The Role of ABC Transporters in Skin Cells Exposed to UV Radiation

ABC transporters are expressed in skin cells to protect them against harmful xenobiotics. Moreover, these transmembrane proteins have a number of additional functions that ensure skin homeostasis. This review summarizes the current knowledge about the role of specific ABC proteins in the skin, including multi-drug resistance transporters (MDR1/3), the transporter associated with antigen processing 1/2 (TAP1/2), the cystic fibrosis transmembrane conductance regulator (CFTR), sulfonylurea receptors (SUR1/2), and the breast cancer resistance protein (BCRP). Additionally, the effect of UV radiation on ABC transporters is shown. The exposure of skin cells to UV radiation often leads to increased activity of ABC transporters—as has been observed in the case of MDRs, TAPs, CFTR, and BCRP. A different effect of oxidative stress has been observed in the case of mitochondrial SURs. However, the limited data in the literature—as indicated in this article—highlights the limited number of experimental studies dealing with the role of ABC transporters in the physiology and pathophysiology of skin cells and the skin as a whole. At the same time, the importance of such knowledge in relation to the possibility of daily exposure to UV radiation and xenobiotics, used for both skin care and the treatment of its diseases, is emphasized.


Introduction
Skin, as the most external organ of the human body, is responsible for creating a physical and biochemical barrier to protect the body from harmful environmental factors. Moreover, skin also takes part in the constant interaction between the body and the environment, thus playing a pivotal role in maintaining body homeostasis [1]. For this reason, the skin has developed a number of adaptations that make it easier to perform this role. One of them is its layered structure consisting of various cell types, including keratinocytes, which are the main cells in the outermost layer, i.e., the epidermis, and fibroblasts, i.e., the basic cells of the next layer-the dermis. Despite the advanced differentiation conducted by the skin, keratinocytes and fibroblasts, as well as other less numerous cells present in the skin (melanocytes, nerve, or immune cells), are prepared for the metabolism of xenobiotics which are delivered directly from the external environment or from the bloodstream. However, the level and activity of xenobiotic-metabolizing enzymes in the skin cells are generally much lower than those in, e.g., the liver or the intestine; therefore, it is suggested that the activity of membrane transporters, located in the phospholipid structures of skin cell membranes, is responsible for the influence of exogenous substances on the functioning of the skin and the whole organism [2][3][4][5]. The best-known membrane proteins involved in this process are ABC (ATP-binding cassette) transporters, whose expression has been observed in skin cells such as keratinocytes [6], fibroblasts [7], and melanocytes [8].

ABC Transporters
ABC transporters are expressed in many epithelial and endothelial barrier tissues/cells, limiting the penetration of the xenobiotics between the body's compartments. They are

ABC Transporters in the Skin
The expression and activity of ABC transporters in skin cells is indis

ABC Transporters in the Skin
The expression and activity of ABC transporters in skin cells is indisputably linked to their role in skin protection against harmful xenobiotics and the oxidative stress that they induce [6]. However, the current knowledge concerning these proteins allows for the conclusion that the activity of ABC transporters is dependent on numerous factors and that they have a much wider range of action in relation to skin cells ( Figure 2

ABC Transporters in the Skin
The expression and activity of ABC transporters in skin cells is indisputably linked to their role in skin protection against harmful xenobiotics and the oxidative stress that they induce [6]. However, the current knowledge concerning these proteins allows for the conclusion that the activity of ABC transporters is dependent on numerous factors and that they have a much wider range of action in relation to skin cells ( Figure 2).

