D-amino acid-substituted analogs of corticotropin-releasing hormone (CRH) and urocortin with selective agonist activity at CRH1 and CRH2beta receptors

Key role of CRF in the skin stress response system

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.

Corticotropin-releasing factor reduces tumor volume, halts further growth, and enhances the effect of chemotherapy in 4T1 mammary carcinoma in mice

The present study examines the effect of the endogenous neuroendocrine factor, corticotropin-releasing factor (CRF), alone or in combination with 5-fluorouracil (5-FU), on 4T1 mammary tumor cells in vitro and in vivo. CRF has been detected in breast cancer tissues; however, the biological effects reported in the literature are sparse and variable. We found that exogenously administered CRF significantly reduced tumor growth without influencing angiogenesis or cell death. Furthermore, CRF reduced tumor interstitial fluid pressure (Pif) and potentiated the effect of 5-FU. These results show that CRF has antitumor effect on mammary carcinoma in mice.

Seasonal changes in CRF-I and urotensin I transcript levels in masu salmon: correlation with cortisol secretion during spawning

Pacific salmon employ a semelparous reproductive strategy where sexual maturation is followed by rapid senescence and death. Cortisol overproduction has been implicated as the central physiologic event responsible for the post-spawning demise of these fish. Cortisol homeostasis is regulated through the action of hormones of the hypothalamus-pituitary-interrenal (HPI) axis. These include corticotropin-releasing factor (CRF) and urotensin-I (UI). In the present study, masu salmon (Oncorhynchus masou) were assayed for changes in the levels CRF-I and UI mRNA transcripts by quantitative real-time PCR (qRT-PCR). These results were compared to plasma cortisol levels in juvenile, adult, and spawning masu salmon to identify specific regulatory factors that appear to be functionally associated with changes in cortisol levels. Intramuscular implantation of GnRH analog (GnRHa) capsules was also used to determine whether GnRH influences stress hormone levels. In both male and female masu salmon, spawning fish experienced a 5- to 7-fold increase in plasma cortisol levels relative to juvenile non-spawning salmon. Changes in CRF-I mRNA levels were characterized by 1-2 distinctive short-term surges in adult masu salmon. Conversely, seasonal changes in UI mRNA levels displayed broad and sustained increases during the pre-spawning and spawning periods. The increases in UI mRNA levels were positively correlated (R(2)=0.21 male and 0.26 female, p<0.0001) with levels of plasma cortisol in the pre-spawning and spawning periods. Despite the importance of GnRH in sexual maturation and reproduction, the administration of GnRHa to test animals failed to produce broad changes in CRF-I, UI or plasma cortisol levels. These findings suggest a more direct role for UI than for CRF-I in the regulation of cortisol levels in spawning Pacific salmon.

Proopiomelanocortin (POMC), the ACTH/melanocortin precursor, is secreted by human epidermal keratinocytes and melanocytes and stimulates melanogenesis

Proopiomelanocortin (POMC) can be processed to ACTH and melanocortin peptides. However, processing is incomplete in some tissues, leading to POMC precursor release from cells. This study examined POMC processing in human skin and the effect of POMC on the melanocortin-1 receptor (MC-1R) and melanocyte regulation. POMC was secreted by both human epidermal keratinocytes (from 5 healthy donors) and matched epidermal melanocytes in culture. Much lower levels of alpha-MSH were secreted and only by the keratinocytes. Neither cell type released ACTH. Cell extracts contained significantly more ACTH than POMC, and alpha-MSH was detected only in keratinocytes. Nevertheless, the POMC processing components, prohormone convertases 1, 2 and regulatory protein 7B2, were detected in melanocytes and keratinocytes. In contrast, hair follicle melanocytes secreted both POMC and alpha-MSH, and this was enhanced in response to corticotrophin-releasing hormone (CRH) acting primarily through the CRH receptor 1. In cells stably transfected with the MC-1R, POMC stimulated cAMP, albeit with a lower potency than ACTH, alpha-MSH, and beta-MSH. POMC also increased melanogenesis and dendricity in human pigment cells. This release of POMC from skin cells and its functional activity at the MC-1R highlight the importance of POMC processing as a key regulatory event in the skin.

