 |
By Jamshed Arslan, Pharm. D., PhD.
Generating energy (ATP) from nutrients is a recipe for life. One of the sensors of cellular energy levels is the serine/threonine kinase AMPK. This enzyme facilitates lipid oxidation and glucose uptake when energy levels fall, thereby promoting ATP production. AMPK comprises a catalytic alpha subunit with two regulatory subunits, beta and gamma. How the subunit isoforms are selectively modulated in metabolic diseases is largely unknown.
UBE2O is a large ubiquitin-conjugating enzyme upstream of MAPK pathway. UBE2O takes part in adipogenesis and other cellular processes. Its preferential expression in metabolic tissues like skeletal muscles, liver and adipose tissues raises the question of a tissue-specific role of UBE2O in metabolic homeostasis. To answer this, researchers from different countries including South Korea , France and the US , generated knockout mice that lacked Ube2o in liver, skeletal muscle, adipose tissue or the whole body. These in vivo experiments led them to discover a skeletal muscle-specific UBE2O/AMPK alpha 2 axis that paves the way for
metabolic syndrome .
Discover Metabolism Resources »
The researchers selected UBE2O, among a list of 30 ubiquitin-conjugating enzymes, based on its highly potent negative regulatory effect on insulin-induced glucose uptake . This was corroborated by the overexpression of Ube2o in skeletal muscles of the mice fed with high-fat diet (HFD), which predisposed the mice to obesity and diabetes. In contrast, HFD had little impact on the body weight and fat mass of the Ube2o-knockout mice. These knockout mice also exhibited improved metabolic profile in serum and histochemical tests. In other words, whole body knockout of Ube2o protected against HFD-induced metabolic diseases.
Knowledge about Ube2o's preferential expression in skeletal muscle, fat, and liver, motivated the researchers to explore tissue-specific roles of UBE2O. The team generated conditional knockouts of Ube2o using Cre-loxP system and found that extirpating Ube2o in skeletal muscles, but not in liver or adipose tissue, improves metabolic profile and protects against diet-induced insulin resistance. The next step was to identify downstream effectors behind this observation that benefit from muscle-specific knockout of Ube2O.
Left- AMPK alpha 1 expression in cell lysates of: (1), Hela (2), HepG2 (3), MCF-7 (4), Cos7 (5), NIH/3T3 (6), K562 (7), HEK293 (8), and PC-12 (9), detected by Western blot with AMPK alpha 1 Antibody (2B7) [NBP2-22127]. Right- AMPK alpha 2 expression was detected in the nuclei and cytoplasm of HEK293 cells by immunocytochemistry with Goat Anti-Human/Mouse/Rat AMPK alpha 2 Antigen Affinity-purified Polyclonal Antibody (AF2850) and NorthernLights™ 557-conjugated Anti-Goat IgG Secondary Antibody (red, NL001).
AMPK pathway is downstream of Ube2O in cancers. To explore this Ube2O-AMPK interaction in non-cancerous situations, the lysates of skeletal muscles of Ube2O-deficient and control (Ube2O-proficient) mice were immunoprecipitated. High levels of AMPK alpha 2, but not AMPK alpha1, in Ube2O-deficient muscles, and the observation that Ube2O mutants have reduced Ube2O-mediated AMPK alpha 2 ubiquitination, validated Ube2O-AMPK alpha 2 interaction in non-cancerous contexts.
To study how AMPK alpha 2 contributes to Ube2O-mediated effects on metabolism, Ube2O was either exogenously expressed or blocked by shRNA in cultured myotubes. Immunoblot analysis of the myotube lysates indicated that AMPK alpha 2 expression and the phosphorylation of AMPK alpha 2 substrates, acetyl-CoA carboxylase and TBC1D1, were suppressed in exogenously expressed cases. Opposite trends were seen with Ube2O knockdown.
As expected, HFD feeding had little effect on the weight and fat mass of muscle-specific Ube2o-knockout mice. When these mice were crossed with mice heterozygous for Prkaa2 (catalytic subunit of AMPK alpha 2) in skeletal muscles, the protective effects of Ube2o-knockout were abolished. In other words, metabolically beneficial effects of Ube2O loss depend on muscular AMPK alpha 2 activation.
This study identifies Ube2O/AMPK alpha 2 axis as a therapeutic target for treating a global health crisis called metabolic syndrome. Increasing rates of obesity worldwide make this research worthy of our attention since it sheds light on how diet-induced metabolic problems can be ameliorated, if not cured.
Jamshed Arslan, Pharm D, PhD
Dr. Arslan is an Assistant Professor at Dow University of Health Sciences, Pakistan,
where he teaches Pharmacology to future pharmacists.
References
Vila, I. K., Park, M. K., Setijono, S. R., Yao, Y., Kim, H., Badin, P.-M., … Song, M. S. (2019). A muscle-specific UBE2O/AMPKα2 axis promotes insulin resistance and metabolic syndrome in obesity. JCI Insight. https://doi.org/10.1172/jci.insight.128269
