Adenosine and adenine nucleotides stimulation of skin (epidermal) adenylate cyclase

Chemical Probes for the Adenosine Receptors

Research on the adenosine receptors has been supported by the continuous discovery of new chemical probes characterized by more and more affinity and selectivity for the single adenosine receptor subtypes (A₁, A_2A, A_2B and A₃ adenosine receptors). Furthermore, the development of new techniques for the detection of G protein-coupled receptors (GPCR) requires new specific probes. In fact, if in the past radioligands were the most important GPCR probes for detection, compound screening and diagnostic purposes, nowadays, increasing importance is given to fluorescent and covalent ligands. In fact, advances in techniques such as fluorescence resonance energy transfer (FRET) and fluorescent polarization, as well as new applications in flow cytometry and different fluorescence-based microscopic techniques, are at the origin of the extensive research of new fluorescent ligands for these receptors. The resurgence of covalent ligands is due in part to a change in the common thinking in the medicinal chemistry community that a covalent drug is necessarily more toxic than a reversible one, and in part to the useful application of covalent ligands in GPCR structural biology. In this review, an updated collection of available chemical probes targeting adenosine receptors is reported.

Effect of propylthiouracil on adenosine deaminase activity and thyroid function in patients with psoriasis

BACKGROUND

T-cell activation has been implicated in the pathogenesis of psoriasis; adenosine deaminase (ADA) activity has been considered as a marker of T-cell activation. The antithyroid drug propylthiouracil (PTU) has recently been shown to have beneficial effects on psoriatic lesions, probably by acting on the immune system.

OBJECTIVES

To investigate whether ADA activity may be related to psoriasis and whether oral PTU affects ADA activity and gives clinical improvement in psoriatic patients.

METHODS

ADA activities were measured in plasma, erythrocyte and tissue samples of patients with psoriasis before and after 2 months of treatment with either PTU 100 mg three times daily or PTU plus thyroxine 25 microg once daily (to prevent possible hypothyroidism, which may be induced by PTU) as well as in healthy controls. The severity of the disease was evaluated before and after treatment according to Psoriasis Area and Severity Index (PASI) scores. Routine analyses and thyroid function tests were also carried out during the study.

RESULTS

All patients showed significant clinical improvement in their lesions and decreased PASI scores after the treatments. Elevated baseline ADA activities in skin and plasma were found to be lower, and decreased baseline erythrocyte ADA was higher, after the treatments in all patients, and they were not different from control values. Although thyroid function tests were not affected by the treatments, serum thyroid-stimulating hormone levels were found to be higher after the treatments, and there was a larger increase in patients treated with PTU alone. However, none of the patients had clinical hypothyroidism or cytopenia.

CONCLUSIONS

ADA activity may be clinically useful for indicating T-cell activation in psoriasis. Because of its antiproliferative and immunomodulatory effects, antioxidant potential and low toxicity, PTU may be an effective agent in the treatment of psoriasis.

Adenosine- and adenine-nucleotide-mediated inhibition of normal and transformed keratinocyte proliferation is dependent upon dipyridamole-sensitive adenosine transport

Extracellular adenosine and its related nucleotides have been referred to as retaliatory metabolites that can be released into the extracellular environment during inflammation, wounding, and other pathologic states. We have previously reported that these compounds reversibly inhibit the proliferation of normal keratinocyte cultures and we now demonstrate that these compounds also arrest the proliferation of transformed keratinocytes. Although our study shows that keratinocytes express mRNA corresponding to the A2B purinoreceptors and that adenosine or AMP treatment elevates intracellular cAMP in these cells, our study also demonstrates that dipyridamole-inhibitable transport of adenosine into the keratinocyte is central to the mechanism by which adenosine and adenine nucleotides arrest proliferation in these cells. In support of this mechanism, our results demonstrate that human keratinocytes express mRNA corresponding to the recently cloned dipyridamole-sensitive human equilibrative nucleoside transporter. Interestingly, coincubation with adenosine deaminase reverses the antiproliferative action of adenosine and exerts no effect on the antiproliferative activity of the adenine nucleotides, thus supporting a model in which adenine nucleotides are enzymatically converted to adenosine and transported into the keratinocyte in a tightly coupled and adenosine-deaminase-resistant manner. Analysis of adenosine- and adenosine-monophosphate-treated keratinocytes demonstrated that quiescence is induced within 12-24 h, and fluorescence-activated cell sorter analysis suggests that treatment with these compounds may result in the inhibition of keratinocyte proliferation at both G1 and S phases of the cell cycle. In addition to their documented antiproliferative action on other cell types, adenosine, adenine nucleotides, and related analogs may also represent a potential new class of pharmacologic regulators of keratinocyte proliferation in vivo.

Neuroendocrinology of the skin

The classical observations of the skin as a target for melanotropins have been complemented by the discovery of their actual production at the local level. In fact, all of the elements controlling the activity of the hypothalamus-pituitary-adrenal axis are expressed in the skin including CRH, urocortin, and POMC, with its products ACTH, alpha-MSH, and beta-endorphin. Demonstration of the corresponding receptors in the same cells suggests para- or autocrine mechanisms of action. These findings, together with the demonstration of cutaneous production of numerous other hormones including vitamin D3, PTH-related protein (PTHrP), catecholamines, and acetylcholine that share regulation by environmental stressors such as UV light, underlie a role for these agents in the skin response to stress. The endocrine mediators with their receptors are organized into dermal and epidermal units that allow precise control of their activity in a field-restricted manner. The skin neuroendocrine system communicates with itself and with the systemic level through humoral and neural pathways to induce vascular, immune, or pigmentary changes, to directly buffer noxious agents or neutralize the elicited local reactions. Therefore, we suggest that the skin neuroendocrine system acts by preserving and maintaining the skin structural and functional integrity and, by inference, systemic homeostasis.

