Mitotic response of normal and psoriatic keratinocytes in vitro to compounds known to affect intracellular cyclic AMP

Chronic fluoride ingestion decreases 45Ca uptake by rat kidney membranes

High exposures to fluoride (F-) may occur in environments rich in F- from natural or industrial sources and from misuse of F--containing dental care products, particularly by children. Both acute and chronic exposures to elevated levels of F- have negative effects on several calcium-dependent processes, including kidney glomerular and tubular function. We examined the effect of chronic F- ingestion on ATP-dependent 45Ca uptake by rat kidney membrane vesicles to characterize the mechanism by which high F- alters Ca++ transport in the kidney. Twenty weanling female Sprague-Dawley rats were raised on low-F- (0.9 mg/L), semi-purified diet with a Ca++ concentration of 400 mg/100g diet. Rats were divided into four groups and were fed ad libitum deionized water containing F- at 0, 10, 50, or 150 mg/L added as NaF for 6 wk. This consumption produced plasma F- levels of <0.4, 2, 7, or 35 micromol/L, respectively. ATP-dependent 45Ca uptake was significantly lower in the 150 mg F-/L exposure group than in the 0 mg F-/L controls (P < 0.05). Studies with thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca++-pump, showed that the lower uptake was associated with significantly lower activities of both the plasma membrane Ca++-pump (P < 0.05, 150 mg F-/L group versus control) and endoplasmic reticulum Ca++-pump (P < 0.05 for both the 50 and 150 mg F-/L groups versus control). Slot blot analysis of kidney homogenates with specific Ca++-pump antibodies showed less (P < 0.05) endoplasmic reticulum Ca++-pump protein and plasma membrane Ca++-pump protein in all treatment groups than controls. Both Ca++-pumps are transport molecules of great importance in the regulation of Ca++ homeostasis. Our study suggests that chronic, high F- ingestion producing high plasma F- levels may occur in humans and may affect Ca++ homeostasis by increasing the turnover or breakdown or decreasing the expression of plasma membrane and endoplasmic reticulum Ca++-pump proteins.

Genetic toxicity of fluoride

F- is not mutagenic in standard bacterial systems, but produces chromosome aberrations and gene mutations in cultured mammalian cells. Although there is disagreement in the literature concerning the ability of F- to induce chromosome aberrations in cultured human and rodent cells, the weight of the evidence leads to the conclusion that F- exposure results in increased chromosome aberrations in these test systems. NaF induced primarily chromatid gaps and chromatid breaks, indicating that the rodent cells are responsive in the G2 stage of the cell cycle. In contrast, studies with synchronized human cells indicated that the S phase was the most sensitive. If F- does have a cell cycle-specific effect, it could be expected that differences in the cell treatment and harvest protocols could lead to conflicting results for the induction of chromosome aberrations. Gene mutations were produced in cultured rodent and human cells in the majority of the studies. Unfortunately, a number of the in vitro and in vivo cytogenetic studies are of questionable utility because of the protocols used, the quality of the responses reported, or the interpretations of the data. The conflicting results in the in vivo cytogenetic studies are difficult to reconcile. There are reports of increased chromosome aberrations in rat bone marrow and testes, but other studies, using similar protocols and dose ranges, have reported no induced chromosome damage. Although some of the studies were performed at toxic levels of F-, other studies, including those that showed positive results, were at F- concentrations (1-5 ppm) equivalent to human exposure levels. In the majority of studies that were reported to be positive, there were high background frequencies, or the investigators reported categories of nuclear or chromosome damage that are difficult to interpret. Interestingly, many of the positive results were obtained when anaphase cells were scored, whereas similar treatment protocols in other laboratories yielded negative results when metaphase cells were the only cell type examined. It is difficult, without additional data, to determine the reasons for finding chromosome breaks in anaphase, but not metaphase, cells. Other reports have presented insufficient information to allow adequate evaluations. Therefore, at this time, the question of whether F- produces chromosome damage in vivo should be considered unresolved.

Epinephrine synthesis in rat skin by an N-methyltransferase

Homogenates of rat skin N-methylated norepinephrine to form epinephrine. In the brain and adrenal medulla the enzyme phenylethanolamine-N-methyltransferase synthesizes epinephrine, but the skin epinephrine forming enzyme was an N-methyltransferase distinct from phenylethanolamine-N-methyltransferase. Skin N-methyltransferase was not inhibited by the phenylethanolamine-N-methyltransferase inhibitor SKF 29661. Unlike phenylethanolamine-N-methyltransferase, skin readily methylated dopamine to form epinine. Sympathetic denervation by superior cervical ganglionectomy had no effect on skin N-methyltransferase levels. Procedures that reduced skin norepinephrine levels to 2% of control left skin epinephrine levels at 38% of control even when plasma epinephrine levels were very low. Skin contains an extraneuronal enzyme that synthesizes epinephrine in vitro and appears to synthesize part of the epinephrine normally present in skin. The enzyme can synthesize epinephrine and epinine, both of which can regulate epidermal proliferation, skin blood flow, and atopic responses.

