Neuroendocrine signaling in the skin with a special focus on the epidermal neuropeptides

Abstract

The skin, which is comprised of the epidermis, dermis, and subcutaneous tissue, is the largest organ in the human body and it plays a crucial role in the regulation of the body’s homeostasis. These functions are regulated by local neuroendocrine and immune systems with a plethora of signaling molecules produced by resident and immune cells. In addition, neurotransmitters, endocrine factors, neuropeptides, and cytokines released from nerve endings play a central role in the skin's responses to stress. These molecules act on the corresponding receptors in an intra-, juxta-, para-, or autocrine fashion. The epidermis as the outer most component of skin forms a barrier directly protecting against environmental stressors. This protection is assured by an intrinsic keratinocyte differentiation program, pigmentary system, and local nervous, immune, endocrine, and microbiome elements. These constituents communicate cross-functionally among themselves and with corresponding systems in the dermis and hypodermis to secure the basic epidermal functions to maintain local (skin) and global (systemic) homeostasis. The neurohormonal mediators and cytokines used in these communications regulate physiological skin functions separately or in concert. Disturbances in the functions in these systems lead to cutaneous pathology that includes inflammatory (i.e., psoriasis, allergic, or atopic dermatitis, etc.) and keratinocytic hyperproliferative disorders (i.e., seborrheic and solar keratoses), dysfunction of adnexal structure (i.e., hair follicles, eccrine, and sebaceous glands), hypersensitivity reactions, pigmentary disorders (vitiligo, melasma, and hypo- or hyperpigmentary responses), premature aging, and malignancies (melanoma and nonmelanoma skin cancers). These cellular, molecular, and neural components preserve skin integrity and protect against skin pathologies and can act as “messengers of the skin” to the central organs, all to preserve organismal survival.

THE SKIN IN A NUTSHELL

The skin is the largest organ in the body and is strategically located as the interface between external and internal environments. As such, it plays a crucial role in the regulation and preservation of the body’s homeostasis (1–5). It is composed of the epidermis, dermis, and hypodermis, which are, respectively, of ectodermal and mesodermal origin (6, 7). It is a multifunctional self-regulating organ the functional integrity of which is sustained by the anatomical connections between the three compartments and the mutual signaling between them and other skin cohabitants (1, 3, 4). The skin protects against physical and chemical insults, invasion by microorganisms, injury caused by mechanical stress, retention of body fluids, regulation of body temperature, storage and production of energy, insulation against temperature variation and mechanical insults, sensation, and production of vitamin D, folate, and other factors. In addition, it has immune functions, is important for temperature regulation in the body, and is a source of social communication and camouflage (1, 4, 7–15).

The epidermis represents the outermost layer of the skin, which is predominantly formed by proliferating and differentiating keratinocytes forming, accordingly, stratum basale, suprabasale, spinosum, granulosum, and, most superficially, the acellular stratum corneum and stratum lucidum (1, 3). The latter is present only on acral skin. The epidermis contains melanocytes and Merkel cells of neural crest origin, and Langerhans cells of mesodermal lineage (6, 7). Having no blood supply, the epidermis is nourished through the diffusion of nutrients from the dermis through the basement membrane, which separates it from the dermis. The epidermis is oxygenated by diffusion from the air. It also contains sensory nerves network that can even penetrate to the stratum corneum (16–19). This is in addition to classical sensory functions including the detection of pain, temperature, touch, and vibration (20–22).

The dermis is divided into papillary (adjacent to the epidermis) and reticular components. It comprises fibroblasts/fibrocytes, myofibroblasts, resident and circulating immune cells including mast cells. It contains hair follicles, sebaceous and sweat glands, smooth muscle, nerve bundles and ending, and superficial vascular plexuses and lymphatics. Adjacent to the dermis is the lowermost component of the skin, the hypodermis, which is predominantly composed of adipocytes with other components including fibroblasts, histiocytes, nerve bundles, and deep vascular plexuses and lymphatics. It also contains distal aspect of anagen hair follicles (23).

AN OVERVIEW OF THE NEUROENDOCRINE FUNCTIONS OF THE SKIN

Neuro-Immuno-Endocrinology of the Skin

It has been over two decades since the formulation of the concept that the skin acts as a neuroendocrine organ (24) equipped with corticotropin-releasing hormone (CRH) and proopiomelanocortin (POMC) signaling systems to regulate skin responses to stress (25). A myriad of papers has confirmed the original hypotheses as reviewed in Refs. 2, 4, 26–44. This concept is consistent with other theories on neuroendocrine self-regulating pathways in peripheral organs, including the immune system (28, 45–51). The skin cells produce classic hormones and neurotransmitters including neuropeptides that act on their receptors through para-, auto-, and intracrine mechanisms, which in turn regulate local homeostasis (Fig. 1A). These neuroendocrine mediators with their receptors are organized into epidermal and dermal (including hypodermis) units, which communicate with each other (Fig. 1B) and systemically through humoral and neural pathways (Fig. 1C) to induce local homeostatic self-regulating activities, to preserve and maintain the skin structural and functional integrity under environmental pressure and, by inference, participate in systemic homeostasis (24, 25). Examples of such neuropeptides expressed in the skin are CRH (52–60), urocortins (58, 61), and POMC-derived peptides (44, 62–68). In addition, immune cells primed in the skin by neuroendocrine mediators released by skin cells or neurotransmitters released locally from nerve endings serve as additional cutaneous neuroendocrine messengers regulating systemic homeostasis (24, 28). Finally, neuromediators can be produced by the skin microbiome (69).