Adenosine and adenine nucleotides inhibit the autonomous and epidermal growth factor-mediated proliferation of cultured human keratinocytes

Previous investigations have shown disparate effects of adenine nucleotides on epidermal cell proliferation. Our present study demonstrates that adenosine and its related nucleotides (ATP, ADP, AMP) are antiproliferative for normal human epidermal keratinocytes cultured in the absence or presence of exogenous epidermal growth factor. Furthermore, the inhibitory effects of these compounds occur at concentrations less than 100 microM, are reversible, and do not affect the viability of the keratinocyte cultures. Our current investigation also demonstrates that both selective and nonselective adenosine receptor agonists are themselves approximately as potent as keratinocyte proliferation inhibitors, but are all less potent inhibitors than adenosine. These observations are consistent with the theory that adenosine mediates its antiproliferative response via a novel or more poorly characterized adenosine purinoreceptor subclass. Moreover, our present study demonstrates that ATP and ATP-gamma-S are significantly more potent antiproliferative agents than either alpha,beta-methylene ATP or beta,gamma-methylene ATP. Based on previous studies that have demonstrated that P2y purinoreceptors possess this type of ligand specificity and that the P2y purinoreceptor may be expressed by keratinocyte cultures, we propose that ATP may mediate its antiproliferative effects via this purinoreceptor. Collectively, our results indicate that adenosine and adenine nucleotides abrogate exogenous epidermal growth factor-dependent and -independent keratinocyte proliferation at submillimolar concentrations and may be important physiologic regulators of keratinocyte growth in vivo. Further, these results suggest that these or related compounds may have application as treatments for epidermal growth factor receptor-signaling pathway has been activated.

Direct effects of cutaneous neuropeptides on adenylyl cyclase activity and proliferation in a keratinocyte cell line: stimulation of cyclic AMP formation by CGRP and VIP/PHM, and inhibition by NPY through G protein-coupled receptors

Many neuropeptides are present in the peripheral nerves of human skin and are distributed from the intraepidermis to subcutaneous appendages, and those peptides are considered to be involved in the pathogenesis of various inflammatory dermatoses. In this investigation, we determined the effects of various neuropeptides on intracellular cyclic adenosine-5'-monophosphate (AMP) formation in cultured human keratinocytes. Among the many peptides tested, calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), peptide histidine-methionine (PHM), and growth hormone releasing factor (GRF) stimulated a rapid and marked formation of intracellular cyclic AMP in keratinocytes in a dose-dependent manner. The direct association of the receptors for CGRP and VIP with adenylyl cyclase in keratinocytes was confirmed by the findings that CGRP and VIP stimulated the enzyme activity in membrane preparations derived from cultured keratinocytes in the presence of guanosine triphosphate (GTP). On the other hand, neuropeptide Y (NPY) showed an inhibitory effect on forskolin-induced cyclic AMP accumulation in keratinocytes. This inhibitory effect of NPY was completely eliminated by glucocorticoid pretreatment of cultured keratinocytes. Furthermore, the presence of peptides that substantially increase intracellular cyclic AMP accumulation also stimulated DNA synthesis and proliferation in a human keratinocyte cell line in a dose-dependent manner. These results suggest that neuropeptides work directly as biologic modulators of keratinocytes through the cyclic AMP cascade.

Cyclic AMP as a negative regulator of DNA synthesis in FRSK cells, a fetal rat epidermal cell line

In FRSK cells, a cell line derived from fetal rat epidermal cells, cyclic AMP-elevating agents forskolin (10 microM) and cholera toxin (10 ng/ml) increased cellular cyclic AMP content and suppressed [3H] thymidine incorporation. These effects of forskolin were enhanced by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.1 mM). Dibutyryl cyclic AMP (1 mM), an analog of cyclic AMP, decreased not only basal but also both tumor promoter 12-O-tetradecanoylphorbol-13-acetate- and epidermal growth factor-stimulated [3H] thymidine incorporation. From these results, we suggest that cyclic AMP may be a negative regulatory factor of DNA synthesis in FRSK cells.

Adenylate cyclase system of human trichilemmoma cell line

The adenylate cyclase system of an established human trichilemmoma cell line was investigated. Stimulators of human epidermal adenylate cyclase system, epinephrine, histamine, adenosine, and prostaglandin E increased cyclic AMP levels of the trichilemmoma cells. The effects of epinephrine, histamine, and adenosine were inhibited by the addition of propranolol (a beta-adrenergic antagonist), cimetidine (histamine H2-antagonist), and theophylline (adenosine-receptor antagonist), respectively. The epinephrine, histamine, and prostaglandin E effects were augmented by the addition of cyclic AMP (cAMP) phosphodiesterase inhibitor, isobutylmethylxanthine (IBMX); the adenosine effect was augmented by another phosphodiesterase inhibitor, papaverine. Without the addition of these phosphodiesterase inhibitors, the maximal accumulations were observed at 3 min incubation. Following this, the cAMP content returned to the basal level, and the cells did not respond to repeated stimulations with the same initial stimulator. This fact indicates receptor-specific refractoriness. For example, epinephrine-pretreated cells did not respond to epinephrine, but retained their sensitivity to histamine. It has been known that normal epidermal keratinocytes are regulated in vitro by glucocorticoids, colchicine, and retinoids, resulting in the augmentation of their beta-adrenergic response. Only hydrocortisone treatment on the trichilemmoma cells resulted in the augmentation of the beta-adrenergic response. Although the established human trichilemmoma cell line has similar adenylate cyclase systems as normal epidermis, it apparently has lost some of the regulatory mechanism of the beta-adrenergic response.

