Bone marrow adipose tissue is a unique adipose subtype with distinct roles in systemic glucose homeostasis

SUMMARYBone marrow adipose tissue (BMAT) represents >10% of total adipose mass, yet unlike white or brown adipose tissues (WAT or BAT), its role in systemic metabolism remains unclear. Using transcriptomics, we reveal that BMAT is molecularly distinct to WAT but is not enriched for brown or beige adipocyte markers. Instead, pathway analysis indicated altered glucose metabolism and decreased insulin responsiveness in BMAT. We therefore tested these functions in mice and humans using positron emission tomography–computed tomography (PET/CT) with 18F-fluorodeoxyglucose, including establishing a new method for BMAT identification from clinical CT scans. This revealed that BMAT resists insulin- and cold-stimulated glucose uptake and is thus functionally distinct to WAT and BAT. However, BMAT displayed greater basal glucose uptake than axial bones or subcutaneous WAT, underscoring its potential to influence systemic glucose homeostasis. These PET/CT studies are the first to characterise BMAT function in vivo and identify BMAT as a distinct, major subtype of adipose tissue.HIGHLIGHTSBone marrow adipose tissue (BMAT) is molecularly distinct to other adipose subtypes.BMAT is less insulin responsive than WAT and, unlike BAT, is not cold-responsive.Human BMAT has greater basal glucose uptake than axial bone or subcutaneous WAT.We establish a PET/CT method for BMAT localisation and functional analysis in vivo.

Effect of Sucralose on Glucose Uptake in Rat L6 Myotubes

ABSTRACT Introduction With growing awareness of the link between diet and health and the problem of obesity, public concern over sugar levels in the diet is forcing a worldwide trend toward cutting down on sugar by using artificial sweeteners (AS). Aim To study the effect of increasing concentrations of sucralose (an AS) on glucose uptake in rat L6 myotubes. Materials and methods The L6 cell line from American type cell culture (ATCC) was grown in Dulbecco's Modified Eagle's Medium (DMEM) and differentiated into myotubes. The wells were exposed to either 0, 1 nM, 1 μM, or 1 mM of sucralose alone or with 10 nM insulin for 24 hours. Glucose uptake was studied after this period. Results Significant decrease was seen between the insulin-stimulated basal glucose uptake and insulin-stimulated glucose uptake across all the concentrations of sucralose treatment. Conclusion Increased concentration of sucralose appears to decrease glucose uptake even on insulin stimulation. Clinical significance It may not be beneficial to use sucralose in certain groups of people who have insulin resistance or are prone to it. How to cite this article Prakash SN, Shanthakumari J, Devanath A. Effect of Sucralose on Glucose Uptake in Rat L6 Myotubes. Indian J Med Biochem 2017;21(2):162-165.

Amelioration of palmitate-induced metabolic dysfunction in L6 muscle cells expressing low levels of receptor-interacting protein 140

We have shown that reduced expression of receptor-interacting protein 140 (RIP140) alters the regulation of fatty-acid (FA) oxidation in muscle. To determine whether a high level of FA availability alters the effects of RIP140 on metabolic regulation, L6 myotubes were transfected with or without RNA interference oligonucleotide sequences to reduce RIP140 expression, and then incubated with high levels of palmitic acid, with or without insulin. High levels of palmitate reduced basal (53%–58%) and insulin-treated (24%–44%) FA uptake and oxidation, and increased basal glucose uptake (88%). In cells incubated with high levels of palmitate, low RIP140 increased basal FA uptake and insulin-treated FA oxidation and glucose uptake, and decreased basal glucose uptake and insulin-treated FA uptake. Under basal conditions, low RIP140 increased the mRNA content of FAT/CD36 (159%) and COX4 (61%), as well as the protein content of Nur77 (68%), whereas the mRNA expression of FGF21 (50%) was decreased, as was the protein content of CPT1b (35%) and FGF21 (44%). Under insulin-treated conditions, low RIP140 expression increased the mRNA content of MCAD (84%) and Nur77 (84%), as well as the protein content of Nur77 (23%). Thus, a low level of RIP140 restores the rates of FA uptake in the basal state, in part via a reduction in upstream insulin signaling. Our data also indicate that the protein expression of Nur77 may be modulated by RIP140 when muscle cells are metabolically challenged by high levels of palmitate.

