title: Signaling mechanism of tumor cell-induced up-regulation of E3 ubiquitin ligase UBR2.
abstract: The N-end rule pathway contributes significantly to accelerated muscle proteolysis mediated by the ubiquitin-proteasome pathway in various catabolic conditions. UBR2 (aka E3α-II) is the only known E3 ubiquitin ligase of the N-end rule pathway that is up-regulated by cachectic stimuli including proinflammatory cytokines and tumors. However, the signaling mechanism through which UBR2 is up-regulated remains undetermined. Here we identify a signaling pathway that mediates tumor cell-induced up-regulation of UBR2. UBR2 expression in C2C12 myotubes was up-regulated by conditioned medium from Lewis lung carcinoma cells or C26 colon adenocarcinoma cells, which was blocked by a pharmacological inhibitor of p38α/β mitogen-activated protein kinase (MAPK), SB202190. Similarly, SB202190 administration (i.p.) abolished UBR2 up-regulation in the tibialis anterior of LLC tumor-bearing mice. Genetic gain and loss of function assays in C2C12 myotubes indicated that tumor-induced activation of the p38β isoform is sufficient and necessary for UBR2 up-regulation. In addition, UBR2 up-regulation required p38β-mediated phosphorylation of CCAAT/enhancer binding protein (C/EBP)-β Thr-188, which was critical to C/EBPβ binding to the UBR2 promoter. Furthermore, luciferase reporter assay revealed that the C/EBPβ binding motif in the UBR2 promoter is a functional C/EBPβ-responsive cis-element that enhances the promoter activity on activation by p38β. Finally, genetic ablation of C/EBPβ blocked UBR2 up-regulation in LLC tumor-bearing mice. These results suggest that UBR2 up-regulation in cachectic muscle is mediated by the p38β-C/EBPβ signaling pathway responsible for the bulk of tumor-induced muscle proteolysis.
authors.affiliation: Department of Integrative Biology and Pharmacology, University of Texas Health Science Center, Houston, TX 77030, USA.
authors.forename: Guohua
authors.initials: G
authors.lastname: Zhang
authors.forename: Ren-Kuo
authors.initials: RK
authors.lastname: Lin
authors.forename: Yong Tae
authors.initials: YT
authors.lastname: Kwon
authors.forename: Yi-Ping
authors.initials: YP
authors.lastname: Li
mesh_terms: D000818:Animals; D015153:Blotting, Western; D022782:CCAAT-Enhancer-Binding Protein-beta / Q000235:genetics / Q000378:metabolism; D018827:Carcinoma, Lewis Lung / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D002460:Cell Line; D045744:Cell Line, Tumor; D017077:Culture Media, Conditioned / Q000494:pharmacology; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D007093:Imidazoles / Q000494:pharmacology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D048290:Mitogen-Activated Protein Kinase 11 / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism; D032446:Myoblasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D010766:Phosphorylation; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding; D011725:Pyridines / Q000494:pharmacology; D034622:RNA Interference; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000378:metabolism*; D015854:Up-Regulation
chemical_list: D022782:CCAAT-Enhancer-Binding Protein-beta; D017077:Culture Media, Conditioned; D007093:Imidazoles; D011725:Pyridines; C511160:UBR2 protein, mouse; D044767:Ubiquitin-Protein Ligases; D048290:Mitogen-Activated Protein Kinase 11; C090942:4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)imidazole
keywords: C/EBPβ; MAPK; cachexia; gene regulation; p38β