Activation and Suppression According to Oxidative Conditions
It is generally assumed that the appearance of an agonistic xenobiotic in the cytoplasm of the cell activates the ABC transporters and induces the efflux of this potentially harmful compound outside the cell [6]. Exposure to these compounds is very often accompanied by oxidative stress (the oxidative effect of these compounds or a side effect of their metabolism). It has not been shown that free radicals formed at the time of exposure directly affect the functioning of ABC transporters; however, there are many pathways linking oxidative stress with these transporters [17,18]. Reactive oxygen species (ROS) conjugated with GSH, glucuronide, and sulphate only are agonistic molecules for ABC transporters [19]. However, in the case of lung cancer cells, it was found that low doses of anticancer drugs, by inducing a moderate increase in ROS levels (approximately a 3-4-fold increase of the control levels), promote a defense response which results in an increase in the expression of ABC proteins, thus providing these cells with drug resistance [18]. This might be connected with an ROS-induced activity of transcription factors, such as nuclear factor-κB (NFκB), responsible for the formation of inflammation, and nuclear factor E2-related factor-2 (Nrf2), responsible for the biosynthesis of antioxidant proteins [20]. Therefore, NFκB increases the expression of ABC transporters during inflammation [21,22], while Nrf2 initiates ABC transporters in response to oxidative stress [23,24].
Oxidative stress arising from, e.g., exposure of cells/organism to pathogenic factors (exogenous and endogenous), often leads to the activation of kinases involved in intracellular signal transduction, including mitogen-activated protein kinases (MAPKs) [25]. As a result, numerous proteins are phosphorylated, including ABC transporters. The data in the literature indicate that the phosphorylation of ABC proteins is often a constitutive element of the functioning of transporters, and is necessary for their full activity, especially under oxidative conditions [26]. Moreover, MAPKs activation by ROS additionally induces NFκB and Nrf2 activity, thus favoring the expression of ABC transporters [24,27].
It is known that, while oxidative stress activates most ABC transporters, antioxidants such as vitamin C, flavonoids, or phytocannabinoids are able to suppress their activity [28][29][30][31]. Due to the recent increased public interest in aging and disease prevention, the use of herbal preparations, especially those containing high doses of natural antioxidants, has become very popular, raising the potential for interactions with the implemented drug therapies. In relation to the influence of antioxidants on ABC transporters, their action is not only based on ROS scavenging, but they are also able to inhibit drug interaction with ABC transporters during therapy, as well as prevent nucleotide hydrolysis, thus limiting the access of transporters to the energy from ATP hydrolysis [30]. Therefore, antioxidants could be considered as potential modulators of multidrug resistance and as therapeutic agents to suppress ABC transporter activity under drug-induced oxidative conditions.

Main Functions in the Skin
It has been reported that ABC transporters in the skin have different intensities of distribution in the epidermis compared to the dermis. For example, MRP1 has a strong expression in whole skin specimens and the dermis, and a weak expression in the epidermis [32]. This leads to the uptake of compounds from the epidermal compartment and their secretion into the deeper layers of the skin. Moreover, by coordinating the efflux of steroid hormones from normal human epidermal keratinocytes, ABC transporters ensure proper hormonal balance in the skin [33]. ABC transporters' expression in human skin biopsies has been correlated with sweat metabolites, which indicates their role in sweat secretion and, thus, an indirect effect on body thermoregulation [34]. By removing contact allergens and exogenous compounds, such as fragments of pathogens, outside the cell, ABC transporters also play an important role in the migration of Langerhans cells and help maintain a healthy immune response in the skin [35,36]. ABC transporters also translocate lipid metabolites between cell organelles in order to regulate lipid homeostasis and prevent disease development [37]. It has been found that a dysfunction of ABCA12, which is responsible for the translocation of glucosylceramides (GlcCer) into lamellar granules, leads to a disturbance of the skin's barrier functions and is even co-responsible for the development of a rare skin disease called harlequin ichthyosis [38].
ABC transporters also have play a significant role in melanoma, as well non-melanoma skin cancers, where the expression of these molecules is always present in a high level compared to non-cancerous human skin cells [39][40][41][42]. The exact mechanism of the ABC proteins expression in skin cancer cells is not known; however, ABC-dependent drug efflux in these cells leads to cancer multidrug resistance by decreasing intracellular drug accumulation [41,[43][44][45]. Moreover, ABC transporters additionally protect the mitochondrial genome of melanoma cells against drug-induced DNA damage [43]. It has also been observed that high levels of ABC transporters in melanoma cells favors their migration and invasion, being a prognosis of numerous metastases and failure of anticancer therapy [8,46].
The presented examples are only a fragment of ABC transporters' role in the skin that is currently known. However, it can already be seen at this stage how important they are in the functioning of cells both in normal physiology and in pathological states ( Figure 2). Therefore, due to the constant exposure of skin cells to the UV radiation naturally contained in the Sun's rays, the following question arises: what effect does UV radiation have on the expression and activity of these proteins in skin cells?

