Abstract B9: Activated MAPK Pathway Mediates Resistance to Romidepsin via Bim Degradation in Romidepsin-Selected HuT 78 Cells

Abstract

Abstract Acquired and intrinsic resistance to histone deacetylase inhibitors (HDIs), a new targeted group of anti-tumor agents, limits their clinical efficacy. A detailed understanding of the mechanisms of resistance to HDIs may lead to strategies designed to increase clinical efficacy. To investigate molecular mechanisms of resistance to the HDI romidepsin (Dp), we studied a cutaneous T-cell lymphoma (CTCL) cell line, HuT 78, independently selected in verapamil (Vp) or valspodar (PSC833) to prevent the emergence of P-glycoprotein (Pgp), a known resistance mechanism. The HuT 78 sublines DpVp 50 and DpP 75 display 100-200-fold resistance to romidepsin, not due to Pgp expression. A custom-made Taqman low density gene expression array detected increased expression of insulin receptor (IR) in the resistant cells. Real-time PCR analysis confirmed the results of gene array and detected more than 50-fold upregulation of IR in the romidepsin-selected cells compared to the parental cells. Increased phosphorylation (5- to 8- fold) of mitogen activated protein kinase kinase (MEK), a downstream effector of the IR pathway, was also observed in the resistant cells compared to the parental cells. While HuT 78 cells were insensitive to MEK inhibition, resistant cells were found to be exquisitely sensitive to MEK inhibition (IC50 < 10 nM) but not to phosphatidylinositol 3-kinase (PI3K) inhibition. Combined treatment of romidepsin with low concentrations (1- 3 nM) of MEK inhibitor also resulted in increased cell death in romidepsin-resistant HuT 78 cells. The exquisite sensitivity to MEK inhibition in the resistant sublines was found to correlate with restoration of the expression of Bim (BCL2L11), a Bcl-2-homology domain-3 only (BH-3) proapoptotic protein. In the resistant cells, the MAPK pathway appeared to regulate Bim expression posttranslationally as we did not detect any gene induction of BCL2L11 or FOXO3, a transcription factor known to activate BCL2L11 expression, following MEK inhibition. These findings implicate increased activation of the MAPK pathway as a mechanism of resistance to romidepsin, and suggest that combining romidepsin with MEK inhibitors may be an effective strategy to overcome resistance.