Characterization of the purine-reactive site of the rat testis cytosolic adenylate cyclase

Naturally soluble rat germ cell adenylate cyclase was inhibited by adenosine and the adenosine analogs, 9-beta-D-arabinofuranosyl adenine (AFA) and 2',5'-dideoxyadenosine (DDA), all of which inhibited hormone-sensitive adenylate cyclases at the "P" site. The IC50 values for adenosine and DDA were approximately 0.1 and for AFA, 4.0 mM. The onset of adenosine inhibition was very rapid whether adenosine was added to the enzyme reactant mixture at time zero concomitantly with the addition of substrate or after the enzyme had been activated by the addition of substrate. The adenosine analogs, N6-methyladenosine (MeA) and N6-phenylisopropyl adenosine (PIA), which interact with plasma membrane receptors ("R" receptors) for hormone-sensitive adenylate cyclase, had little effect on the activity of the cytosolic adenylate cyclase. Additionally, aminophylline, which has been shown to competitively antagonize adenosine interactions with the plasma membrane "R" receptors but not "P" site interactions, had no effect upon substrate activation of the soluble enzyme and did not prevent adenosine from inhibiting the activity of the enzyme. These data provide evidence for an adenosine regulatory site on the cytosolic enzyme which resembles the "P" site described for membrane bound-adenylate cyclase.

Cytochemical localization of VIP-stimulated adenylate cyclase activity in human sweat glands

Vasoactive intestinal polypeptide (VIP)-stimulated adenylate cyclase was demonstrated cytochemically in human sweat glands. Biopsies containing axillary sweat glands were incubated with 5 micron VIP and adenylate cyclase was demonstrated using the cerium-salt technique. Reaction products were localized in the plasma membranes of glandular and myoepithelial cells. Thus, VIP appears to stimulate adenylate cyclase activity without acetylcholine. VIP and acetylcholine in human sweat glands most probably act through a receptor complex, where adenylate cyclase is coupled to that part of the receptor stimulated by VIP.

Dimethyl sulfoxide-induced augmentation of adenosine-adenylate cyclase response of pig skin epidermis

Adenosine-adenylate cyclase response in pig skin epidermis showed a specific increase after long-term (24 h) incubation in the presence of 0.5%-1% dimethyl sulfoxide (DMSO). There was no significant difference between control and DMSO-treated epidermis with regard to cyclic AMP (cAMP) phosphodiesterase activity. DMSO had no effect on the basal cAMP levels of epidermis; beta-adrenergic and histamine-adenylate cyclase responses were not affected. The direct addition of DMSO at the time of incubation with various adenylate cyclase stimulators (adenosine, epinephrine, and histamine) had no effect on agonist-induced cAMP accumulation effects. It was concluded that DMSO affected epidermal keratinocytes during long-term incubation, resulting in a specific increase in the adenosine-adenylate cyclase response. Although the biological significance of this DMSO effect remains to be determined, it should be kept in mind when using DMSO as a solvent for various chemicals in the experiments dealing with epidermal keratinocytes in vitro.

Methylxanthine and non-xanthine phosphodiesterase inhibitors. Their effects on adenosine uptake and the low Km cyclic AMP phosphodiesterase in intact rat adipocyte

The effects of methylxanthines and non-xanthine phosphodiesterase-inhibitors on the low Km cyclic AMP phosphodiesterase of intact rat adipocytes were studied. Methylxanthines and papaverine stimulated rather than inhibited the enzyme when intact adipocytes were incubated in their presence. The effect of papaverine was not abolished by adenosine deaminase and was enhanced by adenosine. On the other hand, the effect of xanthine inhibitors and adenosine do not enhance each other. The difference in behaviour of these inhibitors could not be explained by their effects on adenosine uptake at the concentrations studied. Both agents inhibited adenosine uptake when measured after 15 sec and 10 min, with methylisobutylxanthine (MIX) having a greater inhibitory effect than papaverine only if uptake was measured after 15 sec. Effects similar to that of adenosine with the inhibitors on phosphodiesterase were obtained with insulin, which has been shown to act through a similar or related mechanism to that of adenosine. This was not the case with lipolytic agents whose effects were not potentiated by either MIX or papaverine. Under certain conditions the degree of stimulation of the enzyme was in fact decreased. Thus lipolytic and antilipolytic agents probably stimulated phosphodiesterase through distinct mechanisms.

Glucocorticoid-induced alteration of beta-adrenergic adenylate cyclase response of epidermis

It has been reported that the beta-adrenergic adenylate cyclase system of the pig epidermis is regulated by glucocorticoids, resulting in the augmentation of epinephrine-induced cyclic-AMP accumulations. Using this phenomenon, we compared the glucocorticoidal potency of three typical glucocorticoids: hydrocortisone, prednisolone and dexamethasone. There was a considerable variation in the magnitude of the glucocorticoid-induced augmentation of the beta-adrenergic response when pig skin that had been obtained on different occasions was used. In each experimental series (using the same pig skin), however, the maximal augmentation effects obtained with these glucocorticoids were approximately the same. The potent glucocorticoid, dexamethasone, demonstrated its effect at lower concentrations than were required for prednisolone, while hydrocortisone required a much higher concentration before its effect was detectable. Thus, despite considerable variations in the magnitude of the glucocorticoid effects, the concentrations required for the glucocorticoid effect were closely associated with the established glucocorticoidal potency which has previously been described.

