Objective·To investigate the molecular mechanisms and biological functions of the E3 ubiquitin ligase membrane-associated RING-CH 9(MARCH9)in regulating the ubiquitination of receptor protein tyrosine phosphatase alpha(RPTPα).Methods·Western blotting was employed to identify the ubiquitination type of RPTPα and to evaluate the regulatory effect of MARCH9 on its ubiquitination level;Comparative analysis of RPTPα protein stability was performed among wild-type MARCH9,catalytically inactive MARCH9 mutants(MARCH9 S198A or MARCH9-HC/CC),and endogenous MARCH9 knockdown via shRNA.Proteasome inhibitor MG132,autophagy inhibitor 3-MA,and lysosomal inhibitor chloroquine(CQ)were used to determine the degradation pathway of MARCH9-mediated RPTPα ubiquitination.The mechanism underlying 43℃heat shock-induced RPTPα degradation was explored.Stable lung cancer cell lines with MARCH9 single-knockdown(H1299-sh MARCH9)and MARCH9/RPTPα double-knockdown(H1299-sh MARCH9-sh RPTPα)were established using lentiviral vectors.CCK-8 proliferation assay,colony formation assay,and soft agar assay were conducted to evaluate the effects of MARCH9 or RPTPα on lung cancer cell proliferation and clonogenicity.Vasculogenic mimicry formation assay and scratch wound healing assay were performed to assess the impacts on tumor cell invasion and migration.Subcutaneous xenograft models in nude mice were established to examine in vivo tumorigenicity.Bioinformatics analysis was used to compare the expression differences and prognostic correlations of MARCH9 and RPTPα in lung cancer patients.Results·RPTPα predominantly underwent K63-linked poly-ubiquitination,which was significantly enhanced by MARCH9 overexpression.Wild-type MARCH9,but not its catalytic mutants,markedly reduced RPTPα protein stability,while endogenous MARCH9 knockdown increased RPTPα levels.CQ,not MG132 or 3-MA,restored RPTPα stability,indicating that MARCH9 mediated lysosomal degradation of RPTPα through ubiquitination.Heat shock at 43℃specifically enhanced MARCH9-RPTPα interaction,promoting RPTPα degradation.Functional assays revealed that,compared to control H1299 cells,MARCH9-knockdown cells exhibited elevated RPTPα levels,accelerated proliferation,enhanced clonogenicity and invasive capacity,and increased tumorigenicity in nude mice.These phenotypes could be reversed by double knockdown of MARCH9/RPTPα.Bioinformatics analysis demonstrated that high RPTPα expression correlated with poor prognosis and tumor metastasis in lung cancer patients,while MARCH9 showed inverse correlations.Conclusion·MARCH9 mediates K63-linked ubiquitination-dependent lysosomal degradation of phosphatase RPTPα,providing new insights into developing RPTPα-targeted cancer therapeutic strategies.

Objective·To investigate the molecular mechanisms and biological functions of the E3 ubiquitin ligase membrane-associated RING-CH 9(MARCH9)in regulating the ubiquitination of receptor protein tyrosine phosphatase alpha(RPTPα).Methods·Western blotting was employed to identify the ubiquitination type of RPTPα and to evaluate the regulatory effect of MARCH9 on its ubiquitination level;Comparative analysis of RPTPα protein stability was performed among wild-type MARCH9,catalytically inactive MARCH9 mutants(MARCH9 S198A or MARCH9-HC/CC),and endogenous MARCH9 knockdown via shRNA.Proteasome inhibitor MG132,autophagy inhibitor 3-MA,and lysosomal inhibitor chloroquine(CQ)were used to determine the degradation pathway of MARCH9-mediated RPTPα ubiquitination.The mechanism underlying 43℃heat shock-induced RPTPα degradation was explored.Stable lung cancer cell lines with MARCH9 single-knockdown(H1299-sh MARCH9)and MARCH9/RPTPα double-knockdown(H1299-sh MARCH9-sh RPTPα)were established using lentiviral vectors.CCK-8 proliferation assay,colony formation assay,and soft agar assay were conducted to evaluate the effects of MARCH9 or RPTPα on lung cancer cell proliferation and clonogenicity.Vasculogenic mimicry formation assay and scratch wound healing assay were performed to assess the impacts on tumor cell invasion and migration.Subcutaneous xenograft models in nude mice were established to examine in vivo tumorigenicity.Bioinformatics analysis was used to compare the expression differences and prognostic correlations of MARCH9 and RPTPα in lung cancer patients.Results·RPTPα predominantly underwent K63-linked poly-ubiquitination,which was significantly enhanced by MARCH9 overexpression.Wild-type MARCH9,but not its catalytic mutants,markedly reduced RPTPα protein stability,while endogenous MARCH9 knockdown increased RPTPα levels.CQ,not MG132 or 3-MA,restored RPTPα stability,indicating that MARCH9 mediated lysosomal degradation of RPTPα through ubiquitination.Heat shock at 43℃specifically enhanced MARCH9-RPTPα interaction,promoting RPTPα degradation.Functional assays revealed that,compared to control H1299 cells,MARCH9-knockdown cells exhibited elevated RPTPα levels,accelerated proliferation,enhanced clonogenicity and invasive capacity,and increased tumorigenicity in nude mice.These phenotypes could be reversed by double knockdown of MARCH9/RPTPα.Bioinformatics analysis demonstrated that high RPTPα expression correlated with poor prognosis and tumor metastasis in lung cancer patients,while MARCH9 showed inverse correlations.Conclusion·MARCH9 mediates K63-linked ubiquitination-dependent lysosomal degradation of phosphatase RPTPα,providing new insights into developing RPTPα-targeted cancer therapeutic strategies.