The significance of CYP11A1 expression in skin physiology and pathology

Abstract

CYP11A1, a member of the cytochrome P450 family, plays several key roles in the human body. It catalyzes the first and rate-limiting step in steroidogenesis, converting cholesterol to pregnenolone. Aside from the classical steroidogenic tissues such as the adrenals, gonads and placenta, CYP11A1 has also been found in the brain, gastrointestinal tract, immune systems, and finally the skin. CYP11A1 activity in the skin is regulated predominately by StAR protein and hence cholesterol levels in the mitochondria. However, UVB, UVC, CRH, ACTH, cAMP, and cytokines IL-1, IL-6 and TNFα can also regulate its expression and activity. Indeed, CYP11A1 plays several critical roles in the skin through its initiation of local steroidogenesis and specific metabolism of vitamin D, lumisterol, and 7-dehydrocholesterol. Products of these pathways regulate the protective barrier and skin immune functions in a context-dependent fashion through interactions with a number of receptors. Disturbances in CYP11A1 activity can lead to skin pathology.

1. Introduction to CYP11A1

CYP11A1, also known as cytochrome P450scc, is a member of the cytochrome P450 family of heme-containing enzymes and catalyzes the first and rate-limiting step in steroidogenesis (Goursaud et al., 2018; Miller and Auchus, 2011; Rone et al., 2012). It is located in the inner mitochondrial membrane of steroid producing cells (Goursaud et al., 2018; Strushkevich et al., 2011; Tuckey, 2005). CYP11A1 converts cholesterol to pregnenolone through three sequential hydroxylation reactions: hydroxylation at the C22 position of the cholesterol side chain to produce 22R-hydroxycholesterol, hydroxylation at C20 of 22R-hydroxycholesterol to produce 20R,22R-dihydroxycholesterol, and the subsequent oxidative cleavage of the C20-C22 bond in 20R,22R-dihydroxycholesterol to produce pregnenolone (Fig. 1). The two electrons required for each of these reactions are supplied by NADPH. They are initially transferred to the FAD-containing adrenodoxin reductase and then one at a time to the iron-sulfur protein adrenodoxin, which in turn passes them to the CYP11A1 to activate the oxygen substrate. Adrenodoxin reductase and adrenodoxin are located in the mitochondrial matrix (Miller and Auchus, 2011; Strushkevich et al., 2011; Tuckey, 2005).

Figure 1.

CYP11A1 activities in the skin. A. CYP11A1 receives electrons form its redox partners to cleave the side chain of cholesterol producing pregnenolone which is converted to other steroids by cell/tissue specific pathways. B. CYP11A1 can act on endogenous sterols and secosteroids in the skin. The major products from cholesterol and 7DHC have their side chain removed whereas little cleavage of the side chain occurs for lumisterol and no cleavage occurs for vitamin D3. Major products from each substrate are shown in bold font. The stereochemistry of products is shown, where known. 7DHC, 7-dehydrocholesterol; lumisterol, lumisterol3. More details on these reactions can be found in (Slominski et al., 2015a; 2020a; Tuckey et al., 2011; Tuckey et al., 2019).

It is well established that CYP11A1 is expressed at relatively high levels in the classical steroidogenic tissues such as the adrenal cortex (fasciculata, reticularis and glomerulosa zones), testis, ovary and placenta. However, CYP11A1 is also expressed at lower levels in a range of other tissues including the brain, skin, thymus, lung, and even T lymphocytes (Slominski et al., 2004a; Slominski RM et al., 2020a). CYP11A1 has been shown to function as a critical regulator of Th2 and Tc2 cell differentiation and type 2 cytokine production (Gelfand et al., 2017, Wang et al., 2020).

Although cholesterol is the major and best characterized substrate for CYP11A1, CYP11A1 also acts on a range of other sterols and secosteroids including: 7-dehydrocholesterol (7DHC), lumisterol, hydroxysterols, plant and fungal sterols such as camposterol and ergosterol, as well as vitamins D2 and D3 (Slominski et al., 2014a; 2020a; Tuckey et al., 2019). Figure 1 summarizes the various biochemical pathways in which CYP11A1 is involved. These alternative pathways appear to be important in the skin and are further discussed below.

The structure of the CYP11A1 gene was discovered by Morohashi et al in 1987, and it was found to be at least 20 kb long with 9 exons and 8 introns (Morohashi et al., 1987). Human CYP11A1 is located on chromosome 15q23–24 (Lara-Velazquez et al., 2017). The gene is only expressed in vertebrates, including fish, birds, amphibians, and mammals (Slominski et al., 2015a).

