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Circadian clock confers temporal control in metabolism, with its disruption leading to the development of insulin resistance. Metabolic substrate utilization in skeletal muscle is coordinated with diurnal nutrient cycles. However, whether the molecular clock is involved in this coordination is largely unknown. Using a myocyte-selective genetic ablation mouse model of the essential clock activator Bmal1, here we identify muscle-intrinsic clock as a sensor of feeding cues to orchestrate skeletal muscle oxidation required for global nutrient flux. Bmal1 in skeletal muscle responds robustly to feeding in vivo and insulin induces its expression. Muscle Bmal1 deficiency impaired the transcriptional control of glucose metabolic pathway, resulting in markedly attenuated glucose utilization and fasting hyperglycemia. Notably, the loss of Bmal1 response to feeding abolished fasting-to-feeding metabolic fuel switch from fatty acids to glucose in skeletal muscle, leading to the activation of energy-sensing pathways for fatty acid oxidation. These altered metabolic substrate oxidations in Bmal1-deficient muscle ultimately depleted circulating lipid levels that prevented hepatic steatosis. Collectively, our findings highlight the key role of the metabolic-sensing function of skeletal muscle clock in partitioning nutrient flux between muscle and liver to maintain whole-body lipid and glucose homeostasis.

Biguanides such as metformin are one of the most widely administered anti-diabetic drugs. Biguanides can suppress OXPHOS by inhibiting the complex-I activity of the mitochondrial electron transport chain (ETC). Though metformin recently generated lots of hope in cancer therapy, several clinical trials showed only limited advantages for metformin in breast cancer patients. Considering its long-term observed and widely accepted safety parameters, we investigated the possible mechanisms of the lack of in vivo anticancer effect of metformin and its potential combination therapies. While metformin is an ETC inhibitor, metformin can activate AMPK, leading to ACC phosphorylation, independently of ETC inhibition. Since ACC is the upstream regulator of mitochondrial fatty acid β-oxidation (FAO), lower concentrations of biguanides can also activate FAO. Thus, biguanides can play opposing roles in mitochondrial metabolism by FAO activation, and ETC inhibition. Considering the hybrid metabolic phenotype and increased dependency of TNBC on FAO, we analyzed the effect of biguanides in TNBC progression and metastasis. While higher concentrations of biguanides suppress ETC complex-1 function, their lower concentrations activate AMPK-FAO pathway-driven ETC activity in TNBC. When the mice with TNBC tumors were treated with a clinically relevant dose of metformin, the tumor concentration of metformin after a few hours of treatment was much lower to inhibit ETC effectively. Since low concentrations of metformin can activate FAO, we did in vitro analysis of the combination of metformin and FAO inhibitor, etomoxir, in TNBC cell lines. As expected, this combination synergistically inhibited tumor properties in TNBC cells. We have published that FAO activates c-Src, one of the frequently upregulated oncopathways in TNBC. Thus, we analyzed the role of metformin concentration in Src activation in TNBC. In alignment with FAO, lower metformin concentrations activate, and high concentrations inhibit Src phosphorylation. We then investigated the significance of the combination of Src inhibitor dasatinib with metformin. As observed with FAO inhibitor, Src inhibitor synergistically enhanced the anticancer effect of metformin. To understand the translational significance of this combination, we performed ex vivo studies using TNBC organoids and PDX models. Interestingly, the combination of metformin and dasatinib synergistically inhibited the tumor growth, and metastasis and enhanced survival potential. Our results suggest that the combination of metformin and dasatinib significantly inhibits lung metastasis and modulates the immune microenvironment of the lungs. Since both dasatinib and metformin are clinically approved drugs, our findings provide a clinically translatable option for treating TNBC patients who currently lack targeted therapy. Citation Format: Junhyoung Park, Kwang Hwa Jung, Dongya Jia, Sukjin Yang, Kuldeep S. Attri, Songyeon Ahn, Divya Murthy, Meron Ghidey, Somik Chatterjee, Diego A. Pedroza, Abha Tiwari, Suna Kim, Chad C. Creighton, Nagireddy Putluri, Jeffrey M. Rosen, José N. Onuchic, Andrei Goga, Benny A. Kaipparettu. Tumor concentration of metformin is a determinant factor of its regulation of fatty acid β-oxidation and c-Src pathway in triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 294.

