Targeting the epigenome in malignant melanoma: Facts, challenges and therapeutic promises

Malignant melanoma is the most lethal type of skin cancer with high rates of mortality. Although current treatment options provide a short-clinical benefit, acquired-drug resistance highlights the low 5-year survival rate among patients with advanced stage of the disease. In parallel, the involvement of an aberrant epigenetic landscape, (e.g., alterations in DNA methylation patterns, histone modifications marks and expression of non-coding RNAs), in addition to the genetic background, has been also associated with the onset and progression of melanoma. In this review article, we report on current therapeutic options in melanoma treatment with a focus on distinct epigenetic alterations and how their reversal, by specific drug compounds, can restore a normal phenotype. In particular, we concentrate on how single and/or combinatorial therapeutic approaches have utilized epigenetic drug compounds in being effective against malignant melanoma. Finally, the role of deregulated epigenetic mechanisms in promoting drug resistance to targeted therapies and immune checkpoint inhibitors is presented leading to the development of newly synthesized and/or improved drug compounds capable of targeting the epigenome of malignant melanoma.

Oxidative stress based-biomarkers in oral carcinogenesis: how far have we gone?

Oral cancer accounts for 2-3% of all malignancies and according to the World Health Organization (WHO) is the fifth most common cancer worldwide. On the other hand, "oxidative stress" implies a cellular state whereby reactive oxygen species (ROS) production exceeds its metabolism resulting in excessive ROS accumulation and overwhelmed cellular defenses. Such a state has been shown to be involved in the multistage process of human carcinogenesis (including oral cancer) via many different mechanisms. Amongst them are ROS-induced oxidative modifications on major cellular macromolecules like DNA, proteins and lipids with the resulting byproducts being involved in the pathophysiology of human oral malignant and pre-malignant lesions. Throughout this manuscript, we review the current state of knowledge on the role of these oxidative-modified cellular byproducts in serving as reliable biomarkers for oral cancer detection, prognosis and diagnosis.

Biomarkers of protein oxidation in human disease

Oxidative stress is caused by an imbalance between the production of reactive species of oxygen and nitrogen (RS) and the ability to either detoxify the reactive intermediates produced or repair the resulting damage. Ultimately, oxidative stress conveys the alteration in cellular function caused by the reaction of RS with cellular constituents. Oxidative stress has been extensively reported to participate in the progression of a variety of human diseases including cancer, neurodegenerative disorders and diabetes. Oxidation of proteins is thought to be one of the major mechanisms by which oxidative stress is integrated into cellular signal transduction pathways. Thus, recent research efforts have been aimed to identify the role of specific oxidative protein modifications in the signal transduction events mediating the etiology of human diseases progression. The identification of these oxidative modifications has also raised the possibility of using this knowledge to develop new methods to diagnose diseases before they are clinically evident. In this work, we summarize the mechanisms by which RS generate distinct oxidative modifications. Furthermore, we also review the potential of these oxidative modifications to be used as early biomarkers of human disease.

Sulfur-containing compounds in protecting against oxidant-mediated lung diseases

Over 95% of the oxygen we metabolize undergoes a four-electron reduction to produce two molecules of water. Whenever electrons escape from the mitochondrial electron-transport chain and pass directly onto oxygen, oxidants that can cause cytotoxicity are generated. The lung being constantly exposed to atmospheric oxygen is more susceptible to oxidant-induced cellular damage. For instance, increased generation of oxidants is implicated in many pulmonary pathological conditions including emphysema, adult respiratory distress syndrome, idiopathic pulmonary fibrosis and asthma. Sulfur is an essential major inorganic element with a recently described protective cellular role. One of its many biologically important functions is the formation of disulfide bridges between two cysteine molecules thus stabilizing protein conformation. Also, it provides the site for attachment and transfer of 1-C methyl groups via formation of S-adenosylmethionine, and most importantly it is an essential constituent of the antioxidant tripeptide, glutathione, and vitamins like thiamin and biotin. However, its protective role emanates from its antioxidant properties in the context of sulfur-containing compounds (S-adenosylmethionine, cysteine, taurine, glutathione etc) that are known to act in protecting against oxidant-induced lung disease. The efficacy of these sulfur-containing compounds in scavenging oxidants directly or indirectly and consequently protecting against lung diseases is discussed herein.

The central role of glutathione in the pathophysiology of human diseases

Reduced glutathione (L-gamma-glutamyl-L-cysteinyl-glycine, GSH) is the prevalent low-molecular-weight thiol in mammalian cells. It is formed in a two-step enzymatic process including, first, the formation of gamma-glutamylcysteine from glutamate and cysteine, by the activity of the gamma-glutamylcysteine synthetase; and second, the formation of GSH by the activity of GSH synthetase which uses gamma-glutamylcysteine and glycine as substrates. While its synthesis and metabolism occur intracellularly, its catabolism occurs extracellularly by a series of enzymatic and plasma membrane transport steps. Glutathione metabolism and transport participates in many cellular reactions including: antioxidant defense of the cell, drug detoxification and cell signaling (involved in the regulation of gene expression, apoptosis and cell proliferation). Alterations in its concentration have also been demonstrated to be a common feature of many pathological conditions including diabetes, cancer, AIDS, neurodegenerative and liver diseases. Additionally, GSH catabolism has been recently reported to modulate redox-sensitive components of signal transduction cascades. In this manuscript, we review the current state of knowledge on the role of GSH in the pathogenesis of human diseases with the aim to underscore its relevance in translational research for future therapeutic treatment design.

On the mechanism of ionic regulation of apoptosis: would the Na+/K+-ATPase please stand up?

Apoptosis is an active process with distinct features including loss of cell volume, chromatin condensation, internucleosomal DNA fragmentation, and apoptotic body formation. Among the classical characteristics that define apoptosis, the loss of cell volume has become a very important component of the programmed cell death process. Changes in cell volume result from alterations in the homeostasis of ions and in particular the movement of Na+ and K+ ions. Most living cells have a high concentration of intracellular K+ and a low concentration of intracellular Na+. This is in contrast to the outside of the cell, where there is a high concentration of extracellular Na+ and a low concentration of extracellular K+. Thus a concentration gradient exists for the loss and gain of intracellular K+ and Na+, respectively. This gradient is maintained through the activity of various ionic channels and transporters, but predominantly the activity of the Na+/K+-ATPase. During apoptosis, there is compelling evidence indicating an early increase in intracellular Na+ followed by a decrease in both intracellular K+ and Na+ suggesting a regulatory role for these cations during both the initial signalling, and the execution phase of apoptosis. Recent studies have shown that the Na+/K+-ATPase is involved in controlling perturbations of Na+ and K+ homeostasis during apoptosis, and that anti-apoptotic Bcl-2 and Bcl-XL molecules influence these ionic fluxes. Finally, understanding the regulation or deregulation of ionic homeostasis during apoptosis is critical to facilitate the treatment of cardiovascular, neurological, and renal diseases where apoptosis is known to play a major role.