Mystixin-7 and mystixin-11 increase cytosolic free Ca2+ and inositol trisphosphates in human A-431 cells

Expression of molecular equivalent of hypothalamic-pituitary-adrenal axis in adult retinal pigment epithelium

We have investigated expression of molecular elements of the hypothalamic-pituitary-adrenal (HPA) axis in the human retinal pigment epithelium (RPE) cells. The presence of corticotropin-releasing factor (CRF); urocortins I, II and III; CRF receptor type 1 (CRFR1); POMC and prohormone convertases 1 and 2 (PC1 and PC2) mRNAs were shown by RT-PCR; the protein products were detected by ELISA, western blot or immunocytochemical methods in an ARPE-19 cell line derived from an adult human donor. CRFR2 was below the level of detectability. The CRFR1 was functional as evidenced by CRF stimulation of cAMP and inositol triphosphate production as well as by ligand induction of transcriptional activity of inducible cis-elements cAMP responsive element (CRE), activator protein 1 responsive element (AP-1) and POMC promoter) in ARPE-19 using luciferase reporter assay. Immunoreactivities representative of CRF, pre-urocortin, CRFR1 receptor and ACTH were also detected in mouse retina by in situ immunocytochemistry. Finally, using RT-PCR, we detected expression of genes encoding four key enzymes participating in steroids synthesis (CYP11A1, CYP11B1, CYP17 and CYP21A2) and showed transformation of progesterone into cortisol-immunoreactivity in cultured ARPE-19 cells. Therefore, we suggest that ocular tissue expresses CRF-driven signalling system that follows organisational structure of the HPA axis.

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.

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.

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.

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-like peptides increase cytosolic [Ca2+] in human epidermoid A-431 cells

This study investigated whether sauvagine and urotensin I change [Ca2+]i in human epidermoid A-431 cells and whether these changes are correlated with their anti-edema properties in vivo. A-431 cells were used because they possess the corticotropin-releasing factor (CRF) receptor 2. Treatment with either sauvagine or urotensin I led to an immediate increase in [Ca2+]i, the magnitude of which depended on the concentration of the drug. Sauvagine was more effective than urotensin I, with a median effective concentration (EC50) of 1.4 +/- 0.2 fM, compared to an EC50 of 66 +/- 7 fM for urotensin I. Both were more effective at stimulating increases in [Ca2+]i than CRF (EC50 of 6.8 +/- 0.1 pM). There was a correlation between the EC50 for increasing [Ca2+]i and the median effective dose (ED50) for inhibiting edema induced by heating rat paw (r = 0.99). Removal of extracellular Ca2+ or incubation with La3+ eliminated the increase in [Ca2+]i stimulated by either peptide. Pretreatment with a CRF receptor antagonist reduced the increase in [Ca2+]i by these peptides. This occurred in an antagonist concentration-dependent manner, with median inhibitory concentrations (IC50) of 1.99 +/- 0.04 nM and 0.85 +/- 0.04 nM, respectively. Both pertussis toxin (an inhibitor of G proteins) and U-73122 (an inhibitor for inositol trisphosphate (InsP3) production) partially inhibited the increases. InsP3 was measured to determine whether these peptides mobilized Ca2+ from an InsP3-sensitive store. Both sauvagine and urotensin I increased InsP3. The InsP3 increases were inhibited by U-73 122 and CRF antagonist, but not by removal of external Ca2+. Both peptides elevated protein tyrosine phosphorylation. In summary, these peptides increase [Ca2+]i as a result of Ca2+ influx via CRF receptor-operated Ca2+ channels coupled to pertussis toxin-sensitive G proteins and a Ca2+ mobilization from InsP3-sensitive Ca2+ pools. Their in vivo effectiveness at inhibiting edema is related to their respective capacities to stimulate elevations of [Ca2+]i, supporting a role for intracellular Ca2+ in this process.