CYP11A1 activity is stimulated directly by ACTH, LH, and FSH and indirectly by CRH, and proinflammatory cytokines such as IL-1, Il-6, and TNF-α (Guo et al., 2007a; Guo et al., 2007b; Huang et al., 2014; Ruggiero and Lalli, 2016; Slominski et al., 2020a). The steroidogenic acute regulatory (StAR) protein mediates the transport of cholesterol from the outside of the mitochondria to the inner mitochondrial membrane (Chien et al., 207; Issop et al., 203; Manna et al., 2016). ACTH mediates its actions by binding to the melanocortin 2 receptor (MC2R), thereby stimulating adenylyl cyclase and inducing cAMP production (Ruggiero and Lalli, 2016). Subsequently, cAMP increases the availability of cholesterol via rapid synthesis of the StAR protein and directly increases the CYP11A1 mRNA and protein levels (Guo et al., 2007b; Miller and Auchus, 2011)

In the adrenals and gonads, CYP11A1 transcription is regulated by a 2.3 kb promotor containing binding sites for multiple transcription factors including cAMP- response element binding proteins, steroidogenic factor 1 (SF-1), and an Sp-1 (Guo et al., 2007a; 2007b; Ruggiero and Lalli, 2016). SF-1 was discovered nearly thirty years ago by Parker and Morohashi and was shown to play a key role in the regulation of the CYP11A1 gene (Lala et al., 1992; Morohashi et al., 1992; Ruggiero and Lalli, 2016). The two cAMP-responsive sequences (CRS) in CYP11A1: P-CRS and U-CRS contain an SF-1 binding site (Guo et al., 2007a). Both SF-1 and activating protein-1 (AP-1) play a role in the activation of the CYP11A1 gene (Guo et al., 2007a). Runx2, an osteogenic transcription factor, also plays a role in the regulation of the CYP11A1 gene regulation (Teplyuk et al., 2009). Teplyuk et al discovered that Runx2 stimulates CYP11A1 gene expression in osteoblasts (Teplyuk et al., 2009). 1,25(OH)₂D3 via binding to the VDR appears to be a negative regulator of CYP11A1 expression in CD8⁺ cells (Schedel et al., 2016). Whether this relationship (and hence feedback regulation) also holds for 20(OH)D3, the major product of CYP11A1 action on vitamin D3 that can similarly act through the VDR, remains to be established.

The structure of the CYP11A1 protein displays the typical P450 like folds with a heme group at the protein core (Strushkevich et al., 2011). CYP11A1 has also been found to have a monotopic association with the matrix side of the inner mitochondrial membrane that is mediated primarily via the F-G loop region (Headlam et al., 2003). This forms part of a substrate access channel permitting cholesterol to enter the active site from the membrane phase (Strushkevich et al., 2011).

The CYP11A1 enzyme plays an essential role in steroidogenesis and thus complete loss of expression is incompatible with life. Defects in CYP11A1 are one of the causes of primary adrenal insufficiency (PAI) which can range from classical PAI that is characterized by salt loosing adrenal insufficiency and gonadal insufficiency to more mild cases that present itself as glucocorticoid insufficiency (Maharaj et al., 2019). Congenital lipoid adrenal hyperplasia (CLAH) is primarily due to defects in StAR, which delivers cholesterol to the CYP11A1 in the inner mitochondrial membrane, however, mutations in the CYP11A1 gene can also cause the disorder (Goursaud et al., 2018; al Kandari et al., 2006). A case study done by al Kandari et al found that a case of adrenal insufficiency, hypogonadism, and agenesis of the corpus callosum was due to a homozygous point mutation in the CYP11A1 gene (al Kandari et al., 2006). Although rare, mutations in the CYP11A1 gene can be shown to have clinically significant effects (Kim et al., 2008).

Aberrant/alternative splicing in the CYP11A1 gene can cause a deficiency in the expression of active enzyme (Goursaud et al., 2018) or production of protein that has a lower molecular weight (MW) that appears to play a different function to the full-length enzyme (Teplyuk et al., 2009; Annalora et al., 2017). An alternatively spliced CYP11A1 isoform has been detected in human epidermal keratinocytes and melanoma cells (Slominski et al., 2004a).

2. Skin as a stress response organ:

2.1. Overview of the skin:

The skin together with the subcutaneous adipose tissue is the largest organ in the body and plays many roles in preserving the homeostasis of the human body (Slominski et al, 2012a). These roles include, but are not limited to, acting as a physical barrier against microorganisms and physical and chemical insults, regulating body temperature, retaining body fluid, protecting against UV light, and acting as a sensory, immune, endocrine and even steroidogenic organ (Boer et al., 2016; Elias, 2012; Fuchs, 2016; Racine et al., 2020; Slominski et al., 2012a; 2015b; 2018a).

The skin is divided into three layers: the epidermis, dermis, and subcutaneous fat. The epidermis (from the external to internal layers) is subdivided into the stratum corneum, stratum lucidum (only found in thick skin), stratum granulosum, stratum spinosum, and stratum basale. The epidermis contains keratinocytes as well as Langerhans cells, Merkel cells, and melanocytes (Boer et al., 2016; Gallo and Hooper, 2012). The dermis is divided into two layers, papillary and reticular (Brown and Krishnamurthy, 2020). The reticular dermis is thicker than the papillary layer and contains hair follicles, sensory nerves, and sebaceous and sweat glands (Nguyen and Soulika, 2019). Both layers of the dermis are comprised of fibroblasts and myofibroblasts, as well as resident immune cells (Gallo and Hooper, 2012; Nguyen and Soulika, 2019). The immune cells in the dermis play an important role in both innate and adaptive immunity. Dendritic cells, macrophages, mast cells, eosinophils, innate lymphoid cells (ILC), and B and T lymphocytes have all been found in the dermis (Nguyen and Soulika, 2019).

