ABSTRACT
The HSP40 cochaperone SEC63 is associated with the SEC61 translocon complex in the ER. Mutations in the gene encoding SEC63 cause polycystic liver disease in humans; however, it is not clear how altered SEC63 influences disease manifestations. In mice, loss of SEC63 induces cyst formation both in liver and kidney as the result of reduced polycystin-1 (PC1). Here we report that inactivation of SEC63 induces an unfolded protein response (UPR) pathway that is protective against cyst formation. Specifically, using murine genetic models, we determined that SEC63 deficiency selectively activates the IRE1α-XBP1 branch of UPR and that SEC63 exists in a complex with PC1. Concomitant inactivation of both SEC63 and XBP1 exacerbated the polycystic kidney phenotype in mice by markedly suppressing cleavage at the G protein-coupled receptor proteolysis site (GPS) in PC1. Enforced expression of spliced XBP1 (XBP1s) enhanced GPS cleavage of PC1 in SEC63-deficient cells, and XBP1 overexpression in vivo ameliorated cystic disease in a murine model with reduced PC1 function that is unrelated to SEC63 inactivation. Collectively, the findings show that SEC63 function regulates IRE1α/XBP1 activation, SEC63 and XBP1 are required for GPS cleavage and maturation of PC1, and activation of XBP1 can protect against polycystic disease in the setting of impaired biogenesis of PC1.
Subject(s)
DNA Helicases/physiology , DNA-Binding Proteins/physiology , Endoribonucleases/metabolism , Polycystic Kidney, Autosomal Dominant/genetics , Polycystic Kidney, Autosomal Recessive/genetics , Protein Serine-Threonine Kinases/metabolism , TRPP Cation Channels/deficiency , Transcription Factors/physiology , Unfolded Protein Response/physiology , Animals , Cell Line , DNA Helicases/deficiency , DNA Helicases/genetics , DNA-Binding Proteins/deficiency , DNA-Binding Proteins/genetics , Disease Models, Animal , Female , Glucosidases/deficiency , Glucosidases/genetics , Intracellular Signaling Peptides and Proteins/deficiency , Intracellular Signaling Peptides and Proteins/genetics , Kidney/metabolism , Kidney/pathology , Male , Mice , Mice, Inbred C57BL , Mice, Knockout , Mice, Transgenic , Molecular Chaperones , Polycystic Kidney, Autosomal Dominant/metabolism , Polycystic Kidney, Autosomal Recessive/metabolism , Protein Structure, Tertiary , RNA Splicing , RNA, Small Interfering/genetics , RNA-Binding Proteins , Receptors, G-Protein-Coupled/metabolism , Recombinant Fusion Proteins/metabolism , Regulatory Factor X Transcription Factors , TRPP Cation Channels/biosynthesis , TRPP Cation Channels/genetics , Transcription Factors/deficiency , Transcription Factors/genetics , Transfection , X-Box Binding Protein 1ABSTRACT
Receptor tyrosine kinases have become important therapeutic targets for anti-neoplastic molecularly targeted therapies. c-Met is a receptor tyrosine kinase shown to be over-expressed and mutated in a variety of malignancies. Stimulation of c-Met via its ligand hepatocyte growth factor also known as scatter factor (HGF/SF), leads to a plethora of biological and biochemical effects in the cell. There has been considerable knowledge gained on the role of c-Met-HGF/SF axis in normal and malignant cells. This review summarizes the structure of c-Met and HGF/SF and their family members. Since there are known mutations of c-Met in solid tumors, particularly in papillary renal cell carcinoma, we have summarized the various mutations and over-expression of c-Met known thus far. Stimulation of c-Met can lead to scattering, angiogenesis, proliferation, enhanced cell motility, invasion, and eventual metastasis. The biological functions altered by c-Met are quite unique and described in detail. Along with biological functions, various signal transduction pathways, including the cytoskeleton are altered with the activation of c-Met-HGF/SF loop. We have recently shown the phosphorylation of focal adhesion proteins, such as paxillin and p125FAK in response to c-Met stimulation in lung cancer cells, and this is detailed here. Finally, c-Met when mutated or over-expressed in malignant cells serves as an important therapeutic target and the most recent data in terms of inhibition of c-Met and downstream signal transduction pathways is summarized.
