ABSTRACT
Lysosomal degradation of the endoplasmic reticulum (ER) via autophagy (ER-phagy) is emerging as a critical regulator of cell homeostasis and function. The recent identification of ER-phagy receptors has shed light on the molecular mechanisms underlining this process. However, the signaling pathways regulating ER-phagy in response to cellular needs are still largely unknown. We found that the nutrient responsive transcription factors TFEB and TFE3-master regulators of lysosomal biogenesis and autophagy-control ER-phagy by inducing the expression of the ER-phagy receptor FAM134B. The TFEB/TFE3-FAM134B axis promotes ER-phagy activation upon prolonged starvation. In addition, this pathway is activated in chondrocytes by FGF signaling, a critical regulator of skeletal growth. FGF signaling induces JNK-dependent proteasomal degradation of the insulin receptor substrate 1 (IRS1), which in turn inhibits the PI3K-PKB/Akt-mTORC1 pathway and promotes TFEB/TFE3 nuclear translocation and enhances FAM134B transcription. Notably, FAM134B is required for protein secretion in chondrocytes, and cartilage growth and bone mineralization in medaka fish. This study identifies a new signaling pathway that allows ER-phagy to respond to both metabolic and developmental cues.
Subject(s)
Autophagy , Basic Helix-Loop-Helix Leucine Zipper Transcription Factors/metabolism , Cell Nucleus/metabolism , Endoplasmic Reticulum/metabolism , Intracellular Signaling Peptides and Proteins/metabolism , Membrane Proteins/metabolism , Signal Transduction , Active Transport, Cell Nucleus , Animals , Basic Helix-Loop-Helix Leucine Zipper Transcription Factors/genetics , Cell Nucleus/genetics , Endoplasmic Reticulum/genetics , HeLa Cells , Humans , Intracellular Signaling Peptides and Proteins/genetics , Mechanistic Target of Rapamycin Complex 1/genetics , Mechanistic Target of Rapamycin Complex 1/metabolism , Membrane Proteins/genetics , Mice , OryziasABSTRACT
Oxidative damage triggers micronuclear membrane rupture and defective repair.
Subject(s)
Neoplasms , Oxidative Stress , Humans , Neoplasms/genetics , DNA Repair , Nuclear Envelope/metabolism , DNA DamageABSTRACT
Protein biogenesis within the endoplasmic reticulum (ER) is crucial for organismal function. Errors during protein folding necessitate the removal of faulty products. ER-associated protein degradation and ER-phagy target misfolded proteins for proteasomal and lysosomal degradation. The mechanisms initiating ER-phagy in response to ER proteostasis defects are not well understood. By studying mouse primary cells and patient samples as a model of ER storage disorders (ERSDs), we show that accumulation of faulty products within the ER triggers a response involving SESTRIN2, a nutrient sensor controlling mTORC1 signaling. SESTRIN2 induction by XBP1 inhibits mTORC1's phosphorylation of TFEB/TFE3, allowing these transcription factors to enter the nucleus and upregulate the ER-phagy receptor FAM134B along with lysosomal genes. This response promotes ER-phagy of misfolded proteins via FAM134B-Calnexin complex. Pharmacological induction of FAM134B improves clearance of misfolded proteins in ERSDs. Our study identifies the interplay between nutrient signaling and ER quality control, suggesting therapeutic strategies for ERSDs.
Subject(s)
Endoplasmic Reticulum , Mechanistic Target of Rapamycin Complex 1 , Protein Folding , X-Box Binding Protein 1 , Animals , Endoplasmic Reticulum/metabolism , Humans , Mice , Mechanistic Target of Rapamycin Complex 1/metabolism , X-Box Binding Protein 1/metabolism , X-Box Binding Protein 1/genetics , Signal Transduction , Membrane Proteins/metabolism , Membrane Proteins/genetics , Intracellular Signaling Peptides and Proteins/metabolism , Intracellular Signaling Peptides and Proteins/genetics , Nuclear Proteins/metabolism , Nuclear Proteins/genetics , Lysosomes/metabolism , Endoplasmic Reticulum Stress , Sestrins/metabolism , Sestrins/genetics , Phosphorylation , Proteostasis , Basic Helix-Loop-Helix Leucine Zipper Transcription FactorsABSTRACT
Selective degradation of the endoplasmic reticulum (ER) via autophagy (ER-phagy) is initiated by ER-phagy receptors, which facilitate the incorporation of ER fragments into autophagosomes. FAM134 reticulon family proteins (FAM134A, FAM134B, and FAM134C) are ER-phagy receptors with structural similarities and nonredundant functions. Whether they respond differentially to the stimulation of ER-phagy is unknown. Here, we describe an activation mechanism unique to FAM134C during starvation. In fed conditions, FAM134C is phosphorylated by casein kinase 2 (CK2) at critical residues flanking the LIR domain. Phosphorylation of these residues negatively affects binding affinity to the autophagy proteins LC3. During starvation, mTORC1 inhibition limits FAM134C phosphorylation by CK2, hence promoting receptor activation and ER-phagy. Using a novel tool to study ER-phagy in vivo and FAM134C knockout mice, we demonstrated the physiological relevance of FAM134C phosphorylation during starvation-induced ER-phagy in liver lipid metabolism. These data provide a mechanistic insight into ER-phagy regulation and an example of autophagy selectivity during starvation.