The cyclic AMP system in normal and psoriatic epidermis

Circulating Monocytes Are Predictive and Responsive in Moderate-to-Severe Plaque Psoriasis Subjects Treated with Apremilast

Monocytes play a critical role in the inflammation associated with psoriasis, and their abnormalities have been reported as biomarkers of cardiovascular event risk, a psoriasis comorbidity. Monocytic cells in chronic inflammatory disorders express elevated levels of cAMP phosphodiesterase. Restoring cAMP levels using the oral cAMP phosphodiesterase-4 inhibitor, apremilast, improves clinical outcomes for a subset of patients with psoriasis. We asked whether aberrant monocyte subsets or transcriptomic pathways can function as biomarkers of psoriasis endotypes that can predict enhanced clinical responses to cAMP phosphodiesterase inhibition. A 16-week open-label study of 22 patients with monocyte flow cytometric and transcriptomic analysis was performed. Subjects with elevated hyperadhesive monocyte doublets at baseline were more likely to be responders to apremilast (P < .0001); 82% of subjects with elevated hyperadhesive monocyte doublets achieved 50% reduction in PASI compared with 46% in those without elevated doublets. We observed a significant reduction in hyperadhesive monocyte-containing doublets and monocyte-platelet aggregates, suggesting an effect of apremilast on the adhesiveness of blood monocytes during chronic inflammation. Monocyte differentially expressed gene transcripts predictive of clinical response uncovered pharmacoendotypes with distinct patterns of nucleotide metabolism, energetics, and differentiation. Further study to understand the basis of drug responsiveness and to develop an apremilast psoriasis treatment algorithm using monocyte-refined gene expression is required to validate and become practical in clinical use, offering patients a test that personalizes their likelihood of clinical response.

A Tissue-Engineered Human Psoriatic Skin Model to Investigate the Implication of cAMP in Psoriasis: Differential Impacts of Cholera Toxin and Isoproterenol on cAMP Levels of the Epidermis

Pathological and healthy skin models were reconstructed using similar culture conditions according to well-known tissue engineering protocols. For both models, cyclic nucleotide enhancers were used as additives to promote keratinocytes’ proliferation. Cholera toxin (CT) and isoproterenol (ISO), a beta-adrenergic agonist, are the most common cAMP stimulators recommended for cell culture. The aim of this study was to evaluate the impact of either CT or ISO on the pathological characteristics of the dermatosis while producing a psoriatic skin model. Healthy and psoriatic skin substitutes were produced according to the self-assembly method of tissue engineering, using culture media supplemented with either CT (10⁻¹⁰ M) or ISO (10⁻⁶ M). Psoriatic substitutes produced with CT exhibited a more pronounced psoriatic phenotype than those produced with ISO. Indeed, the psoriatic substitutes produced with CT had the thickest epidermis, as well as contained the most proliferating cells and the most altered expression of involucrin, filaggrin, and keratin 10. Of the four conditions under study, psoriatic substitutes produced with CT had the highest levels of cAMP and enhanced expression of adenylate cyclase 9. Taken together, these results suggest that high levels of cAMP are linked to a stronger psoriatic phenotype.

Mechanisms of Beta-Blocker Induced Psoriasis, and Psoriasis De Novo at the Cellular Level

Beta-blockers are a commonly prescribed medication, but the increase in use goes hand in hand with increasing side effects; one of particular interest lately has been its dermatological reactions. Although rare, beta-blockers can exacerbate pre-existing psoriasis and also cause de novo psoriasis in patients naïve to the disease. The mechanism by which this occurs is still unclear, although numerous articles have been published throughout the years as to how this unusual effect takes place. The most common mechanism suggests that beta-blockers cause intracellular changes in calcium, affecting both keratinocyte proliferation and granulocyte function via decreased cyclic adenosine monophosphate (cAMP) levels. Several inflammatory mediators are known to play a role, as well as reduced expression and desensitization of the beta-adrenergic receptor itself. We discuss these posed pathways in-depth and how each contributes to the worsening or formation of new psoriasis. With this knowledge, future physicians may be more mindful of this side effect should it occur, and why they occur, to better manage our patients on this widely used medication.