Corticotropin releasing hormone and the skin

Cotricotropin-releasing hormone (CRH) and related peptides are produced in skin that is dependent on species and anatomical location. Local peptide production is regulated by ultraviolet radiation (UVR), glucocorticoids and phase of the hair cycle. The skin also expresses the corresponding receptors (CRH-R1 and CRH-R2), with CRH-R1 being the major receptor in humans. CRH-R1 is expressed in epidermal and dermal compartments, and CRH-R2 predominantly in dermal structures. The gene coding for CRH-R1 generates multiple isoforms through a process modulated by UVR, cyclic adenosine monophosphate (cAMP) and phorbol 12-myristate 13-acetate. The phenotypic effects of CRH in human skin cells are largely mediated by CRH-R1alpha through increases in concentrations of cAMP, inositol triphosphate (IP3), or Ca2+ with subsequent activation of protein kinases A (PKA) and C (PKC) dependent pathways. CRH also modulates the activity of nuclear factor of kappa light polypeptide gene enhancer in B-cells (NF-kappaB), activator protein 1 (AP-1) and cAMP responsive element binding protein (CREB). The cellular functions affected by CRH depend on cell type and nutritional status and include modulation of differentiation program(s), proliferation, viability and immune activity. The accumulated evidence indicates that cutaneous CRH is also a component of a local structure organized similarly to the hypothalamo-pituitary-adrenal axis.

Modulation of the human hair follicle pigmentary unit by corticotropin-releasing hormone and urocortin peptides

Human skin is a local source of corticotropin-releasing hormone (CRH) and expresses CRH and CRH receptors (CRH-R) at mRNA and protein levels. Epidermal melanocytes respond to CRH by induction of cAMP with up-regulation of pro-opiomelanocortin gene expression and subsequent production of adrenocorticotropin hormone. However, the role of CRH/CRH-R in melanocyte biology is complicated by the significant heterogeneity of cutaneous melanocyte subpopulations, from continuously active and UV-responsive melanocytes in epidermis to UV nonresponsive, hair growth cycle-coupled melanogenesis in hair follicles. In the present study we report that normal human scalp hair follicle melanocytes express CRH at the mRNA level. Furthermore, CRH, urocortin and CRH-R 1 and 2 were differentially expressed in follicular melanocytes, fibroblasts, and keratinocytes depending on anatomic location and differentiation status in situ and in vitro. Stimulation of follicular melanocytes with CRH and CRH peptides, modified for selectivity for CRH-R1 and/or CRH-R2, variably induced cell melanogenesis, dendricity, and proliferation. CRH-peptides also stimulated the expression and activity of Tyrosinase, and expression of Tyrosinase-related protein-1 and-2. However, a modified urocortin peptide highly selective for CRH-R2 down-regulated melanocyte differentiation phenotype. This study indicates that CRH peptides can differentially influence hair follicle melanocyte behavior not only via CRH-R1 signaling but also by complex cross-talk between CRH-R1 and CRH-R2.

CRH functions as a growth factor/cytokine in the skin

We tested the effect of CRH and related peptides in a large panel of human skin cells for growth factor/cytokine activities. In skin cells CRH action is mediated by CRH-R1, a subject to posttranslational modification with expression of alternatively spliced isoforms. Activation of CRH-R1 induced generation of both cAMP and IP3 in the majority of epidermal and dermal cells (except for normal keratinocytes and one melanoma line), indicating cell type-dependent coupling to signal transduction pathways. Phenotypic effects on cell proliferation were however dependent on both cell type and nutrition conditions. Specifically, CRH stimulated dermal fibroblasts proliferation, by increasing transition from G1/0 to the S phase, while in keratinocytes CRH inhibited cell proliferation. In normal and immortalized melanocytes CRH effect showed dichotomy and thus, it inhibited melanocyte proliferation in serum-containing medium CRH through G2 arrest, while serum free media led instead to CRH enhanced DNA synthesis (through increased transition from G1/G0 to S phase and decreased subG1 signal, indicating DNA degradation). CRH also induced inhibition of early and late apoptosis in the same cells, demonstrated by analysis with the annexin V stains. Thus, CRH acts on epidermal melanocytes as a survival factor under the stress of starvation (anti-apoptotic) as well as inhibitor of growth factors induced cell proliferation. In conclusion, CRH and related peptides can couple CRH-R1 to any of diverse signal transduction pathways; they also regulate cell viability and proliferation in cell type and growth condition-dependent manners.

