Insights into the roles of Apolipoprotein E in adipocyte biology and obesity.

Apolipoprotein E (APOE) is a multifunctional protein expressed by various cell types, including hepatocytes, adipocytes, immune cells of the myeloid lineage, vascular smooth muscle cells, astrocytes, etc. Initially, APOE was discovered as an arginine-rich peptide within very-low-density lipoprotein, but it was subsequently found in triglyceride-rich lipoproteins in humans and other animals, where its presence facilitates the clearance of these lipoproteins from circulation. Recent epidemiolocal studies and experimental research in mice suggest a link between ApoE and obesity. The latest findings highlight the role of endogenous adipocyte ApoE in regulating browning of white adipose tissue, beige adipocyte differentiation, thermogenesis and energy homeostasis. This review focuses on the emerging evidence showing the involvement of ApoE in the regulation of obesity and its associated metabolic diseases.

Elicitation of potent neutralizing antibodies in obese mice by ISA 51-adjuvanted SARS-CoV-2 spike RBD-Fc vaccine.

Vaccination is considered to be the most effective countermeasure to prevent and combat the global health threats of COVID-19. People with obesity are at a greater risk of hospitalization, life-threatening illness, and adverse outcomes after having COVID-19. Therefore, a safe and effective COVID-19 vaccine for obese individuals is urgently needed. In the study, the vaccine composed of the ISA 51 adjuvant and the SARS-CoV-2 spike (S) receptor-binding domain (RBD) in conjugation with the human IgG1 Fc fragment (named as ISA 51-adjuvanted RBD-Fc vaccine) was developed and inoculated in the regular chow diet (RCD) lean mice and the high-fat diet (HFD)-induced obese mice. The S protein-specific IgG titers were largely induced in an increasing manner along with three doses of ISA 51-adjuvanted RBD-Fc vaccine without causing any harmful side effect. In the HFD mice, the S protein-specific IgG titers can be quickly observed 2 weeks post the first inoculation. The antisera elicited by the ISA 51-adjuvanted RBD-Fc vaccine in the RCD and HFD mice exhibited potent SARS-CoV-2 neutralizing activities in the plaque reduction neutralization test (PRNT) assays and showed similar specificity for recognizing the key residues in the RBD which were involved in interacting with angiotensin-converting enzyme 2 (ACE2) receptor. The immune efficacy of the ISA 51-adjuvanted RBD-Fc vaccine in the HFD mice can be sustainably maintained with the PRNT50 values of 1.80-1.91×10-3 for at least 8 weeks post the third inoculation. Collectively, the RBD-Fc-based immunogen and the ISA 51-adjuvanted formulation can be developed as an effective COVID-19 vaccine for obese individuals. KEY POINTS: • The ISA 51-adjuvanted RBD-Fc vaccine can induce potent SARS-CoV-2 neutralizing antibodies in the obese mouse • The antibodies elicited by the ISA 51-adjuvanted RBD-Fc vaccine can bind to the key RBD residues involved in interacting with ACE2 • The immune efficacy of the ISA 51-adjuvanted RBD-Fc vaccine can be sustainably maintained for at least 8 weeks post the third inoculation.

Glucose transporter 10 modulates adipogenesis via an ascorbic acid-mediated pathway to protect mice against diet-induced metabolic dysregulation.

The development of type 2 diabetes mellitus (T2DM) depends on interactions between genetic and environmental factors, and a better understanding of gene-diet interactions in T2DM will be useful for disease prediction and prevention. Ascorbic acid has been proposed to reduce the risk of T2DM. However, the links between ascorbic acid and metabolic consequences are not fully understood. Here, we report that glucose transporter 10 (GLUT10) maintains intracellular levels of ascorbic acid to promote adipogenesis, white adipose tissue (WAT) development and protect mice from high-fat diet (HFD)-induced metabolic dysregulation. We found genetic polymorphisms in SLC2A10 locus are suggestively associated with a T2DM intermediate phenotype in non-diabetic Han Taiwanese. Additionally, mice carrying an orthologous human Glut10G128E variant (Glut10G128E mice) with compromised GLUT10 function have reduced adipogenesis, reduced WAT development and increased susceptibility to HFD-induced metabolic dysregulation. We further demonstrate that GLUT10 is highly expressed in preadipocytes, where it regulates intracellular ascorbic acid levels and adipogenesis. In this context, GLUT10 increases ascorbic acid-dependent DNA demethylation and the expression of key adipogenic genes, Cebpa and Pparg. Together, our data show GLUT10 regulates adipogenesis via ascorbic acid-dependent DNA demethylation to benefit proper WAT development and protect mice against HFD-induced metabolic dysregulation. Our findings suggest that SLC2A10 may be an important HFD-associated susceptibility locus for T2DM.