Figure 1.

Neuroendocrine organization of the skin. A: components of the exocrine and endocrine units of the skin produce hormones, neural mediators, and cytokines that act upon the corresponding receptors expressed on skin cells. B: the communication between epidermal and dermal neuroendocrine units and the systemic level. C: the cutaneous neuroendocrine system communicates with the central nervous, endocrine, and immune system or other organs through humoral or neural pathways. Reprinted from Ref. 24 with the permission of the publisher.

The elements of the nervous, immune, and endocrine systems present in the skin can be organized into different regulatory axes, circuits, and pathways, which not only regulate skin’s homeostasis but due to its rich innervation and vasculature can regulate other body organs, including the brain (12, 24). Examples include the cutaneous equivalent of the hypothalamus-pituitary-adrenal axis (HPA) (70–75), which is organized in a similar manner as the central HPA (76–82), with some local specificity (78) to be discussed later. The skin also contains its own cholinergic (83, 84) and catecholaminergic (84–88) systems with l-tyrosine and l-DOPA (L-3,4-dihydroxyphenylalanine) also functioning as bioregulators (89–91). It contains local and autonomous serotoninergic (92–98), melatoninergic (92, 99–106), and cannabinoid (33, 107–109) systems.

Small molecules synthesized in skin play important roles not only for epidermal homeostasis, but also for cutaneous inflammation, sensory perception, and vascular permeability. For example, nitric oxide (NO) synthesized by epidermal keratinocytes play a crucial role in endothelial cells by regulating vascular permeability in the skin, and ATP and other chemical factors released from keratinocytes contribute to cutaneous sensory and inflammatory responses (22, 110–113). The skin also expresses functional elements of the hypothalamic-pituitary-thyroid axis (114–119) and is a target for prolactin and growth hormone (GH) (120–122). It also produces and responds to leptin (32, 123). Importantly, skin is a recognized steroidogenic organ (34, 124) and can synthesize estrogens, androgens, and corticosteroids including corticosterone and cortisol in a highly organized fashion (27, 28, 34, 51, 73–75, 125–142), which has significant implications for skin immune functions. This can be started de novo by CYP11A1 expressed in skin cells (70, 143) with implications for the production of Δ7-steroids (144, 145) and generation of novel secosteroidal metabolites (15, 146, 147). Finally, it also produces calcitonin gene-related peptide (CGRP), corticosteroids, gonadotrophins, hemokinin-A (HKA), leptin, neurotensin (NT), neurotrophins, parathyroid hormone (PTH), PTH-related peptide (PTHRP), pituitary adenylate cyclase-activating peptide (PACAP), calcitonin gene-related peptide (CGRP), substance P (SP) neuropeptide Y (NPY), and vasoactive intestinal peptide (VIP) among other neuropeptides (24, 29, 32, 40, 148–166).

Cutaneous HPA Axis

The HPA is one of the most important neuroendocrine axes regulating responses to stress (Fig. 2) (79, 82) and its homologue is also expressed locally to coordinate skin responses to stress (71, 72). However, there are differences and similarities between the central and cutaneous HPA (cHPA). Although the central HPA axis involves interactions between different organs (brain, pituitary, and adrenals) organized along the structure CRH→CRHR1→POMC→ACTH→COR (cortisol/corticosterone) with input from cytokines (IL-1, IL-6, TNF), the cutaneous HPA (cHPA) involves the interaction between the epidermis, dermis, hypodermis, or the cells that are in close contact. This close contact and lack of significant spatial separation lead to several scenarios in which elements of the cutaneous HPA can also operate in a coordinated or independent manner in para-, auto-, and intracrine fashions, leading to various phenotypic effects and possible pathology when feedback mechanisms, characteristic for the central HPA, are disconnected (5, 78, 125, 167). The key element is the activation of the CRHR1 by either CRH or urocortin with follow-up signaling cascade leading to production and release of POMC peptides including ACTH with their downstream effects that include corticosteroidogenesis. Different biological and physicochemical stressors such as ultraviolet radiation (UVR) and stress-induced cytokines can activate the entire axis, CRH→CRHR1→POMC→ACTH→COR, or its proximal, intermediate, and distal elements which are cell type or compartment dependent and may lead to opposing biological effects. For example, the direct CRH effect in the periphery (168–170) including skin will be proinflammatory (32, 39, 78, 171–173), which is exactly the opposite as that which occurs in the central HPA, unless attenuated by downstream POMC-derived peptides or glucocorticosteroids (72, 125, 174). In skin, the number of the elements and regulators of cHPA axis includes endorphins and MSH peptides (25, 44, 175–178) as wells as ACTH, CRHR2, and alternatively spliced isoforms of CRHR1 (59, 179–184). These differences with central organization could be secondary to the origin of the primordial HPA axis, which would have developed first in the integument for organismal defense against environmental stressors and pathogens (Fig. 3) (185).