Beta adrenergic receptors in keratinocytes

Beta2 adrenergic receptors were identified in keratinocytes more than 30 years ago, but their function in the epidermis continues to be elucidated. Abnormalities in their expression, signaling pathway, or in the generation of endogenous catecholamine agonists by keratinocytes have been implicated in the pathogenesis of cutaneous diseases such as atopic dermatitis, vitiligo, and psoriasis. New studies also indicate that the beta2AR also modulates keratinocyte migration, and thus can function to regulate wound reepithelialization. This review focuses on the function of these receptors in keratinocytes and their contribution to cutaneous physiology and disease.

Psoriasis: a fresh look

Psoriasis is a disease characterized by scaly skin lesions secondary to keratinocyte hyperplasia. The presence of active T cells in the lesions, experimental observations on disease transfer, and therapeutic efficacy of specific immunosuppressive drugs have led to the identification of the activated T lymphocyte as the primary factor for keratinocyte stimulation. Understanding the pathways of pathogenesis is fundamental in evolving therapies for intervention at different points in the pathogenic model and for curtailing the process. Advances in biotechnological methods have helped to create designer molecules and proteins that specifically recognize target receptors and chemicals that modify their actions. These drugs, termed "biologic response modifiers," are now being studied as specific immunosuppressive agents producing different T-cell and cytokine effects in psoriasis.

A highly decreased binding of cyclic adenosine monophosphate to protein kinase A in erythrocyte membranes is specific for active psoriasis

A cyclic adenosine monophosphate binding abnormality in psoriatic erythrocytes that could be corrected by retinoid treatment has been reported. It was tested whether this binding abnormality is specific for psoriasis and the effects of treatment were compared with etretinate, cyclosporine A, or anthralin on 2-(3)H-8-N(3)-cyclic adenosine monophosphate binding to the regulatory subunit of protein kinase A in erythrocyte membranes. One hundred and fifteen individuals were evaluated, including: (i) 34 healthy persons; (ii) 15 patients with nonatopic inflammatory skin diseases (eczema, erythroderma, tinea, Grover's disease, erysipelas, urticaria); (iii) eight with other dermatoses mediated by immune mechanisms (systemic lupus erythematosus, lichen planus, necrotizing vasculitis, erythema nodosum, systemic sclerosis); (iv) 14 with generalized atopic dermatitis; and (v) 44 with psoriasis vulgaris clinically assessed by Psoriasis Area and Severity Index. In psoriasis, the course of the binding of 2-(3)H-8-N(3)-cyclic adenosine monophosphate to erythrocytes was measured in nine patients during a 10 wk treatment with etretinate, in 21 patients during a 10 wk treatment with cyclosporine A, and one patient under topical treatment with anthralin for 4 wk. We found the following femtomolar binding per mg protein: (i) healthy persons (1064 +/- 124, mean +/- SD); (ii) nonatopic inflammatory skin diseases (995 +/- 103); (iii) immune dermatoses (961 +/- 92); (iv) atopic dermatitis (960 +/- 110); and (v) psoriasis (645 +/- 159; p < 0.0001 compared with nonpsoriatics, Mann-Whitney U test). Treatment of psoriasis with etretinate, cyclosporine A, or anthralin normalized the binding of cyclic adenosine monophosphate, which was inversely correlated to the Psoriasis Area and Severity Index score. It was concluded that the decreased binding of cyclic adenosine monophosphate to protein kinase A in erythrocytes is specific for psoriasis and normalizes after successful treatment.

The mast cell products histamine and serotonin stimulate and TNF-alpha inhibits the proliferation of murine epidermal keratinocytes in situ

To better characterize the proposed growth modulatory functions of mast cells and their products in the regulation of epidermal homeostasis, we have assessed keratinocyte proliferation by incorporation of [3H]thymidine in murine skin organ cultures after incubation with different concentrations of histamine, serotonin or TNF-alpha. Both histamine and serotonin significantly stimulate the proliferation of keratinocytes as compared to controls (P < 0.001), reaching maxima within a range from micro- to nanomolar concentrations. Even at subnanomolar concentration, TNF-alpha significantly inhibits the incorporation of labelled thymidine by epidermal keratinocytes in murine skin organ culture (P < 0.001). These observations support the concept of mast cell products as modulators of keratinocyte proliferation, and suggest a role for mast cells in the regulation of epidermal tissue turnover under physiological conditions.

