Regulation of beta-adrenergic adenylate cyclase responsiveness of pig skin epidermis by suboptimal concentrations of epinephrine

Cyclic AMP differentially regulates cell proliferation of normal human keratinocytes through ERK activation depending on the expression pattern of B-Raf

Intracellular cyclic AMP (cAMP) increased by extracellular stimuli induces various biological effects, such as cell proliferation, differentiation, and migration. Previous reports regarding the effect of cAMP on keratinocyte proliferation are contradictory and indicate that the effect apparently depends on cellular density. Recent studies have revealed that cAMP signaling regulates cell proliferation by modulating mitogen-activated protein kinase (MAPK) activity. The precise mechanism by which cAMP affects keratinocyte proliferation and/or the crosstalk between the cAMP and MAPK signaling pathways, however, remain to be determined. Using normal human keratinocytes (NHK), we investigated the effect of cAMP on keratinocyte proliferation and its molecular mechanism in terms of cellular density. In confluent NHK, cyclic AMP decreased extracellular regulated kinase (ERK) phosphorylation and cell proliferation in a Ras-independent and Rap1-dependent manner. The decreased cell proliferation by cAMP was blocked by the MEK-1 inhibitor, PD98059. In contrast, in subconfluent NHK, cAMP increased ERK phosphorylation and cell proliferation. Western blot analysis revealed that NHK expressed B-Raf and Rap-1. Although both 95 kDa and 62 kDa B-Raf isoforms were expressed in subconfluent NHK, only 62 kDa B-Raf was detected in confluent NHK. Transfection of 95 kDa B-Raf into confluent NHK resulted in a cAMP-dependent increase in ERK phosphorylation and cell proliferation. These findings indicate that differential expression of B-Raf isoforms is critical for cAMP-dependent regulation of NHK proliferation that depends on phosphorylation of ERK.

The mitogen-activated protein kinases system (MAP kinase cascade): its role in skin signal transduction. A review

Mitogen-activated protein (MAP) kinases are proline-directed kinases which are downstream components of a pathway involving p21ras and the serine/threonine kinase Raf-1. They represent an important link between the signal transduction processes at the level of the plasma membrane and the final nuclear events. Not only various growth factors and cytokines, but also other signals such as UV-light or extracellular matrix components are able to activate MAP kinases. We believe that the MAP kinase cascade may play a significant role in regulating cell proliferation and differentiation in human epidermis. In this review we summarize the rapidly increasing knowledge in this field of signal transduction and discuss some very recent results on MAP kinases and their role in skin biology.

1,25-Dihydroxyvitamin D3 increased beta-adrenergic adenylate cyclase response of fetal rat keratinizing epidermal cells (FRSK cells)

Using fetal rat keratinizing epidermal cells (FRSK), the effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on adenylate cyclase system were investigated. The beta-adrenergic adenylate cyclase response was significantly increased by the stimulation of 1 x 10(-7) M 1,25(OH)2D3. The effect was observed by 6 h and continued for at least 48 h. The 1,25(OH)2D3-induced beta-adrenergic augmentation effect was dose-dependent and the maximal response was observed at a concentration of 1 x 10(-7) M 1,25(OH)2D3. Other adenylate cyclase systems (adenosine, prostaglandin E2 and histamine) were not affected by treatment with 1,25(OH)2D3. The thymidine incorporation in FRSK cells was not significantly affected by 1,25(OH)2D3 treatment. Forskolin-induced cyclic AMP accumulation was significantly increased by 1,25(OH)2D3 treatment. Cholera toxin-induced cyclic AMP accumulation was moderately increased, but this was statistically not significant. Northern blot hybridization showed that none of the mRNAs (the beta 2-adrenergic receptor, the alpha subunits of the stimulatory or inhibitory guanine nucleotide binding proteins, Gs alpha, Gi2 alpha, Gi3 alpha) were significantly altered by 1,25(OH)2D3 treatment. We have already reported that the beta-adrenergic response was increased by dexamethasone and inhibited by retinoids in FRSK cells. The addition of both 1,25(OH)2D3 and dexamethasone to the incubation medium resulted in an additive augmentation. On the other hand, the beta-adrenergic augmentation by the 1,25(OH)2D3 treatment was suppressed by the addition of all trans-retinoic acids. Our results indicate that 1,25(OH)2D3 induces beta-adrenergic augmentation without an alternation of thymidine incorporation of FRSK cells.

