Redox balance is important for the homeostasis of normal cells, but also for the proliferation, progression, and survival of cancer cells. Both oxidative and reductive stress can be harmful to cells. In contrast to oxidative stress, reductive stress and the therapeutic opportunities underlying the mechanisms of reductive stress in cancer, as well as how cancer cells respond to reductive stress, have received little attention and are not as well characterized. Therefore, there is recent interest in understanding how selective induction of reductive stress may influence therapeutic treatment and disease progression in cancer. There is also the question of how cancer cells respond to reductive stress. Selenium compounds have been shown to have chemotherapeutic effects against cancer and their anticancer mechanism is thought to be related to the formation of their metabolites, including hydrogen selenide (H2Se), which is a highly reactive and reducing molecule and possible can generate the reductive stress in cells. Here we show the recent reports on the molecular mechanism of how cells recognize and respond to oxidative and reductive stress and selenium compounds with well documented hydrogen selenide release, as compounds possibly useful in study of redox homeostasis by the selective induction of reductive stress in cells and in vivo, as well as possibly their utility in anti-cancer therapy.
When the equilibrium between cellular oxidation and reduction potential is shifted in favor of the oxidizing fraction, a phenomenon called oxidative stress is observed. It occurs when the cell’s ability to defend itself against antioxidants is overwhelmed by the massive production of pro-oxidants such as ROS/RNS/RSS (reactive oxygen species/reactive nitrogen species/reactive sulfur species).[5][6] Redox imbalance towards pro-oxidative conditions is involved in cancer development as oxidative stress causes genomic instabilities, favoring cancer metastasis and progression
On the other hand, if the cellular redox balance is shifted towards reduction, reductive stress is observed, defined as excessive accumulation or production of reducing agents, such as GSH, NADH, NADPH, as well as the free thiol groups in proteins present in cysteine residues. These cysteines in reduced form, which are present in excess in proteins, can lead to the activation of the “Unfolded Protein Response” (UPR),[7] which impairs the activity of endogenous oxidoreductases. In addition, reductive stress lowers cellular ROS levels below their physiological levels, disrupting their signaling functions. From another perspective, reductive stress may also promote the production of ROS, as redox couples can reduce O2 to ●O2 − in an oxygen environment.[3][8][9]
Imbalance of redox status may result from contact with infectious factors or certain diseases. Oxidative stress plays a role in the development of many human diseases, including cancer, as well as aging. Different levels of redox balance affect the regulation of cellular processes in tumors in different ways.[10] Cancer cells generally exhibit a more oxidized environment, meaning that their redox balance is shifted toward higher levels of ROS, which plays a critical role in tumor development, e.g., initiation, progression, migration, invasion, and metastasis. Moderate concentration of ROS promotes the proliferation and metastasis of cancer cells, favoring tumor progression. This is because higher levels of ROS may be the result of more intense oxidative phosphorylation in mitochondria, which means higher ATP production, since cancer cells require a lot of energy for growth. However, oxidative stress, when enormously high, is toxic through oxidative damage to intracellular biomacromolecules in cancer cells.[10] Depending on the stage of cancer development, cells are able to adapt to high ROS levels via altering their metabolism through various mechanisms. These include the activation of antioxidant transcription factors, the elevation of NADPH via the pentose phosphate pathway (PPP), and reductive glutamine and folate metabolism, all of which allow cancer cells to survive.[11] Several antioxidant enzymes and molecules are overexpressed under oxidative stress conditions in cancer and often this metabolic reprogramming leads to a state of ‘pseudohypoxia’.[12]
On the other hand, reductive stress impairs cellular signaling and function and has been associated with cancer, diabetes, and cardiomyopathy.[13] Reductive stress can lead to disruption of mitochondrial homeostasis, decrease metabolism, influence resistance to anti-cancer therapies, alter the formation of disulfide bonds in proteins leading to activation of UPR/ER stress,[3] and finally be harmful to cells. Redox biology, therefore, seeks to understand the mechanisms of regulation and maintenance of homeostasis, as well as the processes that are perturbed in various disease progress where oxidative or reductive stress is a problem.[14]
Chemotherapeutic agents are designed to kill cancer cells because, as antioxidants, they usually act by shifting the redox balance toward reductive stress, which paradoxically can force cells to produce an excess of ROS and, consequently generate oxidative stress.[3][9][15] This mechanism resembles an uncontrolled amplification of cellular antioxidant signaling leading to reductive stress [10]. However, under hypoxic conditions, which are a feature of the microenvironment of solid tumors, only a small amount of O2 is present, limiting the production of ROS.[16]. Therefore, it is of great interest to understand how selective induction of reductive stress may influence therapeutic treatment and disease progression in cancer, particularly under conditions of limited O2 levels. Consequently, it is important to recognize that the effects of chemotherapeutic agents may differ under different oxygen conditions. Over the years, many selenium-containing compounds have been investigated as anticancer chemotherapeutic agents, but the specific mechanism of their anticancer activity has not been fully elucidated.[17][18][19][20] It has been suggested that their metabolites, such as methylselenol (CH3SeH) and/or hydrogen selenide (H2Se), may be responsible for their anticancer effects. What is important, cancer cells have been found to be significantly more sensitive than normal cells to the antiproliferative effects of many selenium-containing compounds.[17] Hydrogen selenide is a highly reactive and reducing molecule and thus can induce a reductive environment in cells. Therefore, selenium compounds that are H2Se donors may selectively induce reductive stress and be useful in anticancer and redox homeostasis research.
