Abstract

Ligands for the transcription factor PPARγ are emerging as a new class of anti-tumor agents. Herein we report that the synthetic triterpenoid CDDO, a PPARγ ligand that induces PPARγ transcriptional activity in human DLCL OCI-Ly-19 cells, also induces cell death in human DLCL of both germinal center (OCI-Ly19) and activated B-cell phenotype (OCI-Ly10), cells which express the PPARγ protein. This effect of CDDO appears to be independent of PPARγ stimulated pathways since the functional antagonist of PPARγ, GW9662, which completely inhibits CDDO induced PPARγ transcriptional activity was unable to prevent CDDO induced cell death. Similar findings were seen using the additional PPARγ antagonists T0070907 and BADGE. CDDO induces cell death by inhibiting cell proliferation and inducing apoptosis as shown by Annexin-V and propidium iodide staining. As we have previously shown that PPARγ ligands inhibit NF-κB activity in B lymphocytes (

J. Immunol
2005
;
174
(7):
4060
–9
), we next examined the effect of CDDO on NF-κB in DLCL cells. Surprisingly, exposure of Ly19 cells to CDDO resulted in a dose-, and time-dependent increase in the activity of both the p50 and p65 subunits of NF-κB as determined by ELISA, by direct visualization of the nuclear translocation of p65 using indirect immunofluorescence assays, and by EMSA. The nuclear translocation of both the p50 and p65 NF-κB subunits was also confirmed by performing immunoblot analyses using nuclear fractions of CDDO-treated Ly19 cells. NF-κB activation was also observed in Ly10 cells exposed to CDDO. Follow-up experiments revealed that the activation of NF-κB in Ly19 cells by CDDO was due to proteolysis of inhibitory IκBα molecules. To determine whether the CDDO-induced NF-κB activation was a pro- survival mechanism, Ly19 and Ly10 cells were pre-treated with the NF-κB inhibitors SN50, helenalin or BAY 11-7082 and then exposed to CDDO for 24 hrs. In all cases, the NF-κB inhibitors significantly enhanced CDDO induced cell death suggesting that NF-κB activation is an anti-apoptotic mechanism elicited to protect the cell against CDDO cytotoxicity. Collectively, these studies suggest that; (a) CDDO (which will shortly be entering clinical trials for patients with acute myeloid leukemia) as a single agent may have significant clinical activity in patients with DLCL and; (b) the combination of CDDO with pharmacological inhibitors of NF-κB would be a rationale combination of novel agents to test in the context of clinical trials for patients with DLCL.

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