2.2. Skin neuroendocrine system:

The concept that the skin is indeed a neuroendocrine organ was presented for the first time in 2000 (Slominski and Wortsman, 2000; Slominski et al., 2000a), and expanded on in reviews concerning cutaneous CRH signaling, cutaneous serotoninergic and melatoninergic systems, complex local and systemic responses to the UVR and microorganisms, and clinical implications of these properties (Racine et al., 2020; Ramot et al., 2020; Slominski et al., 2005a; 2013a; 2018a; 2018b; 2020b).

The hypothalamic-pituitary axis (HPA) plays a critical role in the stress response as well as being a regulator of glucocorticoid production (Chrousos, 2009). Under stressful conditions, corticotropin releasing hormone (CRH) is released from the paraventricular nucleus (PVN), moves into the anterior pituitary and binds to the type 1 CRH receptors (CRH-R1) (Hillhouse and Grammatopoulos, 2006; Slominski et al., 2001; 2013a; Turnbull and Rivier, 1999; Vale et al., 1981). CRH promotes the expression and processing of proopiomelanocortin (POMC) giving rise to ACTH, endorphins, melanotropins (MSH), and lipotropins (LPH)(Cawley et al., 2016; Slominski et al., 2000a; 2013a; Turnbull and Rivier, 1999). In the adrenal gland, ACTH binds to the melanocortin type 2 receptor (MC-2) resulting in the rapid synthesis of the StAR protein which increases the movement of cholesterol into the mitochondria, and also causes the chronic stimulation of the expression of the steroidogenic enzymes such as CYP11A1.

HPA-like axis behavior has been described in the skin (Slominski and Mihm, 1996; Slominski et al., 2007). The skin has been shown to express CRH, urocortins and CRH receptors (Ito et al., 2004; Pisarchik and Slominski, 2001; Slominski et al., 1996a; 1998a; 1999a; 2000b; 2001e; 2004b; 2006a; Zoubouliset al., 2002; Zbytek and Slominski, 2005) as well as POMC and POMC-derived peptides (Bohm et al., 2006; Luger et al., 1999; Schauer et al., 1994; Scholzen et al., 2000; Slominski, 1998; Slominski et al., 1992; 1993; 1998b) and corticosteroids (Hannen et al., 2017; Ito et al., 2005; Sarkar al., 2017; Slominski et al., 1999b; 2000c; 20002; 2005b; 2005c; 2006b; Vukelic et al., 2011). More details on CRH and POMC involvement in the cutaneous response to stress including those describing the HPA axis in the skin can be found in a number of original reviews (Slominski et al., 2006c; 2013a), with one discussing their possible origin (Slominski, 2007). The elements of CRH-POMC signaling systems can regulate CYP11A1 expression and activity in different cutaneous compartments

In addition to expressing an analog of the HPA axis, the skin plays many neuroendocrine roles (Slominski et al., 2012a). The skin has been shown to produce a number of neurohormones and precursors including catecholamines (reviewed in (Gillbro et al., 2004, Grando et al., 2006, Schallreuter, 1997, Schallreuter et al., 1995)), L-DOPA and L-tyrosine (reviewed in (Slominski et al., 2012c)), serotonin and melatonin (Slominski et al., 2005a,Slominski et al., 2018b,Slominski et al., 2020b), acetylcholine (reviewed in (Grando et al., 2006; Grando, 2006)), histamine (reviewed in (Paus et al., 2006)), cannabinoids (reviewed in (Biro et al., 2009)), and vitamin D including products form non-canonical activation pathways (reviewed in (Bikle, 2020; Bikle and Christakos 2020; Slominski et al., 2020a; 2020c)). This provides strong evidence that the skin plays a critical role in both local and systemic neuroendocrine systems with important implications for steroidogenesis.