Electroaffinity in paracellular absorption of hydrophilic d-dipeptides by sparrow intestine

We previously demonstrated size selectivity in the absorption of nonelectrolyte hydrosoluble probes in birds, presumably by the paracellular pathway. Our goal in this study was to determine the charge selectivity in the absorption of hydrosoluble D-dipeptides, because there have been no studies of the electroaffinity of this absorption pathway in birds. For this purpose isosmotic solutions with two hydrophilic D-dipeptides: serine-lysine (positive at pH 7.4) and serine-aspartic (negative at pH 7.4) were gavaged into the stomach in nonanesthetized house sparrows (Passer domesticus), and injected into the pectoralis with a syringe in different trials. Fractional absorption was calculated as F = [AUC by gavage)]/[AUC by injection] (AUC = area under the curve of plasma probe concentration vs. time). Fractional absorption was significantly higher for the positively charged than negatively charged dipeptide (respectively, F=0.30+/-0.05 vs. F=0.17+/-0.03). These findings give the first evidence of cation selectivity by the paracellular route in the absorption of hydrosoluble solutes in the small intestine in birds.

Corticotropin-releasing hormone triggers differentiation in HaCaT keratinocytes

BACKGROUND

Corticotropin-releasing hormone (CRH) is proposed to be involved in the regulation of the proliferative capacity of keratinocytes, based on its significant actions in the skin. These are mediated by CRH-R1alpha and represented by adenylate cyclase activation, Ca2+ influx, inhibition of cell proliferation and modifications in intracellular signal transduction by NF-kappaB.

OBJECTIVES

To define CRH action in the cell cycle we investigated its effects on the differentiation programme using the HaCaT keratinocytes model.

METHODS

HaCaT keratinocytes were incubated with CRH in Dulbecco's modified Eagles's medium (containing 1.8 mmol L(-1) calcium) or EpiLife (containing 0.06 mmol L(-1) calcium) medium. Cell proliferation was assessed with the MTT assay. Flow cytometry was used for the measurement of DNA content, cell size and granularity and the expression of cytokeratin 14, cytokeratin 1 and involucrin. The electrophoretic mobility shift assay was used to determine DNA binding activity by AP-1 transcription factor. Expression of cytokeratin 1 was also assessed with immunofluorescence microscopy.

RESULTS

CRH did produce inhibition of proliferation, which was dose-dependent; the shape of the inhibition curve was determined by the media calcium concentration. CRH action was pinpointed at inhibition of the G0/1 to the S phase transition of the cell cycle. CRH also increased AP-1 binding activity, cell granularity, cytokeratin 1 and involucrin expression, and inhibited cytokeratin 14 expression.

CONCLUSIONS

These results are consistent with CRH induction of the keratinocyte differentiation programme. Thus, the overall CRH cutaneous actions connote protective functions for the epidermis, that appear to include the triggering or acceleration of the differentiation programme.