Effects of insulin, transferrin, cholera toxin, and epidermal growth factor on growth and morphology of human fetal normal colon epithelial cells

Primary and serially passaged human fetal normal colon epithelial cells have been propagated and characterized with regard to their nature and origin. The cells exhibited many characteristics of colonic epithelial cells including the presence of mucopolysaccharides and carcinoembryonic antigen. Serial passaging of the cultures required supplementing the medium with insulin, transferrin, epidermal growth factor, and cholera toxin. This study also shows that these factors have specific roles in the regulation of growth and morphologic differentiation of the cell cultures. Insulin apparently is mainly associated with cell multiplication, whereas transferrin, epidermal growth factor, and cholera toxin are associated with the maintenance of morphologic differentiation status of the cell cultures.

Effects of the anti-inflammatory agents indomethacin, metiamide and acetylsalicylic acid on the induction of hyperplasia in hamster cheek-pouch epithelium and ear epidermis

Previous studies have shown that agents such as indomethacin and hydrocortisone act to reduce the level of epidermal hyperplasia produced by various chemical, but little information is available about the effects of such anti-inflammatory agents on the induction of hyperplasia in oral mucosa. Hamster cheek-pouch epithelium and ear epidermis were treated with indomethacin, metiamide and acetylsalicylic acid prior to treatment with turpentine, podophyllin or TPA. The levels of hyperplasia induced were monitored by rates of epithelial glycolysis, protein synthesis and mitosis. The response of the metabolic assays to epinephrine was also examined. Alone, turpentine, podophyllin and TPA each caused a 3-5 fold increase in the metabolic assays and loss of the normal tissue response to epinephrine. Tissues pretreated with indomethacin showed significant reductions in the levels of hyperplasia produced and they retained a normal response to epinephrine. No reduction in hyperplasia was observed following pretreatment with acetylsalicylic acid, an alternative inhibitor or prostaglandin synthesis or with metiamide, a histamine blocker. The results indicate that the production of epithelial hyperplasia by turpentine, podophyllin and TPA can be inhibited by indomethacin.

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.

Cell cycle kinetics of cultured human epidermal keratinocytes

When stratified epithelia maintained in culture are used for autoradiographic studies of labeling index, the emulsion is usually placed over the uppermost strata of the culture. In many cases the distance from the basal cell nucleus to the emulsion exceeds the average pathlength of beta-particle emissions from 14C or 3H. We describe a technique for inverting the cultures so that the emulsion can be brought into close association with the basal cells. Attempts to label cultured human epidermal keratinocytes using a pulse of [3H]- or [14C]-thymidine produced labeling only at the periphery of the colonies. This was noted when emulsion was laid on top of the colonies but also when the emulsion was in close contact with the "basal cells" adhering to the plastic culture vessel. Continuous labeling of the cultures produced nearly 100% labeling of all the basal layer, i.e., central and peripheral, indicating that the central cells were also in rapid cell cycle. The results are interpreted as indicating the presence of an efficient barrier to free diffusion over the center of the colonies, presumably due to the presence of several layers of corneocytes. Percent labeled mitoses (PLM) studies produced an unusual PLM curve with a well-defined third peak which showed a higher PLM than the second peak. These results may indicate that the cultures contain discrete cell populations with different cell kinetic phase durations.

Reversal of beta-agonist-induced refractoriness in skin by tetracaine and mepacrine

We previously reported that in skin slices stimulated by a beta-adrenergic agonist, the intracellular cyclic AMP level increased transiently. The level returned to a low steady state in 20-30 min and further stimulation by the agonist did not increase the cyclic AMP level. This state of "refractoriness" was found to be specific to the initial stimulator, i.e., histamine but not epinephrine could restimulate the cyclic AMP system after an initial exposure to epinephrine (Biochim Biophys Acta 497:428-436, 1977). We now report that incubation of skin with mepacrine or tetracaine after beta-adrenergic stimulation caused partial recovery from the refractoriness. Neither the simultaneous incubation of skin with epinephrine plus mepacrine (or tetracaine) nor preincubation of skin with mepacrine (or tetracaine) before the beta-adrenergic stimulation prevented the development of the refractoriness. Mepacrine inhibited the skin adenylate cyclase catalytic (or the complex of GTP-regulatory protein and catalytic) unit. The available data suggest that mepacrine and tetracaine interacted with the agonist-receptor complex at the cell membrane.