2.3. The role of UV in the skin:

UV radiation acts as a two-edge sword in its role of regulating skin functions (Bernard et, 2019; De Silva et al., 2020; Slominski et al., 2018a; Wacker and Holick, 2013; Wondrak, 2007). It can play positive roles in the skin such as stimulating the production of antimicrobial peptides, initiating the synthesis of vitamin D (Bikle, 2011; Holick, 2003; Hong et al., 2008; Wacker and Holick, 2013), and causing the elimination of microbes such as the fungus malassezia furfur (plays a role in seborrheic dermatitis) and the bacterium streptococcus aureus (Abhimanyu and Coussens, 2017; Gontijo et al., 2006; Silva et al., 2006; Wikler et al., 1990). However, UV also stimulates the production of pro-inflammatory cytokines, produces radical oxygen species (ROS) and promotes prostaglandin E2 (PGE2) production, and in high doses damages epithelial keratinocytes in the epidermis and stimulates an inflammatory response (Abhimanyu and Coussens, 2017; Bickers and Athar, 2006; Kabashima et al., 2007). It further accelerates skin aging (Bocheva et al., 2019) and plays a critical role in DNA damage and cutaneous carcinogenesis by acting as a full carcinogen ( Athar et al., 2011; Gordon-Thomson et al., 2014; Hocker and Tsao, 2007; Reichrath and Rass, 2014; Schadendorf et al., 2015, Wondrak, 2007; Yang et al., 2020). UV radiation also stimulates the local and systemic HPA axis and the production of corticosterone or cortisol in the skin (Skobowiat et al., 2013a; 2013b; 2017a; Skobowiat and Slominski, 2015; Tiganescu, Hupe, Jiang et al., 2015). Figure 2 illustrates this concept in both human and mouse skin.

Figure 2.

Immunofluorescent staining of CYP11A1 (P450scc) in skin with and without UV treatment. A shows the human skin and the levels of CYP11A1 and its regulators CRH (corticotropin releasing hormone), PC1 (proconvertase-1), ACTH (adrenocorticotropic), B-END (β-endorphin), and GR (glucocorticoid receptor) after treatment with UVA, UVB, and UVC. Image taken from (Skobowiat et al., 2011) with permission from the publisher. B shows the CYP11A1 levels in the skin of C57BL6 mice and DBA/2J mice before and after treatment with UVB. Image taken from (Skobowiat et al., 2013a) with permission from the publisher.

2.4. The skin immune system:

The skin contains a unique assortment of immunocompetent cells and soluble mediators that play a key role in its protective function against exogenous physical insults, microbes and toxins as well as against endogenous mutations and malignancies (Bernard et al., 2019; Xu, et al.,, 2019). In the stratum corneum of the epidermis, cornified keratinocytes known as corneocytes, contain a lipid envelope and are linked by keratin (Boer et al., 2016; Hoath and Leahy, 2003; Nguyen and Soulika, 2019). The stratum corneum has a pH of around 5.5 that makes it inhospitable to microorganisms (Schmid-Wendtner and Korting, 2006). As part of the rapid innate immune response, epidermal keratinocytes express toll-like receptors (TLRs) and cytosolic nucleotide-binding domain, leucine-rich repeat containing receptors (NLRs) that are receptors for microbial products such as lipopolysaccharides (LPS) from gram-negative bacteria, lipoteichoic acid and peptidoglycans from gram positive bacteria, mannans of yeast and fungi, and nucleic acids from pathogens and the host (McInturff et al.,, 2005; Nestle et al., 2009).

Other components of the cutaneous innate immune response in the skin are the antimicrobial peptides (Schauber and Gallo, 2008) of which cathelicidins and β-defensins are the two best characterized. They are synthesized by keratinocytes, cells of sebaceous and eccrine glands, and mast cells (Gallo and Hooper, 2012; Sanford and Gallo, 2013). The cathelicidins and defensin class of antimicrobial proteins act by disrupting the bacterial and fungal membranes, and viral envelopes (Gallo and Hooper, 2012; Ordonez et al., 2014). Cathelicidin and B-defensin expression is normally low in the skin but is markedly increased when the skin barrier is disrupted (Gallo and Hooper, 2012).

The skin is also a rich source of cytokines and chemokines, which manipulate the extent and characteristics of the immune responses qualitatively and quantitatively (Bernard et al., 2019, Xu et al., 2019; Tan et al., 2015). Cytokines and chemokines also modulate the systemic host response. Their cutaneous production involves several different cell types including keratinocytes, melanocytes, dendritic cells, fibroblasts and mast cells. While cytokines produced by skin cells have similar actions as those secreted by other cell types, they can have distinctive effects on cutaneous tissues. For example, IL-1 stimulates matrix metalloproteinase synthesis in the dermis (Schonbeck et al.,, 1998), while at the same time contributing to wound repair by augmenting collagen production.

Adaptive immunity provides antigen specific protection against intracellular and extracellular pathogens and can be both initiated and expressed in the skin (Nestle et al., 2009; Xu et al., 2019). This is accomplished primarily by a variety of T cell subsets and by IgA, IgM, and IgE antibodies (Debes and McGettigan, 2019). The T cells that carry out these defenses reside within the skin and preferentially recirculate between the skin and regional lymph nodes (Clark, 2015). T-cell mediated immunity is initiated by antigen-presenting myeloid dendritic cells (DCs). There are multiple distinct types of DCs in skin (Clausen and Kel, 2010; Henri et al., 2010). Those that reside within the epidermis are call Langerhans cells (Nguyen and Soulika, 2019; Sanford and Gallo, 2013). The diverse array of dendritic cells enables a more precise activation of different T-cell subpopulations. Plasmacytoid dendritic cells reside within the dermis and are a potent source of the cytokine interferon-γ (Xu et al., 2019).