Rev-erbα is a ligand-dependent nuclear receptor and a key repressor of the molecular clock transcription network. Accumulating evidence indicate that the circadian clock machinery governs diverse biological processes in skeletal muscle, including muscle growth, repair and mass maintenance. The physiological function of Rev-erbα in myogenic regulation remains largely unknown. Here we show that Rev-erbα exerts cell-autonomous inhibitory effects on proliferation and differentiation of myogenic precursor cells, and these actions concertedly inhibit muscle regeneration in vivo. Mechanistic studies reveal Rev-erbα direct transcriptional control of two major myogenic mechanisms, proliferative pathway and the Wnt signaling cascade. Consistent with this finding, primary myoblasts lacking Rev-erbα display significantly enhanced proliferative growth and myogenic progression. Furthermore, pharmacological activation of Rev-erbα activity attenuates, whereas its inhibition by an antagonist promotes these processes. Notably, upon muscle injury, the loss-of-function of Rev-erbα in vivo augmented satellite cell proliferative expansion and regenerative progression during regeneration. Collectively, our study identifies Rev-erbα as a novel inhibitory regulator of myogenic progenitor cell properties that suppresses postnatal myogenesis. Pharmacological interventions to dampen Rev-erbα activity may have potential utilities to enhance regenerative capacity in muscle diseases.

The transformation of prostatic epithelial cells to prostate cancer (PCa) has been characterized as a transition from citrate secretion to citrate oxidation, from which one would anticipate enhanced mitochondrial complex I (CI) respiratory flux. Molecular mechanisms for this transformation are attributed to declining mitochondrial zinc concentrations. The unique metabolic properties of PCa cells have become a hot research area. Several publications have provided indirect evidence based on investigations using pre-clinical models, established cell lines, and fixed or frozen tissue bank samples. However, confirmatory respiratory analysis on fresh human tissue has been hampered by multiple difficulties. Thus, few mitochondrial respiratory assessments of freshly procured human PCa tissue have been published on this question. Our objective is to document relative mitochondrial CI and complex II (CII) convergent electron flow to the Q-junction and to identify electron transport system (ETS) alterations in fresh PCa tissue. The results document a CII succinate: quinone oxidoreductase (SQR) dominant succinate oxidative flux model in the fresh non-malignant prostate tissue, which is enhanced in malignant tissue. CI NADH: ubiquinone oxidoreductase activity is impaired rather than predominant in high-grade malignant fresh prostate tissue. Given these novel findings, succinate and CII are promising targets for treating and preventing PCa.

Diastolic dysfunction (DD) is prevalent in elderly post-menopausal women. In addition to impaired cardiac relaxation and output, DD is associated with loss of skeletal muscle mass, muscle strength and function, and exercise capacity. Understanding the mechanisms of estrogen (E2)-mediated regulation of skeletal muscle structure and function in DD can have broader implications in understanding the progression of heart disease in post-menopausal women. In muscle, E2 regulates autophagy and mitophagy, processes that orchestrate degradation and recycling of damaged cellular components to maintain healthy tissue and conserve energy. However, this role of E2 under a physiological stress such as DD, is largely unknown. Our study aimed to understand the relationship between E2, autophagy/mitophagy and muscle function in DD. We hypothesized that estrogen receptor (ER) signaling plays a direct role in regulation of autophagy and mitophagy-mediated maintenance of muscle mass and muscle mitochondrial function in DD. Sham or ovariectomy (OVX) surgeries were performed to induce E2-deficiency, followed by induction of DD by administering hypertension agents L-NAME and angiotensin II. NMR analysis revealed significantly reduced total lean mass in OVX mice with DD, further confirmed by direct measurement of gastrocnemius muscle mass. Expression of ERα target-genes PDK4 and STAT3 were reduced in muscle of OVX mice with DD. DD significantly reduced LC3B II/I ratio in muscle, indicative of reduced autophagy. Consistent with this, expression of transcription factor FoxO3, a master-regulator of multiple autophagy and mitophagy genes, was reduced in OVX mice with DD compared to sham control mice. Expression of SDHA gene coding for mitochondrial respiratory chain subunit II, was reduced in sham and OVX mice with DD compared to sham control mice. Mitochondrial respiratory function measured from isolated muscle mitochondria, was reduced in sham and OVX mice with DD compared to sham control mice, and in OVX mice with DD compared to OVX control mice, indicating compromised skeletal muscle mitochondrial function. Our findings indicate that OVX-induced E2-deficiency and DD leads to muscle impairments, including defects in mitochondrial function.