Figure 2.

The organization of central hypothalamus-pituitary-adrenal (HPA) axis that can be activated at different levels by stress originating in the skin. CRH, corticotropin releasing hormone; POMC, proopiomelanocortin; URC, urocortin. Figure was created using BioRender.

Figure 3.

The evolutionary origin of the hypothalamus-pituitary-adrenal (HPA) axis. Right: organization of the central HPA axis; left: primordial integumental HPA axis. CRH, corticotropin releasing hormone; POMC, proopiomelanocortin. Reprinted from Ref. 185 with the permission of the publisher.

The cHPA plays an important role in the regulation of biological functions of the skin (26, 75, 78, 139, 186, 187). Its deregulation or pathological overexpression or downregulation of its functional elements can lead to skin pathology including inflammatory disorders (32, 36, 130, 132, 188–196), wound healing (134, 197–199), skin aging (31, 35), or skin cancers (200–204). Importantly, skin factors can activate the central HPA after cutaneous stressors such as ultraviolet B (UVB) (205). UVB can also trigger other neuroendocrine mechanisms affecting the body’s homeostasis (206–209), which has recently been discussed (12, 210, 211).

Nerve Fibers

An important element of the skin neuroendocrine system is its innervation composed of networks of somatosensory and autonomic nerve fibers (20, 111) terminating in each skin layer, which contributes to the physiological skin functions or development of disease (2, 16, 24, 26, 29, 32, 37, 40, 78, 155, 156, 188, 212, 213). This is in addition to detecting sensory inputs such as pain, touch, temperature, or vibration (20, 21). Cutaneous nerve fibers derived from neurons localized either in the dorsal root ganglia (DRG), the trigeminal ganglion (face and neck), or autonomic nerve centers (19, 20, 37). Depending on their nature they can release neurotransmitters including amines, neuropeptides, neurotrophins, bioactive lipids, and even gases to facilitate neural transmission through the synapses and to act on corresponding receptors present on effector skin cells. Thus, cutaneous nerve fibers can release classical neurotransmitters such as epinephrine and nonepinephrine, dopamine, acetylcholine (ACh), γ-aminobutyric acid (GABA), glutamate, serotonin, and perhaps melatonin; neuropeptides including CRH, urocortins, substance P (SP), PACAP, calcitonin gene-related peptide (CGRP), galanin (GAL), somatostatin (SOM), vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), α-, β-, and γ-MSH, β-endorphin, dynorphins (DYN), enkephalins; bioactive lipids like cannabinoids; gases: NO, CO; and neurotrophins: like nerve growth factor (NGF). The same molecules released not only by cutaneous nerves but depending on the context, are also produced by resident skin cells and circulating immune cells to act as neurotransmitters, hormones, cytokines, or biological modulators (2, 16, 24, 25, 30, 37, 78, 97, 162, 188, 214, 215). A good example of such a molecule is CRH which in the skin is released from cutaneous nerve fibers as well as produced by keratinocytes, melanocytes, fibroblasts, mast cells, and other skin immune cells (53, 56, 58, 59, 216). The important feature of sensory nerve fibers is the ability to transmit signals through orthodromic (afferently to the spinal cord and brain), and antidromic (efferently, releasing neurotransmitters into effector skin cells), which is important for homeostatic communication between different skin components (24) (Fig. 1B and Fig. 4). Examples of mediators used by sensory nerves in the skin including epidermis include CRH, NPY, SP, VIP, GAL, CGRP, NGF, serotonin, β-endorphin, and other opioids and MSH peptides (37, 155, 217, 218). Cutaneous sensory receptors can be activated by physicochemical and biological factors such as UVR, visible light, pollutants, changes in proton concentrations, reactive oxygen species, toxins, allergens, and mechanical stimuli such as trauma, touch, or vibration (12, 16, 20, 37, 219, 220), This can lead to orthodromic and antidromic signals to regulate skin and systemic homeostasis (12, 24, 221).

Figure 4.

Skin neuroimmunoendocrine system encompassing the epidermal neuroendocrine system (A), and interactions with the central systems and organs (B) in a coordinated stress response mode. A: epidermal cells are not only are sensitive to neurohormonal regulation but also produce elements of hypothalamus-pituitary-adrenal (HPA) or hypothalamus-pituitary-thyroid (HPT) axes, other neuropeptides, biogenic amines, serotonin, melatonin, nitric oxide, opioids, cannabinoids, catecholamines, acetylcholine, steroids, secosteroids, neurotrophins, and cytokines. B: the skin neuroimmunoendocrine system can activate the central responses with direct homeostatic, metabolic, and phenotypic consequences. The constant exchange of neuroendocrine mediators between skin and other organs is responsible for the maintenance of local and global homeostasis. BM, basement membrane; F, fibrocytes/fibroblasts; IC, immune cells; K, keratinocytes; LC, Langerhans cells; M, melanocytes. Reprinted from Ref. 12 with the permission of the publisher.