Apolipoprotein E deficiency activates thermogenesis of white adipose tissues in mice through enhancing ß-hydroxybutyrate production from precursor cells.

White adipose tissue (WAT) has the capacity to undergo a white-to-beige phenotypic switch, known as browning, in response to stimuli such as cold. However, the mechanism underlying beige adipocyte formation is largely unknown. Apolipoprotein E (ApoE) is highly induced in WAT and has been implicated in lipid metabolism. Here, we show that ApoE deficiency in mice increased oxygen consumption and thermogenesis and enhanced adipose browning pattern in inguinal WAT (iWAT), with associated characteristics such as increased Ucp1 and Pparγ expression. At the cellular level, ApoE deficient beige adipocytes had increased glucose uptake and higher mitochondrial respiration than wild-type cells. Mechanistically, we showed that ApoE deficient iWAT and primary adipose precursor cells activated the thermogenic genes program by stimulating the production of ketone body ß-hydroxybutyrate (ßHB), a novel adipose browning promoting factor. This was accompanied by increased expression of genes involved in ketogenesis and could be compromised by the treatment for ketogenesis inhibitors. Consistently, ApoE deficient mice show higher serum ßHB level than wild-type mice in the fed state and during cold exposure. Our results further demonstrate that the increased ßHB production in ApoE deficient adipose precursor cells could be attributed, at least in part, to enhanced Cd36 expression and CD36-mediated fatty acid utilization. Our findings uncover a previously uncharacterized role for ApoE in energy homeostasis via its cell-autonomous action in WAT.

Elp1 facilitates RAD51-mediated homologous recombination repair via translational regulation.

BACKGROUND: RAD51-dependent homologous recombination (HR) is one of the most important pathways for repairing DNA double-strand breaks (DSBs), and its regulation is crucial to maintain genome integrity. Elp1 gene encodes IKAP/ELP1, a core subunit of the Elongator complex, which has been implicated in translational regulation. However, how ELP1 contributes to genome maintenance is unclear. METHODS: To investigate the function of Elp1, Elp1-deficient mouse embryonic fibroblasts (MEFs) were generated. Metaphase chromosome spreading, immunofluorescence, and comet assays were used to access chromosome abnormalities and DSB formation. Functional roles of Elp1 in MEFs were evaluated by cell viability, colony forming capacity, and apoptosis assays. HR-dependent DNA repair was assessed by reporter assay, immunofluorescence, and western blot. Polysome profiling was used to evaluate translational efficiency. Differentially expressed proteins and signaling pathways were identified using a label-free liquid chromatography-tandem mass spectrometry (LC-MS/MS) proteomics approach. RESULTS: Here, we report that Elp1 depletion enhanced genomic instability, manifested as chromosome breakage and genotoxic stress-induced genomic DNA fragmentation upon ionizing radiation (IR) exposure. Elp1-deficient cells were hypersensitive to DNA damage and exhibited impaired cell proliferation and defective HR repair. Moreover, Elp1 depletion reduced the formation of IR-induced RAD51 foci and decreased RAD51 protein levels. Polysome profiling analysis revealed that ELP1 regulated RAD51 expression by promoting its translation in response to DNA damage. Notably, the requirement for ELP1 in DSB repair could be partially rescued in Elp1-deficient cells by reintroducing RAD51, suggesting that Elp1-mediated HR-directed repair of DSBs is RAD51-dependent. Finally, using proteome analyses, we identified several proteins involved in cancer pathways and DNA damage responses as being differentially expressed upon Elp1 depletion. CONCLUSIONS: Our study uncovered a molecular mechanism underlying Elp1-mediated regulation of HR activity and provides a novel link between translational regulation and genome stability.