Although not specifically made in the skin, IgE antibodies play an important role in host defenses against parasites. IgE antibodies bind to mast cells, which express the high affinity surface receptor for IgE (Longley et al.,, 1995). As a consequence of antigen binding to IgE molecules on mast cells, degranulation occurs with the release of potent prostanoids, histamines and cytokines which help to attract additional inflammatory cells to the skin (Xu et al., 2019).

Aberrant T-cell mediated immunity can result in increased susceptibility to infections and skin cancer if the immune response underperforms, as well as to allergic and atopic dermatitis and autoimmune diseases such as psoriasis, alopecia areata and vitiligo when there is excessive activation. Increased IgE and mast cell activity is considered to be an essential element of urticaria and angioedema, atopic dermatitis, hyperimmunoglobulin E syndrome and bullous pemphigoid.

3. CYP11A1 and the skin

3.1. Classical function

It is now well established that the skin can produce cortisol which can be derived from cholesterol via a complete steroidoidogenic pathway. This starts with the StAR protein that delivers cholesterol to the inner mitochondrial membrane which is subsequently converted to pregnenolone by CYP11A1 (Slominski et al., 2004a; 2013b).The cutaneous rate of steroid synthesis is much lower than in classical steroidogenic tissues such as the adrenal cortex and placenta (Slominski et al., 2013b). Since the initial discovery of CYP11A1 gene expression in the human skin (Slominski et al., 1996b), a number of papers have documented CYP11A1 enzyme expression in keratinocytes, melanocytes and dermal fibroblasts (Hannen et al., 2011; Skobowiat and Slominski, 2015; Skobowiat et al., 2011; 2013a; 203b; Slominski et al., 2004a; 2013b; 2017; Thiboutot et al., 2003, Tiala et al., 2007; Tongkao-On et al., 2015), as well as in immune cells (Slominski RM et al., 2020a). This indicates that a variety of cell types in the skin are potentially capable of de novo steroid synthesis. CYP11A1 plays several key roles in the skin of which the most important is that it catalyzes the conversion of cholesterol to pregnenolone, which serves as the precursor to cutaneous cortisol, estrogens, and androgens. The pregnenolone is initially converted to either progesterone by 3β-hydroxysteroid dehydrogenase (3-βHSD) or to 17β-hydropregnenolone by CYP17A1. The skin expresses all the downstream CYP enzymes required to convert the progesterone into cortisol (Slominski et al., 2013b; 2015b)(Fig. 3). The skin can also convert the 17β-hydropregnenolone into androgens and estrogens, but these can also be derived from circulating DHEA-sulfate (Nikolakis et al., 2016; Slominski et al., 2013b; 2015b)

Figure 3.

Production of cortisol in human skin cells. A. Corticotropin-releasing hormone (CRH) were shown to stimulate proopiomelanocortin (POMC) in melanocytes in a time dependent manner. The top graph shows CRH stimulating mRNA production of POMC. The white bar shows negative control while the black bar shows treatment with CRH. The bottom graph shows CRH stimulating ACTH production in a concentration dependent manner after 24 hours of treatment. B. Melanocytes were shown to produce cortisol. The top graph shows liquid chromatography-mass spectrometry (LC/MS) of cortisol control vs melanocyte extract. Notice that both of them have a peak of [M+H]+ at mass to charge ratio of 363 with retention time at 11 minutes. The last two graphs come from mass spectrometry fragmentation analysis and shows similarities between melanocytes extract (middle graph) and cortisol standard (bottom graph). C. CRH was shown to promote cortisol production in melanocytes in POMC and CRH-R1 dependent manner. D. Fibroblasts were shown to produce cortisol. The top graph shows liquid chromatography-mass spectrometry (LC/MS) of cortisol standard vs fibroblasts extract. Notice that both of them have a peak of [M+H]+ at mass to charge ratio of 363 with retention time at 11 minutes. The last two graphs come from mass spectrometry fragmentation analysis and shows similarities between fibroblast containing media (middle graph) and cortisol standard (bottom graph). Panels A, B, and C were taken with permission from the following article (Slominski at al. 2005c) and panel D was taken with permission from the publisher (Slominski et al., 2006b).

The role of steroidogenesis in the skin includes countering the inflammatory responses in the skin and preventing hyperproliferation of keratinocytes (Bigas et al., 2018; Phan et al., 2021; Slominski and Zmijewski, 2017; Vukelic et al., 2011). In fact, dysregulation of steroid synthesis in the skin has been associated with many skin disorders (Hannen et al., 2011; 2017;Nikolakis et al., 2016; Phan et al., 2021; Ramot et al., 2020; Zmijewski, 2017; Slominski et al., 2014c; 2015b; 2017a), which are discussed further below.