NEUROENDOCRINOLOGY OF THE EPIDERMIS

Epidermis as a Unique Neuroendocrine Unit

Being directly exposed to the environment, the epidermis not only serves as a physical barrier against environmental stressors, but it also senses and responds to the environmental signals through a sophisticated stress response system with a computing capability to regulate local and systemic homeostasis involving communication with the central nervous and endocrine systems (Fig. 4) (2, 4, 12, 24, 26, 78). Neurohormonal mediators and cytokines produced in the epidermis are predominantly used by the neuroendocrine system in that location since epidermal soluble factors would need to travel through the basement membrane and diffuse efficiently without degradation through an acellular dermal matrix component to reach dermal, hypodermal, or systemic components unless there was a specific transport system for these neuroendocrine mediators (24). Therefore, their concerted activity is predominantly to regulate epidermal functions, and forming an autonomous epidermal neuroendocrine system, which can communicate with dermal or hypodermal systems via nerve endings, unless there is sufficiently intense stress allowing the release of large amounts of mediators that could diffuse to other components of the skin (Fig. 4) (24, 25). In addition, the epidermal surface serves as an ecosystem for a variety of microorganisms forming the microbiome (69, 222, 223) that can affect skin physiology in a positive or negative manner. This microbiome keeps the skin immune system in equilibrium with its metabolic activity involving the various functions of the skin (69, 223). It can also regulate the neuroendocrine activity of the skin (69).

Epidermal cholinergic (83, 84, 224, 225), catecholaminergic that include dopamine and adrenergic receptors (84–86, 88, 135, 226–230), serotoninergic (97), melatoninergic (101), canabinoidogenic (107, 109), steroidogenic (27, 34, 124, 126, 231–233), vitamin D (15, 234, 235), nitric oxide (NO) (110), mitochondrial ATP (111), purinergic P2X (236–239), γ-aminobutyric acid (GABA) (240, 241), glutamatergic (242–245), parathyroid hormone, and calcitonin and related peptides (157, 160, 161, 214), and other local neuropeptide signaling systems (26, 29, 156, 157, 214, 218, 241, 246); all have also been cited in several reviews listed earlier (Fig. 4A). All of these neuromodulators are produced by keratinocytes and melanocytes and to some degree by Langerhans cells or they are released by sensory nerve endings entering the epidermis. They act on the corresponding membrane-bound receptors, which depending on the chemical structure and target receptor, stimulate downstream signaling systems such as cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), inositol trisphosphate (IP3), diacylglycerol (DAG), or calcium (Ca⁺²) or are coupled with ion channels. Different locally produced steroids and secosteroids act on corresponding receptors to regulate epidermal functions (8, 34, 126, 198, 234, 247). They act in a para-, juxta-, endo-, or autocrine fashions depending on the distance from the target cell. With the exception of Langerhans cells, human epidermis does not have other immune cells under physiological conditions. However, keratinocytes and melanocytes act as epidermal immunocompetent cells producing and releasing cytokines in response to different stimuli including allergens (11, 14, 78, 133, 177, 248–251). In the epidermis, melanocytes act as sensory cells, detecting environmental changes directly or indirectly through extensive dendrite processes and then after computation sending regulatory signals to multiple target to counteract the environmental damage through mechanisms described previously(12, 252, 253).

The epidermal neuroendocrine system can be affected by chemical, biological, and physical factors. As relates to the different wave length of solar radiation including visible spectrum and ultraviolet B (UVB) and A (UVA), each will have different mechanisms of action with UVB acting through chromophores and UVA or visible light predominantly through reactive oxygen species (ROS) or other mechanisms (11, 12, 235, 254). This raises the question whether there are specific photoreceptors in the skin. This concept was first formulated in relation to regulation of melanin pigmentation and was further advanced by Pawelek group (220, 255). Previous reports demonstrated that rhodopsin, opsin, and the biochemical cascade for the signal transduction were expressed in the epidermal keratinocytes (256, 257). Recent data also indicate that opsins can act as receptors for blue light (258, 259) or UVA (260, 261). The aforementioned advancements represent confirmation of original studies on light detection by melanophores (262, 263).

CRH SIGNALING

CRH and related peptides are produced by human keratinocytes and melanocytes under physiological and pathological conditions or after stimulation by UVB or microbial antigens (52, 54, 58, 59, 61, 139, 195, 201, 264–267) and by active forms of vitamin D (268). Also, their receptors including CRHR1 and CRHR2 are expressed on human keratinocytes and melanocytes affected by their level of differentiation (59, 171, 179, 181, 216, 264, 269–274). Both CRH and related peptides including urocortin 1, 2, and 3 as well as CRHR1 and CRHR2 are also expressed on follicular keratinocytes and melanocytes (57, 58, 61, 75, 264, 266, 270, 274), which are mentioned here because of their direct connection between the epidermis and outer rout sheath of the hair follicle. It must be noted that CRH and urocortin 1 have a high affinity for CRHR1 with urocortin also having a high affinity for CRHR2 and urocortin 2 having a high affinity only for CRHR2.