Effects of adenosine and 2'-deoxyadenosine on epidermal keratinocyte proliferation: its relation to cyclic AMP formation

Although it has been reported that adenosine has an inhibitory effect on keratinocyte proliferation at both G2 and S phases of the cell cycle, its relation to cyclic AMP formation through the adenylate cyclase system has been less well characterized. In order to determine the precise mechanism of the adenosine effect, another physiologic adenine nucleoside, 2'-deoxyadenosine was employed. 2'-Deoxyadenosine was shown to be remarkably different from adenosine in its ability to stimulate the epidermal adenylate cyclase; whereas adenosine markedly increased cyclic AMP levels of pig epidermis, deoxyadenosine had a much weaker effect on the cyclic AMP levels of the skin. Using several parameters of cell proliferation, comparison was made between the effects of these two compounds. Pig keratinocyte explant culture system was employed for the measurement of outgrowth and mitosis. Mitosis was determined after 72-h incubation (to monitor the overall cell proliferation inhibition) and 4-h incubation (to monitor G2 phase inhibition) with the chemicals. Pig skin keratome slice system was employed for [3H]thymidine uptake measurement. Both adenosine and deoxyadenosine were shown to have marked inhibitory effects on keratinocyte out-growth, [3H]thymidine uptake, and keratinocyte mitosis. The effects of deoxyadenosine on outgrowth and [3H]thymidine uptake were greater than that of adenosine. The inhibitory effect of adenosine and deoxyadenosine on mitosis were about the same in both 4-h and 72-h incubation systems. Thus deoxyadenosine, which is a much weaker stimulator of epidermal adenylate cyclase, was also shown to be as potent an inhibitor of keratinocyte proliferation as adenosine. These results further substantiate the view that cyclic AMP elevating agents (such as adenosine and deoxyadenosine) might not necessarily reveal their inhibitory effects on keratinocyte proliferation through their effects of cyclic AMP formation.

Agents that activate cyclic AMP-dependent protein kinase inhibit explant culture growth and mitotic activity

Epidermal cells contain 4 separate surface receptors which are linked to adenylate cyclase. Activation of any one of these receptors leads to the accumulation of cAMP within the cell which in turn leads to the activation of cAMP-dependent protein kinase. The levels of cAMP accumulation within the cell caused by the 4 activators are not the same. Epinephrine, histamine, adenosine, and prostaglandins of the "E" series cause easily measurable concentrations of cAMP within 5 min of exposure. Prostaglandin F2 alpha causes only a small nonsignificant increase. Similarly, 2 phosphodiesterase inhibitors, which inhibit the breakdown of cAMP formed within the cell, differ in their ability to accumulate cAMP when cells are exposed to these agents alone. Isobutylmethylxanthine causes a measurable increase in cAMP, while theophylline, a weak inhibitor of phosphodiesterase, gives a nonsignificant increase in cAMP. Recently, experiments have shown that agents that give only slight increases in cAMP by biochemical measurements, that is, prostaglandins F2 alpha and theophylline, are equally able to activate protein kinase within the cell. Since activation of protein kinase is the only mechanism for an increase in cAMP to have a physiologic effect, all of these agents that do activate protein kinase should cause physiologic effects. Using an explant culture system, we show in this paper that this supposition is correct and that all agents that activate protein kinase do result in inhibition of mitotic activity regardless of whether or not they are able to raise cAMP to a level that can be biochemically measured as being significantly different from the baseline value.

Adenosine deaminase in human epidermis from healthy and psoriatic subjects

Adenosine deaminase, which catalyzes the irreversible hydrolytic deamination of adenosine and deoxyadenosine to inosine and deoxyinosine respectively, plays an important role in the degradation of adenine nucleotide and purine nucleotide salvage pathway metabolism. We investigated human epidermal adenosine deaminase activity using a radiochemical method, which enabled us to measure the adenosine deaminase activity of protein samples as small as several micrograms. We measured adenosine deaminase activity of microdissected pure epidermis of the healthy skin and the psoriatic affected and unaffected skin. It was shown that psoriatic affected epidermis had increased adenosine deaminase activity compared with the healthy epidermis (P less than 0.05) and the unaffected epidermis (P less than 0.01). There was no difference in enzyme activity between healthy and psoriatic unaffected epidermis. The increased adenosine deaminase activity in the psoriatic affected epidermis may reflect the accelerated salvage pathway of the nucleic acid metabolism probably associated with the hyperproliferative condition of the psoriatic epidermis.

Forskolin activates adenylate cyclase activity and inhibits mitosis in in vitro in pig epidermis

The novel adenylate cyclase activator forskolin caused rapid and high intracellular accumulation of cyclic AMP in a floating skin (epidermal) slice system. Increased cAMP levels were also detected in the media. Addition of a phosphodiesterase inhibitor to forskolin-containing medium caused only a slight increase in the intracellular cAMP level and forskolin itself did not inhibit phosphodiesterase activity. Ka of forskolin for epidermal adenylate cyclase was about 2-3 X 10(-5) M. This forskolin activation was rapidly reversed after washing. The forskolin stimulation (Ka 5 X 10(-5) M) was also found when tested with an epidermal membrane preparation which contained the catalytic unit of adenylate cyclase but lacked either the GTP or receptor stimulation. With the epidermal slice system, the combination of forskolin and epinephrine (or histamine) stimulated adenylate cyclase synergistically. The data suggest that forskolin activates not only the catalytic unit but also the nucleotide regulatory protein or the receptor-regulatory protein complex of the adenylate cyclase system. The cAMP accumulation caused by forskolin produced a dose-dependent mitotic inhibition of epidermal cells in an in vitro outgrowth system. This inhibitory effect was reversible 48 h after washing out the forskolin.