Effects of epinephrine on glucose-1,6-bisphosphate and carbohydrate metabolism in skin

1. Injection of epinephrine induced in skin a decrease in the level of glucose-1,6-bisphosphate (Glc-1,6-P2), which was accompanied by correlated changes in the activities of several enzymes which are modulated by this regulator. 2. These effects were blocked by the alpha adrenergic blocker phentolamine, in contrast to muscle where the hormone increases Glc-1,6-P2, acting through beta receptors. 3. The changes in the enzymes' activities, as well as in glycogen and lactate content induced by epinephrine, reveal that the hormone causes, in skin, a stimulation of glycogenolysis and glycolysis, as well as an acceleration of pentose phosphate pathway. 4. The reduction in glycogen content induced by epinephrine, was blocked by the beta adrenergic blocker propranolol, whereas the hormone's effects on the other processes were mainly mediated through alpha receptors.

Changes in the rate of epithelial proliferation of rat oral mucosa in response to acute inflammation induced by turpentine

Inflammatory lesions were produced in the buccal mucosa by a subepithelial injection of turpentine; animals were killed 24 h later, 1 h after an intravenous injection of tritiated thymidine ( [3H]-Tdr). Control rats were given [3H]-Tdr but no turpentine. Lesions comprised a turpentine pool surrounded by a dense layer of inflammatory cells, beyond which the tissues were more diffusely inflamed. The labelling index (L.I.) for mitotic activity in overlying epithelium was determined in a region (C) close to the layer of dense infiltration and in a region (D) more distant. The L.I. in region D was over four times greater than in region C, and nearly four times greater than that of the contralateral, uninjected cheek. The L.I. in the uninjected cheek was significantly lower than that in controls, which may indicate a systemic depression of proliferative activity in the experimental animals, probably due to stress. Thus mild inflammatory injury stimulates epithelial proliferation, whereas more severe inflammation depresses it, perhaps due to more extensive progenitor-cell damage.

Modulation of adenosine 3', 5'-cyclic monophosphate-dependent protein kinase in skin

Incubation in vitro of full-thickness guinea-pig skin with epinephrine or histamine resulted in the formation of adenosine 3', 5'-cyclic monophosphate (cyclic AMP) and increase in the cyclic AMP-dependent protein kinase activity ratio. The effect was dose-dependent between 10(-5) and 10(-3) M concentration of the agents. Histamine antagonists, diphenhydramine (H1) and cimetidine (H2) prevented stimulation by histamine in a dose-dependent manner suggesting the presence of both types of receptors in full-thickness skin. Cyclic AMP accumulation with a concomitant increase in protein kinase activity denotes that phosphoproteins may have an important role in skin biology.

Histaminergic mediation of the response of rat skin to topical fluorides

The topical application of sodium fluoride to abraded rat skin produced several morphological and biochemical effects. Related to the degranulation of dermal mast cells, skin histamine concentration was increased, fluorides were absorbed into the skin, and deposited mainly kin mitochondria. Dermal histamine binding was decreased for both H1 and H2 receptors with reduced binding sites, but epidermal adenyl cyclase was activated by fluorides. The response of the rat skin to fluorides involves a sequence of changes by which the potentiation of an inflammatory response also involves alterations in specific histamine receptors and a histamine-specific adenyl cyclase system.

Stimulated mouse ear epidermis in explant culture- The effect of retinoic acid and hexadecane

Ear skin of adult mice was used as a source of explant cultures of epidermal cells and the requirements for growth were defined. Two growth phases were observed, the first more rapid than the second. All-trans-retinoic acid or n-hexadecane was applied topically for up to 9 days and the skin was then used for explant cultures. During both treatments the ear became red and swollen. The epidermis became hyperkeratotic or deprived of its horny layer. The thickness of the stratum Malpighii increased to up to nine layers, from the normal of two or three layers. The stratum granulosum became multilayered and keratohyalin granules were abundant. Explants of n-hexadecane-treated skin behaved exactly as their normal controls. All-trans-retinoic acid included an increase in migratory cell activity during the first growth phase. In the second growth phase the growth rate was similar or lower than that of the controls.

