Arginine And Lysine Methylation Of MRPS23 Promotes Breast Cancer Metastasis Through Regulating OXPHOS

The distant metastasis has been reported to be the major cause of mortality in breast cancer patients.Mitochondrial oxidative phosphorylation(OXPHOS)plays an important role in the regulation of cancer metastasis.Recent evidence provides a metabolic mode in which inhibition of OXPHOS moderately promotes mt ROS generation to promote tumor metastasis.Besides,OXPHOS is essential for the synthesis of necessary macromolecules(nucleotides,lipids and amino acids),as well as ATP and NADPH,which are required for cell proliferation.Therefore,studying the relationship between OXPHOS and breast cancer is helpful to reveal new mechanisms of breast cancer metastasis.The mitochondrial ribosomal protein S23(MRPS23)is a component of the highly conserved mitochondrial ribosome small subunit.Early research discovered that when MRPS23 is mutated,enzyme activities in respiratory chain Complexes I and IV are decreased in metabolic syndrome of the heart disease.These data point to the possibility that MRPS23 may regulate OXPHOS.In addition,high MRPS23 expression contributes to cell proliferation in hepatocellular carcinoma and breast cancer.However,the association between MRPS23 and metastasis of human breast cancer has not been investigated so far.Methylation of arginine and lysine residues on proteins has emerged as a prevalent post-translational modification(PTM)involved in OXPHOS dysregulation during tumorigenesis.The protein arginine methyltransferase 7(PRMT7)belongs to the type III arginine methyltransferase capable of generating mono-methylarginine(MMA)of proteins.Our previous work also demonstrated that PRMT7 and its automethylation promote breast cancer metastasis by inducing the epithelial-to-mesenchymal transition(EMT).The SET-domain-containing protein 6(SETD6)is a key regulator of proliferation.Silencing of SETD6 in several breast carcinoma cell lines induces cellular proliferation defects and induction of apoptosis.However,whether PRMT7 coordinates with SETD6 to regulate breast cancer metastasis currently remains unclear.In this study,we performed the mass spectrometry of 3×Flag-PRMT7 complex purified from stable MCF7-3×Flag-PRMT7 cell lines,in an attempt to discover PRMT7-interacting protein.Noticeably,MRPS23 was a putative PRMT7-interacting protein.Further,we performed glutathione S-transferase(GST)pull-down and the results revealed that MRPS23 was able to bind PRMT7 directly,suggesting that PRMT7 might methylate MRPS23.To prove this assumption,we performed mass spectrum analysis and in vitro methylation assays,and the rusults revealed that arginine 21(R21)of MRPS23 was methylated by PRMT7.R21 methylation accelerated the poly-ubiquitin-dependent degradation of MRPS23 to a low level.The MRPS23 degradation inhibited OXPHOS with elevated mt ROS level,which consequently increased breast cancer cell migration and invasion.Subsequently,lung metastasis in nude mice experiment demonstrates MRPS23 R21 methylation promotes breast cancer metastasis.On the other hand,our mass spectrometry analysis showed that in addition to arginine methylation,MRPS23 lysine methylation and SETD6 were also detected.We further discovered that SETD6 methylated MRPS23 at lysine 108(K108).K108 methylation increased MRPS23 stability,and K108 methylation coordinated with R21 methylation to maintain a low level of MRPS23,which was in favor of supporting breast cancer cell survival through regulating OXPHOS.Consistently,R21 and K108 methylation was correlated with the malignant breast carcinoma.In conclusion,we found the arginine and lysine methylation of MRPS23 mediates MRPS23 protein stability to a low level,leading to the inhibition of OXPHOS to promote breast cancer cell invasion and metastasis.These findings provide mechanistic insights into breast cancer metastasis controlled by MRPS23 arginine and lysine methylation,implicating that inhibition of MRPS23 arginine and lysine methylation may have therapeutic applications.