Molecular and functional characterization of novel CRFR1 isoforms from the skin

In our continued studies on corticotropin releasing factor receptor (CRFR1) signaling in the skin, we tested functional activity of CRFR1alpha, e, f, g and h isoforms after transfection to COS cells. Both membrane-bound and soluble variants are translated in vivo into final protein products that undergo further post-translational modifications. CRFR1alpha was the only isoform coupled directly to adenylate cyclase with the exception of an artificial isoform (CRFR1h2) with the insertion of 37 amino acids between the ligand binding domain and the first extracellular loop that was capable of producing detectable levels of cyclic AMP (cAMP). Soluble isoforms could modulate cell response with CRFR1e attenuating and CRFR1h amplifying CRFR1alpha-coupled cAMP production stimulated by urocortin. Testing with plasmids containing the luciferase reporter gene, and inducible cis-elements (CRE, CaRE, SRE, AP1 or NF-kappaB) demonstrated that only CRFR1alpha was involved directly in the transcriptional regulation, while CRFR1g inhibited CRE activity. Significantly higher reporter gene expression by CRF was observed than that mediated by 4beta-phorbol 12-myristate 13-acetate and forskolin alone, being compatible with the concomitant treatment by phorbol 12-myristate 13-acetate and forskolin. This suggests that both protein kinase A and C can be involved in CRF-dependent signal transduction.

Intracellular calcium measurements of single human skin cells after stimulation with corticotropin-releasing factor and urocortin using confocal laser scanning microscopy

Using confocal laser scanning microscopy we investigated the Ca(2+) distribution in single corticotropin releasing factor- and urocortin-stimulated human skin cells. The models tested included melanoma cells, neonatal melanocytes and keratinocytes, and immortalized HaCaT keratinocytes. The changes in intracellular Ca(2+) signal intensities observed after stimulation of different cell types with corticotropin releasing factor and urocortin showed that: (1) the increase of intracellular Ca(2+) concentration was caused by a Ca(2+) influx (inhibition by EGTA); (2) this Ca(2+) influx took place through voltage-activated Ca(2+) ion channels (inhibition by d-cis-diltiazem, verapamil) and (3) cyclic nucleotide-gated ion channels were not involved in this process (no effect of Mg(2+)). The effects were also observed at very low peptide concentrations (10(-13) M) with no apparent linear correlation between peptide dosage and increase of fluorescence intensity, which implied co-expression of different corticotropin releasing factor receptor forms in the same cell. Immortalized (HaCaT) keratinocytes exhibited the strongest differential increases of a Ca(2+) fluorescence after peptide-stimulation. Corticotropin releasing factor induced Ca(2+) flux into the cytoplasm, while urocortin Ca(2+) flux into the nucleus with a remarkable oscillatory effect. The latter indicated the presence of an intracellular urocortin-induced signal transduction pathway that is unique to keratinocytes.

Electrophysiological effects of sustained delivery of CRF and its receptor agonists in hippocampal slices

The corticotropin-releasing factor (CRF) is a hypothalamic peptide that regulates the release of adrenocorticotropic hormone (ATCH) and of beta-endorphin. It has been suggested that it modulates learning and memory processes in rat. However, the electrophysiological effects that CRF produces on hippocampal neurons have been so far little investigated. In particular, the effects of CRF on long-term potentiation (LTP), a phenomenon which is thought to be the substrate of memory processes, are unknown. We studied the effects of sustained administration of CRF and of two of its receptor agonists on basal neuronal activity and on in vitro hippocampal LTP. The two receptor agonists were D-Glu-20-CRF and D-Pro-5-CRF, selective for the CRF-R1 and the CRF-R2 receptors, respectively. We found that CRF, D-Pro-5-CRF and D-Glu-20-CRF at the concentration of 1 nM diminish the amplitude of hippocampal population spike and prevent the onset of LTP. Higher concentrations of CFR have less depressing effects on neuronal activity, yet they still prevent the occurrence of LTP.

Cutaneous expression of corticotropin-releasing hormone (CRH), urocortin, and CRH receptors

Studies in mammalian skin have shown expression of the genes for corticotropin-releasing hormone (CRH) and the related urocortin peptide, with subsequent production of the respective peptides. Recent molecular and biochemical analyses have further revealed the presence of CRH receptors (CRH-Rs). These CRH-Rs are functional, responding to CRH and urocortin peptides (exogenous or produced locally) through activation of receptor(s)-mediated pathways to modify skin cell phenotype. Thus, when taken together with the previous findings of cutaneous expression of POMC and its receptors, these observations extend the range of regulatory elements of the hypothalamic-pituitary-adrenal axis expressed in mammalian skin. Overall, the cutaneous CRH/POMC expression is highly reactive to common stressors such as immune cytokines, ultraviolet radiation, cutaneous pathology, or even the physiological changes associated with the hair cycle phase. Therefore, similar to its central analog, the local expression and action of CRH/POMC elements appear to be highly organized and entrained, representing general mechanism of cutaneous response to stressful stimuli. In such a CRH/POMC system, the CRH-Rs may be a central element.