The in vivo effects of lymphokines on mitotic activity and keratinization in guinea pig epidermis

Lymphokines may alter epidermal growth and differentiation contributing to changes such as acanthosis and hyperkeratosis. The main in vivo effects of lymphokines on epidermal mitotic activity were therefore investigated. Guinea pigs were injected intradermally with antigen-stimulated lymphocyte culture supernatants and a partially purified lymphokine preparation in phosphate buffered saline (PBS) 18, 24, 36 and 48 hr prior to biopsy. Control sites were injected with unstimulated supernatants and PBS respectively and the mitotic activity determined by use of a stathmokinetic agent. Both lymphokine injected areas and controls showed significantly increased mitotic indices compared to untreated skin which was apparent only at 24 hr. However mitotic activity in lymphokine lesions was significantly higher than in control lesions. There was no difference in the effect on mitotic activity between PBS and unstimulated culture supernatants. Lymphokine lesions at 24 hr also exhibited marked epidermal edema and acanthosis compared to minimal changes in controls. A variable patchy parakeratosis developed between 18 and 24 hr in areas injected with partially purified lymphokine but not in control sites or after injection with unpurified supernatants. The lymphokine-induced inflammatory infiltrate was mild and consisted mainly of neutrophils not differing significantly from that of the control lesions. This strongly suggests that lymphokines induce an alteration in epidermal kinetics and keratinization by a direct effect on keratinocytes and not indirectly via the dermal inflammatory infiltrate.

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.

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.

Adenyl-cyclase activity in rat-hair-cycle

In the skin of rat, during telogen phase, a high adenyl-cyclase activity coincides with maximal catecholamines content, minimal histamine level and blocking of mitoses in epithelial cells. These events suggest that in telogen the control of mitotic activity in rat hair follicles may operate through a block of G1 cells as already suggested for epidermal cells.

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

Adenosine, AMP, ADP and ATP activated adenylate cyclase in pig skin (epidermis) slices resulting in the accumulation of cyclic AMP. This effect was highly potentiated by the addition of the cyclic AMP-phosphodiesterase inhibitor, papaverine. But another inhibitor, theophylline, strongly blocked the activation of adenylate cyclase by adenosine and adenine nucleotides. Theophylline apparently competed with adenosine for the cell surface receptor. Like theophylline, the addition of adenine alone caused no accumulation of cyclic AMP, but it significantly inhibited the stimulatory effect of adenosine. Guanosine, or guanine, cytidine, uridine, or thymidine nucleotides had no effect on the accumulation of cyclic AMP. Among other adenine nucleotides we tested, adenosine 5'-monophosphoramidate, but not adenosine 5'-monosulfate significantly increased cyclic AMP especially with the addition of papaverine. Neither 2'- nor 3'-adenylic acid were effective. Our data indicate that pig epidermis has four specific and independent adenylate cyclase systems for adenosine (and adenine nucleotides), histamine, epinephrine and prostaglandin E.

Extracellular stimulation of epidermal DNA synthesis

The removal of the topmost cell layers of the epidermal stratum corneum by stripping initiates a series of biochemical events which alters the normal homeostatic control of and results in the acceleration of the cell cycle in basal cells which are ten to twenty cell layers removed from the site of stripping. One measure of accelerated events is a stimulation of thymidine incorporation into epidermal DNA at time intervals following stripping. Two peaks of maximal stimulation occur between 12 and 24 hr and 48 and 54 hr after stripping. Stimulation of thymidine incorporation into epidermal DNA by limited stripping is a useful technique for studying the stripping-initiated signal at the stratum corneum and its subsequent translation at the proliferative cell receptor site.

