Rational therapeutic combinations with histone deacetylase inhibitors for the treatment of cancer

Abstract

Histone deacetylases (HDACs) regulate the acetylation of a variety of histone and nonhistone proteins, controlling the transcription and regulation of genes involved in cell cycle control, proliferation, survival, DNA repair and differentiation. Unsurprisingly, HDAC expression is frequently altered in hematologic and solid tumor malignancies. Two HDAC inhibitors (vorinostat and romidepsin) have been approved by the US FDA for the treatment of cutaneous T-cell lymphoma. As single agents, treatment with HDAC inhibitors has demonstrated limited clinical benefit for patients with solid tumors, prompting the investigation of novel treatment combinations with other cancer therapeutics. In this article, the rationales and clinical progress of several combinations with HDAC inhibitors are presented, including DNA-damaging chemotherapeutic agents, radiotherapy, hormonal therapies, DNA methyltransferase inhibitors and various small-molecule inhibitors. The future application of HDAC inhibitors as a treatment for cancer is discussed, examining current hurdles to overcome before realizing the potential of this new approach.

Figure 1. Histone deacetylase classification and inhibition

HDACs (boxes) are categorized based on their structure, and divided into four separate classes. HDAC inhibitors (upper lines) target specific classes of HDACs. Class I, II and IV HDACs are Zn⁺-dependent enzymes. Class III HDACs (sirtuins 1–7) are NAD⁺-dependent enzymes and are not depicted. HDAC: Histone deacetylase.

Figure 2. Role of acetylation in the regulation of chromatin and gene transcription

Acetylation and methylation work in concert to regulate chromatin structure and gene transcription. Methylation of DNA by DNMTs and deacetylation of nucleosome histone tails by HDACs leads to a compact chromatin structure and gene silencing. Proper maintenance of chromatin further relies on other components, such as HP-1 and SMC1–5, whose expression is regulated by HDAC activity. 5-aza-cytidine and 5-aza-2′-deoxycytidine inhibit DNMT and promote a more open DNA structure, which is conducive for gene transcription. Therefore, combining a HDAC inhibitor with a DNMT inhibitor may lead to greater expression of a silenced gene. In addition, a HDAC inhibitor combined with other chromatin maintenance inhibitors, such as topoisomerase inhibitors, may lead to greater DNA damage and cell death. Ac: Acetylation; DNMT: DNA methyltransferase; HAT: Histone acetyltransferase; HDAC: Histone deacetylase; Me: Methyl group.

Figure 3. Histone deacetylase inhibitors downregulate DNA repair

HDAC proteins interact with a variety of vital DNA repair components including ATM, ATR, BRCA1 and p53. Inhibitors of HDACs increase acetylation of Ku70 and p53 and alter their function. Combined treatment with HDAC inhibitors and DNA-damaging agents results in a synergistic increase in DNA damage and reduced repair kinetics. Ac: Acetylation; ATM: Ataxia telangiec tasia-mutated; ATR: Ataxia telangiectasia and Rad3-related; HDAC: Histone deacetylase.

Figure 4. Role of acetylation in microtubule-targeted therapy

Microtubule dynamicity is, in part, regulated by tubulin acetylation, which promotes stabilization. HDAC6 deacetylates microtubules and promotes their depolymerization. Chemotherapeutic taxanes bind to tubulin and inhibit depolymerization and thus microtubule dynamicity, which is essential for chromosome capture and mitosis. Therefore, combined taxanes (dark-colored balls) and HDAC inhibitors would lead to greater disruption of microtubule function and more cell death as a result. Ac: Acetylation; HAT: Histone acetyltransferase; HDAC: Histone deacetylase

Figure 5. Role of acetylation in hormone therapy

Hormones (e.g., estrogen and androgen) mediate activity through HRs, which regulate the transactivation of various target genes. HRs are maintained in a ligand-binding conformation by the HSP90 chaperone complex. This function is regulated by Ac and HDAC6. Acetylation of HSP90 promotes dissociation with HRs and their subsequent degradation via the proteasome. HRs are further regulated by direct acetylation, in part by the histone acetylase p300. Two types of hormone therapy have been developed to disrupt hormone-mediated signaling. Aromatase inhibitors and ADT inhibit the production of estrogen and androgen, respectively. Tamoxifen and bicalutamide compete with hormones for binding to the estrogen and androgen receptors, respectively. Therefore, combining hormone therapy with a HDAC inhibitor would lead to further perturbation of tumorigenic signaling. Ac: Acetylation; ADT: Androgen deprivation therapy; HDAC: Histone deacetylase; HR: Hormone receptor.

Figure 6. Role of acetylation in therapies targeting receptor tyrosine kinase signaling

Mitogenic signaling involves ligand-induced dimerization and autophosphorylation of receptor tyrosine kinases (e.g., HER2/3 and EGF receptor), kinase signal transduction and target gene transcription, such as upregulation of c-Myc and cyclin D1, and downregulation of p21. Several therapies target these pathways, including trastuzumab, which inhibits HER2-ligand binding, erlotinib and gefitinib, which inhibit receptor tyrosine kinase autophosphorylation, and sorafenib and everolimus, which inhibit RAF and mTOR, respectively. HDACs also have been shown to mediate transcription of these cell cycle regulators. Therefore, combining HDAC inhibitors with receptor tyrosine kinase signaling inhibitors might further reduce tumor growth. HDAC: Histone deacetylase.