GLUT10 maintains the integrity of major arteries through regulation of redox homeostasis and mitochondrial function.

Glucose transporter 10 (GLUT10) is a member of the GLUT family of membrane transporters, and mutations in this gene cause arterial tortuosity syndrome (ATS). However, the physiological role and regulation of GLUT10 in arteries remains unclear. To further understand its physiological roles in major arteries, we examined the regulatory mechanisms of GLUT10 in ASMCs and aortic tissues. Interestingly, we find that targeting of GLUT10 to mitochondria is increased in ASMCs under both stress and aging conditions, which enhances dehydroascorbic acid (DHA) uptake and maintains intracellular ascorbic acid (AA) levels. We further demonstrate that the targeting of GLUT10 to mitochondria is important to maintain redox homeostasis, mitochondrial structure and mitochondrial function in ASMCs. A missense mutation of GLUT10 (Glut10G128E) impairs mitochondrial targeting in ASMCs. Consequently, ASMCs isolated from Glut10G128E mice exhibit increased reactive oxygen species (ROS) levels, fragmented mitochondria and impaired mitochondrial function, as well as enhanced cell proliferation and migration. In vivo, mitochondrial structure is altered, and ROS levels are heightened in aortic tissues of Glut10G128E mice. Furthermore, increased number and disorganization of ASMCs, along with progressive arterial wall remodeling were observed in aortic tissues of Glut10G128E mice. These defects were coincident with elevated systolic blood pressure in aged Glut10G128E animals. Our results describe a novel mechanism that GLUT10 targeting to mitochondria under stress and aging condition has a critical role in maintaining AA levels, redox homeostasis and mitochondrial structure and function in ASMCs, which is likely to contribute to the maintenance of healthy vascular tissue.

Vitamin C attenuates predisposition to high-fat diet-induced metabolic dysregulation in GLUT10-deficient mouse model.

BACKGROUND: The development of type 2 diabetes mellitus (T2DM) is highly influenced by complex interactions between genetic and environmental (dietary and lifestyle) factors. While vitamin C (ascorbic acid, AA) has been suggested as a complementary nutritional treatment for T2DM, evidence for the significance and beneficial effects of AA in T2DM is thus far inconclusive. We suspect that clinical studies on the topic might need to account for combination of genetic and dietary factors that could influence AA effects on metabolism. In this study, we tested this general idea using a mouse model with genetic predisposition to diet-induced metabolic dysfunction. In particular, we utilized mice carrying a human orthologous GLUT10G128E variant (GLUT10G128E mice), which are highly sensitive to high-fat diet (HFD)-induced metabolic dysregulation. The genetic variant has high relevance to human populations, as genetic polymorphisms in glucose transporter 10 (GLUT10) are associated with a T2DM intermediate phenotype in nondiabetic population. RESULTS: We investigated the impacts of AA supplementation on metabolism in wild-type (WT) mice and GLUT10G128E mice fed with a normal diet or HFD. Overall, the beneficial effects of AA on metabolism were greater in HFD-fed GLUT10G128E mice than in HFD-fed WT mice. At early postnatal stages, AA improved the development of compromised epididymal white adipose tissue (eWAT) in GLUT10G128E mice. In adult animals, AA supplementation attenuated the predisposition of GLUT10G128E mice to HFD-triggered eWAT inflammation, adipokine dysregulation, ectopic fatty acid accumulation, metabolic dysregulation, and body weight gain, as compared with WT mice. CONCLUSIONS: Taken together, our findings suggest that AA has greater beneficial effects on metabolism in HFD-fed GLUT10G128E mice than HFD-fed WT mice. As such, AA plays an important role in supporting eWAT development and attenuating HFD-induced metabolic dysregulation in GLUT10G128E mice. Our results suggest that proper WAT development is essential for metabolic regulation later in life. Furthermore, when considering the usage of AA as a complementary nutrition for prevention and treatment of T2DM, individual differences in genetics and dietary patterns should be taken into account.