Adenylate cyclase activation by cholera toxin in pig epidermis: an obligatory role of the GTP-regulatory protein

Cholera toxin (CT) stimulates the epidermal adenylate cyclase system in vitro. This stimulation was demonstrated in the skin (slice) floating system and the homogenate (membrane) assay system. With the floating system, the addition of CT to the incubation medium caused a marked accumulation of cAMP intracellularly, which was both dose- and time-dependent. A 1-h lag time was present before activation started. Pretreatment of the skin with CT changed the nature of the stimulatory effect caused by epinephrine and histamine, i.e., the transient accumulation of cAMP (a peak at 5 min and subsequent decrease) was no longer observed but the stimulation became persistent. With the membrane assay system in which the receptor components had been uncoupled, adenylate cyclase activities were markedly stimulated by CT (with guanosine-5'-triphosphate, GTP), guanylyl-beta,gamma-imidodiphosphate (GTP-analog, Gpp[NH]p), or sodium fluoride. The stimulation was both dose- and time-dependent without an initial time lag. Either CT or Gpp[NH]p could fully activate adenylate cyclase, and the simultaneous addition of both did not cause further additive stimulation. These data are consistent with the view that the GTP-regulatory protein plays a key role in the activation of adenylate cyclase, and that CT both activates the catalytic unit and modifies the response to receptor hormones through its action on this protein.

Effects of glucocorticoids on the beta-adrenergic adenylate cyclase system of pig skin

Effects of glucocorticoids on the epidermal beta-adrenergic adenylate cyclase system were investigated. Long-term incubation of pig skin slices in RPMI 1640 medium resulted in the gradual decrease in the epinephrine-induced cyclic AMP accumulations of skin. The addition of hydrocortisone (100 microM) in the incubation medium prevented this decrease, and after 24- and 48-h incubation, there was a marked difference in beta-adrenergic responsiveness between control and hydrocortisone-treated skin. The study using other steroid hormones revealed that this effect on the beta-adrenergic system was relatively specific for glucocorticoids. Hydrocortisone, prednisolone, dexamethasone, and beta-methasone-17-valerate were shown to have marked effects on the beta-adrenergic system, while androstenedione, testosterone, dihydrotestosterone, progesterone, estrone, and beta-estradiol had no effect. Cortisone and estriol were shown to have similar but weaker effects than hydrocortisone. The effect of glucocorticoids was also relatively specific to the beta-adrenergic system, since there was no significant difference in adenosine-or histamine-induced cyclic AMP accumulations of skin after long-term incubation with and without hydrocortisone. The mechanism of this glucocorticoid action does not seem to be through the simple protection of the beta-adrenergic system of the skin, since the addition of hydrocortisone in the incubation medium at 24 or 48 h incubation time, when the epinephrine-induced cyclic AMP accumulation was considerably decreased, reversed the epinephrine unresponsiveness of the skin, after the additional 24-h incubation. Furthermore, the effect of hydrocortisone was inhibited by 3 different kinds of inhibitors: (a) progesterone, an inhibitor of intracytoplasmic glucocorticoid receptor binding; (b) actinomycin D, an inhibitor of messenger RNA (mRNA) synthesis; and (c) cycloheximide, an inhibitor of protein synthesis at the translation step. These results are in accordance with the view that glucocorticoids affect the beta-adrenergic system of epidermis by a mechanism requiring mRNA and protein synthesis possibly through the intracytoplasmic glucocorticoid receptor system of epidermis.

Cyclic AMP-dependent protein kinase isozymes of pig skin and human skin from normal and psoriatic subjects

Cyclic AMP-dependent protein kinase isozymes of pig and human skin (epidermis) were separated by DEAE-cellulose column chromatography after micromodification for small biopsy samples. Clear-cut separations of type I and type II isozymes, which were of about equal amounts, could be obtained only when the ischemia effect was avoided by in vivo freezing of skin and homogenization for less than 10 s. Intradermal injections of epinephrine caused dose-dependent activation of type I isozyme, but not of type II. Injections of other skin adenylate cyclase stimulators such as histamine, adenosine, and prostaglandin E2 elevated the local cyclic AMP levels to not more than 5 pmol/mg protein and also stimulated only the type I isozyme. Incubation of keratome-sliced pig skin under various conditions caused both activation by dissociation and inactivation by reassociation of the subunits, which appeared to be dependent on the cyclic AMP content. Epinephrine added to the incubation medium led to complete activation of both type I and type II isozymes (the intraepidermal cyclic AMP contents ranged from 20-50 pmol/mg protein). The isozymes of normal skin and involved skin of psoriatics showed identical peaks of type I and type II isozymes of equal amounts. The data indicate that protein kinase in the involved skin is not in an activated (by cyclic AMP) state.

Retinoids increase the response of guinea pig but not human keratinocytes to agonists of adenylate cyclase in vitro

Primary cultures of adult human and guinea-pig keratinocytes were treated with various retinoids for up to 7 days. The cells were exposed to 3H-adenine and then challenged with several agonists of adenylate cyclase. 3H-labeled adenine nucleotides were extracted and 3H-cAMP purified chromatographically. All retinoids increased the formation of 3H-ATP from 3H-adenine and the conversion of 3H-ATP to 3H-cAMP by guinea-pig cells, the extent being dependent on the particular retinoid and agonist used. Human cells were relatively sensitive to isoproterenol and not to prostaglandins or histamine, but their response was little affected by retinoids. Guinea-pig cells were most sensitive to prostaglandins. The proliferogenic effects of retinoids on the latter cells may be mediated by increased sensitivity to agonists leading to generation of cAMP.

Epidermal surface receptors which link pharmacological mediators to the adenylate cyclase system

The major purpose of our studies has been to investigate various stimulators of the epidermal adenylate cyclase system. We have recognized the occurrence of four distinct adenylate cyclase systems which respond respectively to catecholamine, histamine, prostaglandin and adenosine. The exposure of floating skin slices in vitro to a stimulator causes a rapid intracellular accumulation of cyclic AMP, which is always transient. Further addition of the same stimulator will not stimulate the same receptor system against the state of 'refractoriness'. The addition of any of the other stimulators can increase the cyclic AMP level. Furthermore, the fact that each stimulator can yield an 'additive' stimulatory effects leads to the conclusion that the epidermis has four distinctly specific and independent adenylate cyclase systems. Our recent investigations have been directed to the analyses of subunits of these skin surface receptor-adenylate cyclase systems. We used two experimental systems, i.e. one being a 'leaky' cell system in which its subunits such as receptor, GTP-regulatory protein and the catalytic unit (adenylate cyclase) are still linked together, and the other being independent preparations of the receptor and catalytic units (with GTP-regulatory protein). These systems allowed us to probe the cell membrane from the inside as well as from the outside. Some of the preliminary kinetic data are herein introduced.

