Dual Role of Caspase 8 in Adipocyte Apoptosis and Metabolic Inflammation.

Caspases are cysteine-aspartic proteases that were initially discovered to play a role in apoptosis. However, caspase 8, in particular, also has additional nonapoptotic roles, such as in inflammation. Adipocyte cell death and inflammation are hypothesized to be initiating pathogenic factors in type 2 diabetes. Here, we examined the pleiotropic role of caspase 8 in adipocytes and obesity-associated insulin resistance. Caspase 8 expression was increased in adipocytes from mice and humans with obesity and insulin resistance. Treatment of 3T3-L1 adipocytes with caspase 8 inhibitor Z-IETD-FMK decreased both death receptor-mediated signaling and targets of nuclear factor κ-light-chain-enhancer of activated B (NF-κB) signaling. We generated novel adipose tissue and adipocyte-specific caspase 8 knockout mice (aP2Casp8-/- and adipoqCasp8-/-). Both males and females had improved glucose tolerance in the setting of high-fat diet (HFD) feeding. Knockout mice also gained less weight on HFD, with decreased adiposity, adipocyte size, and hepatic steatosis. These mice had decreased adipose tissue inflammation and decreased activation of canonical and noncanonical NF-κB signaling. Furthermore, they demonstrated increased energy expenditure, core body temperature, and UCP1 expression. Adipocyte-specific activation of Ikbkb or housing mice at thermoneutrality attenuated improvements in glucose tolerance. These data demonstrate an important role for caspase 8 in mediating adipocyte cell death and inflammation to regulate glucose and energy homeostasis. ARTICLE HIGHLIGHTS: Caspase 8 is increased in adipocytes from mice and humans with obesity and insulin resistance. Knockdown of caspase 8 in adipocytes protects mice from glucose intolerance and weight gain on a high-fat diet. Knockdown of caspase 8 decreases Fas signaling, as well as canonical and noncanonical nuclear factor κ-light-chain-enhancer of activated B (NF-κB) signaling in adipose tissue. Improved glucose tolerance occurs via reduced activation of NF-κB signaling and via induction of UCP1 in adipocytes.

Macrophage Jak2 deficiency accelerates atherosclerosis through defects in cholesterol efflux.

Atherosclerosis is a chronic inflammatory condition in which macrophages play a major role. Janus kinase 2 (JAK2) is a pivotal molecule in inflammatory and metabolic signaling, and Jak2V617F activating mutation has recently been implicated with enhancing clonal hematopoiesis and atherosclerosis. To determine the essential in vivo role of macrophage (M)-Jak2 in atherosclerosis, we generate atherosclerosis-prone ApoE-null mice deficient in M-Jak2. Contrary to our expectation, these mice exhibit increased plaque burden with no differences in macrophage proliferation, recruitment or bone marrow clonal expansion. Notably, M-Jak2-deficient bone marrow derived macrophages show a significant defect in cholesterol efflux. Pharmacologic JAK2 inhibition with ruxolitinib also leads to defects in cholesterol efflux and accelerates atherosclerosis. Liver X receptor agonist abolishes the efflux defect and attenuates the accelerated atherosclerosis that occurs with M-Jak2 deficiency. Macrophages of individuals with the Jak2V617F mutation show increased efflux which is normalized when treated with a JAK2 inhibitor. Together, M-Jak2-deficiency leads to accelerated atherosclerosis primarily through defects in cholesterol efflux from macrophages.

JAK2-IGF1 axis in osteoclasts regulates postnatal growth in mice.

Osteoclasts are specialized cells of the hematopoietic lineage that are responsible for bone resorption and play a critical role in musculoskeletal disease. JAK2 is a key mediator of cytokine and growth factor signaling; however, its role in osteoclasts in vivo has yet to be investigated. To elucidate the role of JAK2 in osteoclasts, we generated an osteoclast-specific JAK2-KO (Oc-JAK2-KO) mouse using the Cre/Lox-P system. Oc-JAK2-KO mice demonstrated marked postnatal growth restriction; however, this was not associated with significant changes in bone density, microarchitecture, or strength, indicating that the observed phenotype was not due to alterations in canonical osteoclast function. Interestingly, Oc-JAK2-KO mice had reduced osteoclast-specific expression of IGF1, suggesting a role for osteoclast-derived IGF1 in determination of body size. To directly assess the role of osteoclast-derived IGF1, we generated an osteoclast-specific IGF1-KO mouse, which showed a similar growth-restricted phenotype. Lastly, overexpression of circulating IGF1 by human transgene rescued the growth defects in Oc-JAK2-KO mice, in keeping with a causal role of IGF1 in these models. Together, our data show a potentially novel role for Oc-JAK2 and IGF1 in the determination of body size, which is independent of osteoclast resorptive function.