The muscle-intrinsic clock machinery is required for the maintenance of muscle growth, remodeling and function. Our previous studies demonstrated that the essential transcription activator of the molecular clock feed-back loop, Brain and Muscle Arnt-Like 1(Bmal1), plays a critical role in myogenic progenitor behavior to promote and regenerative myogenesis. Using genetic approaches targeting Bmal1 in the DMD(mdx) (mdx) dystrophic mouse model, here we report that the loss of Bmall function significantly accelerated dystrophic disease progression. In contrast to the mild dystrophic changes in mdx mice, the genetic loss-of-function of Bmal1 aggravated muscle damage in this dystrophic disease background, as indicated by persistently elevated creatine kinase levels, increased injury area and reduced muscle grip strength. Mechanistic studies revealed that markedly impaired myogenic progenitor proliferation and myogenic response underlie the defective new myofiber formation in the chronic dystrophic milieu. Taken together, our study identified the function of pro-myogenic clock gene Bmal1 in protecting against dystrophic damage, suggesting the potential for augmenting Bmal1 function to ameliorate dystrophic or degenerative muscle diseases.

The circadian clock exerts temporal control in metabolism and its disruption leads to the development of diabetes and obesity. Tissue-intrinsic clock circuits are integral components of global metabolic homeostasis, although how they sense nutrient signals to orchestrate metabolic flux is not clear. Skeletal muscle is major site for metabolic substrate oxidation, and its utilization of glucose and fatty acid depends on availability of nutrients accompanying feeding-fasting transitions. Interestingly, nearly 30% of rhythmic transcripts in skeletal muscle belongs to metabolism. By generating a mouse model with myocyte-selective ablation of the clock transcription activator Bmal1, here we show that cell-autonomous muscle clock plays an essential role in coordinating nutrient utilization with feeding-fasting induced cycles. Bmal1 in skeletal muscle was robustly induced by feeding, and its loss markedly impaired feeding-induced switch to glucose metabolism from fatty acid utilization. As a result, Bmal1-deficient muscle displays nearly 50% reduction of glucose oxidation whereas fatty acid oxidation was enhanced, resembling a constant fasting state. This metabolic shift was accompanied by muscle fiber type switching to an oxidative phenotype, and led to a remarkable resistance to hepatic lipid accumulation induced by prolonged fasting or high-fat diet feeding. In contrast, systemic fasting glucose levels in muscle Bmal1-deficient mice was elevated, which is due to a failure of suppressing hepatic glucose output as revealed by hyperinsulinemic-euglycemic glucose clamp study. Collectively, our results reveal a novel function of the Bmal1-driven muscle clock as a key metabolic sensor that coordinates metabolic fuel oxidation with oscillatory nutrient availability in fasting-feeding cycles. This temporal mechanism in orchestrating global nutrient flux may contribute to metabolic abnormalities induced by circadian misalignment. Disclosure K. Ma: None. S. Chatterjee: None. H. Yin: None.

Isolated mitochondria are polymerically functionalized for transplantation into metabolically compromised cells. In article number 1700530, Elvin Blanco, Dale J. Hamilton, and co‐workers demonstrate that coating mitochondria with a dextran‐triphenylphosphonium (TPP) conjugate results in longer functional survival of the organelle ex vivo, increased cellular uptake, and an enhanced shift in the metabolic phenotype of cells compared to uncoated mitochondria.

Accumulating evidence indicates that the circadian clock, a transcriptional/translational feedback circuit that generates ~24-hour oscillations in behavior and physiology, is a key temporal regulatory mechanism involved in many important aspects of muscle physiology. Given the clock as an evolutionarily-conserved time-keeping mechanism that synchronizes internal physiology to environmental cues, locomotor activities initiated by skeletal muscle enable entrainment to the light-dark cycles on earth, thus ensuring organismal survival and fitness. Despite the current understanding of the role of molecular clock in preventing age-related sarcopenia, investigations into the underlying molecular pathways that transmit clock signals to the maintenance of skeletal muscle growth and function are only emerging. In the current review, the importance of the muscle clock in maintaining muscle mass during development, repair and aging, together with its contribution to muscle metabolism, will be discussed. Based on our current understandings of how tissue-intrinsic muscle clock functions in the key aspects muscle physiology, interventions targeting the myogenic-modulatory activities of the clock circuit may offer new avenues for prevention and treatment of muscular diseases. Studies of mechanisms underlying circadian clock function and regulation in skeletal muscle warrant continued efforts.