Since the predominant CRH receptor in the human epidermis is CRHR1, with CRHR2 expressed predominantly in the dermal compartment or adnexal structures (56, 58, 78, 171, 266), the discussion will focus on epidermal keratinocytes and melanocytes. Activation of CRHR1 on epidermal cells is coupled to cAMP, IP3, DAG, or Ca⁺² signaling (171, 216, 269, 270, 273) with downstream activation of protein kinases A (PKA) and C (PKC) dependent pathways and activator protein-1 including Jun D or cAMP responsive element binding protein (CREB) (171, 272). Ca⁺² signaling is caused by Ca influx from the extracellular space, including through voltage-activated Ca⁺² ion channels, which are cyclic nucleotide-gated ion channels, and/or through its mobilization from the intracellular stores (216, 269, 271, 273). Interestingly, confocal laser scanning microscopy has shown that in HaCaT keratinocytes CRH induces Ca⁺² flux into the cytoplasm, while urocortin induces Ca⁺² flux into the nucleus with a remarkable oscillatory effect (273) (Fig. 5).

Figure 5.

Time course of the changes of intracellular Ca²⁺ after corticotropin-releasing hormone (CRH) or urocortin (URC) stimulation. I): time galleries of fluorescence images of the Ca²⁺ indicators in the HaCaT cells after stimulation with CRF (10⁻⁷ M, left) and urocortin (URC) (10⁻⁷ M, right). The intensity range (8 bit) is indicated below the image galleries. II), A and B: time course of the relative fluorescence intensities (unbroken line, cytosol; broken line, nucleus) of the Ca²⁺ indicators in HaCaT cells after stimulation with CRF (A, 10⁻⁷ M) and URC (B, 10⁻⁷ M). C: dose-dependent frequency of the oscillations in HaCaT-cells after stimulation with URC (means ± SD). Number of cells recorded (n) is shown in the respective columns. D: dose-dependent numbers of HaCaT cells with oscillations after stimulation with URC (percentage of total number of cells recorded). Number of cells recorded with oscillations (n) is shown in the respective columns. Reprinted from Ref. 273 with the permission of the publisher.

CRHR1 and CRHR2 undergo alternative splicing in epidermal keratinocytes and melanocytes with the production of multiple functionally active isoforms (59, 179–181, 183, 184, 275), consistent with the original studies of Hillhouse and Grammatopoulos in other tissues (276–280). In the skin, the alternative splicing and processing of the CRHR1 are subject to environmental regulation, affected by UVB, phorbol esters, factors raising intracellular cAMP, and cell culture conditions and pathology (58, 179–181, 184). The predominant splice form coupled to PKA and PKC signaling is CRHR1α. Other forms, including β, c, d, e, f, g, and h, were detected with the ones missing transmembrane domain serving as soluble forms potentially sequestering CRH or urocortin while others having part of the transmembrane domain with a predominant intracellular location. These could dimerize with CRHR1α affecting its presentation on the cell surface and signal transduction. Co-expression of CRHR1 isoforms, therefore, modulates the sensitivity of cells to the ligands and influences downstream coupling to G-proteins; it may affect fast mRNA and/or protein turnover or have a decoy receptor function of CRHR1 isoforms (179, 180, 184). In the epidermis, a gradient of CRHR1 is also present with the strongest expression at the basal layer (179).

CRH and CRH-related peptides can regulate proliferation and differentiation programs as well as immune and endocrine activities of keratinocytes and melanocytes through interaction with CRHR1 or CRHR2 receptors (57, 74, 75, 171–174, 186, 265, 266, 270–272, 274, 281–286). Specifically, CRH and related peptides can inhibit normal keratinocytes proliferation with the shape of the inhibition curve determined by the media calcium concentration and cellular context (171, 270, 272, 281, 284). It included inhibition of the G0/1 to the S phase transition of the cell cycle, which was accompanied by an increased expression of cdk inhibitor p16 (Ink4a) protein. The antiproliferative effect was attenuated by inhibition of PKC but not when inhibitors of PKA and MAP kinases were used (272). The cell cycle withdrawal was associated with the induction of keratinocyte differentiation with increased expression of CK1, involucrin, decreased expression of CK14, and increased granularity (272, 284). Therefore, it was proposed that the program, triggered by CRH interaction with CRH-R1, includes induction of a transduction pathway involving the sequential activation of phospholipase C, protein kinase C, activator protein-1 (including Jun D), and p16 (272). As relates to regulation of immune functions CRH enhanced the interferon-γ stimulated expression of the homing cell adhesion molecule (HCAM) and intercellular adhesion molecules and of the human leukocyte antigen DR (HLA-DR) antigen (281), rapidly stimulated IL-1β, IL-6, and TNFα genes expression (172) and stimulated nuclear factor-kappa B (NF-κβ) activity (283). In immortalized HaCaT keratinocytes, the effects were dependent on culture conditions, e.g., in serum-free media CRH stimulated NF-κβ activity and IL-1β (171), whereas in serum-containing media CRH inhibited NF-κβ activity and IL-1β but stimulated IL-6 and IL-11 (282, 286). Thus, the local CRH effects on keratinocytes are dependent on coupling to the type of receptor (CRHR1 vs. CRHR2), receptor signaling (coupling to PKA or PKC), or related to stimulation of POMC and corticosteroidogenesis (78, 134). For example, in closely related human follicular keratinocytes CRHR1 agonists inhibit keratinocytes proliferation and hair shaft elongation, whereas CRHR2 agonists maintain follicular keratinocytes in an anagen-like state, which indicates their stimulation of keratinocyte proliferation and retardation of follicular keratinocyte differentiation in undifferentiated hair bulb matrix keratinocytes (78, 171, 186).