Cyclic AMP-phosphodiesterase and epidermal mitosis

The relationship between cyclic AMP-phosphodiesterase (cAMP-PDE) inhibition and inhibition of epidermal mitosis was examined for several compounds using a soluble, low Km PDE activity from hairless mouse skin and the G2 mouse ear mitosis assay. Orders of potency determined at IC50 levels (concentrations required for 50% inhibition) were SQ 20009 greater than RO 20-1724 greater than papaverine greater than bufexamac greater than indomethacin greater than theophylline greater than p-biphenylylacetic acid greater than or less than glycyrrhetinic acid for inhibition of both PDE and mitosis. The disproportionately high antimitotic potency of puromycin relative to PDE inhibition was believed to reflect effects on protein biosynthesis. Activity of the three nonsteroidal anti-inflammatory agents (bufexamac, indomethacin, and p-biphenylylacetic acid) was unrelated to their effect on prostaglandin synthesis in homogenates of hairless mouse skin. The results suggest that the epidermal antimitotic activity of the compounds tested is related to their inhibition of cAMP-PDE and provide additional support for cAMP as a regulator of the G2 stage of the epidermal cell cycle.

Mast cell activation and tissue cell proliferation

The effect of mast cell activation and degranulation on the proliferation in the intact mesentery was studied in Sprague-Dawley rats. Mast cell activation was achieved by a single intraperitoneal injection of Compound 48/80. The proliferation was studied using three independent methods for estimation of cell production and DNA synthesis: 1. the mitotic index, 2. the relative number of cells having a DNA content in the S and G2 regions, by Feulgen photometric measurement in individual cells, and 3. the specific DNA activity, employing a method which combines a liquid scintillation technique after an intravenous injection of 3H-thymidine and Feulgen photometric determination of the DNA content per membrane preparation. It was found that the proliferation of the normal mesenchymal cells adjacent to the activated and degranulated mast cells in the mesentery was significantly increased within 24 and 32 h, the maximum increase being more than 20-fold compared to untreated controls. The results suggest that the common type of mast cell may have a pathophysiological function related to stimulation of local cell proliferation.

[Disturbed distribution of surface glycoconjugates of the plasma cell membrane in psoriasis. Cytochemical detection using the alcianblue-lanthan-reaction (author's transl)]

Lanthanum has been used as a tracer for the intercellular space and the surface coats in different epithelia including human epidermis, however, lanthanum compounds may or may not penetrate into skin specimes and reaction product may or may not be present in the intercellular space. After treatment with alcian blue prior exposure to lanthanum the trilaminar plasma membrane was more regularly stained and a thin coat of reaction product corresponding to superficial glycoconjugates outlined the cell surface and filled the narrow intercellular space in normal epidermis. In psoriatic epidermis the quantity of superficial membrane-bound glycoconjugates was rather reduced, their distribution and compartmentalization was profoundly changed and the outer leaflet of the trilaminar plasma membrane was not always clearly seen, having a "moth-eaten appearance" in some places. These findings show once more that alterations of the cell membranes and their surface architecture are present in psoriasis. They may play a major role in the pathogenesis of the disease, influencing the activities of membrane-bound enzymes and the flow of the epidermal growth control mechanism. The glycocalyx may serve in this mechanism as an accumulator of energy rich compounds or as a receptor site of membrane-bound ezymes. According to our views it represents a "ground substance" of epidermal origin, which controls the social behaviour of the keratinocytes in tissue.

Effects of reserpine, epidermal growth factor, and cyclic nucleotide modulators on epidermal mitosis

The mouse ear G2 mitosis assay was modified for the screening of potential antimitotic agents. An inhibitory adrenergic influence, which maintains mitotic rate at a normally low level, was removed by pretreatment of mice with reserpine. This depletes endogenous catecholamines, produces a state of enhanced mitotic activity, and makes the epidermal cells particularly sensitive to mitotic inhibition by agents which elevate the levels of cyclic AMP. Isoproterenol [IC 50 approximately 1 X 10(-9) M], prostaglandins, dibutyryl cyclic AMP [IC 50 approximately 2 X 10(-5) M], papaverine, theophylline and 5' AMP were inhibitory in the assay, whereas dibutyryl cyclic GMP and the cholinergic stimulator carbamylcholine either stimulated or had no effect on mitosis. Epidermal growth factor was employed as an alternate means of stimulating cell division. Skin fron newborn mice or rats pretreated with this substance had increased epidermal mitotic activity which was inhibited cyclic AMP elevators.