Histamine and epidermal proliferation

Histamine is liberated in the inflammatory reaction and has been reported to inhibit epidermal cell division in vitro. This study has investigated the effects of histamine and HI and H2 antagonists on epidermopoiesis in vivo in man. No direct stimulatory effect of histamine was detected for normal epidermis. A combination of HI antagonist (chlorpheniramine) and H2 antagonist (cimetidine) led to further increases in epidermal labelling indices in mitotically stimulated epidermis. The administration of HI antagonist alone led to a decrease in mean epidermal thickness. These data suggest that histamine release is unlikely to play a major role in the hyperplasia of inflammatory dermatoses, but that it may be possible to influence epidermal reactions via the HI and H2 receptors.

Enhancement of squamous cell development in cultured skin by cyclic adenine nucleotide and prostaglandins

The present study tests the hypothesis that agents known to elevate the level of intracellular cyclic adenine nucleotide may direct different epithelial cells onto a pathway of epidermoid (squamous) development and differentiation. We report here that the mixture of dibutyryl cyclic AMP (dbcAMP), prostaglandins E1, E2 and B1 (PG E1, E2, B1), and papaverine (pap) enhances the rate of normal squamous cell development in organ-cultured skin of chick embryos. The three components may act synergistically to elevate the level of intracellular cyclic adenine nucleotide. We recently reported that the same group of agents induces abnormal development (squamous metaplasia) and aberrant differentiation (keratin production) in the normally cuboidal epithelium of cultured whole mammary glands of mice [1]. Thus, dbcAMP, PG E1, E2, B1, and pap are effective in enhancing normal squamous cell development and also in inducing squamous metaplasia de novo in the epithelial components of two different organs of embryonic and adult animals of two classes of vertebrates. The combined findings are suggestive that cyclic adenine nucleotide together with the prostaglandins may act generally on diverse types of epithelia to bring about squamous cell development and a differentiation marked by keratin production.

The effects of alpha and beta adrenergic agonists and cyclic adenosine 3':5'-monophosphate on epidermal metabolism

Continuously regenerating stratified squamous epithelia form an interesting model for examining mechanisms controlling the balance between rates of cell formation and cell maturation and death. In vitro assays of rates of glycolysis and amino acid incorporation of epidermal sheets free from dermal contamination were used to examine rates of metabolism in both normal and hyperplastic epidermis after treatment with various adrenergic agonists and cAMP. Epinephrine and isoproterenol over the concentration range of 1 x 10(-9) to 1 x 10(-5) M depressed the rates of glycolysis and amino acid incorporation in normal epidermis. Dibutyryl cyclic AMP produced a 73 to 78% depression in metabolic activity and its action was enhanced by the addition of theophylline. The alpha adrenergic agonist norepinephrine produced similar reductions. When epidermal samples were treated with hexadecane to induce a mild hyperplasia, depressant effects of isoproterenol and epinephrine were lost, but dibutyryl cyclic AMP and norepinephrine still reduced metabolic activity. The results suggest that adrenergic agents and their putative second messenger cAMP cause reductions in epidermal metabolic activity, an effect similar to their effects on cell proliferation, and that increased rates of proliferation are associated with loss of beta adrenergic responsiveness of the epidermis.

Effect of hydrocortisone on the adenylate cyclase system of the skin--in vitro explant study

We have previously reported the effect of hydrocortisone (HC) on the adenylate cyclase system of pig epidermis. HC had no effect on the basal level of cyclic AMP but there was an increased adrenaline-induced cyclic AMP accumulation when epidermal slices were incubated for more than 6 h with HC. This incubation did not alter the responsiveness to histamine. We are interested in the effects on epidermal cells of HC, which may act through the adenylate cyclase system. It is well documented that adrenaline, histamine and adenosine stimulate adenylate cyclase and cause an accumulation of cyclic AMP in pig epidermis. An increased cyclic AMP results in the inhibition of mitosis and epidermal outgrowth. Employing the rate of epidermal outgrowth and mitotic index as indicators of cyclic AMP effect, we studied the effect of HC on the adenylate cyclase system. The pretreatment with HC (100 microM) magnified the inhibitory effect of adrenaline on epidermal outgrowth and mitosis, whereas the inhibitory effect of histamine or adenosine was not affected by HC. It is suggested that HC induced an increased responsiveness of adenylate cyclase to adrenaline, resulting in a magnified inhibitory effect of adrenaline on epidermal outgrowth and mitosis.

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.