Activation of cAMP-dependent protein kinase in epidermis by the compounds which increase epidermal cAMP

Pig epidermal slices were incubated with various compounds which increased epidermal cAMP (adenosine 3',5'-monophosphate), and the change in cAMP-dependent protein kinase activity ratio was studied by the method of Cherrington et al (J Biol Chem 251:5209-5218, 1976) with modification. Epinephrine (5 x 10(-5) M), histamine (10(-4) M) and adenosine (10(-3) M), potent agonists of epidermal adenyl cyclase, fully activated the protein kinase (PK) during an incubation of 30 to 45 seconds, that was much shorter than that required for maximal cAMP accumulation under the same conditions (5 min). With such a brief stimulus, the epidermal cAMP-PK system did not become refractory and responded to repeated stimuli. Prostaglandin E2 (PGE2) and isobuthylmethylxanthine (IBMX) and ethanol only partially activated the enzyme. Prostaglandin F2 alpha (PGF2 alpha) and theophylline which were much less effective in increasing epidermal cAMP, activated the enzyme to the same extent as PGE2 and IBMX respectively. These results suggest that protein kinase activation takes place in response to a cAMP increase in small locus of the cell. Such an increase in cAMP can be very small or even not measurable when measured as total cAMP in the tissue homogenate. Also, increases above this level may not be physiologic. It is concluded that measurement of cAMP-dependent protein kinase activity ratio is a more direct and more sensitive way to study the effect of compounds which act through cAMP mediated mechanisms.

Purification and properties of adenylyl sulphate:ammonia adenylyltransferase from Chlorella catalysing the formation of adenosine 5' -phosphoramidate from adenosine 5' -phosphosulphate and ammonia

Extracts of Chlorella pyrenoidosa, Euglena gracilis var. bacillaris, spinach, barley, Dictyostelium discoideum and Escherichia coli form an unknown compound enzymically from adenosine 5'-phosphosulphate in the presence of ammonia. This unknown compound shares the following properties with adenosine 5'-phosphoramidate: molar proportions of constituent parts (1 adenine:1 ribose:1 phosphate:1 ammonia released at low pH), co-electrophoresis in all buffers tested including borate, formation of AMP at low pH through release of ammonia, mass and i.r. spectra and conversion into 5'-AMP by phosphodiesterase. This unknown compound therefore appears to be identical with adenosine 5'-phosphoramidate. The enzyme that catalyses the formation of adenosine 5'-phosphoramidate from ammonia and adenosine 5'-phosphosulphate was purified 1800-fold (to homogeneity) from Chlorella by using (NH(4))(2)SO(4) precipitation and DEAE-cellulose, Sephadex and Reactive Blue 2-agarose chromatography. The purified enzyme shows one band of protein, coincident with activity, at a position corresponding to 60000-65000 molecular weight, on polyacrylamide-gel electrophoresis, and yields three subunits on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis of 26000, 21000 and 17000 molecular weight, consistent with a molecular weight of 64000 for the native enzyme. Isoelectrofocusing yields one band of pI4.2. The pH optimum of the enzyme-catalysed reaction is 8.8. ATP, ADP or adenosine 3'-phosphate 5'-phosphosulphate will not replace adenosine 5'-phosphosulphate, and the apparent K(m) for the last-mentioned compound is 0.82mm. The apparent K(m) for ammonia (assuming NH(3) to be the active species) is about 10mm. A large variety of primary, secondary and tertiary amines or amides will not replace ammonia. One mol.prop. of adenosine 5'-phosphosulphate reacts with 1 mol.prop. of ammonia to yield 1 mol.prop. each of adenosine 5'-phosphoramidate and sulphate; no AMP is found. The highly purified enzyme does not catalyse any of the known reactions of adenosine 5'-phosphosulphate, including those catalysed by ATP sulphurylase, adenosine 5'-phosphosulphate kinase, adenosine 5'-phosphosulphate sulphotransferase or ADP sulphurylase. Adenosine 5'-phosphoramidate is found in old samples of the ammonium salt of adenosine 5'-phosphosulphate and can be formed non-enzymically if adenosine 5'-phosphosulphate and ammonia are boiled. In the non-enzymic reaction both adenosine 5'-phosphoramidate and AMP are formed. Thus the enzyme forms adenosine 5'-phosphoramidate by selectively speeding up an already favoured reaction.

Effects of trypsin on the cyclic AMP system of the pig skin

Trypsin increased cyclic AMP levels of the pig skin. This effect was markedly potentiated by the cyclic nucleotide phosphodiesterase inhibitor theophylline. Without theophylline this trypsin-induced cyclic AMP accumulation was transient and the maximal accumulation was noted by 5 min. Soybean trypsin inhibitor inhibited this trypsin-induced cyclic AMP accumulation. After the trypsin treatment, marked acantholysis was noted histologically and the decreased responsiveness to other adenylate cyclase stimulators was seen. The decrease of the epinephrine response was most marked and that of histamine response was much less. Both low and high Km cyclic nucleotide phosphodiesterase activities were decreased by the trypsin treatment. However, at 5 min incubation time, when the increase in cyclic AMp level was most marked, the decrease in the phosphodiesterase activities was minimal. Trypsin seems to reveal its action through the proteolytic activation of adenylate cyclase system of the skin.

