title: Effects of MdmX on Mdm2-mediated downregulation of pRB.
abstract: Mdm2, a RING-finger type ubiquitin ligase, is overexpressed in a variety of human cancers. It promotes ubiquitination of the tumor suppressor p53 and can function as an oncogene by largely downregulating p53. Recently, we reported that Mdm2 degrades retinoblastoma tumor suppressor protein (pRB) via the ubiquitin-proteasome system. In the present study, we assessed the effects of MdmX, a structural homolog of Mdm2, on the Mdm2-mediated ubiquitination of pRB. MdmX is known to negatively regulate p53 function by enhancing the Mdm2-mediated ubiquitination and degradation of p53. Interestingly, MdmX inhibited the Mdm2-mediated pRB ubiquitination. Furthermore, an MdmX siRNA decreased the endogenous pRB level, while MdmX overexpression stimulated pRB functions in cultured cells. Therefore, MdmX may have different roles in the regulation of Mdm2 activity for ubiquitination of pRB and p53.
authors.affiliation: Department of Biochemistry 1, Hamamatsu University School of Medicine, 1-20-1 Handayama, Hamamatsu, Shizuoka 431-3192, Japan.
authors.forename: Chiharu
authors.initials: C
authors.lastname: Uchida
authors.forename: Seiichi
authors.initials: S
authors.lastname: Miwa
authors.forename: Tomoyasu
authors.initials: T
authors.lastname: Isobe
authors.forename: Kyoko
authors.initials: K
authors.lastname: Kitagawa
authors.forename: Takayuki
authors.initials: T
authors.lastname: Hattori
authors.forename: Toshiaki
authors.initials: T
authors.lastname: Oda
authors.forename: Hideyo
authors.initials: H
authors.lastname: Yasuda
authors.forename: Masatoshi
authors.initials: M
authors.lastname: Kitagawa
mesh_terms: D018797:Cell Cycle Proteins; D002478:Cells, Cultured; D015536:Down-Regulation; D006801:Humans; D009687:Nuclear Proteins / Q000502:physiology*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000502:physiology*; D051736:Proto-Oncogene Proteins c-mdm2 / Q000502:physiology*; D034741:RNA, Small Interfering / Q000494:pharmacology; D016160:Retinoblastoma Protein / Q000235:genetics* / Q000378:metabolism; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism; D025801:Ubiquitin / Q000378:metabolism
chemical_list: D018797:Cell Cycle Proteins; C482775:MDM4 protein, human; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; D034741:RNA, Small Interfering; D016160:Retinoblastoma Protein; D016159:Tumor Suppressor Protein p53; D025801:Ubiquitin; C497206:MDM2 protein, human; D051736:Proto-Oncogene Proteins c-mdm2; D046988:Proteasome Endopeptidase Complex