Unlike oxidative stress, reductive stress is a phenomenon that is not sufficiently described. Little attention has been paid to reductive stress and the therapeutic possibilities underlying the mechanisms of reductive stress in cancer, as well as how cancer cells respond to reductive stress, and there are many conflicting hypotheses in the literature.[2][9] This concept was first described by Gores et al. in 1989.[21] The authors performed experiments in which they induced hypoxia using chemicals and blocked mitochondrial respiration and ATP production in rat hepatocytes. They concluded that inhibition of respiration leads to “reductive stress”, which can contribute to lethal cellular damage due to low oxygen levels and the formation of toxic oxygen species. These findings called for further studies on this new concept and the mechanism of reductive stress.
On the other hand, the term oxidative stress was first used in 1970 by Paniker et al. during studies on GSH/GSSG pairs in H2O2-stimulated normal and GR-deficient human erythrocytes.[22] Since then, the number of reports of oxidative stress in the literature has increased significantly, from 14 in 1980 and 242 in 1990 to 12,356 in 2010 and 29,069 in 2022 (based on PubMed).
Se is essential for cell survival at relatively low concentrations (about 55 µg/day) but is toxic at high doses (> 400µg/day),[17] so the complex role of H2Se in human cancers, require the development of well-defined H2Se donors with controllable release properties. Inorganic selenide salts (e.g., Na2Se and NaHSe) are short-living H2Se donors, that not only can be toxic [64][65] but also fail to mimic slow and well-regulated H2Se formation in vivo. Organic Se compounds as potential H2Se donors are generally considered safer (lower toxicity) than inorganic Se salts.[20]
There is not known many well-characterized and controllable H2Se donors that would act under physiological conditions, and the study of H2Se biology encountered technical difficulties because there was no reliable assay for direct H2Se quantification. Here, we focus on compounds whose mechanism of action is based on hydrogen selenide, and formation of this product is well documented. These compounds are collected in Table 1.
Table 1
. Selenium compounds as H
2
Se donors and studies of their biological effects.
Selenium compound | Detection method of H2Se release | Mechanism of H2Se release | Biological model | Ref |
Na2SeO3 (under clinical trials) [66] |
Fluorescence imaging: NIR-H2Se Fluorescence imaging: Hcy-H2Se Fluorescence imaging: Mito-N-D-MSN (nanoprobes mitochondria-targeted) |
Enzymatic: Grx (GSH), Trx, TrxR (NADPH) |
HepG2 cells (cytotoxicity IC50=5 μM after 24h, reductive stress; H2Se release), mice HepG2 cells (H2Se release) HepG2 cells (H2Se release) |
[68] [69]
|
TDN1042 (P=Se motif) |
31P and 77Se NMR and electrophilic trapping reagent |
Acidic conditions |
no data |
[70] |
2AP-PSe, Cat-PSe (P=Se motif) |
31P and 77Se NMR and electrophilic trapping reagent; colorimetric detection with NBD-CI |
pH 7.2 |
HeLa cells (antioxidant activity) |
[71] |
selenocyclopropenones and arylselenoamides (C=Se motif) |
H2Se-selective gas detector; electrophilic trapping reagent and HRMS analysis; Cy7-CI trapping; fluorescence imaging (NIR-H2Se) |
Cys at pH 7.4 |
HeLa cells (Cys-mediated H2Se release) |
[72] |
dGMPSe (2'-deoxyguanosine selenophosphate) |
Fluorescence imaging: SF7 |
Enzymatic: HINT1 |
HeLa cells (cytotoxicity IC50=8 µM after 24h; H2Se release) |
[73] |
γ-keto selenides |
Trapping reagent and HRMS (high-resolution mass spectrometry) |
neutral to slightly basic conditions |
HeLa and HCT116 cells (cytotoxicity IC50= 3.7–10.6 µM ) |
[74] |
Many pathological conditions, including cancer, may be due to an imbalance of oxidative and reductive byproducts. To protect their cell populations from damage, organisms have conserved stress response pathways that recognize and mitigate a variety of adverse conditions. Because of their rapid activation, stress responses should be terminated soon after cellular homeostasis is restored. Otherwise, the cell is threatened with dire consequences, even death.
Reductive stress is not as well-characterized of a phenomenon as oxidative stress. It has only recently gained more attention, particularly because of its potential importance and therapeutic intervention in cancer. Understanding the regulation of responses to reductive stress both in normal cells and in the tumor microenvironment can provide new therapeutic approaches for cancer treatment and anticancer drug resistance. Therefore, compounds that can selectively and controllably induce reductive stress in cells are being sought. Selenium-containing compounds which are H2Se donors meet these requirements.
However, there are few detailed studies on the chemical biology of H2Se, and there are not many reports about the essential physiological functions of H2Se, the cellular objects, and the therapeutic perspectives. The lack of clarity on these key questions was largely due to the lack of small molecule donors that could effectively enhance the bioavailability of H2Se through continuously releasing the unstable biomolecule under physiologically relevant conditions. Several H2Se donors have been developed for which H2Se release has been well documented, although not all of them have been demonstrated to show anticancer activity in the cellular model. It is too early to compare their efficacy in cancer therapy with other selenium-containing compounds as well the mechanisms responsible for their toxicity in cells as only Na2SeO3 was studied so deeply (apoptosis under oxidative stress and autophagy under reductive stress).
In conclusion, future studies are needed to show the utility of selenium compounds as H2Se donors in cancer therapy and in the selective induction of reductive stress in cells and in vivo, as few data are available to date.