The effect of histamine on epidermal outgrowth: its possible dual role as an inhibitor and stimulator

We have investigated the effect of histamine on pig epidermal cell outgrowths in vitro. Histamine inhibited the epidermal cell outgrowths (and also mitosis). This inhibition was partially counteracted by a specific H2 antagonist, cimetidine. Inhibition was maximal at a histamine concentration of 10(-4) M and was less at 10(-3) M. These histamine concentrations respectively coincide with the optimal concentrations for accumulating intracellular cyclic AMP (via H2 receptors) and cyclic GMP (via H1 receptors) in the same pig epidermal slice system. 4-Methyl-histamine, a pure H2 agonist, which only increased the intracellular cyclic AMP level but not the cyclic GMP level, caused a maximal outgrowth inhibition at 10(-3) M. Attempts to counteract the histamine effects due to cyclic GMP accumulation by various H1 antagonists (so that 10(-3) M histamine would have caused maximal outgrowth inhibition) were unsuccessful, since the addition of each H1 antagonist alone strongly inhibited the outgrowth. These data strongly suggest a dual role of histamine through the cyclic nucleotide system; i.e., histamine inhibits epidermal cell growth by elevating the intracellular cyclic AMP level via an H2 receptor, while histamine at high concentrations (10(-3) M) partially counteracts the inhibition by increasing cyclic GMP via an H1 receptor.

Regional cyclic AMP levels in homogenates of rat brain after ketalar and trifluoperazine

There was a significant fall in cAMP levels after administration of TFP or ketalar. Different amounts of cAMP were present in different regions of rat brain. Concentrations of cAMP in different regions of the rat brain were found to decrease in the following order: cerebrum > thalamus with hypothalamus > midbrain > hippocampus > cerebral cortex.

ATP enhances cyclic AMP accumulation by intact P388 mouse lymphoma cells

One of the characteristics of malignant cells is a poor response to hormones and a low level of cyclic AMP. Whilst this is true of intact P388 mouse lymphoma cells, high levels of adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) activity can be measured in particulate preparations of these cells. When ATP is added to the incubation medium of intact lymphoma cells, the cyclic AMP level is enhanced. This effect of ATP is not mediated by adenosine, nor is it enhanced by NaF. The ATP content of the lymphoma cells is much lower than that of CH23 Chinese hamster fibroblast and PCM3 hybrid cells, whose cyclic AMP levels are not affected by the presence of ATP. This suggests that adenylate cyclase, in the lymphoma cells, is bathed in a pool which is deficient in substrate. The substrate concentration of this pool is thought to be elevated by addition of ATP to the incubation medium with ATP, itself, crossing the plasma membrane.

The influence of adenosine on intermediary metabolism of isolated hepatocytes

The effect of adenosine was tested on the energetic metabolism of fed rat liver cells after isolation. The cells were incubated in a buffered saline medium with glucose (5 mM) and adenosine (1 mM) for 30 minutes at 37 degrees C. This increased the concentration of the adenylic nucleotides ATP (+57 per cent, ADP (+39 per cent). Cyclic AMP was increased (+50 per cent) and the intracellular inorganic phosphate decreased (-22 per cent). These changes were accompaned by a decrease of glycogenolysis, glucose consumption and lactate production. Measurement of glycolytic intermediates showed decreased concentrations of fructose 1,6-bis-phosphate and 3-phosphoglycerate proportional to the increase in ATP concentration. The near-equilibrium of the glyceraldehyde 3-phosphate dehydrogenase-phosphoglycerate kinase system was not modified by adenosine. The decrease of the NAD+/NADH ratio along with the increase of the ATP/ADP X PO4 ratio explains the decrease of 3-phosphoglycerate. The decrease in glucose consumption can be explained by the cross over at the phosphofructokinase stage with the decrease of fructose 1,6-bisphosphate. The major part of adenosine was deaminated as indicated by an increase in the production of ammonia and urea. The effects of inosine, or adenosine along with an inhibitor of adenosine deaminase (pentostatin) suggest that adenosine acts on the glucose consumption through adenylic nucleotides. However the increase of the adenylic nucleotide level cannot totally explain the other metabolic changes: decrease of the NAD+/NADH cytoplasmic ratio, constancy of this ratio in mitochondria, decrease of gluconeogenesis from lactate. A direct action of adenosine can therefore be expected.

Inhibitors of epidermal cell DNA synthesis in surviving pig skin in vitro

Keratome slices of domestic pig skin were used to study the DNA synthesis phase of epidermal cell DNA synthesis. Cyclic AMP and agents which elevate intracellular concentrations of cyclic AMP have no direct effect on the "S" phase of DNA synthesis. Theophylline, isobutylmethylxanthine, and adenosine inhibit DNA synthesis immediately by a mechanism which is reversible and is not dependent on cyclic AMP. This inhibition is not associated with an increase in intracellular thymidine phosphates. Hydroxyurea, however, inhibits DNA synthesis immediately and does produce an elevated pool of thymidine phosphates.

Hemicyst formation stimulated by cyclic AMP in dog kidney cell line MDCK

Certain epithelial cell lines have morphologic, physiologic, biochemical and pharmacologic characteristics of transporting epithelia from intact organs. In this paper we show that dibutyryl cyclic AMP, 5' AMP, adenosine and cyclic AMP phosphodiesterase inhibitors stimulate hemicyst formation by the dog kidney cell line MDCK. It is suggested that this effect is explained by elevation of intracellular cyclic AMP levels by means of an exogenous non-metabolizable source of cyclic AMP, phosphodiesterase inhibition or adenyl cyclase stimulation. Since hemicyst formation is in part due to transepithelial fluid transport, these findings raise the possibility that this fraction might be modulated by cAMP in an established cell line. We believe that cultured epithelial cells may provide an exploitable model system to investigate at the cellular and subcellular levels, the mechanism by which cyclic AMP modifies water and solute movements across epithelia.