The circadian clock is an essential time-keeping mechanism that entrains internal physiology to environmental cues. Despite the well-established link between the molecular clock and metabolic homeostasis, an intimate interplay between the clock machinery and the metabolically active brown adipose tissue (BAT) is only emerging. Recently, we came to appreciate that the formation and metabolic functions of BAT, a key organ for body temperature maintenance, are under an orchestrated circadian clock regulation. Two complementary studies from our group uncover that the cell-intrinsic clock machinery exerts concerted control of brown adipogenesis with consequent impacts on adaptive thermogenesis, which adds a previously unappreciated temporal dimension to the regulatory mechanisms governing BAT development and function. The essential clock transcriptional activator, Bmal1, suppresses adipocyte lineage commitment and differentiation, whereas the clock repressor, Rev-erbα, promotes these processes. This newly discovered temporal mechanism in fine-tuning BAT thermogenic capacity may enable energy utilization and body temperature regulation in accordance with external timing signals during development and functional recruitment. Given the important role of BAT in whole-body metabolic homeostasis, pharmacological interventions targeting the BAT-modulatory activities of the clock circuit may offer new avenues for the prevention and treatment of metabolic disorders, particularly those associated with circadian dysregulation.

Aberrant mitochondrial energy transfer underlies prevalent chronic health conditions, including cancer, cardiovascular, and neurodegenerative diseases. Mitochondrial transplantation represents an innovative strategy aimed at restoring favorable metabolic phenotypes in cells with dysfunctional energy metabolism. While promising, significant barriers to in vivo translation of this approach abound, including limited cellular uptake and recognition of mitochondria as foreign. The objective is to functionalize isolated mitochondria with a biocompatible polymer to enhance cellular transplantation and eventual in vivo applications. Herein, it is demonstrated that grafting of a polymer conjugate composed of dextran with triphenylphosphonium onto isolated mitochondria protects the organelles and facilitates cellular internalization compared with uncoated mitochondria. Importantly, mitochondrial transplantation into cancer and cardiovascular cells has profound effects on respiration, mediating a shift toward improved oxidative phosphorylation, and reduced glycolysis. These findings represent the first demonstration of polymer functionalization of isolated mitochondria, highlighting a viable strategy for enabling clinical applications of mitochondrial transplantation.

The molecular clock is intimately linked with metabolic regulation and brown adipose tissue plays a key role in energy homeostasis. However, whether the cell-intrinsic clock machinery participates in brown adipocyte development is unknown. Here we show that Bmal1, the essential clock transcription activator, inhibits brown adipogenesis to adversely impact brown fat formation and thermogenic capacity. Global ablation of Bmal1 in mice increases brown fat mass and cold tolerance, while adipocyte-selective inactivation of Bmal1 recapitulates these effects and demonstrates its cell-autonomous role in brown adipocyte formation. Further loss- and gain-of function studies in mesenchymal precursors and committed brown progenitors reveal that Bmal1 inhibits brown adipocyte lineage commitment and terminal differentiation. Mechanistically, Bmal1 inhibits brown adipogenesis through direct transcriptional control of key components of the TGF-β pathway together with reciprocally altered BMP signaling, and activation of TGF-β, or blockade of BMP pathways, suppresses enhanced differentiation in Bmal1-deficient brown adipocytes. Collectively, our study demonstrates a novel temporal regulatory mechanism in fine-tuning brown adipocyte lineage progression to impact brown fat formation and thermogenic regulation, which may be targeted therapeutically to combat obesity.

Brown adipose tissue is a major thermogenic organ that plays a key role in maintenance of body temperature and whole-body energy homeostasis. Rev-erbα, a ligand-dependent nuclear receptor and transcription repressor of the molecular clock, has been implicated in the regulation of adipogenesis. However, whether Rev-erbα participates in brown fat formation is not known. Here we show that Rev-erbα is a key regulator of brown adipose tissue development by promoting brown adipogenesis. Genetic ablation of Rev-erbα in mice severely impairs embryonic and neonatal brown fat formation accompanied by loss of brown identity. This defect is due to a cell-autonomous function of Rev-erbα in brown adipocyte lineage commitment and terminal differentiation, as demonstrated by genetic loss- and gain-of-function studies in mesenchymal precursors and brown preadipocytes. Moreover, pharmacological activation of Rev-erbα activity promotes, whereas its inhibition suppresses brown adipocyte differentiation. Mechanistic investigations reveal that Rev-erbα represses key components of the TGF-β cascade, an inhibitory pathway of brown fat development. Collectively, our findings delineate a novel role of Rev-erbα in driving brown adipocyte development and provide experimental evidence that pharmacological interventions of Rev-erbα may offer new avenues for the treatment of obesity and related metabolic disorders.