UV Radiation and ABC Transporters' Activity
The UV radiation that reaches the surface of the Earth (UVA and UVB) is one of the most common harmful environmental factors to which cells of the human skin are daily exposed. So far it is known that UV radiation directly induces oxidative stress, disturbs the cellular lipid metabolism, leads to disorders of the structure and function of proteins, and also damages DNA molecules, thus disrupting the functioning of the exposed cells and even leading to cancer formation [47][48][49]. However, the growing public awareness of these risks means that those substances that protect or reduce the effects of UV radiation are used increasingly often [50]. For effective protection, it is often necessary for these molecules to penetrate inside the cells without their being simultaneously pumped out, e.g., by transmembrane transporters. It has been found that UV radiation (UVA and UVB) significantly increases the permeability of skin cell membranes, both through their oxidative damage and the activation of transmembrane proteins [51]. This has also been observed in the case of transporters from the ABC family [29,52]. However, some data show that UVB radiation, by impairing the generation of ATP, limits its pool in the cell, thus inhibiting the activity of ATP-dependent transporters [53]. An overview of the effects of UV radiation on the basic ABC transporters in skin cells is provided below (Figure 3).

Multi-Drug Resistance Transporters (MDR1/3)
The physiological functions of MDRs, especially in the skin, are poorly defined, while, under stress conditions, these are well-known transporters that mediate the efflux of chemotherapeutic agents from the intracellular space, thus inducing drug resistance [54]. It is known that the activity of MDRs in skin cells can be stimulated, e.g., by factors that induce oxidative stress [54]. Moreover, the induced activity of MDRs in the skin stimulates the migration of mononuclear phagocytes into lymphatic vessels, a process necessary for the body's inflammatory response to a pro-inflammatory factor in the skin [36]. The relationship between UV radiation and potential substrates of ABC transporters, mentioned in Table 1, is also significant for the functioning of the skin cells. It is suggested that, UV-induced protein oxidation, leading to decreased free thiol groups in peptides, may lead to a reduction in ABC transporter activity [54,55]. On the other hand, UV radiation, by decreasing cholesterol synthesis in keratinocytes [56], and increased pumping of it out from cells by the membrane transporters [57], significantly impair the structure of the cell membrane. UV radiation also has a huge impact on lipid peroxidation [58]; however, in the case of substrates for ABC transporters, they are non-oxidized compounds which are metabolized after transmembrane transport, especially in the case of peroxisomal ABC proteins [59,60].

Multi-Drug Resistance Transporters (MDR1/3)
The physiological functions of MDRs, especially in the skin, are poorly defined, while, under stress conditions, these are well-known transporters that mediate the efflux of chemotherapeutic agents from the intracellular space, thus inducing drug resistance [54]. It is known that the activity of MDRs in skin cells can be stimulated, e.g., by factors that induce oxidative stress [54]. Moreover, the induced activity of MDRs in the skin stimulates the migration of mononuclear phagocytes into lymphatic vessels, a process necessary for the body's inflammatory response to a pro-inflammatory factor in the skin [36]. On the other hand, MDRs pump glucocorticosteroids out of skin cells, which is a particularly unfavorable effect during therapies for immune skin diseases, including psoriasis [55]. In the case of psoriasis, the effect of a frequent use of UV radiation as a therapeutic factor or a supporting pharmacotherapy is particularly noteworthy [56]. As reported for both health and psoriatic skin cells, the activity of MDRs during combination therapy (pharmacotherapy with phototherapy) is induced by the used therapeutic chemical, as well as UV radiation [29,52]. On the other hand, UV radiation causes an increase in the total level of oxidized proteins in cancer cells (human colon cancer cells) with enhanced expression of MDRs, compared to MDR non-stimulated cells [57]. However, these oxidative modifications do not initiate DNA repair [57]. Moreover, MDRs are sensitive to different wavelengths of UV radiation to various degrees [58]. In the case of leukemia cell line, it has been found that UVA impairs the activity of MDRs, which has not been observed in the case of UVB or UVC [58]. In all of these treatments, UV doses do not alter cell viability; hence, the authors suggest that MDRs are a physical target for oxidative damage induced directly by UVA [58]. However, the exact mechanism of the influence of UV on MBRs in skin cells is still not fully understood.

