Histone deacetylase inhibitors: emerging mechanisms of resistance

Abstract

The histone deacetylase inhibitors (HDIs) have shown promise in the treatment of a number of hematologic malignancies, leading to the approval of vorinostat and romidepsin for the treatment of cutaneous T-cell lymphoma and romidepsin for the treatment of peripheral T-cell lymphoma by the U.S. Food and Drug Administration. Despite these promising results, clinical trials with the HDIs in solid tumors have not met with success. Examining mechanisms of resistance to HDIs may lead to strategies that increase their therapeutic potential in solid tumors. However, relatively few examples of drug-selected cell lines exist, and mechanisms of resistance have not been studied in depth. Very few clinical translational studies have evaluated resistance mechanisms. In the current review, we summarize many of the purported mechanisms of action of the HDIs in clinical trials and examine some of the emerging resistance mechanisms.

Figure 1

Modification of lysines in histone tails. DNA is wound around four core histone proteins: H2A, H2B, H3 and H4. Each of the histones possess lysine-rich tails and accessibility of the DNA is controlled by modifications to the tail. Lysines can either be multiply methylated or acetylated. Methylation and deacetylation of lysines both contribute to a more condensed chromatin structure, preventing transcription of genes. Demethylation and acetylation promote a more open chromatin structure allowing for increased gene transcription.

Figure 2

Structures of some of the histone deacetylase inhibitors currently in clinical trials.

Figure 3

Histone deacetylase inhibitors all cause increased histone acetylation but differentially cause tubulin acetylation. HEK293 (Flp-In-239) cells were treated with 46 nM romidepsin, 100 nM panobinostat, 10 μM vorinostat or 1 mM valproic acid for 24 h after which protein was extracted, subjected to electrophoresis, and transferred to a PVDF membrane. The membrane was subsequently probed for acetylated histone H3 (AcH3), acetylated α-tubulin (acetyl-α-tub), total α-tubulin (α-tub) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). While all of the HDIs were able to induce histone acetylation, only panobinostat and vorinostat were able to cause increased tubulin acetylation, suggesting that these HDIs also target HDAC6

Figure 4

Resistance to romidepsin is not mediated by Pgp expression in HuT78 DpVp50 cells. HuT78 parental and DpVp50 cells were incubated with the Pgp-specific antibody, MRK-16 (blue histogram), or IgG negative control antibody (red histogram) for 30 min after which cells were washed and incubated with phycoerythrin-labeled secondary antibody (top row, Pgp). While HuT78 parental cells are Pgp negative, the DpVp50 cells express low but detectable levels. Cells were also left untreated (second row, C) or were incubated with 50 ng/mL romidepsin for 48 h in the presence (third row, DP) or absence (bottom row DP + TAR) of 250 nM of the Pgp inhibitor tariquidar, after which cells were incubated with annexin V antibody and propidium iodide. Cells in the lower left quadrant are viable cells, while cells in the lower right quadrant are early apoptotic cells, and cells in the upper right quadrant are late apoptotic or necrotic cells. HuT parental cells readily undergo apoptosis after incubation with romidepsin either in the presence or absence of tariqudiar, so shown by the increase of cells in the upper and lower right quadrants. DpVp50 cells are resistant to romidepsin whether the inhibitor is added or not, suggesting a resistance mechanism that does not involve Pgp.