Yang Cao1, Naveen Bojjireddy1, Maengjo Kim1, Tao Li1, Peiyong Zhai1, Narayani Nagarajan1, Junichi Sadoshima1, Richard D Palmiter1, Rong Tian2. 1. From the Department of Anesthesiology and Pain Medicine, Mitochondria and Metabolism Center (Y.C., N.B., M.K., T.L., R.T.) and Department of Biochemistry (R.D.P.), University of Washington, Seattle; Department of Cell Biology and Molecular Medicine, Rutgers New Jersey Medical School, Newark (P.Z., N.N., J.S.); and Howard Hughes Medical Institute, Seattle, WA (R.D.P.). 2. From the Department of Anesthesiology and Pain Medicine, Mitochondria and Metabolism Center (Y.C., N.B., M.K., T.L., R.T.) and Department of Biochemistry (R.D.P.), University of Washington, Seattle; Department of Cell Biology and Molecular Medicine, Rutgers New Jersey Medical School, Newark (P.Z., N.N., J.S.); and Howard Hughes Medical Institute, Seattle, WA (R.D.P.). rongtian@u.washington.edu.
Abstract
RATIONALE: AMPK (AMP-activated protein kinase) is a heterotrimeric protein that plays an important role in energy homeostasis and cardioprotection. Two isoforms of each subunit are expressed in the heart, but the isoform-specific function of AMPK remains unclear. OBJECTIVE: We sought to determine the role of γ2-AMPK in cardiac stress response using bioengineered cell lines and mouse models containing either isoform of the γ-subunit in the heart. METHODS AND RESULTS: We found that γ2 but not γ1 or γ3 subunit translocated into nucleus on AMPK activation. Nuclear accumulation of AMPK complexes containing γ2-subunit phosphorylated and inactivated RNA Pol I (polymerase I)-associated transcription factor TIF-IA at Ser-635, precluding the assembly of transcription initiation complexes for rDNA. The subsequent downregulation of pre-rRNA level led to attenuated endoplasmic reticulum (ER) stress and cell death. Deleting γ2-AMPK led to increases in pre-rRNA level, ER stress markers, and cell death during glucose deprivation, which could be rescued by inhibition of rRNA processing or ER stress. To study the function of γ2-AMPK in the heart, we generated a mouse model with cardiac-specific deletion of γ2-AMPK (cardiac knockout [cKO]). Although the total AMPK activity was unaltered in cKO hearts because of upregulation of γ1-AMPK, the lack of γ2-AMPK sensitizes the heart to myocardial ischemia/reperfusion injury. The cKO failed to suppress pre-rRNA level during ischemia/reperfusion and showed a greater infarct size. Conversely, cardiac-specific overexpression of γ2-AMPK decreased ribosome biosynthesis and ER stress during ischemia/reperfusion insult, and the infarct size was reduced. CONCLUSIONS: The γ2-AMPK translocates into the nucleus to suppress pre-rRNA transcription and ribosome biosynthesis during stress, thus ameliorating ER stress and cell death. Increased γ2-AMPK activity is required to protect against ischemia/reperfusion injury. Our study reveals an isoform-specific function of γ2-AMPK in modulating ribosome biosynthesis, cell survival, and cardioprotection.
RATIONALE: AMPK (AMP-activated protein kinase) is a heterotrimeric protein that plays an important role in energy homeostasis and cardioprotection. Two isoforms of each subunit are expressed in the heart, but the isoform-specific function of AMPK remains unclear. OBJECTIVE: We sought to determine the role of γ2-AMPK in cardiac stress response using bioengineered cell lines and mouse models containing either isoform of the γ-subunit in the heart. METHODS AND RESULTS: We found that γ2 but not γ1 or γ3 subunit translocated into nucleus on AMPK activation. Nuclear accumulation of AMPK complexes containing γ2-subunit phosphorylated and inactivated RNA Pol I (polymerase I)-associated transcription factor TIF-IA at Ser-635, precluding the assembly of transcription initiation complexes for rDNA. The subsequent downregulation of pre-rRNA level led to attenuated endoplasmic reticulum (ER) stress and cell death. Deleting γ2-AMPK led to increases in pre-rRNA level, ER stress markers, and cell death during glucose deprivation, which could be rescued by inhibition of rRNA processing or ER stress. To study the function of γ2-AMPK in the heart, we generated a mouse model with cardiac-specific deletion of γ2-AMPK (cardiac knockout [cKO]). Although the total AMPK activity was unaltered in cKO hearts because of upregulation of γ1-AMPK, the lack of γ2-AMPK sensitizes the heart to myocardial ischemia/reperfusion injury. The cKO failed to suppress pre-rRNA level during ischemia/reperfusion and showed a greater infarct size. Conversely, cardiac-specific overexpression of γ2-AMPK decreased ribosome biosynthesis and ER stress during ischemia/reperfusion insult, and the infarct size was reduced. CONCLUSIONS: The γ2-AMPK translocates into the nucleus to suppress pre-rRNA transcription and ribosome biosynthesis during stress, thus ameliorating ER stress and cell death. Increased γ2-AMPK activity is required to protect against ischemia/reperfusion injury. Our study reveals an isoform-specific function of γ2-AMPK in modulating ribosome biosynthesis, cell survival, and cardioprotection.
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