title: Kinetics of myc-max-mad gene expression during hepatocyte proliferation in vivo: Differential regulation of mad family and stress-mediated induction of c-myc.
abstract: Mad proteins (Mad1, Mxi1, Mad3, Mad4, Mnt/Rox) are biochemical and biological antagonists of c-Myc oncoprotein. Mad-Max dimers repress the transcription of the same target genes activated by Myc-Max dimers. Despite the critical role of Max and Mad proteins as modulators of c-Myc functions, there are no comparative data on their regulation in vivo. We carried out a systematic analysis of c-myc, max, and mad family expression in a model of synchronized cell proliferation in vivo in adult tissues, that is, rat hepatocytes after partial hepatectomy. We confirmed the previously reported early peak of c-myc expression after hepatectomy but we show that it did not correlate with hepatocyte proliferation as it also occurred in sham-operated animals as a result of surgical stresses. A second peak of c-myc expression was observed later, at the time of the wave of DNA synthesis. No such expression was detected in sham-operated rat quiescent hepatocytes. max expression increased around 4-16 h after hepatectomy, before the peaks of c-myc and DNA synthesis. mxi1 and mad4 were slightly downregulated during liver regeneration. mnt/rox expression did not change. These expression patterns suggest a role of Myc-Max for efficient mitogenic response of hepatocytes. We also analyzed the effects of Myc and Max ectopic expression on the clonogenic growth of the rat hepatoma cells. Expression of c-Myc and Max increased clonogenic growth, whereas the reduction of c-Myc levels by an antisense vector decreased growth. The results suggest nonredundant roles for mad genes in hepatocyte proliferation and point to c-Myc as a putative target for anticancer therapy of liver cancer.
authors.affiliation: Grupo de Biología Molecular del Cáncer, Departamento de Biología Molecular y Unidad de Biomedicina del CSIC, Facultad de Medicina, Universidad de Cantabria, Santander, Spain.
authors.forename: Itsaso
authors.initials: I
authors.lastname: Mauleon
authors.forename: Marie-Noëlle
authors.initials: MN
authors.lastname: Lombard
authors.forename: Maria J
authors.initials: MJ
authors.lastname: Muñoz-Alonso
authors.forename: Matilde
authors.initials: M
authors.lastname: Cañelles
authors.forename: Javier
authors.initials: J
authors.lastname: Leon
mesh_terms: D000818:Animals; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D050976:Basic-Leucine Zipper Transcription Factors; D006528:Carcinoma, Hepatocellular / Q000378:metabolism; D002455:Cell Division / Q000502:physiology; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D015972:Gene Expression Regulation, Neoplastic / Q000502:physiology*; D022781:Hepatocytes / Q000378:metabolism*; D008115:Liver Regeneration / Q000235:genetics / Q000502:physiology; D008297:Male; D005810:Multigene Family; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012097:Repressor Proteins; D014157:Transcription Factors
chemical_list: D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D050976:Basic-Leucine Zipper Transcription Factors; D004268:DNA-Binding Proteins; C499329:Max protein, rat; C067850:Myc associated factor X; D016271:Proto-Oncogene Proteins c-myc; D012333:RNA, Messenger; D012097:Repressor Proteins; D014157:Transcription Factors