Role of Liver CD38 in the Regulation of Metabolic Pathways during Cold-Induced Thermogenesis in Mice

Abstract

Boosting NAD+ levels are considered a promising means to promote healthy aging and ameliorate dysfunctional metabolism. The expression of CD38, the major NAD+-consuming enzyme, is downregulated during thermogenesis in both brown and white adipose tissues (BAT and WAT). Moreover, BAT activation and WAT “browning” were enhanced in Cd38−/− mice. In this study, the role of CD38 in the liver during thermogenesis was investigated, with the liver being the central organ controlling systemic energy metabolism. Wild-type mice and Cd38−/− mice were exposed to cold temperatures, and levels of metabolites and enzymes were measured in the livers and plasma. During cold exposure, CD38 expression was downregulated in the liver, as in BAT and WAT, with a concomitant increase in NAD(H) and a marked decrease in NADPH levels. Glucose-6-phosphate dehydrogenase and the malic enzyme, along with enzymes in the glycolytic pathway, were downregulated, which is in line with glucose-6-P being re-directed towards glucose release. In Cd38−/− mice, the cross-regulation between glycolysis and glucose release was lost, although this did not impair the glucose release from glycogen. Glycerol levels were decreased in the liver from Cd38−/− animals upon cold exposure, suggesting that glyceroneogenesis, as gluconeogenesis, was not properly activated in the absence of CD38. SIRT3 activity, regulating mitochondrial metabolism, was enhanced by cold exposure, whereas its activity was already high at a warm temperature in Cd38−/− mice and was not further increased by the cold. Notably, FGF21 and bile acid release was enhanced in the liver of Cd38−/− mice, which might contribute to enhanced BAT activation in Cd38−/− mice. These results demonstrate that CD38 inhibition can be suggested as a strategy to boost NAD+ and would not negatively affect hepatic functions during thermogenesis.

The TRPM2 ion channel regulates metabolic and thermogenic adaptations in adipose tissue of cold-exposed mice

“…To the best of our knowledge, our study is the first one reporting an induction of PAR, as well as MAR hydrolases, in association to WAT browning and BAT activation, revealing ADPR increase and higher TRPM2 expression in response to cold. It is of note that CD38 plays a crucial role in the cross-regulation of glycolysis and gluconeogenesis in liver in response to cold-induced thermogenesis ( 33 ), which is in line with the importance of CD38-mediated cADPR production and Ca 2+ signaling in glucagon-induced stimulation of gluconeogenesis ( 65 ). Conversely, the ADPR-TRPM2 axis does not seem to be relevant in the regulation of glycolysis and gluconeogenesis, at least during thermogenesis, since Trpm2 -/- ablation does not affect the down-regulation of glycolytic genes and the upregulation of G6P phosphatase ( Figure 6 ).…”

“…In conclusion, our results demonstrate that increased NAD + levels, obtained through physiological downregulation of CD38 in response to cold (as during thermogenesis; 30 , 33 ), or following CD38 pharmacological inhibition, are not impacting TRPM2 activity through a hampered CD38-dependent production of ADPR. This is of great interest in the field of chronic inflammatory diseases, given the numerous efforts of the research community to increase intracellular NAD + levels for the treatment of various pathological conditions.…”

“…Liver, as BAT and WAT, is a relevant organ governing thermogenesis, both by providing energy sources and by releasing stimulatory factors that affect peripheral tissues ( 33 ). We recently demonstrated that in cold-exposed Cd38 -/- mice, the hepatic cross-regulation between glycolysis and glucose release was lost ( 33 ). To investigate whether TRPM2 is relevant in this process, the impact of Trpm2 depletion on hepatic metabolism during thermogenesis was evaluated.…”

“…The cDNA was used as template for real-time PCR analysis: reactions were performed in an iQ5 real-time PCR detection system (Bio-Rad) following the experimental conditions described before ( 32 ). The used specific primers were described in ( 30 , 33 ). The following primers were designed through Beacon Designer 2.0 Software (Bio-Rad): Trpm2 (fw 5’-CCAATCTCCGACGAAGCAATAGC-3’; rv 5’-CATATTGGTGTGCGTGTGTGATGG-3’); MacroD1 (fw 5’-TGAGCACCTCCACCGACT-3’, rv 5’-TGTTCCTCTCTCTGCTTGTCAC-3’) MacroD2 (fw 5’-AGCTGAAGTCCACAAAGATGAA-3’, rv 5’-TTGGGAAGTTCTTGGTCACACA-3’); Oard1 (fw 5’-TGGAGGCCATGAAGTCCCAT-3’; rv 5’-CCGATCCAGACCACATCCAA-3’); Adprs (fw 5’-TCCTGAGTCACGTCGAGAGC-3’, rv 5’-TGGCAGTGTCATCTGTGTAGT-3’); MT-ND1 (fw 5’-CCACGCTTCCGTTACGATCA-3’, rv 5’-GTATGGTGGTACTCCCGCTG-3’).…”