Desensitization of the epidermal adenylate cyclase system: agonists and phorbol esters desensitize by independent mechanisms

Exposure of pig epidermis to adenylate cyclase stimulators results in receptor-specific desensitization. We investigated the nature of the agonist-induced desensitization, which was compared with the phorbol ester-induced, receptor-nonspecific desensitization. Both phorbol ester-induced desensitization and the agonist-induced desensitization were accompanied by an increase in forskolin- and cholera toxin-induced cyclic AMP accumulations. The magnitude of the increase in the agonist-induced desensitization was parallel to the degree of the initial cyclic AMP accumulation; histamine and adenosine, which increase more cyclic AMP than epinephrine, resulted in a more marked increase in forskolin- and cholera toxin-induced cyclic AMP accumulations. Similarly, epidermis desensitized to multiple receptors revealed more marked forskolin- and cholera toxin-induced cyclic AMP accumulations than epidermis desensitized to a single receptor. In contrast to the phorbol ester-induced desensitization, agonist-induced desensitization was not affected by the protein kinase C inhibitors H-7 and staurosporin. Further, agonist-induced desensitization was still inducible in phorbol ester-desensitized epidermis and vice versa. In contrast to the agonist-induced desensitization, which is accompanied by the preceding adenylate cyclase stimulation, no evidence for the stimulation of the adenylate cyclase during phorbol ester treatment was obtained. Neither agonist-induced desensitization nor phorbol ester-induced desensitization affected the content of inhibitory guanine nucleotide binding protein of the epidermis, which was monitored by the pertussis toxin (IAP)-catalyzed ADP ribosylation reaction. Our results indicate that agonist-induced desensitization and the phorbol ester-induced desensitization are independent of each other. Although both processes are characterized by increased forskolin- and toxin-induced cyclic AMP accumulations, the former is accompanied by initial cyclic AMP accumulation; the latter is not.

Adenylate cyclase system in fetal rat keratinizing epidermal cells (FRSK cells) and SV40-transformed human keratinocytes

The adenylate cyclase system of FRSK cells, a cultured cell line of fetal rat epidermal keratinocytes, and SV40-transformed human keratinocytes was investigated. Stimulators of the human epidermal adenylate cyclase, epinephrine, adenosine, and prostaglandin E2 increased cyclic AMP levels of these cells. There were marked differences in the stimulatory effects; while epinephrine revealed a much stronger effect than the other stimulators in FRSK cells, epinephrine and prostaglandin E2 revealed similarly marked effects in SV40-transformed cells. Histamine had little or only slight effect on the cyclic AMP levels of these cells. Cholera toxin and forskolin, which work on the stimulatory guanine nucleotide binding protein (Gs) and the catalytic component of adenylate cyclase, respectively, also increased cyclic AMP levels. Northern blot hybridization analysis revealed that both FRSK cells and SV40-transformed human keratinocytes express mRNAs for the beta 2-adrenergic receptor, as well as the stimulatory and inhibitory guanine nucleotide binding proteins (Gs and Gi, respectively). The presence of Gs as well as Gi were confirmed by cholera toxin-, and pertussis toxin (IAP)-induced ADP-ribosylation of membranous proteins of these cells. Our results indicate that both FRSK cells and SV40-transformed human keratinocytes express the fundamental components of the adenylate cyclase system. These cell lines might be useful tools for the analysis of the adenylate cyclase system in epidermal keratinocytes.

Epidermal adenylate cyclase system is regulated by diacylglycerol-protein kinase C signal, but not by calcium signal

The breakdown of inositol phospholipids is an important transmembrane signalling system that is composed of two kinds of signals: the diacylglycerol-protein kinase C signal, and the inositol trisphosphate-Ca2+ signal. Using membrane-permeable diacylglycerol, I-oleoyl-2-acetylglycerol (OAG), and calcium ionophore, A-23187, the effects of these chemicals on the epidermal adenylate cyclase system were investigated. OAG increased forskolin- and cholera toxin-induced cyclic AMP accumulations, but receptor adenylate cyclase responses were markedly decreased by treatment with OAG. The effects of OAG were inhibited by the protein kinase C inhibitor, H-7. Calcium ionophore, A-23187, had no effect on the epidermal adenylate cyclase responses. Combinations of OAG and A-23187 (as well as the calcium chelator, EGTA), showed that the action of OAG was mostly unaffected by the modulation of intracellular and extracellular Ca2+ concentrations. The results suggest that among the signals triggered by the breakdown of inositol phospholipids, only diacylglycerol-protein kinase C signal is involved in the regulation of the epidermal adenylate cyclase system.