Distinct roles of UVRAG and EGFR signaling in skeletal muscle homeostasis.

OBJECTIVE: Autophagy is a physiological self-eating process that can promote cell survival or activate cell death in eukaryotic cells. In skeletal muscle, it is important for maintaining muscle mass and function that is critical to sustain mobility and regulate metabolism. The UV radiation resistance-associated gene (UVRAG) regulates the early stages of autophagy and autophagosome maturation and plays a key role in endosomal trafficking. This study investigated the essential in vivo role of UVRAG in skeletal muscle biology. METHODS: To determine the role of UVRAG in skeletal muscle in vivo, we generated muscle-specific UVRAG knockout mice using the Cre-loxP system driven by Myf6 promoter that is exclusively expressed in skeletal muscle. Myf6-Cre+ UVRAGfl/fl (M-UVRAG-/-) mice were compared to littermate Myf6-Cre+ UVRAG+/+ (M-UVRAG+/+) controls under basal conditions on a normal chow diet. Body composition, muscle function, and mitochondria morphology were assessed in muscles of the WT and KO mice at 24 weeks of age. RESULTS: M-UVRAG-/- mice developed accelerated sarcopenia and impaired muscle function compared to M-UVRAG+/+ littermates at 24 weeks of age. Interestingly, these mice displayed improved glucose tolerance and increased energy expenditure likely related to upregulated Fgf21, a marker of muscle dysfunction. Skeletal muscle of the M-UVRAG-/- mice showed altered mitochondrial morphology with increased mitochondrial fission and EGFR accumulation reflecting defects in endosomal trafficking. To determine whether increased EGFR signaling had a causal role in muscle dysfunction, the mice were treated with an EGFR inhibitor, gefitinib, which partially restored markers of muscle and mitochondrial deregulation. Conversely, constitutively active EGFR transgenic expression in UVRAG-deficient muscle led to further detrimental effects with non-overlapping distinct defects in muscle function, with EGFR activation affecting the muscle fiber type whereas UVRAG deficiency impaired mitochondrial homeostasis. CONCLUSIONS: Our results show that both UVRAG and EGFR signaling are critical for maintaining muscle mass and function with distinct mechanisms in the differentiation pathway.

The cellular stress response of rat skeletal muscle following lengthening contractions.

The cellular stress response of the rat tibialis anterior (TA) muscle was investigated following 20, 40, or 60 lengthening contractions (LCs) using an in vivo model of electrical stimulation. Muscles were removed at 0, 1, 3, or 24 h after LCs and assessed for heat shock transcription factor (HSF) activation, heat shock protein (HSP) content, and/or morphological evidence of muscle fibre damage. When compared with the first muscle contraction, peak muscle torque was reduced by 26% (p < 0.05) after 20 LCs and further reduced to 56% and 60% (p < 0.001) after 40 and 60 LCs, respectively. Following 60 LCs, HSF activation was detected at 0, 1, and 3 h but was undetectable at 24 h. Hsp72 content was elevated at 24 h after 20 LCs (2.34 ± 0.37 fold, p < 0.05), 40 LCs (3.02 ± 0.31 fold, p < 0.01), and 60 LCs (3.37 ± 0.21 fold, p < 0.001). Hsp25 content increased after 40 (2.36 ± 0.24 fold, p < 0.01) and 60 LCs (2.80 ± 0.37 fold, p < 0.01). Morphological assessment of TA morphology revealed that very few fibres were damaged following 20 LCs while multiple sets of LCs (40 and 60) caused greater amounts of fibre damage. Electron microscopy showed disrupted Z-lines and sarcomeres were detectable in some muscles fibres following 20 LCs but were more prevalent and severe in muscles subjected to 40 or 60 LCs. These results suggest LCs elevate HSP content by an HSF-mediated mechanism (60 LC) and a single set of 20 LCs is capable of increasing muscle HSP content without causing significant muscle fibre damage.