A truncated isoform of c-Mpl with an essential C-terminal peptide targets the full-length receptor for degradation.
Thrombopoietin and its cognate receptor c-Mpl are the primary regulators of megakaryopoiesis and platelet production. They also play an important role in the maintenance of hematopoietic stem cells. Here, we have analyzed the function of a truncated Mpl receptor isoform (Mpl-tr), which results from alternative splicing. The mpl-tr variant is the only alternate mpl isoform conserved between mouse and humans, suggesting a relevant function in regulating Mpl signaling. Despite the presence of a signal peptide and the lack of a transmembrane domain, Mpl-tr is retained intracellularly. Our results provide evidence that Mpl-tr exerts a dominant-negative effect on thrombopoietin-dependent cell proliferation and survival. We demonstrate that this inhibitory effect is due to down-regulation of the full-length Mpl protein. The C terminus of Mpl-tr, consisting of 30 amino acids of unique sequence, is essential for the suppression of thrombopoietin-dependent proliferation and Mpl protein down-regulation. Cathepsin inhibitor-1 (CATI-1), an inhibitor of cathepsin-like cysteine proteases, counteracts the effect of Mpl-tr on Mpl protein expression, suggesting that Mpl-tr targets Mpl for lysosomal degradation. Together, these data suggest a new paradigm for the regulation of cytokine receptor expression and function through a proteolytic process directed by a truncated isoform of the same receptor.
Coers
Jörn
J
Department of Research, Experimental Hematology, Basel University Hospital, Hebelstrasse 20, 4031 Basel, Switzerland.
Ranft
Christina
C
Skoda
Radek C
RC
D017398:Alternative Splicing; D000818:Animals; D017209:Apoptosis; D015704:CD4 Antigens / Q000737:chemistry; D002455:Cell Division; D002460:Cell Line; D002470:Cell Survival; D016207:Cytokines / Q000378:metabolism; D004247:DNA / Q000378:metabolism; D018076:DNA, Complementary / Q000378:metabolism; D004151:Dipeptides / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation; D005434:Flow Cytometry; D005799:Genes, Dominant; D016178:Granulocyte-Macrophage Colony-Stimulating Factor / Q000378:metabolism; D006412:Hematopoietic Stem Cells / Q000378:metabolism; D006801:Humans; D051379:Mice; D008957:Models, Genetic; D009363:Neoplasm Proteins / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry; D010957:Plasmids / Q000378:metabolism; D016133:Polymerase Chain Reaction; D011485:Protein Binding; D020033:Protein Isoforms; D021382:Protein Sorting Signals; D017434:Protein Structure, Tertiary; D011518:Proto-Oncogene Proteins / Q000737:chemistry*; D018121:Receptors, Cytokine / Q000737:chemistry*; D053628:Receptors, Thrombopoietin; D015398:Signal Transduction; D013926:Thrombopoietin / Q000737:chemistry / Q000502:physiology; D014162:Transfection
D015704:CD4 Antigens; D016207:Cytokines; D018076:DNA, Complementary; D004151:Dipeptides; D009363:Neoplasm Proteins; D010455:Peptides; D020033:Protein Isoforms; D021382:Protein Sorting Signals; D011518:Proto-Oncogene Proteins; D018121:Receptors, Cytokine; D053628:Receptors, Thrombopoietin; C412454:phenylalanyl-glycyl-NHO-Bz; C082218:MPL protein, human; D016178:Granulocyte-Macrophage Colony-Stimulating Factor; D004247:DNA; D013926:Thrombopoietin