Transporter Associated with Antigen Processing 1/2 (TAP1/2)The physiological
TAPs, unlike the other ABC transporters, are proteins involved in the pumping of degraded cytosolic peptides across the endoplasmic reticulum into the membranehistocompatibility complex (MHC) class I [59]. As a result, MHC displays its antigenic cargo to cytotoxic T cells on the cell surface; therefore, virus-infected or malignantly-transformed cells can be eliminated. It also induces migration and activation of immune cells (including Langerhans cells) in the skin, as well as effector functions, such as cytokine production and cytotoxicity, and may be used in epicutaneous vaccination approaches [60,61]. Hence, a disruption of the proper functioning of this transporter may lead to skin dysfunction and even disease development [62]. So far, differences in the structure of this protein have been linked to skin diseases such as psoriasis [63,64], skin atopy [65], or vitiligo [66]. Moreover, TAPs deficiency syndrome can be diagnosed based on granulomatous skin lesions before the occurrence of respiratory infectious manifestations [67,68]. Additionally, the down-regulation of TAPs in melanoma is correlated with the development of metastases and might be a marker of a poor prognosis [69].
Despite the significant role of TAPs in the skin, no clear data exist in the literature on how UV radiation affects the activity of these proteins. However, due to the fact that UV radiation induces oxidative stress, changes in TAPs activity following UV irradiation can be assumed theoretically from the data collected in the case of vitiligo [70]. In vitiligo, the described down-regulation of antioxidant enzymes, such as glutathione peroxidase 1 (GPx1), superoxide dismutase (SOD), and catalase (CAT), as well as the direct oxidizing action of UV radiation from the Sun, are the reasons for the shift in the redox balance in the oxidative direction [70]. Under conditions that simulate the effects of skin cells' exposure to UV radiation, TAPs show a high activity, which results in T cell activation [71]. This is undoubtedly a protective reaction of the body; however, without external control, it always leads to disease symptoms.

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)
CFTR is a regulator of salt levels and water balance in relation to numerous body surfaces, including the skin [72]. When the protein in question is not functioning properly, chloride ions become trapped in cells [72]. The secretion of chloride ions with water and sweat outside the cells maintains the thermoregulatory function of the skin; therefore, CFTR is strongly expressed in sebaceous glands and is located on the apical side of the membrane [73]. Due to this localization, in the case of CFTR inactivation, sweat ducts become obstructed and eccrine glands become inflamed. Ultimately, this may lead to salt accumulation, resulting in folliculitis, miliary rubra, and atopic dermatitis-like skin lesions [73]. On the other hand, the up-regulation of CFTR results in Clsecretion, which has been correlated with mucous cell degranulation and the distention of the glandular ducts [74]. CFTR is also overexpressed in multiple layers of keratinocytes in the epidermis, and the protein has been found to play a significant role in skin wound healing [75]. However, the down-regulation of CFTR in in vitro-cultured human keratinocytes promotes cell migration but inhibits differentiation, while the overexpression of CFTR suppresses migration but enhances keratinocyte differentiation, indicating an important role of CFTR in the regulation of wound healing, as well as skin keratinization [75].
CFTR can be activated by many various compounds, including hormones (e.g., norepinephrine or estrogens), as well as xenobiotics (e.g., isoproterenol) [76,77]. Moreover, UV radiation belongs to the group of CFTR-activating factors; however, there are no clear data concerning the mechanism of this action. It can only be suggested that UV-induced activation of tyrosinase and tyrosinase-related proteins in melasma, or even melanoma, indirectly enhances CFTR activity [77,78]. In addition, strong exposure to UV radiation can lead to cystic fibrosis, a disease in which CFTR has been found to be overexpressed [79].

