Epigenetics in Melanoma Development and Drug Resistance

Melanomas, which originate from melanocytic cells, mainly develop in the skin but can also arise at other body sites. The disease accounts for approximately 90% of deaths related to cutaneous tumors with late stage metastatic melanoma having a very poor prognosis of 6–9 month median survival for untreated patients. Research in the last decades resulted in ground-breaking discoveries of melanoma genetics and biology. High frequency mutations in genes like BRAF, NRAS and KIT, which lead to hyper-activation of the MAPK signaling pathway, drive melanoma progression. Targeting the MAPK signaling pathway has successfully been translated into effective therapies that significantly improve patient survival. Despite the unquestionable importance of such genetic events, the involvement of epigenetic alterations for melanoma development, and resistance to aforementioned therapies is becoming increasingly apparent. In this chapter, epigenetic alterations commonly found in melanoma are introduced, with a focus on histone and DNA modifications and their relevance for melanoma development, progression and therapy response. Detailed knowledge about this emerging aspect of melanoma research will help to understand the plastic nature of melanoma and set the foundation for novel treatment strategies that target aberrant gene regulation on genetic and epigenetic levels

Tumor cell-intrinsic phenotypic plasticity facilitates adaptive cellular reprogramming driving acquired drug resistance

The enthusiasm about successful novel therapeutic strategies in cancer is often quickly dampened by the development of drug resistance. This is true for targeted therapies using tyrosine kinase inhibitors for EGFR or BRAF mutant cancers, but is also an increasingly recognized problem for immunotherapies. One of the major obstacles of successful cancer therapy is tumor heterogeneity of genotypic and phenotypic features. Historically, drivers for drug resistance have been suspected and found on the genetic level, with mutations either being pre-existing in a subset of cancer cells or emerging de novo to mediate drug resistance. In contrast to that, our group and others identified a non-mutational adaptive response, resulting in a reversible, drug tolerant, slow cycling phenotype that precedes the emergence of permanent drug resistance and is triggered by prolonged drug exposure. More recently, studies described the importance of initially reversible transcriptional reprogramming for the development of acquired drug resistance, identified factors important for the survival of the slow cycling phenotype and investigated the relationship of mutational and non-mutational resistance mechanisms. However, the connection and relative importance of mutational and adaptive drug resistance in relation to the in vitro models at hand and the clinically observed response patterns remains poorly defined. In this review we focus on adaptive intrinsic phenotypic plasticity in cancer cells that leads to the drug tolerant slow cycling state, which eventually transitions to permanent resistance, and propose a general model based on current literature, to describe the development of acquired drug resistance

Epigenetics and metabolism at the crossroads of stress-induced plasticity, stemness and therapeutic resistance in cancer

Despite the recent advances in the treatment of cancers, acquired drug resistance remains a major challenge in cancer management. While earlier studies suggest Darwinian factors driving acquired drug resistance, recent studies point to a more dynamic process involving phenotypic plasticity and tumor heterogeneity in the evolution of acquired drug resistance. Chronic stress after drug treatment induces intrinsic cellular reprogramming and cancer stemness through a slow-cycling persister state, which subsequently drives cancer progression. Both epigenetic and metabolic mechanisms play an important role in this dynamic process. In this review, we discuss how epigenetic and metabolic reprogramming leads to stress-induced phenotypic plasticity and acquired drug resistance, and how the two reprogramming mechanisms crosstalk with each other

Acetylsalicylic acid governs the effect of sorafenib in RAS- mutant cancers

Identify and characterize novel combinations of sorafenib with anti-inflammatory painkillers to target difficult to treat RAS-mutant cancer.The cytotoxicity of acetylsalicylic acid (aspirin) in combination with the multikinase inhibitor sorafenib (Nexavar) was assessed in RAS-mutant cell lines in vitro. The underlying mechanism for the increased cytotoxicity was investigated using selective inhibitors and shRNA-mediated gene knockdown. In vitro results were confirmed in RAS-mutant xenograft mouse models in vivo.The addition of aspirin but not isobutylphenylpropanoic acid (ibruprofen) or celecoxib (celebrex) significantly increased the in vitro cytotoxicity of sorafenib. Mechanistically, combined exposure resulted in increased BRAF/CRAF dimerization and the simultaneous hyper-activation of the AMPK and ERK pathways. Combining sorafenib with other AMPK activators, like metformin or A769662, was not sufficient to decrease cell viability due to sole activation of the AMPK pathway. The cytotoxicity of sorafenib and aspirin was blocked by inhibition of the AMPK or ERK pathways through shRNA or via pharmacological inhibitors of RAF (LY3009120), MEK (trametinib) or AMPK (compound C). The combination was found to be specific for RAS/RAF-mutant cells and had no significant effect in RAS/RAF-wild type keratinocytes or melanoma cells. In vivo treatment of human xenografts in NSG mice with sorafenib and aspirin significantly reduced tumor volume compared to each single-agent treatment alone.Combined sorafenib and aspirin exerts cytotoxicity against RAS/RAF-mutant cells by simultaneously affecting two independent pathways and represents a promising novel strategy for the treatment of RAS-mutant cancers

Magnolol induces cell death through PI3K/Akt-mediated epigenetic modifications boosting treatment of BRAF- and NRAS-mutant melanoma

Most BRAF-mutant melanoma patients experience a fulminate relapse after several months of treatment with BRAF/MEK inhibitors. To improve therapeutic efficacy, natural plant-derived compounds might be considered as potent additives. Here, we show that magnolol, a constituent of Magnolia officinalis, induced G1 arrest, apoptosis and cell death in BRAF- and NRAS-mutant melanoma cells at low concentration, with no effect in BRAF- and NRAS wild-type melanoma cells and human keratinocytes. This was confirmed in a 3D spheroid model. The apoptosis-inducing effect of magnolol was completely rescued by activating Akt suggesting a mechanism relying primarily on Akt signaling. Magnolol significantly downregulated the PI3K/Akt pathway which led to a global decrease of the active histone mark H3K4me3. Alongside, the repressive histone mark H3K9me3 was increased as a response to DNA damage. Magnolol-induced alterations of histone modifications are reversible upon activation of the Akt pathway. Magnolol-induced a synergistic effect in combination with either BRAF/MEK inhibitors dabrafenib/trametinib or docetaxel at a lower concentration than usually applied in melanoma patients. Combination of magnolol with targeted therapy or chemotherapy also led to analogous effects on histone marks, which was rescued by Akt pathway activation. Our study revealed a novel epigenetic mechanism of magnolol-induced cell death in melanoma. Magnolol might therefore be a clinically useful addition to BRAF/MEK inhibitors with enhanced efficacy delaying or preventing disease recurrence