Synthetic triterpenoids activate a pathway for apoptosis in AML cells involving downregulation of FLIP and sensitization to TRAIL

Abstract

Acute myelogenous leukemia (AML) remains a deadly disease for most adult patients, due primarily to the emergence of chemoresistant cells. Defects in apoptosis pathways make important contributions to chemoresistance, suggesting a need to restore apoptosis sensitivity or to identify alternative pathways for apoptosis induction. Triterpenoids represent a class of naturally occurring and synthetic compounds with demonstrated antitumor activity, including 2-cyano-3,12-dioxoolean-1,9-dien-28-oic acid (CDDO) and its methyl ester (CDDO-m). We explored the effects of CDDO and CDDO-m in vitro on established AML cell lines (HL-60, U937, AML-2) and on freshly isolated AML blasts. CDDO and CDDO-m reduced the viability of all AML cell lines tested in a dose-dependent manner, with effective doses for killing 50% of cells (ED 50 ) within 48 h of B1 and 0.5 lM, respectively. CDDO or CDDO-m also induced substantial increases in cell death in five out of 10 samples of primary AML blasts. Cell death induced by CDDO and CDDO-m was attributed to apoptosis, based on characteristic cell morphology and evidence of caspase activation. Immunoblot analysis demonstrated proteolytic processing of caspase-3, -7, and -8, but not caspase-9, suggesting the involvement of the 'extrinsic' pathway, linked to apoptosis induction by TNF-family death receptors. Accordingly, CDDO and CDDO-m induced concentration-dependent reductions in the levels of FLIP protein, an endogenous antagonist of caspase-8, without altering the levels of several other apoptosis-relevant proteins. Reductions in FLIP were rapid, detectable within 3 h after exposure of AML cell lines to CDDO or CDDO-m. CDDO and CDDO-m also sensitized two of four leukemia lines to TRAIL, a TNF-family death ligand. The findings suggest that synthetic triterpenoids warrant further investigation in the treatment of AML, alone or in combination with TRAIL or other immunebased therapies.

Azadirone, a Limonoid Tetranortriterpene, Induces Death Receptors and Sensitizes Human Cancer Cells to Tumor Necrosis Factor-related Apoptosis-inducing Ligand (TRAIL) through a p53 Protein-independent Mechanism

“…Because overexpression of these proteins has been shown to contribute to TRAIL resistance in tumor cells (9,45,46), it is likely that the sensitization of tumor cells to TRAIL is due to down-regulation in the expression of cell survival proteins by azadirone. In agreement with our observations, previous studies have shown that triterpenes such as nimbolide (37), celastrol (40), and 2-cyano-3,12-dioxoolean-1,9-dien-28-oic acid (CDDO) (47) can sensitize tumor cells to TRAIL by suppressing expression of cell survival proteins. XIAP, a potent inhibitor of caspase-3 activity, has previously been shown to confer resistance to TRAIL (48).…”

Valproic acid inhibits proliferation and induces apoptosis in acute myeloid leukemia cells expressing P-gp and MRP1

“…26 In AML cells, synthetic triterpenoids activate a pathway for apoptosis involving downregulation of Flip and sensitization to TRAIL. 27 We found a downregulation of Flip with a cleavage 38 kDa Flip in K562 cells, but not in HL60 cells. We propose a different mechanism of apoptosis induced by VPA.…”

CDDO induces granulocytic differentiation of myeloid leukemic blasts through translational up-regulation of p42 CCAAT enhancer–binding protein alpha

“…CDDO induces apoptosis through both caspase-independent and -dependent mechanisms, the latter involving caspase-8 activation, Bid cleavage, cytochrome c release, and caspase-3 activation. [3][4][5][6] Furthermore, JNK, p38, and ERK pathways are involved in CDDO-induced apoptosis of tumor cell lines [7][8][9] mediated by disrupted intracellular redox balance and involving decreased glutathione and increased reactive oxygen species [9][10][11][12] CDDO-induced growth arrest of breast cancer cell lines correlates with transactivated PPARgamma and leads to up-regulation of p21 cip1waf1 , GADD153, CCAAT enhancer-binding proteins (CEBPs), and proteasome-regulatory factors, and to down-regulation of cyclin D1, PCNA, and IRS1. 13 CDDO and CDDO-Im activate the TGF␤ pathway through activation of Smad2/3, 14,15 which is required for the repression of inflammatory molecules by CDDO.…”