Key role of CRF in the skin stress response system

Abstract

The discovery of corticotropin-releasing factor (CRF) or CRH defining the upper regulatory arm of the hypothalamic-pituitary-adrenal (HPA) axis, along with the identification of the corresponding receptors (CRFRs 1 and 2), represents a milestone in our understanding of central mechanisms regulating body and local homeostasis. We focused on the CRF-led signaling systems in the skin and offer a model for regulation of peripheral homeostasis based on the interaction of CRF and the structurally related urocortins with corresponding receptors and the resulting direct or indirect phenotypic effects that include regulation of epidermal barrier function, skin immune, pigmentary, adnexal, and dermal functions necessary to maintain local and systemic homeostasis. The regulatory modes of action include the classical CRF-led cutaneous equivalent of the central HPA axis, the expression and function of CRF and related peptides, and the stimulation of pro-opiomelanocortin peptides or cytokines. The key regulatory role is assigned to the CRFR-1α receptor, with other isoforms having modulatory effects. CRF can be released from sensory nerves and immune cells in response to emotional and environmental stressors. The expression sequence of peptides includes urocortin/CRF→pro-opiomelanocortin→ACTH, MSH, and β-endorphin. Expression of these peptides and of CRFR-1α is environmentally regulated, and their dysfunction can lead to skin and systemic diseases. Environmentally stressed skin can activate both the central and local HPA axis through either sensory nerves or humoral factors to turn on homeostatic responses counteracting cutaneous and systemic environmental damage. CRF and CRFR-1 may constitute novel targets through the use of specific agonists or antagonists, especially for therapy of skin diseases that worsen with stress, such as atopic dermatitis and psoriasis.

Figure 1.

Dr Wylie Vale. The photograph was taken by Ms Kristen Peelle on September 4, 2010, at the wedding of Dr Vale's daughter at The Bishop's School Chapel.

Figure 2.

Schematic evolution of CRF and related peptides and receptors. The progenitor peptide for CRF and antidiuretic hormone had evolved before separation of chordates because CRF analogs were not found in tunicates or protostomates. The novel function CRF/DH (diuretic hormone)-like peptides in multicellular organisms required development of specific receptors, and those receptors once appeared evolved with peptides and were subjected to genome duplications (64). In the case of CRF precursor, the duplication occurred twice, whereas for CRFR it occurred only once. UCN, urocortin.

Figure 3.

Alternative splicing of CRFR-1 and CRFR-2 pre-mRNA leads to production of several isoforms of the receptors. A, Human CRFR-1 protein sequence is coded by 14 exons that could be subjected to alternative splicing. Expression of at least 12 isoforms was detected so far in humans and rodents (49, 80, 111). Protein coding exons are shown in violet. Alternative splicing results occasionally in the introduction of termination codon, and some exons no longer code protein (white squares). B, Human CRFR-2 gene contains 15 exons with 3 alternative transcription start codons located in different exons. Three main isoforms were described (CRFR-1α, β, and γ), and at least 7 others were detected, including “headless” isoform. Interestingly, some isoforms (CRFR-2α3, CRFR-2α4, CRFR-2α5) show the retention of intronic segment in the coding sequence. Isoforms CRFR-2α2 and CRFR-2β2 have deletion of 3 nucleotides coding glutamine: CRFR-2β (desQ126) and CRFR-2β-2 (desQ126) (227). Positions of exon 1–14 for CRFR-1 and 1–15 for CRFR-2 are marked on the top of each panel. 7-TM domains are shown as dashed-line squares numbered I–VII (49, 80, 81).

Figure 4.

CRFR-1-coupling to the NF-κκB signaling in normal epidermal keratinocytes and melanocytes. CRF stimulates NF-κB activity in normal human keratinocytes (255) and inhibits it in epidermal melanocytes (257). Of note, in immortalized HaCaT keratinocytes, CRF both inhibits and stimulates NF-κB activity, depending on the environmental context (80, 254).

Figure 5.

SNPs can affect CRFR-1 splicing. Exons are shown in green, introns are in blue, and splicing variants are in pink. The positions of selected SNPs are indicated by superscript letters that correspond to reference numbers for the following studies: ^a ; ^b , , ; ^c ; ^d , –; ^e,f ; ^g , , ; ^h , ; ⁱ , , ; ^j –, , ; ^k , ; ^l ; ^m , ; ⁿ , –, ; and ^o .

Figure 6.

CRF signaling is regulated by different CRFR-1 isoforms. CRFR-1 gene contains 14 exons, and only 1 isoform of the CRFR-1β receptor (also called pro-CRFR-1) is coded by all exons. Depending on external or internal factors, CRFR-1 pre-mRNA might be subjected to alternative splicing that results in the formation of at least 12 isoforms. Those proteins might regulate CRF signaling, which is mainly transduced by CRFR-1α, via modulation of its expression, localization, or activity. Minus signs indicate inhibition, and plus signs indicate stimulation of CRF signaling by expression or coexpression of multiple CRFR-1 isoforms. For details, see Ref. .

Figure 7.

