Hypochlorous Acid: From Innate Immune Factor and Environmental Toxicant to Chemopreventive Agent Targeting Solar UV-Induced Skin Cancer

Abstract

A multitude of extrinsic environmental factors (referred to in their entirety as the 'skin exposome') impact structure and function of skin and its corresponding cellular components. The complex (i.e. additive, antagonistic, or synergistic) interactions between multiple extrinsic (exposome) and intrinsic (biological) factors are important determinants of skin health outcomes. Here, we review the role of hypochlorous acid (HOCl) as an emerging component of the skin exposome serving molecular functions as an innate immune factor, environmental toxicant, and topical chemopreventive agent targeting solar UV-induced skin cancer. HOCl [and its corresponding anion (OCl^-; hypochlorite)], a weak halogen-based acid and powerful oxidant, serves two seemingly unrelated molecular roles: (i) as an innate immune factor [acting as a myeloperoxidase (MPO)-derived microbicidal factor] and (ii) as a chemical disinfectant used in freshwater processing on a global scale, both in the context of drinking water safety and recreational freshwater use. Physicochemical properties (including redox potential and photon absorptivity) determine chemical reactivity of HOCl towards select biochemical targets [i.e. proteins (e.g. IKK, GRP78, HSA, Keap1/NRF2), lipids, and nucleic acids], essential to its role in innate immunity, antimicrobial disinfection, and therapeutic anti-inflammatory use. Recent studies have explored the interaction between solar UV and HOCl-related environmental co-exposures identifying a heretofore unrecognized photo-chemopreventive activity of topical HOCl and chlorination stress that blocks tumorigenic inflammatory progression in UV-induced high-risk SKH-1 mouse skin, a finding with potential implications for the prevention of human nonmelanoma skin photocarcinogenesis.

Keywords: chlorination stress; environmental exposure; hypochlorous acid; inflammation; skin cancer; skin exposome; solar ultraviolet radiation.

Figure 1

The Skin Exposome. A multitude of extrinsic environmental factors (referred to in their entirety as the ‘skin exposome’) impact structure and function of skin and its corresponding cellular components. The complex (i.e. additive, antagonistic, or synergistic) interactions between multiple extrinsic (exposome) and intrinsic (biological) factors are important determinants of skin health outcomes. Unfolding skin pathology can potentiate (+) the cutaneous vulnerability to further environmental exposures or intrinsic factors (fueling a positive feedback loop). (PPCP, pharmaceuticals and personal care products; SABV, sex as a biological variable). Image was created using free imaging software: smart.servier.com.

Figure 2

HOCl/OCl-: Physicochemical Properties, Innate and Environmental Origin, and Formation of HOCl-Derived Secondary Oxidants Under Physiological Conditions (A) pH-dependent speciation (HOCl versus OCl-). At physiological pH, HOCl and OCl- occur at near equimolar ratios (B) Photon Absorptivity. HOCl and its corresponding anion differ with regard to photo-absorptive properties: HOCl (λ_max = 235 nm; є = 101); OCl^- (λ_max = 292 nm; є = 365). OCl^- absorptivity covers the solar UVB (290-320 nm) and UVA-II (320-340 nm) regions. (C) Biological and environmental sources of HOCl formation and HOCl-derived secondary oxidants. Left panel: Innate immune activation causes HOCl production by specific myeloid cells under conditions of inflammation and respiratory burst via the myeloperoxidase (MPO)-catalyzed chlorination cycle that consumes H₂O₂ for Cl^- oxidation. Environmental exposure to HOCl occurs in the context of freshwater disinfection (e.g. drinking water, recreational use, etc.) and topical antimicrobial intervention. Right panel: HOCl-derived secondary oxidants. Apart from acting as potent oxidizing species, HOCl/OCl^- may be involved in a number of biochemically relevant reactions producing secondary oxidants including: (a) chloramine formation; (b) hydroxyl radical formation downstream of (i) Fe(II)-dependent Fenton or (ii) superoxide chemistry; (c) singlet oxygen formation downstream of (i) peroxide or (ii) superoxide chemistry; (d) molecular chlorine formation with involvement of Cl^- at low pH; (e) formation of nitryl chloride and chlorine nitrite upon reaction with nitrite; and (f) formation of hydroxyl and chlorine radicals as a result of UV-driven photolysis.