3.2. Non-classical functions

CYP11A1 can also use lumisterol, vitamins D2 and D3, ergosterol and 7-DHC as substrates hydroxylating their side chain at different positions, and in some cases cleaving it (Guryev et al., 2003; Slominski et al., 2004a; 2005d; 2005e; 2006d; 2009a; 2011a; 2012c; 2012d; 2017a; Tuckey et al., 2008; 2011; 2014)(Fig. 1). It should be noted that the vitamin D3, lumisterol and tachysterol are derived from UV irradiation of 7-dehydrocholesterol in the skin (Bikle, 2020; Holick et al., 1981; Wacker and Holick, 2013). Thus, the skin is likely to be a primary site of their metabolism to biologically active hydroxyderivatives, supported by the detection of many of these metabolites in the skin (Slominski et al., 2012d; 2013b; 2015d; 2017c). The hydroxyvitamin D products act as biased agonists on the VDR displaying many but not all the effects of 1,25(OH)₂D3, and also act to some extent through other receptors expressed in the skin including the retinoic acid-related orphan receptors, RORα and γ (Slominski et al., 2014c; 2017b), and the aryl hydrocarbon receptor (AhR) (Slominski et al., 2018c; 2020a).

The above CYP11A1-derived vitamin D hydroxyderivatives have been proposed to play key roles in the maintenance of skin physiology. At least 10 different CYP11A1-derived vitamin D3 metabolites have been discovered with 20(OH)D3 and 20,23(OH)₂D3 being the major ones (Slominski et al., 2014a). In keratinocytes, 20(OH)D3 and 20,23 (OH)₂D3 inhibit DNA synthesis and cause cell cycle arrest. The CYP11A1-derived vitamin D hydroxyderivatives have been found to stimulate differentiation and inhibit proliferation and NF-κB activity in human keratinocytes (Chaiprasongsuk et al., 2020a; Janjetovic et al., 2009; 2010; Slominski et al., 2014a; Zbytek et al., 2008). They also show photoprotective properties which are also shared by the CYP11A1-derived hydroxylumisterols (Chaiprasongsuk et al., 2019; 2020a; 2020b; Slominski et al., 2015c; 2017c; 2020; Tongkao-On et al., 2015). Thus, products of CYP11A1 action appear to protect the skin against the harmful effects of UVR with responses that include increased expression of DNA repair enzymes (Slominski et al., 2020a). This is particularly relevant to the photoprotective properties of the hydroxylumisterols, since lumisterol is a product of excessive UV radiation (Holick et al., 1981). Furthermore, UVB is able to upregulate CYP11A1 expression (see Figure 2) which presumably results in increased CYP11A1-mediated hydroxyvitamin D3 and hydroxylumisterol production in the skin to help combat the harmful effects of radiation.

CYP11A1-derived vitamin D metabolites hold therapeutic potential. Besides their photoprotective role, they have been found to inhibit the growth of normal and malignant melanocytes and display anti-fibrotic activity on dermal fibroblasts (Slominski et al., 2011a; 2011b; 2012e; 2013c; 2013d). Treatment with the CYP11A1-derived vitamin D compounds inhibited the growth of the SKMEL-188 melanoma cells in vivo as well as downregulating NF-κB activity (Janjetovic et al., 2011; Skobowiat et al., 2017b). Thus, the CYP11A1-derived vitamin D compounds are candidates for the treatment of skin cancers including melanoma (Slominski et al., 2018d; 2020c). Importantly, while showing a lack of (20(OH)D3) or low (1,20(OH)₂D3) calcemic effects (Chen et al., 2014; Slominski et al., 2010; 2013c, Wang et al., 2012), CYP11A1-derived secosteroids also express potent anti-inflammatory effects (Chaiprasongsuk et al., 2020b; Janjetovic et al., 2009; 2010; Lin et al., 2017; 2018; Slominski et al., 2014a; 2014c). Thus, CYP11A1 derived secosteroids are also excellent candidates for treatment of autoimmune or inflammatory disorders of different etiology (Slominski RM et al., 2020a; 2020b).

3.3. Regulation of CYP11A1 expression in the skin

The delivery of cholesterol resulting from the activity of the StAR protein appears to play the most important role in the acute regulation of CYP11A1 activity in the skin (Slominski et al., 2014a; 2014d). Other factors that regulate CYP11A1 including CRH, ACTH, POMC, cAMP, and cytokines IL-1, particularly Il-1β, IL-6, and TNF-α, are likely to operate in the skin, although more detailed studies remain to be performed. Tkachanko et al (2011) found that IL-1α and β stimulate StAR protein, CYP17A1, and 3β-hydroxysteroid dehydrogenase 2 (3βHSD2) expression at the mRNA level in adrenal cells, with increased androgen and cortisol production (Tkachenko et al., 2011). UVB and UVC have been found to stimulate CYP11A1 and cortisol production by skin; however, UVA seems to have no effect on cortisol production (Skobowiat et al., 2011a,; 2013b;). Huang et al (2014) reported that TNF-α suppresses steroidogenesis and CYP11A1 expression in intestinal epithelial cells by activating c-Jun and NF-κB (Huang et al., 2014). When the dominant-negative form of c-Jun amino-terminal kinase 1 (JNK1), an NF-κB inhibitor was injected into mice, steroidogenesis in the intestinal epithelial cells was restored (Huang et al., 2014). Toll-like receptors (TLRs), notably TLR3, can play a role in the regulation of steroidogenesis in the skin. Shimada-Omori et al found that Polyinosinic:polycytidylic acid (Poly(I:C)), a known ligand for TLR3, stimulates glucocorticoid production and CYP11A1 expression in rosacea epidermis (Shimada et al., 2020). Also, other environmental stressors leading to barrier disruption can stimulate epidermal cortisol production (Takei et al., 2013; Zhu et al., 2014).