Topical PUVA treatment increases epidermal beta-adrenergic adenylate cyclase responsiveness

The effects of topical PUVA treatment on the epidermal cyclic AMP system were investigated. 8-methoxypsoralen (8-MOP), 0.3% in ethanol was applied to the backs of pigs which were then irradiated with UVA. A significant increase in the epidermal beta-adrenergic adenylate cyclase response was observed 24 h after low (1.1 J/cm2) and moderate (2.1 J/cm2) dose irradiation. There was no significant change in the adenosine- or histamine-mediated adenylate cyclase responses. 8-MOP application or UVA irradiation alone had no effect on the beta-adrenergic adenylate cyclase response. PUVA treatment with a higher irradiation dose (4.2 J/cm2) produced no increase in the beta-adrenergic response and adenosine- and histamine-mediated adenylate cyclase responses were decreased. Cyclic AMP phosphodiesterase activity was decreased by PUVA treatments using UVA doses of 1.1 and 2.1 J/cm2; however, the change was not statistically significant. The increased beta-adrenergic response was also observed in the presence of the cyclic AMP phosphodiesterase inhibitor, isobutylmethylxanthine. These results indicate that epidermal adenylate cyclase responsiveness is affected by topical PUVA treatment in vivo.

"Ischemic" rise of epidermal cyclic AMP is a beta-adrenergic adenylate cyclase-dependent process

The endogenous level of epidermal cyclic AMP does not remain constant but increases rapidly and transiently after removal of the tissue; this is known as the "ischemia" effect. UVB-irradiated epidermis which shows increased beta-adrenergic response revealed an increased ischemia effect, while psoriatic involved epidermis which shows decreased beta-adrenergic response revealed a decreased ischemia effect. Because of the similar rise-and-fall pattern between the ischemia effect and the beta-adrenergic response, the mechanism of the ischemia effect was investigated, especially in terms of the beta-adrenergic relationship. The ischemic rise of epidermal cyclic AMP was well preserved after 6 h pretreatment at 4 degrees C, and, following the pretreatment, the skin markedly increased its cyclic AMP level by the 37 degrees C treatment with 1 mM isobutylmethyl xanthine. The addition of propranolol or cimetidine at the time of 37 degrees C treatment (following the 4 degrees C pretreatment) had no effect on the ischemia effect; both skin groups markedly increased their cyclic AMP levels to an extent similar to that of the control skin. However, the addition of propranolol at the time of both preincubation (at 4 degrees C) and incubation (at 37 degrees C) markedly decreased the ischemic rise of cyclic AMP. Similar treatment by cimetidine had no effect on the ischemia effect. There was no significant difference in cyclic AMP phosphodiesterase activities among skin groups by propranolol or cimetidine pretreatment. These results indicate that the so-called ischemic rise of epidermal cyclic AMP is actually the beta-adrenergic adenylate cyclase-dependent process. Our results also indicate that the magnitude of the "ischemic" rise of cyclic AMP is generally parallel to the beta-adrenergic responsiveness of epidermis.

Effects of retinoids on the cyclic AMP system of pig skin epidermis

Although retinoids reveal various biologic and biochemical activities on epidermal keratinocytes, their effects on the epidermal cyclic AMP (cAMP) system has been less well characterized. In order to elucidate the relation between them, an in vitro pig skin-slice incubation system was employed. After a long-term (up to 24 h) incubation in vitro, control skin responded to epinephrine only slightly. The addition of Ro 10-1670, an active derivative of Ro 10-9359 (etretinate) in the incubation medium, resulted in an increase of the beta-adrenergic adenylate cyclase response of epidermis. On the other hand, histamine-induced cAMP accumulation was decreased by the retinoid treatment after long-term incubation. The augmentation of the beta-adrenergic response was observed at 1 microM concentration and the maximal effect was observed at 10 microM. There was no significant difference in cAMP phosphodiesterase activities between the control and retinoid-treated skin. The effect was also observed by the addition of all-trans-retinoic acid, retinol, and Ro 10-9359; the latter two compounds revealed much lesser effects. The addition of combinations of various drugs (Ro 10-1670 and hydrocortisone; Ro 10-1670 and colchicine) resulted in more marked (additive or synergistic) effects than the single addition of each chemical. On the other hand, the addition of Ro 10-1670 and all-trans-retinoic acid resulted in neither additive nor synergistic effect, suggesting that they probably work on the same site. Our data indicate that the epidermal beta-adrenergic adenylate cyclase response is modulated by retinoids probably as an independent mechanism stimulated by glucocorticoids or colchicine.