Subcellular distribution of phosphatases, proteinases, and ribonucleases in normal human stratum corneum and psoriatic scales

The subcellular distribution of phosphatases, proteinases, and ribonucleases of normal human stratum corneum and psoriatic scales was determined after differential centrifugation. All psoriatic enzymes showed much increased activities as compared to the normal stratum corneum enzymes. The highest activities of alkaline phosphatase from psoriatic scales could be detected in the nuclear fraction. The main activities of all other tested phosphatases and proteinases were present in the cytoplasmatic fraction. The subcellular distribution of the ribonucleases varied according to the pH value.

Absorption and biotransformation of topically applied 8-(methylthio) cyclic AMP. Studies in dogs and humans

8-(Methylthiocyclic AMP-35S was applied topically to the intact or abraded skin of dogs at a dose of about 2-5 mg/kg. In this species, an average of 7% of the dose was absorbed through the intact and 62% through abraded skin. Half-lives for radioactivity in plasma averaged 3-6 h. After the topical application of the drug to the intact skin of dogs, the average concentration in plasma of glucose, but not of insulin, was increased by as much as 21%; after application to abraded skin, glucose and insulin were increased, on average, by as much as 84 and 221%, respectively. 8-(methylthio)adenosine and two unidentified compounds were present as metabolites in the urine of dogs; some unchanged 8-(methylthio)cyclic AMP-35S was also excreted. 8-(methylthio)cyclic AMP-35S was also applied topically (25 mg; 0-32 mg/kg) to the intact and stripped skin of normal human subjects under conditions similar to those used for dogs. Three subjects with intact skin did not absorb any of the drug, whereas three subjects with stripped skin absorbed 0-5, 8-3, and 23-3% of the dose. The half-life of radioactivity in the plasma of the subject with the greatest absorption was 0-5 h during the first 2 h, and 11 h for the next 10 h. During the first 2 h, this same subject excreted unchanged drug and 8-(methylthio)adenosine in his urine. No changes in glucose or insulin concentrations in plasma were observed in any of the subjects nor was there any apparent irritation of the skin.

Effect of pharmacologic agents on human keratinocyte mitosis in vitro. III. Inhibition by histamine and methylated analogs

Keratinocytes from normal human skin were propagated in vitro. Histamine at 2 times 10(-6) M inhibited mitosis 68%. The side-chain methylated analogs, n-methylhistamine and n,n-dimethylhistamine, produced an inhibitory response of 53 and 60%, respectively, at 1 times 10(-6) M. The side-chain acetylated analog, n-acetylhistamine, did not elicit an inhibitory response in concentrations as high as 1 times 10(-4) M. The ring-methylated analogs, 1-methylhistamine and 4-methylhistamine, produced some degree of mitotic inhibition at 1 times 10(-4) M. When the H1-blocking agent, pyrilamine, or the H2-blocking agent, metiamide, was added in conjunction with histamine, the histamine-induced mitotic inhibition was abolished. These data indicate that human keratinocytes may possess receptors for histamine which could play a role in the regulation of human keratinocyte proliferation in vivo.

Effect of pharmacological agents on human keratinocyte mitosis in vitro. II. Inhibition by catecholamines

Catecholamines produce mitotic inhibition in primary cell cultures of human keratinocytes probably via a block in the G2 part of the cell cycle. Epinephrine produced significant mitotic inhibition (49%) at a concentration as low as 4.5 X 10(-10) M, while its analog, isoproterenol, produced 47% inhibition at 1 X 10(-10) M. Norepinephrine elicited a 49% inhibitory response at 1 X 10(-8) M. One other catecholamine, dopamine, caused a 53% decrease in mitosis at 1 X 10(-6) M. Other structurally related amines to exhibit mitotic inhibition were phenylephrine, 58% at 1 X 10(-7) M; octopamine, 47% at 1 X 10(-5) M; and tyramine, 52% at 1 X 10(-4) M. Serotonin showed no mitotic inhibition at 1 X 10(-4) M. Various alpha and beta adrenergic blocking agents were added to the cell system. The alpha blocking agent, phentolamine, had no effect on mitosis. When added in conjunction with epinephrine or norepinephrine, no reduction of the catecholamine-induced mitotic inhibition was observed. The beta blocking agent, propranolol, by itself showed slight mitotic inhibition at 1 X 10(-6) M. When added along with epinephrine or noreinephrine, propranolol reduced the catecholamine-induced mitotic inhibition approximately 65%. In addition, propranolol blocked mitotic inhibition caused by phenylephrine, an alpha adrenergic agent. However, another beta blocking agent, dichloroisoproterenol, showed strong mitotic inhibition (53%) when added to the cultures at a concentration of 1 X 10(-8) M. The effect was reduced to zero in the presence of propranolol. These data suggest that while beta receptors may be involved in the catecholamine-induced mitotic inhibition of human keratinocytes in vitro, the nature of the receptor-molecule interaction may be complex.