Figure 3

Biomolecular Targets of Chlorination Stress (A) Amino acids, peptides, and protein targets of chlorination stress. Theoretical heptapeptide [H₂N-Tyr-Trp-His-Lys-Met-Cys-Arg-COOH (1)] illustrating the range of possible amino acid modifications (2) induced by HOCl (from amino- to carboxyterminus): Dichloro-tyrosine, hydroxy-tryptophan, histidine chloramine, lysine mono- or dichloramine, methionine sulfoxide, cysteine sulfenic/sulfinic/sulfonic acid, arginine chloramine. (B) Fatty acids, lipids, and lipoproteins as targets of chlorination stress. HOCl-mediated modification of cholesterol (3) forms a number of cholesterol chlorohydrin stereoisomers: 5,6-dichloro cholesterol (4); (5R,6R)-5-chloro-6-hydroxy cholesterol (5); (5R,6R)-6-chloro-5-hydroxy cholesterol (6); (5S,6S)-6-chloro-5-hydroxy cholesterol (7), among others. For phospholipids, HOCl may either exert its effects near the head group, or at sites of unsaturation: HOCl-mediated modification of phosphatidylserine (6) results in phosphatidylserine chloramine (7), with further oxidation/decarboxylation to phosphatidyl glycoaldehyde (8). HOCl-mediated modification of phosphatidylethanolamine (9) results in the respective dichloramine (10) subsequently forming N-centered radicals acting as long-lived mediators. For simplicity, for both phospholipids each lipid moiety is stearate. Fatty acids possessing greater degrees of unsaturation are more prone to modification by HOCl: HOCl-mediated modification of arachidonic acid (12) results in the formation of arachidonic acid chlorhydrins such as 8,14-dichloro-9,15 dihydroxy arachidonic acid bis-chlorohydrin (13). (C) Nucleic acids as targets of chlorination stress. HOCl may modify DNA, RNA, and free nucleobases. (For simplicity, only HOCl-mediated modification of deoxyribosides is shown.) HOCl-modification of deoxyadenosine (14) forms 8-chlorodeoxyadenosine (15). HOCl-modification of deoxyguanosine (16) forms 8-chlorodeoxyguanosine (17). HOCl-modification of deoxycytidine (18) forms 5-chlorodeoxycytidine (19), followed by spontaneous deamination forming 5-chlorodeoxyuridine (20).

Figure 4

Biochemical, Natural Product, and Synthetic Hocl Scavengers and MPO Antagonists. (A) Endogenous chemical entities: 1. Taurine, 2. Glutathione, vitamins: 3. Vitamin B₆, 4. Pyridoxamine; neurotransmitters: 6. Serotonin, 7. Melatonin 8. Carnosine; 9. Ergothioneine. Natural products: 10. Ovothiol, 11. Gallic acid, 12. Nordihydroguaiaretic acid, 13. Quercetin. Endogenous and synthetic MPO antagonists: 5. Uric acid, 7. Melatonin, 14. AZD3241 (Verdiperstat). (B) Endogenous production of taurine as a possible sink for HOCl. First, L-cysteine is oxidized by cysteine dioxygenase (CDO1) to form L-cysteine sulfinic acid, which in turn is decarboxylated by the enzyme cysteine sulfinic acid decarboxylase (CSAD) forming hypotaurine. Hypotaurine may undergo spontaneous oxidation to form taurine, acting as a sacrificial quencher of HOCl/OCl^- forming taurine chloramine as a moderately active chloramine with attenuated chlorination reactivity. Interestingly, taurine chloramine has been demonstrated to exert control over downstream signaling pathways including downregulation of NFκB, and upregulation of KEAP1-NRF2. As such, it has been hypothesized that taurine chloramine acts through posttranslational modification of distinct amino acid residues on transcription factors, among other effects.

Figure 5

Chlorination Stress: Molecular Inducers, Signaling Pathways, Human Target Organs and Therapeutic Opportunities In Skin. (A) Direct and indirect chlorination stress inducers. 1. Hypochlorous acid, 2. Monochloramine, 3. Dichloramine, 4. Nitrogen Trichloride, 5. Trichloroisocyanuric acid. Upon chlorination of fresh water, chlorination byproducts (CBPs) are formed due to the presence of dissolved organic matter: Haloacetic acids: 6. Chloroacetic acid, 7. Dichloroacetic acid, 8. Trichloroacetic acid; Trihalomethanes: 9. Trichloromethane, 10. Bromodichloromethane, 11. Chlorodibromomethane, 12. Chlorite, all of which are subject to governmental regulation. Remarkably, numerous major chlorinated byproducts remain largely unexplored (and unregulated) such 3-chloro-4-(dichloromethyl)-5-hydroxy-5H-furan-2-one (13, commonly referred to as ‘mutagen X’). Additionally, pharmaceuticals and personal care products (PPCPs) introduced into the water supply are subject to HOCl mediated chlorination. As shown, the common UVA-sunscreen avobenzone (14) is chlorinated to produce a dichloro-species (15). (B) Chlorination stress signaling pathways. It has been demonstrated that chlorination stress may impact genotoxic, proteotoxic, inflammation and redox responses including p53, Keap1/NRF2, IKK/NFκB, and AP-1. (C) Human target organs of chlorination stress. Chlorination stress impacts multiple organ systems causing specific functional outcomes as discussed. (D) HOCl: Therapeutic opportunities in skin. HOCl may be used as a topical agent for therapeutic induction of chlorination stress in the context of antimicrobial intervention, impaired barrier function, wound healing, pruritus, atopic dermatitis, psoriasis, skin cancer, and prevention of photocarcinogenesis.