3.4. CYP11A1 in pathological skin or skin tumors

The synthesis of CYP11A1-derived steroids is a two-edged sword. Although they can play a positive role in maintaining homeostasis, some can also play an enhancing role in certain pathologies. Figure 4 illustrates this concept. CYP11A1 can also be used as an indicator of autoimmune diseases (Slominski RM et al., 2020a). Its expression is decreased in both atopic dermatitis and psoriasis (Hannen et al., 2011; 2017). Metabolomic and transcriptomic profiling of psoriatic skin vs healthy control revealed that deficiencies in cortisol and cortisone production in the psoriatic skin lesions could lead to production of proinflammatory cytokines (Sarkar et al., 2017). These findings support our previous theory that deficient feedback of POMC and glucocorticoids on cutaneous immunity contributes to inflammatory and autoimmune dermatoses, and that restoration of these endogenous deficiencies represents a realistic goal in treating psoriasis and inflammatory disorders (Slominski, 2009; 2013a; 2017a).

Figure 4.

Summary of the two-edged role that CYP11A1-derived compounds play in the human skin.

CYP11A1 as well as other CYP enzymes can play a role in skin cancer. CYPs such as CYP1A1, CYP1B1, CYP2B6, CYP2E1, and CYP3A5, have been found in keratinocytes (Baronet al., 2001). Keratinocytes play an important role in the most common types of skin neoplasms, such as basal and squamous cell carcinomas (Ratushny et al., 2012; Slominski et al., 2014d). CYPs in keratinocytes metabolize hazardous materials and carcinogens (Slominski et al., 2014d). Cortisol and cortisone immunosuppression can cause progression of basal and squamous cell carcinoma. However, corticosteroids can inhibit melanoma directly (Horn and Buzard, 1981) or indirectly by inhibiting the production of POMC or NF-κB activity (Bohm et al., 2006; Slominski et al., 2014d). Normal melanocytes and melanoma cells can produce their own cortisol and corticosteroids (Fig 3) (Slominski et al., 1999b;, 2005b; 2005c). This can lead to local immunosuppression allowing tumor cells to escape from immune attack leading to tumor growth and melanoma progression (Slominski and Carlson, 2014). This concept appears to be supported by in vivo experiments in hamsters showing that adrenalectomy significantly inhibited melanoma growth, while use of a synthetic glucocorticoid accelerated it (Stanberry et al., 1982).

Some of the immune cells residing in the skin can affect tumor behavior. Mahata et al used two groups of transgenic mice, mCherry (florescence reporter line) and a conditional CYP11A1 knockout, to prove that T cell steroid biosynthesis could aid tumor growth and progression, and that targeting the rate limiting steps in the steroid synthesis of T cells can prevent tumor metastasis, particularly melanoma (Mahata et al., 2020).

Changes in CYP11A1 levels in cancer tissues relative to normal tissue have been found at several sites in the body. CYP11A1 expression was downregulated in 6 different cancer types, colon adenocarcinoma, kidney renal clear cell carcinoma, liver hepatocellular carcinoma, lung squamous cell carcinoma, prostate adenocarcinoma, and uterine corpus endometrial carcinoma (Fan et al., 2016). In addition, CYP11A1 is expressed in many tumor lines of different lineage including melanomas (Slominski et al., 1996b; 2004a; 2012e; 2014d).

Other steroidogenic enzymes can also contribute to the behavior of skin cancer. Levels of mRNA for 11β-hydroxysteroid dehydrogenase (11β-HSD) type 1 (produces cortisol) and 2 (deactivates cortisol) vary between healthy and malignant tissues (Cirillo et al., 2017). Squamous cell carcinoma (SCC) samples showed significantly lower expression of 11β-HSD2 than healthy control cells (Cirillo et al., 2017).

4. Role of steroid signaling in skin physiology and pathology

4.1. Glucocorticoid and mineralocorticoid signaling

Corticosteroids produced endogenously by the skin can affect various signaling pathways. For example, corticosteroids have been shown to influence NF-κB and AP-1, and STAT3 signaling pathways (Sevilla and Perez, 2018). Corticosteroids can also affect gene expression in the skin. Lili et al has found that the synthetic glucocorticoid, clobetasol, upregulates 1607 genes and downregulate 1917 genes from RNA sequencing analysis (Lili et al., 2019). They also found that when given clobetasol, glucocorticoid receptor (GR) targets such as GILZ were more active in African Americans, while females have a stronger shift towards the IFN-α /IFN-γ and IL-6/Jak/STAT3 signaling pathway, which is pro-inflammatory (Lili et al., 2019). Glucocorticoids have been found to activate REDD1, a mTOR inhibitor (Baida et al., 2015). REDD1 mediates many of the adverse effects of glucocorticoids including skin atrophy. Mice that were KO for REDD1 maintained many of the anti-inflammatory effects of glucocorticoids (Baida et al., 2015). Most recent studies show that keratinocytes control skin immune homeostasis through de novo-synthesized glucocorticoids (Phan et al, 2021).