In vitro analysis of the control of keratinocyte proliferation in human epidermis by physiologic and pharmacologic agents

Human keratinocytes grown in vitro as epithelial outgrowths or as organ cultures maintain many of their normal functions such as proliferation and keratinization. These in vitro systems have been used to analyze the effect of various agents on proliferation. All adenine nucleotides, including dibutyryl cyclic AMP, blocked mitosis in the G2 part of the cell cycle at concentrations of 1 times 10(-4) M. Some nonadenine nucleotides also showed this effect, but only at higher concentrations, an indication that the effect was specific for adenine nucleotides. Dibutyryl cyclic AMP and theophylline both depressed the incorporation of [3H]thymidine into DNA. Catecholamines such as isoproterenol, epinephrine, and norepinephrine were also potent inhibitors of mitosis (G2 block) at concentrations of 1 times 10(-8) to 1 times 10(-10) M. The fact that the effect could be blocked by the beta-blocking agent, propranolol, suggests the existence of specific membrane receptor sites. However, dichloroisoproterenol, another beta blocker, had distinct inhibitory properties in itself and thus indicated that the mechanism of action of catecholamines in human keratinocytes is complex and may involve more than binding to specific receptor sites. Histamine at a concentration of 2 times 10(-6) M was also a strong mitotic inhibitor. This finding is directly opposed to that in rat skin where mitosis is stimulated. Imidazole acetate, a histamine breakdown product, was found to be a striking mitotic stimulator in organ culture. A water-extractable protein (chalone) from human skin also caused a block in G2. Most of the substances tested occur naturally in the cell or organism and their ability to stimulate or depress proliferation in vitro suggests that they play a regulatory role in vivo.

Cyclic AMP in skin: effects of acute ischaemia

The cyclic adenosine 3',5'-monophosphate (cyclic AMP) content of pig skin was measured several seconds to minutes after removal of the skin from the body. It increased very rapidly, reached a maximum by 2 min after removal (4 times higher than the initial level), then decreased very slowly. Propranolol injected into the animal before or added after the removal of the skin did not suppress this phenomenon. The practical significance of this finding (increase of cyclic AMP level in skin after ischaemia) is obvious-in order to measure cyclic AMP level in vivo, the sample must be frozen immediately to avoid an 'artificial' increase in cyclic AMP.

Changes in the mitotic cycle induced by alpha-solanine

Low concentrations of alpha-solanine stimulated the growth of cultured human fibroblasts, while higher concentrations (greater 30 mug ml-1) had a markedly inhibitory effect. Autoradiographic studies indicated that the stimulation of cell growth was due to a shortening of the G1 phase. Feulgen microdensitometry of cells treated with high doses of alpha-solanine revealed an abnormal accumulation of cells in G2. The response of cultured human fibroblasts to low doses of alpha-solanine is comparable to that of sex hormones on target tissues. It is concluded that by virtue of either its stimulatory or its inhibitory effect on cell growth, alpha-solanine could act as a human teratogen.

Mechanism of action of granulopoiesis inhibiting factor (chalone). I. Evidence for a receptor protein on bone marrow cells

Rat bone marrow cells respond to granulopoiesis inhibiting factor by a reduced incorporation of tritiated thymidine. Cells treated with low concentrations of trypsin lose their ability to respond to the factor if protein synthesis is partially inhibited by low doses of cycloheximide. Responsiveness is retained if protein synthesis is permitted after enzyme treatment. The data suggest that a protein receptor on the external surface of the target cells is required for the action of granulopoiesis inhibiting factor on bone marrow cells.