Circadian clock is an evolutionarily conserved timing mechanism governing diverse biological processes and the skeletal muscle possesses intrinsic functional clocks. Interestingly, although the essential clock transcription activator, Brain and muscle Arnt-like 1 (Bmal1), participates in maintenance of muscle mass, little is known regarding its role in muscle growth and repair. In this report, we investigate the in vivo function of Bmal1 in skeletal muscle regeneration using two muscle injury models. Bmal1 is highly up-regulated by cardiotoxin injury, and its genetic ablation significantly impairs regeneration with markedly suppressed new myofiber formation and attenuated myogenic induction. A similarly defective regenerative response is observed in Bmal1-null mice as compared to wild-type controls upon freeze injury. Lack of satellite cell expansion accounts for the regeneration defect, as Bmal1(-/-) mice display significantly lower satellite cell number with nearly abolished induction of the satellite cell marker, Pax7. Furthermore, satellite cell-derived primary myoblasts devoid of Bmal1 display reduced growth and proliferation ex vivo. Collectively, our results demonstrate, for the first time, that Bmal1 is an integral component of the pro-myogenic response that is required for muscle repair. This mechanism may underlie its role in preserving adult muscle mass and could be targeted therapeutically to prevent muscle-wasting diseases.

The ICP34.5 gene from HSV-2 strain 333 was cloned and, when expressed in Vero cells, enhanced the efficiency and extent of glycoprotein processing of glycoprotein C (gC1), a representative viral glycoprotein, during infection with HSV-1 SP7. The ICP34.5 from HSV-1 SP7 limits the extent and efficiency of viral glycoprotein processing. The ability of the HSV-2 ICP34.5 protein to enhance the efficiency and extent of HSV-1 SP7 glycoprotein processing indicates that modulation of viral glycoprotein processing is also a property of the HSV-2 ICP34.5 protein.

Accumulating evidence indicates that the circadian clock, a transcriptional/translational feedback circuit that generates ~24-hour oscillations in behavior and physiology, is a key temporal regulatory mechanism involved in many important aspects of muscle physiology. Given the clock as an evolutionarily-conserved time-keeping mechanism that synchronizes internal physiology to environmental cues, locomotor activities initiated by skeletal muscle enable entrainment to the light-dark cycles on earth, thus ensuring organismal survival and fitness. Despite the current understanding of the role of molecular clock in preventing age-related sarcopenia, investigations into the underlying molecular pathways that transmit clock signals to the maintenance of skeletal muscle growth and function are only emerging. In the current review, the importance of the muscle clock in maintaining muscle mass during development, repair and aging, together with its contribution to muscle metabolism, will be discussed. Based on our current understandings of how tissue-intrinsic muscle clock functions in the key aspects muscle physiology, interventions targeting the myogenic-modulatory activities of the clock circuit may offer new avenues for prevention and treatment of muscular diseases. Studies of mechanisms underlying circadian clock function and regulation in skeletal muscle warrant continued efforts.

Circadian clocks in adipose tissue are known to regulate adipocyte biology. Although circadian dysregulation is associated with development of obesity, the underlying mechanism has not been established. Here we report that disruption of the clock gene, brain and muscle Arnt-like 1 (Bmal1), in mice led to increased adipogenesis, adipocyte hypertrophy, and obesity, compared to wild-type (WT) mice. This is due to its cell-autonomous effect, as Bmal1 deficiency in embryonic fibroblasts, as well as stable shRNA knockdown (KD) in 3T3-L1 preadipocyte and C3H10T1/2 mesenchymal stem cells, promoted adipogenic differentiation. We demonstrate that attenuation of Bmal1 function resulted in down-regulation of genes in the canonical Wnt pathway, known to suppress adipogenesis. Promoters of these genes (Wnt10a, β-catenin, Dishevelled2, TCF3) displayed Bmal1 occupancy, indicating direct circadian regulation by Bmal1. As a result, Wnt signaling activity was attenuated by Bmal1 KD and augmented by its overexpression. Furthermore, stabilizing β-catenin through Wnt ligand or GSK-3β inhibition achieved partial restoration of blunted Wnt activity and suppression of increased adipogenesis induced by Bmal1 KD. Taken together, our study demonstrates that Bmal1 is a critical negative regulator of adipocyte development through transcriptional control of components of the canonical Wnt signaling cascade, and provides a mechanistic link between circadian disruption and obesity.—Guo, B., Chatterjee, S., Li, L., Kim, J. M., Lee, J., Yechoor, V. K., Minze, L. J., Hsueh, W., Ma, K. The clock gene, brain and muscle Arnt-like 1, regulates adipogenesis via Wnt signaling pathway.