Oxytocin Increases Glucose Uptake in Neonatal Rat Cardiomyocytes

We have recently shown that an entire oxytocin (OT) system, a peptide and its cognate receptors, is synthesized in the heart. In fetal and newborn hearts, OT exists in its extended three-amino acid form, OT-Gly-Lys-Arg (OT-GKR). OT translocates glucose transporter type 4 to the plasma membrane in human endothelial cells. Therefore, we hypothesized that the cardiac OT/OT-GKR system may be involved in the regulation of myocardial glucose uptake in physiological conditions and during metabolic stress such as hypoxia. Primary cultures of neonatal rat cardiomyocytes (CM) and cardiac progenitor cells expressing ATP-binding cassette efflux transporter G2 transporter (stem cell marker) were studied. OT (10 nm) increased basal glucose uptake in CM to 4.0 ± 0.2 fmol/mg protein, with OT-GKR (10 nm) elevating it to 5.3 ± 0.4 fmol/mg protein (P < 0.001) in comparison with 2.2 fmol/mg in control cells. OT had a moderate synergistic effect with 0.1 mm 2,4-dinitrophenol, augmenting basal glucose uptake to 5.5 ± 0.5 fmol/mg. OT-GKR (10 nm) was even more potent in combination with 2,4-dinitrophenol, increasing glucose uptake to 9.0 ± 1.0 fmol/mg. Wortmannin (0.1 μm), an inhibitor of phosphatidylinositol-3-kinase, significantly suppressed the effect of OT and insulin (10 nm) (P < 0.001), indicating common pathways. Our data suggest that OT and OT-GKR influence glucose uptake in neonatal rat CM and may thus play a role in the maintenance of cardiac function and cell survival during metabolic stress.

Muscle inactivation of mTOR causes metabolic and dystrophin defects leading to severe myopathy

Mammalian target of rapamycin (mTOR) is a key regulator of cell growth that associates with raptor and rictor to form the mTOR complex 1 (mTORC1) and mTORC2, respectively. Raptor is required for oxidative muscle integrity, whereas rictor is dispensable. In this study, we show that muscle-specific inactivation of mTOR leads to severe myopathy, resulting in premature death. mTOR-deficient muscles display metabolic changes similar to those observed in muscles lacking raptor, including impaired oxidative metabolism, altered mitochondrial regulation, and glycogen accumulation associated with protein kinase B/Akt hyperactivation. In addition, mTOR-deficient muscles exhibit increased basal glucose uptake, whereas whole body glucose homeostasis is essentially maintained. Importantly, loss of mTOR exacerbates the myopathic features in both slow oxidative and fast glycolytic muscles. Moreover, mTOR but not raptor and rictor deficiency leads to reduced muscle dystrophin content. We provide evidence that mTOR controls dystrophin transcription in a cell-autonomous, rapamycin-resistant, and kinase-independent manner. Collectively, our results demonstrate that mTOR acts mainly via mTORC1, whereas regulation of dystrophin is raptor and rictor independent.

Involvement of the p66Shc protein in glucose transport regulation in skeletal muscle myoblasts

The p66Shc protein isoform regulates MAP kinase activity and the actin cytoskeleton turnover, which are both required for normal glucose transport responses. To investigate the role of p66Shc in glucose transport regulation in skeletal muscle cells, L6 myoblasts with antisense-mediated reduction (L6/p66Shc as) or adenovirus-mediated overexpression (L6/p66Shc adv) of the p66Shc protein were examined. L6/Shc as myoblasts showed constitutive activation of ERK-1/2 and disruption of the actin network, associated with an 11-fold increase in basal glucose transport. GLUT1 and GLUT3 transporter proteins were sevenfold and fourfold more abundant, respectively, and were localized throughout the cytoplasm. Conversely, in L6 myoblasts overexpressing p66Shc, basal glucose uptake rates were reduced by 30% in parallel with a ∼50% reduction in total GLUT1 and GLUT3 transporter levels. Inhibition of the increased ERK-1/2 activity with PD98059 in L6/Shc as cells had a minimal effect on increased GLUT1 and GLUT3 protein levels, but restored the actin cytoskeleton, and reduced the abnormally high basal glucose uptake by 70%. In conclusion, p66Shc appears to regulate the glucose transport system in skeletal muscle myoblasts by controlling, via MAP kinase, the integrity of the actin cytoskeleton and by modulating cellular expression of GLUT1 and GLUT3 transporter proteins via ERK-independent pathways.