Impaired Ribosomal Biogenesis by Noncanonical Degradation of β-Catenin during Hyperammonemia

ABSTRACT

Increased ribosomal biogenesis occurs during tissue hypertrophy, but whether ribosomal biogenesis is impaired during atrophy is not known. We show that hyperammonemia, which occurs in diverse chronic disorders, impairs protein synthesis as a result of decreased ribosomal content and translational capacity. Transcriptome analyses, real-time PCR, and immunoblotting showed consistent reductions in the expression of the large and small ribosomal protein subunits (RPL and RPS, respectively) in hyperammonemic murine skeletal myotubes, HEK cells, and skeletal muscle from hyperammonemic rats and human cirrhotics. Decreased ribosomal content was accompanied by decreased expression of cMYC, a positive regulator of ribosomal biogenesis, as well as reduced expression and activity of β-catenin, a transcriptional activator of cMYC. However, unlike the canonical regulation of β-catenin via glycogen synthase kinase 3β (GSK3β)-dependent degradation, GSK3β expression and phosphorylation were unaltered during hyperammonemia, and depletion of GSK3β did not prevent ammonia-induced degradation of β-catenin. Overexpression of GSK3β-resistant variants, genetic depletion of IκB kinase β (IKKβ) (activated during hyperammonemia), protein interactions, and in vitro kinase assays showed that IKKβ phosphorylated β-catenin directly. Overexpressing β-catenin restored hyperammonemia-induced perturbations in signaling responses that regulate ribosomal biogenesis. Our data show that decreased protein synthesis during hyperammonemia is mediated via a novel GSK3β-independent, IKKβ-dependent impairment of the β-catenin–cMYC axis.

KEYWORDS:

• β-catenin
• GSK3β
• IKKβ
• hyperammonemia
• protein synthesis
• ribosomal biogenesis

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at https://doi.org/10.1128/MCB.00451-18.

ACKNOWLEDGMENTS

This work was supported in part by NIH grants RO1 GM119174 (S.D. and G.R.S.), RO1 DK 113196 (S.D.), P50 AA024333 (S.D.), R21 AR 71046 (S.D., G.D., and O.W.), and UO1 AA021893 (S.D.) and by the Mikati Foundation (G.D. and S.D.). The Fusion Lumos instrument was purchased via an NIH shared-instrument grant, 1S10OD023436-01.

Guidance for polysome profiling was provided by Scott Kimball from Penn State University, Hershey, PA. Technical assistance and support were provided by Varalakshmi Veera.