Sulfonylurea Receptors (SUR1/2)
SURs are transmembrane proteins that are subunits of the potassium ion channels, responsible for their opening or closing according to ATP availability. Therefore, the primary function of SURs is to sense intracellular levels of the nucleotides ATP and ADP, and monitor the energy balance within the cell [80,81]. The main molecular targets of SURs are antidiabetic drugs, the mechanism of which is action is to promote insulin release from pancreatic beta cells. High levels of glucose lead to the increased production of ATP, which, in turn, opens potassium ion channels. The resulting membrane depolarization opens voltage-dependent calcium channels, thus increasing intracellular calcium concentrations, which triggers exocytosis of insulin [82]. Data obtained in vitro suggest that SURs are also abundantly expressed in skin cells such as fibroblasts and keratinocytes [83,84]. It has been shown that chemical blocking of SUR1, as well as SUR2 action, affects hair growth, thus causing hypertrichosis [85]. Moreover, SURs have been found in the mitochondria of fibroblasts, where the regulation of potassium ion channels influences oxygen consumption, the respiratory chain, membrane potential, and the efflux of pro-apoptotic factors [86].
Due to the very small amount of data in the literature on the action of such proteins in the skin, it is even more difficult to find data on their activity under UV exposure. It can only be assumed that, after exposure to UV, these receptors behave simiarily to other cells under oxidative stress. It is known that oxidative conditions are associated with SUR suppression [87]. Therefore, it can be suggested that a malfunction of these receptors may lead to the development of type 2 diabetes, a disease additionally accompanied by oxidative stress [88]. However, that SUR reactions which are identical to oxidative stress may be occurring in the skin is only a supposition. There are also reports, however, that increased production of ROS induces different potassium channel responses (open or close), depending on the tissue [89].

Breast Cancer Resistance Protein (BCRP)
Another extremely important transporter protein from the ABC family is BCRP. The name itself, however, may be misleading as far as its location and function are concerned. BCRP is a transmembrane transporter of xenobiotics; however, only some of them are chemotherapeutic agents, e.g., mitoxantrone and camptothecin analogues [90]. BCRP is expressed not only in breast cancer cells, but also in the gut, the bile canaliculi, the placenta, the skin, and the blood-testis and blood-brain barriers [29,91]. Toxins and xenobiotics pumped out by these molecules limit the absorption of potentially toxic substances in cells, thus contributing to the natural resistance and longevity of normal health cells. However, malignant tissues can exploit the properties of BCRP to survive hypoxia and evade exposure to chemotherapeutic drugs [91]. In the skin, in addition to its primary role in protecting against toxins, BCRP also significantly stimulates the differentiation of activation of immune cells in response to harmful environmental factors [92]. Moreover, the action of this protein action is UV-sensitive, being activated by it [93]. Additionally, the obtained data show that UV irradiation does not cause phototoxicity nor, surprisingly, hepatotoxicity in BCRP-knockdown animals, which indicates the crucial role of this transporter not only in the skin, but also in the whole organism [93]. On the other hand, UV-induced activity of BCRP reduces the protective effect of the applied therapeutic compounds; those with antioxidant properties as well, have been observed in human keratinocytes treated with cannabidiol [29]. A similar effect has been observed in cannabidiol-treated keratinocytes isolated from psoriatic patients, which, as in the case of cancer, interferes with therapy [52].

Conclusions
The present review shows the importance of ABC transporters for the proper functioning of the human body-individual cells/tissues/organs as well as the whole organismand the dangers to human health posed by improper control of the activity of these transporters. Exposure of skin cells to UV radiation, or the related oxidative stress, often leads to increased activity of ABC transporters, as has been observed in the case of MDRs, TAPs, CFTR, and BCRP. This is not only conducive to the pumping out of toxins, but also of protective compounds, and even drugs. A different effect of oxidative stress has been observed in the case of mitochondrial SURs, with the regulation of these channels influencing oxygen consumption, respiratory chain reactions, membrane potential, and the efflux of pro-apoptotic factors into cytoplasm. However, the limited amount of knowledge cited in this paper highlights the lack of sufficient experimental studies of ABC transporters in skin cells that would make it possible to formulate unambiguous hypotheses. At the same time, the authors show the potential importance of this knowledge in relation to healthy skin exposed to solar radiation, as well as in relation to the pharmacotherapy of various skin diseases.