Activation of peroxisome proliferator-activated receptor gamma inhibits cell growth via apoptosis and arrest of the cell cycle in human colorectal cancer.

OBJECTIVE

To investigate the expression of peroxisome proliferator-activated receptors (PPAR)gamma and the effects of PPARgamma ligands on cells growth in colorectal cancer (CRC) cell line HT-29, and to explore whether the activation of PPARgamma by its selective ligand can induce apoptosis and the arrest of the cell cycle in these cells.

METHODS

A CRC cell line, HT-29, was used in this study. PPARgamma mRNA and the protein expressions were measured by reverse transcriptase-polymerase chain reaction and Western blot. The HT-29 cells were treated with two specific PPARgamma ligands: rosiglitazone and 15-d-PGJ2. The effects of PPARgamma activated by rosiglitazone and 15-d-PGJ2 on the anchorage-dependent and anchorage-independent growth of the cells were assessed by methylthiazolyl terazolium (MTT) and soft agar colony assay, respectively. Apoptosis was measured by TUNEL staining and flow cytometry (FCM) assay by CaspSCREEN Flowcytometric Apoptosis Detection Kit (BioVision, Palo Alto, USA). Furthermore, the caspase-3 expression was determined by a immunocytochemical staining method before and after treatment with rosiglitazone and 15-d-PGJ2 for 48 h. The cell cycles were measured by flow cytometric analysis using propidium iodide (PI).

RESULTS

PPARgamma mRNA and protein expressions were observed in the HT-29 cells. The MTT assay showed that treatment of these cells with 0, 0.1, 1 or 10 micromol/L PPARgamma activators rosiglitazone or 15-d-PGJ2 for 0, 24, 48 or 72 h resulted in the inhibition of anchorage-dependent cell growth in a dosage- and time-dependent way. Rosiglitazone treatment during cell growth resulted in the reduction of colony formation and the effects were not immediately reversible in the cell culture. TUNEL staining showed DNA fragmentation in positive cells after treatment with rosiglitazone and 15-d-PGJ2 for 48 h. In addition, FCM showed that the apoptosis rates were 14.8+/-0.8% and 28.5+/-1.3% or 15+/-0.7% and 40+/-1.2% after the cells were incubated with 10 micromol/L rosiglitazone or 15-d-PGJ2 for 24 h and 48 h, while the apoptosis rates of cells without treatment were 3.8+/-0.4% and 8.8+/-0.4%, respectively. Consistent with these results, the positivity rates of caspase-3 expression in cells treated with rosiglitazone or 15-d-PGJ2 increased significantly when compared with the control group. To explore whether the regulation of the cell cycle was involved in the effect of PPARgamma ligands on cell growth, FCM using PI staining was assessed. The ratio of G0/G1 phase cells increased after incubated with 10 micromol/L rosiglitazone or 15-d-PGJ2 for 24 h and 48 h.

CONCLUSIONS

Our results showed that PPARgamma was expressed in HT-29 cells and PPARgamma activation could inhibit cell growth through inducing apoptosis and suppressing the cell cycle. PPARgamma may be considered as a new therapeutic target for colon cancer in humans.