CRH and related urocortins also regulate proliferation, viability, differentiation program, and endocrine activities of human melanocytes (74, 78, 171, 266, 267, 271). In normal and immortalized epidermal melanocytes, CRH inhibited melanocyte proliferation in growth factors containing medium through G2 arrest, whereas in growth factor-depleted media it enhanced DNA synthesis (through increased transition from G1/G0 to S phase and decreased subG1 component, indicating DNA degradation) and inhibited early and late apoptosis in the same cells, indicating that on epidermal melanocytes CRH acts as a survival factor under the stress of starvation and as an inhibitor of growth factors induced cell proliferation (271). Activation of CRHR1 induced generation of both cAMP and IP3 in epidermal melanocytes (271). In follicular melanocytes, CRH and specific CRHR1 agonists stimulated dendricity and melanogenesis, whereas CRHR2 agonists inhibited melanocyte differentiation program (266). In addition, CRH stimulated POMC expression with production of POMC peptides in normal and malignant melanocytes (74, 174, 267). In addition, UVB stimulated POMC expression and production of ACTH was dependent on CRH production and stimulation of CRHR1 (265). Furthermore, CRH stimulated cortisol production through action on CRHR1 coupled to cAMP signaling and production of POMC and its product ACTH (74). It also inhibited NF-κβ activity through similar mechanisms secondary to POMC expression and processing to final peptides (174).

In summary, CRH and related urocortin can regulate functions of the epidermis through receptor and context-dependent mechanisms. Induction of differentiation programs in epidermal keratinocytes and melanocytes as well as of immune and endocrine activity would strengthen its barrier function as discussed previously and the responsiveness to stress with a feedback mechanism to terminate such responses (5, 78, 167, 185). The differences and similarities in CRH-mediated regulation of keratinocytes and melanocytes functions are presented in Fig. 6.

Figure 6.

Differential and overlapping phenotypic effects secondary to activation of corticotropin-releasing hormone receptor 1 (CRHR1) in human keratinocytes and melanocytes. A: activation of CRHR1 in normal epidermal keratinocytes and melanocytes, respectively stimulates and inhibits NF-κB activity. In immortalized HaCaT keratinocytes, CRF both inhibits and stimulates NF-κB activity, depending on the environmental context (171). B: in keratinocytes, activation of CRHR1 directly inhibits proliferation and stimulates differentiation and immune activity via stimulation of NF-κB. This enhances protective epidermal barrier function. In melanocytes, CRHR1 directly and indirectly [through proopiomelanocortin (POMC) peptides] stimulates differentiation and melanin production leading to enhancement of protective barrier function. In contrast to keratinocytes, there is an indirect (through POMC peptides) inhibition of NF-κB with subsequent suppression of immune activity, which can be amplified by production of cortisol by melanocytes. Reprinted from Ref. 78 with the permission of the publisher.

POMC SIGNALING

Since their initial detection in epidermal keratinocytes and melanocytes (65, 200, 287–289), there have been a number of reports that have addressed this issue. Their activities in the epidermal response system to the environmental stressors will therefore be discussed only briefly. For further information, the reader is referred to in-depth reviews on this topic reported elsewhere (2, 12, 25, 26, 41, 44, 68, 78, 177, 178, 188, 290, 291). POMC is expressed in human epidermal and follicular keratinocytes and melanocytes and is processed to ACTH, ACTH1-13, α-MSH, β-MSH, γ_1–3MSH, β-LPH, and β-endorphin. These products together with POMC precursor are being released into extracellular space (44, 55, 65, 67, 75, 287, 292). This process follows the molecular principles operating at the central level (brain and pituitary) (76) and involves the action of prohormone convertases 1, -2 and regulatory protein 7B2 (44, 292–294). A similar process occurs in murine skin (294), dermal human skin cells (295–297), and human retinal pigment epithelium (298). Local expression of the POMC gene and protein and production of POMC peptides can be triggered by external stressors including UVB (139, 265, 289, 299–301), or other bioregulatory factors such as cytokines (25, 68, 134), active forms of vitamin D (268), and CRH (73–75, 174, 265, 267). The induction of POMC expression by UVB in epidermal melanocytes involves the activation of CRH and the PKA signaling pathway (265).