Effect of in vitro cultivation and Mycoplasma pneumoniae infection on intracellular cyclic AMP levels in hamster tracheal organ cultures

Exogenous cyclic AMP and dibutyryl cyclic AMP decreased the relative ciliary activity values of tracheal organ cultures. In contrast, theophylline and cholera toxin were not ciliostatic. The use of a radioimmunoassay for cyclic AMP indicated that all of the tested substances increased intracellular cyclic AMP levels to some extent (from 3-fold for cholera toxin to almost 40-fold for dibutyryl cyclic AMP). Physical inactivation of explants by either freeze-thaw or heat destroyed all ciliary activity and greatly decreased intracellular cyclic AMP levels. Cyclic AMP levels of explants remained relatively constant during in vitro cultivation. Three strains of Mycoplasma pneumoniae were found to contain extremely low amounts of cyclic AMP. Infection of tracheal explants produced a significant decrease in relative ciliary activity, but only a slight decline in organ-culture cyclic AMP levels.

Effect of prostaglandins on cyclic nucleotide levels in cultured keratinocytes

Ginea pig ear epidermal cells (keratinocytes) were established in primary cultures using trypsin, and treated in their proliferative phase of growth with prostaglandins E1, D1, F1 alpha, E2, D2, or F2 alpha. This phase is induced by the addition of retinoic acid during cell plating. Intracellular content of cAMP and cGMP was measured by radioimmunoassay at various times after treatment. Maximum stimulation of cAMP levels was observed with PGD2, smaller increases with PGE2 and relatively transient rises with PGF2 alpha which were of low significance, but confirm earlier data. Similar results were observed with PGD1, PGE1, and PGF1 alpha with smaller increases. The effects of D and E PGs were biphasic. Significant increases in cGMP were immediately observed with PGD2 and PGE2. With PGF2 alpha, maximum cGMP levels were noted after some delay. All PGs tested showed some effect in elevating cyclic nucleotides in keratinocytes. The most striking result was the increase in cAMP on PGD2 treatment.

Cyclic AMP accumulation in psoriatic skin: differential responses to histamine, AMP, and einephrine by the uninvolved and involved epidermis

Using the uninvolved and involved skin from psoriatic patients, we investigated the effects of histamine and AMP (or adenosine) in vitro on the intracellular cyclic AMP levels. Both agents activated adenylate cyclase of the uninvolved and involved resulting in the accumulation of cyclic AMP. Without a cyclic nucleotide phosphodiesterase (PDE) inhibitor, these responses were biphasic and the maximal accumulation was observed in 5 min. With the PDE inhibitor both responses were markedly potentiated and high levels of cyclic AMP were observed for more than 20 min. The response to histamine by the involved skin was much greater than that by the uninvolved. The degree of the response to adenosine was approximately equal. In accordance with our previous work, the response to epinephrine by the involved skin was much less than that by the uninvolved. Thus adenylate cyclases of involved skin from psoriatic patients exhibit a markedly diminished response to epinephrine while at the same time exhibiting a markedly enhanced response to histamine. This precludes the possibility that the unresponsiveness to epinephrine can be due to a generalized inability of the epidermal psoriatic plaque cell to make a functioning cell membrane.

Epinephrine activation of pig skin adenylate cyclase in vivo and subsequent refractoriness to activation

Epinephrine injected intradermally activated pig skin adenylate cyclase and increased the epidermal cyclic AMP level in vivo. This biphasic response reached a maximum in 5 min and gradually decreased thereafter. The simultaneous injection of a cyclic AMP phosphodiesterase inhibitor, isobutyl methyl xanthin (IBMX) potentiated the increase. The simultaneous injection of a specific beta-adrenergic receptor inhibitor, propranolol, inhibited this accumulation of cyclic AMP. After the first activation by epinephrine in vivo, there was a marked refractoriness of the skin (epidermal) adenylate cyclase to subsequent epinephrine stimulation vivo and in vitro. This refractoriness was specific for catecholamine stimulation as responses to histamine were normal. Recovery from refractoriness started at 48 hr and was completed at 1 week after the injection of epinephrine.

Epidermal adenylate cyclase: stimulation of the histamine (H2) receptor by tolazoline

Tolazoline (2-benzyl-2-imidazoline) activated adenylate cyclase in pig epidermal slices resulting in the accumulation of cyclic AMP. This effect was highly potentiated by the addition of the cyclic AMP-phosphodiesterase inhibitor, theophylline. Specific histamine (H2) receptor inhibitors (metiamide and cimetidine) completely blocked the tolazoline activation of adenylate cylase. At low concentrations (10-100 micrometer), a histamine (H1) receptor inhibitor (diphenhydramine) and a beta-adrenergic blocker (propranolol) did not inhibit this effect. The stimulation of cyclic AMP formation by the combination of tolazoline and histamine was about the same as the stimulation by histamine alone (nonadditive), whereas the stimulatory effects by tolazoline and epinephrine were additive. These data suggest that tolazoline, an alpha-adrenergic blocker, also activates adenylate cyclase at the histamine (H2) receptor site which is distinct from the beta-adrenergic receptor site. Another alpha-adrenergic blocker, phentolamine, did not have this effect.

Epidermal adenylate cyclase systems: the retention of hormone responsiveness after enzymatic separation of pure epidermis

Although it has been shown that keratome-sliced skin contains active adenylate cyclase systems which respond to various hormones and drugs, unequivocal proof that the epidermis contains these hormone-responsive systems is still lacking. We demonstrate in this study that "pure" epidermis obtained after either collagenase or trypsin treatment does contain the hormone-sensitive adenylate cyclase systems.