CNC-bZIP protein NFE2L1 regulates osteoclast differentiation in antioxidant-dependent and independent manners.

Fine-tuning of osteoclast differentiation (OD) and bone remodeling is crucial for bone homeostasis. Dissecting the mechanisms regulating osteoclastogenesis is fundamental to understanding the pathogenesis of various bone disorders including osteoporosis and arthritis. Nuclear factor erythroid 2-related factor 1 (NFE2L1, also known as NRF1), which belongs to the CNC-bZIP family of transcription factors, orchestrates a variety of physiological processes and stress responses. While Nfe2l1 gene may be transcribed into multiple alternatively spliced isoforms, the biological function of the different isoforms of NFE2L1 in bone metabolism, osteoclastogenesis in particular, has not been reported. Here we demonstrate that knockout of all isoforms of Nfe2l1 transcripts specifically in the myeloid lineage in mice [Nfe2l1(M)-KO] results in increased activity of osteoclasts, decreased bone mass and worsening of osteoporosis induced by ovariectomy and aging. In comparison, LysM-Cre-mediated Nfe2l1 deletion has no significant effect on the osteoblast and osteocytes. Mechanistic investigations using bone marrow cells and RAW 264.7 cells revealed that deficiency of Nfe2l1 leads to accelerated and elevated OD, which is attributed, at least in part, to enhanced accumulation of ROS in the early stage of OD and expression of nuclear factor of activated T cells, cytoplasmic, calcineurin dependent 1α (Nfatc1/α). In addition, NFE2L1 regulates the transcription of multiple antioxidant genes and Nfatc1/α and OD in an isoform-specific manner. While long isoforms of NFE2L1 function as accelerators of induction of Nfatc1/α and antioxidant genes and OD, the short isoform NFE2L1-453 serves as a brake that keeps the long isoforms' accelerator effects in check. These findings provide a novel insight into the regulatory roles of NFE2L1 in osteoclastogenesis and highlight that NFE2L1 is essential in regulating bone remodeling and thus may be a valuable therapeutic target for bone disorders.

Rifampicin impairs adipogenesis by suppressing NRF2-ARE activity in mice fed a high-fat diet.

Prolonged treatment with rifampicin (RFP), a first-line antibacterial agent used in the treatment of drug-sensitive tuberculosis, may cause various side effects, including metabolic disorders. The nuclear factor (erythroid-derived 2)-like 2 (NFE2L2, also known as NRF2) plays an essential regulatory role in cellular adaptive responses to stresses via the antioxidant response element (ARE). Our previous studies discovered that NRF2 regulates the expression of CCAAT-enhancer-binding protein ß (Cebpb) and peroxisome proliferator-activated receptor gamma (Pparg) in the process of adipogenesis. Here, we found that prolonged RFP treatment in adult male mice fed a high-fat diet developed insulin resistance, but reduced fat accumulation and decreased expression of multiple adipogenic genes in white adipose tissues. In 3 T3-L1 preadipocytes, RFP reduced the induction of Cebpb, Pparg and Cebpa at mRNA and protein levels in the early and/or later stage of hormonal cocktail-induced adipogenesis. Mechanistic investigations demonstrated that RFP inhibits NRF2-ARE luciferase reporter activity and expression of NRF2 downstream genes under normal culture condition and in the early stage of adipogenesis in 3 T3-L1 preadipocytes, suggesting that RFP can disturb adipogenic differentiation via NRF2-ARE interference. Taken together, we demonstrate a potential mechanism that RFP impairs adipose function by which RFP likely inhibits NRF2-ARE pathway and thereby interrupts its downstream adipogenic transcription network.

Nrf2 in adipocytes.

White adipocytes play a key role in maintaining whole body energy homeostasis by forming white adipose tissue (WAT). The impairment of WAT formation or WAT dysfunction is clearly associated with severe metabolic disorders. Mature adipocytes are derived from differentiated preadipocytes and are pivotal in energy storage and metabolism. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a member of a family of CNC-bZIP proteins which exert their transcriptional control on genes harboring antioxidant response elements (ARE) in partnership with small musculoaponeurotic fibrosarcoma proteins. The activation of Nrf2-ARE coordinated by specific repressor Kelch-like ECH-associated protein 1 (Keap1) regulates networks of genes controlling diverse homeostatic processes involving adaptive antioxidant response and detoxification among many other adaptive responses. Interestingly, accumulating evidence indicates that Nrf2 may act as a transcription factor in regulating the formation and function of adipose tissues, including adipogenesis, lipid metabolism and insulin sensitivity. In this mini-review, an overview on the distinct roles of Nrf2 in adipocytes is provided. While highlighting the regulatory role of Nrf2 in adipogenesis, recent key findings on Nrf2 in insulin signal transduction and energy metabolism of adipocytes are also summarized.

Prolonged inorganic arsenic exposure via drinking water impairs brown adipose tissue function in mice.

Although epidemiologic studies show an association between long-term environmental inorganic arsenic (iAs) exposure and various disorders of glucose and lipid metabolism, the mechanisms of these ailments remain unclear. While white adipose tissue (WAT) essentially acts as a storage tissue for energy and is key to energy homeostasis, brown adipose tissue (BAT) consumes excess energy via uncoupling protein 1-mediated non-shivering thermogenesis in mitochondria and helps maintain the steady state of glucose and lipid metabolism. Our previous in vitro work found that iAs may inhibit adipogenesis and glucose uptake in adipocytes, leading us to hypothesize that chronic exposure to iAs in vivo may also affect the development and function of BAT, which plays a part in iAs-induced metabolic disorders. Thus, adult C57BL/6J female mice were provided drinking water containing 5 or 20 ppm of inorganic arsenicals (iAs3+ and iAs5+) for 17 weeks and control mice were given unaltered water. In these mice, iAs exposure induced cold intolerance and lipid accretion in BAT. In addition, iAs exposure impaired expression of various genes related to thermogenesis, mitochondrial function, adipocyte differentiation, as well as lipolysis in BAT of the exposed mice. These findings suggest a novel toxicity of iAs in BAT occurring via induction of BAT malfunction and impairment of thermogenesis. This novel toxicological linkage helps explain the mechanisms linking iAs exposure to increased risk of disorders of glucose and lipid metabolism.

Adipocyte-specific deficiency of Nfe2l1 disrupts plasticity of white adipose tissues and metabolic homeostasis in mice.

The maintenance of healthy adipose tissues is essential for efficient regulation of energy homeostasis. Nuclear factor-erythroid 2-related factor 1 (NFE2L1, also known as Nrf1), a CNC-bZIP protein, is a master regulator of the cellular adaptive response to stresses. To investigate the role of NFE2L1 in adipocytes, we bred a line of mice with adipocyte-specific Nfe2l1 knockout (Nfe2l1(f)-KO), and found that Nfe2l1(f)-KO mice exhibited a dramatically reduced subcutaneous adipose tissue (SAT) mass, insulin resistance, adipocyte hypertrophy, and severe adipose inflammation. Mechanistic studies revealed that Nfe2l1 deficiency may disturb the expression of lipolytic genes in adipocytes, leading to adipocyte hypertrophy followed by inflammation, pyroptosis, and insulin resistance. Our findings reveal a novel role for NFE2L1 in regulating adipose tissue plasticity and energy homeostasis.