The phenotypic effects of POMC peptides on keratinocytes and melanocytes are mediated through their interaction with corresponding melanocortin or opioid receptors (4, 25, 26, 41, 67, 177, 178, 255, 291, 292, 302–310). The best characterized is the regulation of melanin skin pigmentation and melanocytic activity by MSH through an interaction with the melanocortin type 1 (MC1) receptors coupled to the cAMP signaling cascade, reviewed in Refs. 177, 255, 290, 291, 306, and 309. MC1 can also be activated by ACTH and to lesser degree by other MSH peptides. In normal human melanocytes, these molecules stimulate melanogenesis, dendrite formation, cell proliferation as well as melanosomes transfer to epidermal keratinocytes. An additional function of ligand-induced MC1 activity is stimulation of antiapoptotic, cytoprotective, and antioxidative cellular mechanisms that include DNA damage repair, making this axis important in protection against UVR-induced damage and carcinogenesis (254, 304, 305, 309–312). In addition, this pathway inhibits the immune activities of melanocytes including inhibition of the NF-κβ signaling (313–315). Additional MC receptors that are targets for MSH and ACTH peptides are expressed on melanocytes and keratinocytes include MC2, MC3, and MC4 (70, 177, 178, 268). MC2 is also expressed in murine skin (316). However, their functional activity in epidermal skin cells remains to be established. The role of γ_1–3-MSH in the regulation of melanin pigmentation also remains to be defined (317, 318). POMC-derived and locally produced β-endorphin also regulates the melanocyte differentiation program through the μ-opioid receptor system with potent melanogenic, mitogenic, and dendritogenic effects (44, 67).

Human keratinocytes also express receptors for MSH, ACTH peptides, and β-endorphin (41, 68, 178, 270, 274, 300, 303, 307, 319–323). Through their actions on these membrane-bound receptors, the POMC-derived peptides can regulate keratinocytes proliferation and differentiation, as well as immune activities, wound healing, and cytoprotection (41, 176, 178, 303, 321–323). It should be noted that β-endorphin and MSH can be released by sensory nerve endings in the skin or be produced by circulating immune cells (24, 25, 37, 68, 176). In conclusion, the local POMC system operates to preserve barrier function of the epidermis with POMC peptides acting directly on keratinocytes or indirectly through stimulation of epidermal melanocytes to enhance the protective barrier through production of melanin pigment. Melanin and melanogenesis due to its diverse bioregulatory activity (324), in addition to its light scattering property, can strengthen the barrier function (252, 325). The anti-inflammatory activities of β-endorphin, ACTH, and MSH peptides and their indirect effects through stimulation of cortisol production are also important for epidermal physiology. The local HPA axis can operate at the cellular level in a para- and autocrine fashion as has been shown for melanocytes (74).

OTHER NEUROPEPTIDES AND NEUROTROPHINS

Enkephalins

Enkephalins can either be produced locally in the epidermis, released from sensory nerve endings, or produced by circulating immune cells (211, 218, 326–328). Epidermal melanocytes and keratinocytes express proenkephalin (PENK) with production of the corresponding Met- and Leu-enkephalin peptides (218). UVB, Toll-like receptor (TLR)4, and TLR2 agonists stimulated PENK gene expression in melanocytes and keratinocytes in a time- and dose-dependent manner and Met/Leu-ENK peptides are expressed in differentiating keratinocytes of the epidermis in the outer root sheath of the hair follicle, in myoepithelial cells of the eccrine gland, and in the basement membrane/basal lamina separating epithelial and mesenchymal components (218). These enkephalins are altered in a variety of hyperproliferative and inflammatory cutaneous diseases, especially psoriasis (218, 326, 328, 329). Its expression was also noted in epidermal skin tumors including squamous and basal cell carcinoma (218, 327). Through the interaction of the corresponding receptors, they can regulate the proliferation and differentiation of keratinocytes and inflammatory skin responses (326, 328, 330). Thus, enkephalins together with other opioid peptides can affect epidermal homeostasis and improper signaling can lead to hyperproliferative or inflammatory disorders (41, 175, 176, 302, 303, 326, 328–330).

Thyroid Stimulating Hormone and Thyroid Releasing Hormone

Since initial detection of functional thyroid stimulating hormone (TSH) receptors and genes expressing TSH and thyroid releasing hormone (TRH) in human skin (114), significant progress was made to define their function in that location (115–119, 331–333). It is thought that they regulate epidermal and follicular functions through corresponding receptors in a direct or indirect manner. TRH can also be involved in progression of melanocytic tumors (334) and its role in regulation of hair pigmentation was proposed (335). Furthermore, skin-derived TRH can stimulate anterior pituitary to release TSH with a downstream stimulation of thyroid gland to release thyroid hormones.

Pituitary and Hypothalamic Hormones

An important role for prolactin in skin physiology and pathology was initially proposed by Paus (121). This is supported by detection of prolactin in human scalp and discovery of prolactin receptors in skin cells, identification of its regulators, and defining its phenotypic effects (117, 120, 336).

The epidermis also responds to the action of growth hormone (GH) directly through GH receptor expressed on keratinocytes or indirectly via induction of IGF-1 production in dermis (IGF-1 receptor is expressed by epidermal cells) (337, 338). However, the GH is not expressed in the human epidermis (122).