Many of the actions of glucocorticoids in the skin are mediated by the GR expressed in different cutaneous compartments and involve different mechanisms of action (Aberg et al., 2007; Baida et al., 2015; Chebotaev, Yemelyanov et al., 2007, Jozic et al., 2017; Lili et al., 2019; Nikolakis et al., 2016; Sarkar et al., 2017; Sevilla and Perez, 2018; Slominski and Zmijewski, 2017; Slominski et al., 2017a). Mice in which GR and MC were partially knocked out exhibited more severe inflammation and hyperproliferation of the skin after imiquimod treatment when compared to healthy controls (Bigas et al., 2018). The GR also plays many important roles in skin development. Immunohistostaining on mice embryos that had the GR gene completely knocked out (GR−/−) showed that the skin of these mice had incomplete epidermal stratification as well as significantly less fillagrin and desmosomes, thus incompatible with life (Bayo et al., 2008). In addition, non-genomic actions of GR in the skin have been described concerning the inhibition of wound healing (Jozic et al., 2017; Slominski and Zmijewski, 2017).

Mineralocorticoids mediate their signaling action through the mineralocorticoid receptor (MR). MR antagonism can help counter some of the effects of glucocorticoid-induced skin atrophy and delayed wound healing (Maubec et al., 2015; Nguyen et al., 2016; Stojadinovic et al., 2016). Mice that overexpress MR show skin atrophy and alopecia (Sainte Marie et al., 2007). Thus, research on both GR and MR may yield promising new therapies for treating various skin ailments.

4.2. Estrogen and Androgen signaling

There are two major types of estrogen receptors that have been discovered, estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) (Bakry et al., 2014,; Cutolo and Straub, 2020; Hall and Phillips, 2005; Ohnemus et al., 2006; Thornton et al., 2003), with ERβ playing a more important role in the skin (Ahn et al., 2020; Thornton et al., 2003). ERα is barely detectable in the skin, while ERβ was found in the sebaceous glands, hair follicles, epidermis, and dermal fibroblasts (Thornton et al., 2003). Estrogens play several key roles in the skin which include acting as a regulator of the immune system (Cutolo and Straub, 2020; Ohnemus et al., 2006), increasing collagen production in the skin and modulating hair growth (Ohnemus et al., 2006). Patients with estrogen deprivation have symptoms that include dryness, atrophy and wrinkling of the skin, as well as delayed wound healing (Hall and Phillips, 2005). Most recent studies show that photoprotective responses to 1,25(OH)₂D3 in mice are modulated by the estrogen receptor-β (Tonkgao-on et al., 2021).

Two androgen receptor (AR) isoforms have been identified, AR A and AB (Liegibel et al., 2003). ARs are expressed in epidermal and follicular keratinocytes and melanocytes, dermal fibroblasts, vascular endothelium, and dermal papilla cells (DPC) in hair follicles (Ceruti et al.,, 2018; Chang et al., 2013). Androgens play many roles in skin disorders such as acne, androgenic alopecia, and delayed wound healing (Lai et al., 2012). In castrated mice, accelerated wound healing and decreased inflammation were observed compared to control mice (Ashcroft and Mills, 2002). Thus, androgens show the opposite effect to estrogens and delay wound healing (Lai et al., 2012). Further research into the role of androgens and estrogens in the skin could pay dividends for treating skin disorders.

5. Conclusions and future directions.

CYP11A1 plays many important roles in maintaining the physiology of the body. It catalyzes the first and rate-determining step in steroidogenesis, the conversion of cholesterol to pregnenolone. It is involved in the production of steroids such as cortisol, pregnenolone, estrogens and androgens. Local production of corticosteroids in the skin, initiated by CYP11A1, appears to play important roles in skin physiology and pathology. CYP11A1 can also metabolize vitamin D and lumisterol into non-calcemic hydroxymetabolites which have potential therapeutic benefits. These reactions appear to occur in the skin, which has a high concentration of these UV-derived substrates, but their exact physiological roles remains to be further investigated. Additional research into cutaneous CYP11A1 derived steroids and secosteroids and their biological effects may yield future therapies for many dermatological and systemic diseases.

Highlights.

• CYP11A1 is expressed and enzymatically active in the skin

• CYP11A1 initiates local steroidogenesis, secosteroidogenesis and 5,7-diene metabolism

• Cutaneous CYP11A1 is regulated by UVB, UVC, CRH, ACTH, c-AMP and cytokines

• CYP11A1 plays a key role in regulation of the protective barrier and immune functions

• Disturbances to CYP11A1 catalytic activity can lead to skin pathology