Ultraviolet radiation augments epidermal beta-adrenergic adenylate cyclase response

Pig skin was irradiated in vivo with fluorescent sunlamp tubes (peak emission at 305 nm). A significant increase in epidermal beta-adrenergic adenylate cyclase response was observed as early as 12 h following 1-2 minimum erythema doses (MEDs) UVB exposure, which lasted at least 48 h. The augmentation of adenylate cyclase response was relatively specific to the beta-adrenergic system and there was no significant difference in either adenosine- or histamine-adenylate cyclase response of epidermis. The increased beta-adrenergic adenylate cyclase response was less marked at higher doses of UVB exposure (5 MEDs); in the latter condition, a significant reduction in adenosine- or histamine-adenylate cyclase response was observed. There was no significant difference in either low- or high-Km cyclic AMP phosphodiesterase activity between control and UVB-treated skin at 1-2 MEDs. Our data indicate that the epidermal adenylate cyclase responses are affected in vivo by UVB irradiation, which might be a significant regulatory mechanism of epidermal cyclic AMP systems.

Modulation of pig epidermal adenylate-cyclase responses by protein-synthesis inhibitors: its relation to glucocorticoid and colchicine effects

The effects of protein-synthesis inhibitors (actinomycin D, puromycin, and cycloheximide) on epidermal adenylate-cyclase responses were investigated. When pig skin (epidermis) was incubated in RPMI-1640 medium, the beta-adrenergic adenylate-cyclase response (epinephrine-induced cyclic-AMP accumulations) decreased, whereas the adenosine and histamine responses increased after long-term (up to 48 h) incubation. The addition of actinomycin D or puromycin to the incubation medium resulted in a marked increase in epinephrine-induced cyclic-AMP accumulations and a decrease in adenosine- and histamine-induced cyclic-AMP accumulations. Cycloheximide had a weak effect on the epinephrine response, and had apparently stronger effects on the adenosine and histamine responses than actinomycin D or puromycin. Histologically, various degenerative changes of keratinocytes (with or without acantholytic changes) were observed after long-term incubation with these protein-synthesis inhibitors. Both low- and high-Km cyclic-AMP phosphodiesterase activities were moderately decreased by the protein-synthesis inhibitors. However, augmentation effects on the beta-adrenergic response were also observed in the presence of the cyclic-AMP phosphodiesterase inhibitor, theophylline. We have described previously similar augmentation effects on the beta-adrenergic response caused by glucocorticoids and colchicine. Comparison of the effects of these chemicals with those of protein-synthesis inhibitors revealed that the most marked effects on the beta-adrenergic response were produced by actinomycin D, puromycin and colchicine; glucocorticoid had a moderate effect (hydrocortisone), while cycloheximide had only a weak effect.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Colchicine-induced alteration of hormone-stimulated cyclic AMP synthesis in pig skin (epidermis)

Effects of colchicine on the epidermal adenylate cyclase systems were investigated. When pig skin (epidermis) was incubated in RPMI 1640 medium without the addition of serum, the beta-adrenergic adenylate cyclase response (epinephrine-induced cyclic AMP accumulations) gradually decreased, whereas adenosine and histamine responses remained high or increased during the long-term (up to 48 h) incubation period. The addition of colchicine (1 mumol/liter) in the incubation medium resulted in an increase in the beta-adrenergic responsiveness and a decrease in adenosine and histamine responsivenesses. The effects of colchicine were both time- and concentration-dependent; they could be observed after 9-12 h incubation, and the maximal effect was obtained at a concentration of 0.1 mumol/liter. Similar effects were observed by the addition of another microtubule-disruptive agent, vinblastine. On the other hand, cytochalasin B, which affects the microfilament system, apparently decreased the beta-adrenergic response and increased adenosine and histamine responses during the long-term incubation period. The addition of serum in the incubation medium resulted in essentially the same effect as that of colchicine; in the presence of serum, colchicine-treated skin responded much more markedly to epinephrine (and much less to adenosine and histamine) than the control skin after 24- and 48-h incubation. Previously we reported that hydrocortisone has similar potentiating effects on the beta-adrenergic system of epidermis. The comparison of the effects of both compounds revealed that colchicine had a stronger effect than hydrocortisone, and furthermore, the simultaneous addition of both compounds (colchicine and hydrocortisone) in the incubation medium resulted in the more marked increase of beta-adrenergic response than the single addition of each chemical. Our overall results, coupled with the finding that hydrocortisone has no toxic effects on the adenosine- or histamine-adenylate cyclase system of epidermis, suggest that colchicine affects epidermal adenylate cyclase systems probably through a mechanism that is independent of glucocorticoid (hydrocortisone) effect.