PPARgamma ligands inhibit telomerase activity and hTERT expression through modulation of the Myc/Mad/Max network in colon cancer cells.
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.
Toaldo
Cristina
C
Department of Medicine and Experimental Oncology, University of Turin, Turin, Italy. cristina.toaldo@unito.it
Pizzimenti
Stefania
S
Cerbone
Angelo
A
Pettazzoni
Piergiorgio
P
Menegatti
Elisa
E
Daniela
Berardi
B
Minelli
Rosalba
R
Giglioni
Barbara
B
Dianzani
Mario Umberto
MU
Ferretti
Carlo
C
Barrera
Giuseppina
G
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*
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