Therapeutic potential of CRF receptor antagonists: a gut-brain perspective

Activation of the corticotropin-releasing factor (CRF) family of neuropeptide receptors in the brain and periphery appears to mediate stress-related changes in a variety of physiological and functional domains. Comparative pharmacology of CRF receptor agonists suggests that CRF, urocortin, sauvagine and urotensin consistently mimic, and conversely peptide CRF receptor antagonists lessen, the functional consequences of stressor exposure. Together with the development of novel non-peptide CRF receptor antagonists, a growing number of CRF receptor selective ligands are available to elucidate the neurobiology and physiological role of CRF systems. The present review considers available preclinical evidence as well as results from one Phase II clinical trial which address the hypothesis that CRF receptor antagonists may represent a new option for pharmacotherapy of stress-related disorders.

Corticotropin releasing hormone and proopiomelanocortin involvement in the cutaneous response to stress

The skin is a known target organ for the proopiomelanocortin (POMC)-derived neuropeptides alpha-melanocyte stimulating hormone (alpha-MSH), beta-endorphin, and ACTH and also a source of these peptides. Skin expression levels of the POMC gene and POMC/corticotropin releasing hormone (CRH) peptides are not static but are determined by such factors as the physiological changes associated with hair cycle (highest in anagen phase), ultraviolet radiation (UVR) exposure, immune cytokine release, or the presence of cutaneous pathology. Among the cytokines, the proinflammatory interleukin-1 produces important upregulation of cutaneous levels of POMC mRNA, POMC peptides, and MSH receptors; UVR also stimulates expression of all the components of the CRH/POMC system including expression of the corresponding receptors. Molecular characterization of the cutaneous POMC gene shows mRNA forms similar to those found in the pituitary, which are expressed together with shorter variants. The receptors for POMC peptides expressed in the skin are functional and include MC1, MC5 and mu-opiate, although most predominant are those of the MC1 class recognizing MSH and ACTH. Receptors for CRH are also present in the skin. Because expression of, for example, the MC1 receptor is stimulated in a similar dose-dependent manner by UVR, cytokines, MSH peptides or melanin precursors, actions of the ligand peptides represent a stochastic (predictable) nonspecific response to environmental/endogenous stresses. The powerful effects of POMC peptides and probably CRH on the skin pigmentary, immune, and adnexal systems are consistent with stress-neutralizing activity addressed at maintaining skin integrity to restrict disruptions of internal homeostasis. Hence, cutaneous expression of the CRH/POMC system is highly organized, encoding mediators and receptors similar to the hypothalamic-pituitary-adrenal (HPA) axis. This CRH/POMC skin system appears to generate a function analogous to the HPA axis, that in the skin is expressed as a highly localized response which neutralizes noxious stimuli and attendant immune reactions.

Corticotropin-releasing factor antagonists, binding-protein and receptors: implications for central nervous system disorders

Corticotrophin-releasing factor (CRF; interchangeable with corticotrophin-releasing hormone, CRH) is a neurohormone family of peptides which implements endocrine, physiological and behavioural responses to stressor exposure. Built-in biological diversity and selectivity of CRF system function is provided by multiple endogenous ligands and receptors which are heterogeneously distributed in both brain and peripheral tissues across species. At present, there are at least five distinct targets for CRF with unique cDNA sequences, pharmacology and localization. These fall into three distinct classes, encoded by three different genes and have been termed the CRF1 and CRF2 receptors and the CRF-binding protein. Significant gains in knowledge about the physiological role of CRF binding sites in brain have emerged recently due to the proliferation of novel, high-affinity, receptor-selective pharmacological tools as well as multiple knock-out and knock-in mutant mouse models. These results support a role for CRF binding sites in co-ordinating stress reactivity, emotionality and energy balance over the life-span of the organism.