Maternal hypercholesterolemia exacerbates atherosclerosis lesions in female offspring through potentiating macrophage polarization toward an inflammatory M1 phenotype.

Maternal hypercholesterolemia induces early onset of cardiovascular diseases in offspring; however, its underlying mechanism remains poorly understood. We hypothesized that maternal hypercholesterolemia increases offspring susceptibility to atherosclerosis in adulthood through developmental modifications of macrophages. Female apolipoprotein E (ApoE)-deficient mice were fed a Western-type diet (WD) or a control diet (CD) prior to and throughout gestation and lactation. The offspring were all fed a WD after weaning. Sixteen-week-old female offspring of WD-fed dams showed a significant increase in atherosclerotic lesions of the aorta and aortic root compared with those of CD-fed dams. This effect was associated with increased macrophage accumulation within lesions, macrophage inflammation and an increase in circulating Ly6Chigh monocyte and F4/80 macrophage counts. We further evidenced that in utero WD exposure promoted macrophage polarization toward the M1 phenotype by elevating M1 markers (Cd86, Inos/Nos2) without affecting M2 markers (Cd206, Arg1). Proinflammatory cytokine synthesis was also enhanced in response to LPS. Finally, maternal WD intake strongly inhibited the macrophage expression of Pparg and Lxra, which was associated with aberrant DNA methylation of Lxra promoter. Our findings demonstrate that maternal hypercholesterolemia exacerbates atherosclerosis, in part by altering the epigenetic state of the macrophage genome of the offspring, imprinting gene expression, and changing macrophage polarization, which ultimately contributes to plaque macrophage burden.

Ribonucleotide reductase small subunit p53R2 facilitates p21 induction of G1 arrest under UV irradiation.

p53R2, which is one of the two known ribonucleotide reductase small subunits (the other being M2), is suggested to play an important role in supplying deoxynucleotide triphosphates (dNTP) for DNA repair during the G(1) or G(2) phase of the cell cycle. The ability of p53R2 to supply dNTPs for repairing DNA damages requires the presence of a functional p53 tumor suppressor. Here, we report in vivo physical interaction and colocalization of p53R2 and p21 before DNA damage. Mammalian two-hybrid assay further indicates that the amino acids 1 to 113 of p53R2 are critical for interacting with the NH(2)-terminal region (amino acids 1-93) of p21. The binding between p21 and p53R2 decreases inside the nucleus in response to UV, the time point of which corresponds to the increased binding of p21 with cyclin-dependent kinase-2 (Cdk2), and the decreased Cdk2 activity in the nucleus at G(1). Interestingly, p53R2 dissociates from p21 but facilitates the accumulation of p21 in the nucleus in response to UV. On the other hand, the ribonucleotide reductase activity increases at the corresponding time in response to UV. These data suggest a new function of p53R2 of cooperating with p21 during DNA repair at G(1) arrest.

Loss of Ikbkap/Elp1 in mouse oocytes causes spindle disorganization, developmental defects in preimplantation embryos and impaired female fertility.

Elongator complexes are well known to be involved in a wide variety of cellular processes; however, their functions in mammalian oocytes have not been characterized. Here, we demonstrated in mice that specific deletion of one of the core subunits, Ikbkap/Elp1, in oocytes resulted in spindle defects and chromosome disorganization without affecting folliculogenesis. In accordance with these findings, we observed that Ikbkap mutant female mice are subfertile. Further analyses uncovered that kinetochore-microtubule attachments are severely compromised in Ikbkap-deficient oocytes. Moreover, we revealed that Ikbkap modulates the acetylation status of α-tubulin in oocytes, which may at least in part mediate the meiotic phenotypes described above by affecting microtubule dynamics and kinetochore function. Finally, we showed that embryos derived from Ikbkap-deficient oocytes exhibit an increased frequency of aneuploidy, digyny, progressive delays in preimplantation development, and severe degeneration before reaching the blastocyst stage. In summary, we identify Ikbkap as an important player in regulating oocyte meiosis by modulating tubulin acetylation for chromosome/spindle organization.