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.