis and bone resorption. We have demonstrated previously that the up-regulation of heme oxygenase-1 (HO-1) attenuated osteoclastogenesis in bone marrow-derived macrophages (BMMs). RANKL-induced osteoclastogenesis elevates microRNA-183 (miR-183) in BMM. We show here that HO-1 is a target gene of miR-183 and that this miRNA binds to the 3'-UTR of HO-1. We find that a synthetic inhibitor that binds to miR-183 decreases osteoclast (OC) differentiation and increases the expression of HO-1, while a mimic of endogenous mature miR-183 has the opposite effect. Moreover, the HO-1 inducers, resveratrol and piceatannol decrease the expression of miR-183, resulting in attenuated osteoclastogenesis. Our findings reveal how miR-183 affects OC formation.