Somatostatin is also expressed in the epidermis and hair follicle. It is thought to have a regulatory effect on keratinocyte proliferation and differentiation (339, 340). However, the mechanisms regulating its cutaneous expression and definitive functions remain to be investigated.

Oxytocin, a hypothalamic hormone released from the posterior pituitary, is also produced by human keratinocytes (341). It regulates proliferation of keratinocytes as well as having immunomodulatory and antioxidative activities in these cells (342). Its exact role in epidermal homeostasis remains to be investigated.

Other Neuropeptides

A variety of neuropeptides are produced in the human epidermis locally, in addition to their release from nerve endings (2, 24, 26, 37, 39, 157–162, 188, 215, 339, 343, 344). Examples include dynorphins, CGRP, NT, PTH, PTHRP, PACAP, CGRP, SP, NPY, PPY, and VIP. Their functions in skin physiology and pathology have been extensively reviewed previously (2, 24, 26, 37, 39, 156–166, 188, 215, 339, 343, 344). Briefly, they can affect keratinocytes proliferation, production of cytokines, and epidermal immune activities and they play a role in inflammatory skin diseases. However, their exact role in regulation of epidermal barrier or regulation of melanocyte pigmentary activity remains to be investigated.

Neurotrophins

Neurotrophins including NGF are recognized as neurotrophic factors supporting the synthesis and development of sensory neurons in the central and peripheral nervous system. They are also produced by keratinocytes (148, 149, 345–349). Neurotrophins including NGF act on the corresponding membrane-bound receptors present on epidermal and follicular keratinocytes and melanocytes to regulate their proliferation, survival, and other functions (148, 149, 325, 349–355). They also regulate the innervation of skin by sensory and sympathetic neurons (148, 348, 351). NGF also protects keratinocytes and melanocytes from UVR-induced damage and in the regulation of epidermal barrier functions.

CONCLUDING REMARKS

Because of its strategic location at the interface with the external environment, the skin, including its epidermal compartment, is crucial for organismal survival. These functions are regulated by local neuroendocrine, immune, pigmentary, and adnexal systems that detect, integrate, and respond to a diverse range of stressors to protect skin integrity and functions. This is achieved through precise calibration of stress responses with a high degree of local autonomy by sophisticated sensory, computing, and signaling systems that differentially respond to environmental changes. The skin constituents communicate cross-functionally through local nervous, immune, and endocrine systems to maintain essential epidermal functions. In fact the skin has been recognized as a brain on the outside (24, 43) and stress organ (25).

The epidermis also has its own autonomous self-regulating neuroendocrine system composed of hormonal and neuropeptide networks with corresponding receptors produced and expressed by keratinocytes, melanocytes, Langerhans cells, and Merkel cells that strengthen its barrier function and set the homeostatic responses in the most optimal mode. This system includes epidermal sensory nerves acting in a coordinated manner to orchestrate homeostatic responses and to interact with the local microbiome. Melanocytes, because of their secretory and pigmentary functions, through communication via an extensive dendrite network, represent an additional coordinator of the local stress responses. The skin is also tightly networked to central stress axes, which contribute to the maintenance of body homeostasis (Fig. 1C and Fig. 4B).

In addition, recent advances in genomics have provided big data analysis opportunities for skin research. For example, genomic landscapes of melanocytes from human skin were characterized (356). A large amount of data collected should allow to ask important questions on the gene signatures that may serve as disease biomarkers for screening, diagnostic, therapeutic, and prognosis solutions (357). The database PAGER, which is continuously developing, contains large number of gens related to the skin (358, 359), which forms the background for computational approaches using systems biology or artificial intelligence (AI). Thus, the computational biology represents a future for modeling of neuroendocrine connections in the skin with implications for cutaneous physiology and pathology that would also increase the precision of the diagnosis and therapy.

GRANTS

We acknowledge the support of NIH Grants 1R01AR073004-01A1, R01AR071189-01A1, and a VA Merit Grant No. 1I01BX004293-01A1 (to A.T.S.); R21 AI149267-01A1 (to C.R. and A.T.S.); R21ES034595 and RO1CA1933885 (to C.E.); and U54TR001005, U01CA223976 Grants (to J.Y.C.), U01 AR 078544 (to M.A.). The cited work was also supported by NSF Grants IOS-0918934, IBN-9604364, 9896030, and 049087 and NIH Grants RO1AR052190, 1R01AR056666, R21AR0665051, and AR-047079 (to A.T.S.).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

A.T.S. conceived and designed research; A.T.S. and R.M.S. performed experiments; A.T.S., R.M.S., C.R., J.Y.C., M.A., and C.E. analyzed data; A.T.S., R.M.S., C.R., J.Y.C., M.A., and C.E. interpreted results of experiments; A.T.S. prepared figures; A.T.S., R.M.S., C.R., J.Y.C., M.A., and C.E. drafted manuscript; A.T.S., R.M.S., C.R., J.Y.C., M.A., and C.E. edited and revised manuscript; A.T.S., R.M.S., C.R., J.Y.C., M.A., and C.E. approved final version of manuscript.