Stressed skin regulates the central HPA axis. Signals generated in stressed skin are delivered either by ascending nerve routes to the brain or by circulation to the hypothalamus, pituitary, or adrenal gland, which would depend on the nature and intensity of the stressor and on skin anatomy/histology. Furthermore, UVR production and secretion of final effectors of the HPA (glucocorticoids) is activated by sequential and/or alternative modes of action originating in the skin that will depend on the wavelength and dose of solar electromagnetic energy.

Figure 8.

Differential phenotypic effects of CRF1 signaling in keratinocytes and melanocytes with secondary impact on skin barrier formation. In keratinocytes, CRF1 directly inhibits proliferation and stimulates differentiation plus stimulation of immune activity via stimulation of NF-κB. This enhances protective epidermal barrier function. In melanocytes, CRF1 directly and indirectly (through POMC peptides) stimulates differentiation and melanin production, and the latter enhances protective barrier function. In contrast to keratinocytes, CRF1 signaling leads indirectly (through POMC peptides) to inhibition of NF-κB with subsequent suppression of immune activity. This immunosuppressive effect can be amplified by production of cortisol by melanocytes.

Figure 9.

Expression of CRF, CRFR-1, and CRFR-2 in hair follicle melanocytes and the effect of CRF on follicular melanocytes. a, Human hair follicles express CRF and cognate receptors CRFR-1 (CRF1) and CRFR-2 (CRF2) (red fluorescence). These proteins were also detected in a subpopulation of melanocytes (yellow) located in the proximal/peripheral matrix region and in the outer root sheath (see arrowheads in enlargements of insets) but apparently were down-regulated in the melanogenic zone of anagen VI hair follicles. Cytoplasmic expression of CRF and its receptors was present in hair bulb keratinocytes and less so in dermal papilla cells (FP). b, CRF (10⁻⁸ m) stimulated dendricity in cultured hair follicle melanocytes. Cell dendricity was assessed by counting cells with three or more dendrites before and after stimulation of cells. c, CRF (10⁻⁷ m) stimulated melanogenesis in hair follicle melanocytes in culture. CRF modulated the expression and activity of melanogenic enzymes in hair follicle melanocytes in culture, including: d, tyrosinase protein expression and activity (dopa oxidase); e, TRP-1; and f, DCT (TRP-2). Lane 1, Molecular weight markers; lane 2, CRF 10⁻⁷ m; lane 3, unstimulated control; lane 4, negative control. [Individual panels were reproduced from S. Kauser et al: Modulation of the human hair follicle pigmentary unit by corticotropin-releasing hormone and urocortin peptides. FASEB J. 2006;20:882–895 (213), with permission. © Federation of American Societies for Experimental Biology.]

Figure 10.

Effects of CRF (b) and CRF-related peptides (c) on human scalp hair fiber elongation at day 0 (D0), day 3 (D3), day 6 (D6), and day 9 (D9) and compared with hair follicles grown in the vehicle control (a). Anagen or growing scalp hair follicles were microdissected from human scalp and placed in organ culture as previously described (504).

Figure 11.

Expression of CRF and CRFR-1 in the normal skin, melanoma cells, and effects of CRF1 agonist on proliferation of melanoma cells. A, CRF1 is expressed in normal structures of human skin such as epidermis, blood vessels, eccrine glands, and smooth muscle as well as in malignant melanoma cells (MM). B, CRF is expressed in normal structures of human skin such as epidermis, eccrine glands, and hair follicle as well as in malignant melanoma cells (MM). The slides in A and B were stained with antibodies as described previously (209). Magnification, ×20; insets, ×200. C, CRFR-1 agonist AWS-1 inhibits proliferation of AbC1 hamster melanoma cells. Cells were incubated with the peptide for 48 hours in the 154 medium (Cascade Biologics, Inc) containing growth factors. The DNA synthesis was measured with titrated thymidine incorporation, and data were analyzed as described previously (117). D, CRFR-1 selective agonist AWS-1 inhibits proliferation of Melan A mouse immortalized melanocytes. Cells were incubated with the peptide for 48 hours in the F10 medium containing fetal calf serum (Invitrogen, Inc). The cell viability was measured with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, and data were analyzed as described previously (117). The difference between control and treatments was analyzed with one-way ANOVA (P < .005) as described previously (117). Panels A and B were prepared by Dr Diane Kovacic, a dermatopathology fellow at the University of Tennessee Health Science Center.

Figure 12.

Proposed evolution of the HPA axis organization.

Figure 13.

Members of the team that discovered CRF, left to right: Joachim Spiess, Catherine Rivier, Jean Rivier, and Wylie Vale.

Figure 14.

CRF and skin diseases. Several skin diseases are associated with CRF dysfunction. Dermal inflammatory infiltrate, illustrated in the background, is frequently seen in autoimmune diseases including lupus erythematosus. Please note that lupus erythematosus can be present as a systemic or cutaneous form (discoid). RA is a systemic autoimmune disease affecting the joints. Psoriasis is predominantly a cutaneous disease, although it can often include the joints (psoriatic arthritis). There are many forms of alopecia, including inflammatory/autoimmune alopecia such as AA, lichen planopilaris, and lupus alopecia. The category of dermatitis (inflammatory skin disorders) is represented by allergic contact dermatitis, atopic dermatitis, and nummular dermatitis. The other entities include acne, chronic urticarial, melanoma, squamous cell carcinoma, and basal cell carcinoma.