Histone Deacetylase Inhibitors to Overcome Resistance to Targeted and Immuno Therapy in Metastatic Melanoma

Abstract

Therapies that target oncogenes and immune checkpoint molecules constitute a major group of treatments for metastatic melanoma. A mutation in BRAF (BRAF V600E) affects various signaling pathways, including mitogen activated protein kinase (MAPK) and PI3K/AKT/mammalian target of rapamycin (mTOR) in melanoma. Target-specific agents, such as MAPK inhibitors improve progression-free survival. However, BRAF^V600E mutant melanomas treated with BRAF kinase inhibitors develop resistance. Immune checkpoint molecules, such as programmed death-1 (PD-1) and programmed death ligand-1(PD-L1), induce immune evasion of cancer cells. MAPK inhibitor resistance results from the increased expression of PD-L1. Immune checkpoint inhibitors, such as anti-PD-L1 or anti-PD-1, are main players in immune therapies designed to target metastatic melanoma. However, melanoma patients show low response rate and resistance to these inhibitors develops within 6-8 months of treatment. Epigenetic reprogramming, such as DNA methylaion and histone modification, regulates the expression of genes involved in cellular proliferation, immune checkpoints and the response to anti-cancer drugs. Histone deacetylases (HDACs) remove acetyl groups from histone and non-histone proteins and act as transcriptional repressors. HDACs are often dysregulated in melanomas, and regulate MAPK signaling, cancer progression, and responses to various anti-cancer drugs. HDACs have been shown to regulate the expression of PD-1/PD-L1 and genes involved in immune evasion. These reports make HDACs ideal targets for the development of anti-melanoma therapeutics. We review the mechanisms of resistance to anti-melanoma therapies, including MAPK inhibitors and immune checkpoint inhibitors. We address the effects of HDAC inhibitors on the response to MAPK inhibitors and immune checkpoint inhibitors in melanoma. In addition, we discuss current progress in anti-melanoma therapies involving a combination of HDAC inhibitors, immune checkpoint inhibitors, and MAPK inhibitors.

Keywords: HDACs; MAPK; anti-cancer drug resistance; immune checkpoint; melanoma.

FIGURE 1

The mechanisms of anti-cancer drug resistance. (A) Drug efflux by ABC transporter activity, drug inactivation, and alterations in drug targets leads to anti-cancer drug resistance. Increased DNA damage repair also leads to anti-cancer drug resistance. (B) Cancer stem cells survive anti-cancer drug treatment. Mutations (point mutations, gene amplifications etc.) in these cancer stem cells lead to anti-cancer drug resistant phenotypes. Cancer stem cells that survive anti-cancer drug treatment proliferates and lead to anti0cancer drug resistance (intrinsic resistance). CSC denotes cancer stem cell. (C) Slow-cycling drug-tolerant cells are selected on treatment by reversible epigenetic reprogramming. Further epigenetic reprogramming give rise to re-proliferating drug-resistant cells. Genetic mutation in slow-cycling drug-tolerant cells also give rise to permanent drug-resistant cells. HATs denote histone acetyl transferases. (D) Mesenchymal transition is closely related to increased drug resistance and invasiveness. MET denotes mesenchymal-epithelial transition. (E) Repeated exposure to BRAF inhibitors spurs resistance. BRAF inhibitor resistance develops from gene amplification, gene overexpression, genetic mutations, activation of signaling pathways, and upregulation of HDACs.

FIGURE 2

Classification of HDACs, functional domains, and HDAC inhibitors. TSA denotes trichostatin A. AA denotes amino acids.

FIGURE 3

Effects of HDACs on the responses to anti-cancer drugs and melanoma growth. (A) HDAC2 binds to cancer/testis antigen CAGE and directly regulates the expression of p53 to confer resistance to various anti-cancer drugs in melanoma cells (upper). (B) In Malme3M Cells, HDAC3 decreases the expression levels of HDAC6, MDR1, and tubulin β3 (upper). In Malme3M^R cells, HDAC6 interacts with tubulin β3 and confers resistance to anti-cancer drugs (lower). HDAC3 negatively regulates angiogenic potential by decreasing the expression levels of PAI-1 and VEGF (lower). (C) HDAC3 forms a negative feedback loop with miR-326 and regulates the response to anti-cancer drugs as well as the tumorigenic and metastatic potential of melanoma cells. HDAC3 forms positive feedback loops with miR-200b, miR-217, and miR-335 in Malme3M cells. These miRNAs negatively regulate the expression of CAGE. CAGE interacts with EGFR and HER2 and confers resistance to anti-cancer drugs.

FIGURE 4

The expression and regulation of PD-L1and the role of PD-L1 in anti-cancer drug resistance. (A) Regulation of PD-L1 expression occurs at different levels. HIF-1α directly increases the expression of PD-L1 by binding to the promoter sequences of PD-L1. Toll-like receptor signaling increases the expression of PD-L1 by NF-kB. PI3K/AKT/mTOR and RAS/RAF/MEK/ERK signaling increase the expression of PD-L1 by activating C-Jun and STAT3. JAK/STAT signaling activated by IFN-γ increases the expression of PD-L1. (B) Treatment of metastatic melanomas with BRAF inhibitors or a combination of BRAF/MEK inhibitors leads to immune evasion (left). Increased expression of PD-L1 increases resistance to MEK inhibitors and EGFR-TKIs (left). MEKi denotes MEK inhibitor. EGFR-TKIs denote EGFR-tyrosine kinase inhibitors. Repeated exposure to vemurafenib increases the expression level of PD-L1, which in turn confers resistance to vemurafenib (right).

FIGURE 5

HDAC inhibitors enhance sensitivity to immune checkpoint inhibitors by regulating anti-tumor immune responses. (A) PD-1/PD-L1 interactions between cancer cells and CD8⁺ T cells suppress T cell activation, leading to tumor tolerance (upper). Ipilimumab, an anti-CTLA-4 antibody, disrupts the interaction between CTLA-4 and CD80/CD86, increasing production of pro-inflammatory cytokines and inducing T cell activation. MDSCs (middle) and TAMs (lower) suppress T cell activation via PD-1/PD-L1 interactions. MDSCs inhibit the function of CD8⁺ T cells by secreting TGF-β and IL-10. (B) HDAC inhibitors enhance CTL and NK cell activity, induce M1 macrophage polarization, and suppress the immune regulatory function of MDSCs. (C) HDACs regulate the PD-L1 expression to induce CTL activity or apoptosis. BRD4 denotes bromo domain protein 4. (D) HDAC inhibitors enhance sensitivity to PD-L1 blockade by activating CD8⁺T and NK cells while inactivating MDSCs and M2 macrophages. TAAs denote tumor associated antigens.