title: PPARgamma ligands inhibit telomerase activity and hTERT expression through modulation of the Myc/Mad/Max network in colon cancer cells.
abstract: In human cells the length of telomeres depends on telomerase activity. This activity and the expression of the catalytic subunit of human telomerase reverse transcriptase (hTERT) is strongly up-regulated in most human cancers. hTERT expression is regulated by different transcription factors, such as c-Myc, Mad1 and Sp1. In this study, we demonstrated that 15d-PG J2 and rosiglitazone (an endogenous and synthetic peroxisome proliferators activated receptor gamma (PPARgamma) ligand, respectively) inhibited hTERT expression and telomerase activity in CaCo-2 colon cancer cells. Moreover, both ligands inhibited c-Myc protein expression and its E-box DNA binding activity. Additionally, Mad1 protein expression and its E-box DNA binding activity were strongly increased by 15d-PG J2 and, to a lesser extent, by rosiglitazone. Sp1 transcription factor expression and its GC-box DNA binding activity were not affected by both PPARgamma ligands. Results obtained by transient transfection of CaCo-2 cells with pmaxFP-Green-PRL plasmid constructs containing the functional hTERT core promoter (including one E-box and five GC-boxes) and its E-box deleted sequences, cloned upstream of the green fluorescent protein reporter gene, demonstrated that 15d-PG J2, and with minor effectiveness, rosiglitazone, strongly reduced hTERT core promoter activity. E-boxes for Myc/Mad/Max binding showed a higher activity than GC-boxes for Sp1. By using GW9662, an antagonist of PPARgamma, we demonstrated that the effects of 15d-PG J2 are completely PPARgamma independent, whereas the effects of rosiglitazone on hTERT expression seem to be partially PPARgamma independent. The regulation of hTERT expression by 15d-PG J2 and rosiglitazone, through the modulation of the Myc/Max/Mad1 network, may represent a new mechanism of action of these substances in inhibiting cell proliferation.
authors.affiliation: Department of Medicine and Experimental Oncology, University of Turin, Turin, Italy. cristina.toaldo@unito.it
authors.forename: Cristina
authors.initials: C
authors.lastname: Toaldo
authors.forename: Stefania
authors.initials: S
authors.lastname: Pizzimenti
authors.forename: Angelo
authors.initials: A
authors.lastname: Cerbone
authors.forename: Piergiorgio
authors.initials: P
authors.lastname: Pettazzoni
authors.forename: Elisa
authors.initials: E
authors.lastname: Menegatti
authors.forename: Berardi
authors.initials: B
authors.lastname: Daniela
authors.forename: Rosalba
authors.initials: R
authors.lastname: Minelli
authors.forename: Barbara
authors.initials: B
authors.lastname: Giglioni
authors.forename: Mario Umberto
authors.initials: MU
authors.lastname: Dianzani
authors.forename: Carlo
authors.initials: C
authors.lastname: Ferretti
authors.forename: Giuseppina
authors.initials: G
authors.lastname: Barrera
mesh_terms: D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors / Q000378:metabolism*; D015153:Blotting, Western; D018938:Caco-2 Cells; D003110:Colonic Neoplasms / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D004273:DNA, Neoplasm / Q000378:metabolism; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects; D006801:Humans; D008024:Ligands; D047495:PPAR gamma / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics; D015230:Prostaglandin D2 / Q000031:analogs & derivatives* / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000077154:Rosiglitazone; D019098:Telomerase / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism; D045162:Thiazolidinediones / Q000494:pharmacology*
chemical_list: C477819:15-deoxyprostaglandin J2; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D004273:DNA, Neoplasm; D008024:Ligands; C499330:MAX protein, human; C079439:MXD1 protein, human; D047495:PPAR gamma; D016271:Proto-Oncogene Proteins c-myc; D012333:RNA, Messenger; D012097:Repressor Proteins; D045162:Thiazolidinediones; D000077154:Rosiglitazone; C509186:TERT protein, human; D019098:Telomerase; D015230:Prostaglandin D2