Effects of theophylline and dibutyryl cyclic AMP on proliferation and keratinization of human keratinocytes

Recent evidence suggested that cyclic adenosine 3',5'-monophosphate (cAMP) may be involved in the regulation of cell proliferation and differentiation. The effects of theophylline and dibutyryl cAMP on cell division and keratinization of human epidermal cultures were examined. Nine to 12-day-old cultures were treated with these agents, separately and in combination, for various intervals. Both agents, either singly or in combination, depressed mitoses. The maximum mitotic inhibition was obtained in cultures treated with theophylline or theophylline plus dibutyryl cAMP. Tritiated thymidine studies showed that the test agents had no effect on labelling index (LI) at 4 h, but a 74% inhibition of LI was observed at 24 h. The maximum inhibition of LI (93%) occurred at 96 hours. In contrast to the control cultures, which rarely contain keratohyaline granules (KG), a marked increase in the production of these granules occurred in cultures treated with dibutyryl cAMP plus theophylline. The KG were present over the whole outgrowth. Theophylline alone also stimulated the production of KG, whereas dibutyryl cAMP had no effect. These data show that these agents inhibit cell division and this inhibition may be accompanied by an increased production of KG.

Timing of mitosis in Physarum polycephalum: effects of agents affecting cyclic AMP concentrations

Cultures of Physarum polycephalum incubated with caffeine or theophylline for over 100 min prior to mitosis exhibited mitotic delay proportional to the time of treatment before 100 min. Starved cultures exhibited mitotic delay at times of starvation longer than 180 min and slight stimulation from 100-180 min. Dibutyryl cAMP appeared to accelerate reconstruction of the nucleus following mitosis.

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.

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.

Elevated spermidine and spermine levels in the blood of psoriasis patients

Samples of blood, anticoagulated with EDTA, from 11 patients with psoriasis and 11 individuals without psoriasis were analyzed for their polyamine content. The average spermidine level in patients with psoriasis was approximately twice that of the controls and the average spermine level was three times that of the controls. The level of spermidine and spermine in the skin of two patients with psoriasis were significantly depressed as compared to those of controls.

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.

Cyclic AMP, cyclic GMP, and glucocorticoids as potential metabolic regulators of epidermal proliferation and differentiation

The two cyclic nucleotides, cyclic AMP and cyclic GMP, appear to be central to the metabolic regulation of cell proliferation and differentiation in various cells. Moreover, in many systems glucocorticoids appear to act in concert with or parallel to cyclic AMP. The available evidence suggests that these three molecular species--cyclic AMP, cyclic GMP, and glucocorticoids--may be essential to the normal regulation of epidermal proliferation and differentiation. In 1970, we suggested that perturbed epidermal homeostasis, exemplified by psoriasis, might be associated with low cellular levels of cyclic AMP and, in 1972, with high levels of cyclic GMP as well. Subsequent measurements of these two cyclic nucleotides in our laboratory showed a probable reduction in the cyclic AMP/cyclic GMP ratio in lesional psoriatic tissue. This led to the hypothesis that the cardinal features of psoriatic epidermis--glycogen accumulation, excessive proliferation, and reduced cell specialization--are the results of this reduced ratio. A corollary of this hypothesis was that a psoriatic lesion could not begin or exist without this altered cyclic nucleotide ratio. Recently, four different agents--lithium, a beta adrenergic blocking agent, antimalarials, and iodide--have been found to exacerbate psoriasis and to reduce the formation of cyclic AMP in various tissues. Consequently we believe that cyclic nucleotides are of central importance in the pathogenesis of the epidermal component of psoriasis.

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 and psoriasis

Evidence that an adenyl cyclase system is present in all mammalian epidermis is reviewed. This adenyl cyclase is stimulated by at least two separate types of chemicals: catecholamines, which act at a beta-adrenergic receptor site, and prostaglandins of the E series, which act at a separate site. In the psoriatic lesion, the response to these stimulators, especially to the catecholamines, is reduced. Despite this lack of response to external agents which elevate cyclic AMP, the concentration of cyclic AMP within the epidermis of the psoriatic lesion is no lower than in noninvolved skin. How cyclic nucleotides act to control cell proliferation and cell differentiation remains unclear.

Prostaglandin and DNA synthesis in human skin: possible relationship to ultraviolet light effects

The effect of prostaglandin E2 (PGE2) on DNA synthesis in human skin was evaluated. PGE2 (1 mug) was infected intradermally into normal buttock skin of 15 volunteers followed by tritiated thymidine for autoradiographic quantitation of DNA synthesizing cells. Controls of normal saline, histamine (50 mug), and lower doses of PGE2 were also injected into 8 of the volunteers. Forty-eight hours after injection of 1 mug and 0.1 mug PGE2 there was a 264% and 62% increase, respectively, in the number of DNA synthesizing epidermal cells/high-power field as compared to saline controls. These differences were statistically significant (p smaller than 0.01). Histamine (50 mug) produced a statistically significant 36% higher labeling index compared to its saline controls (p smaller than 0.05). Many types of skin injury, including ultraviolet light (UVL) irradiation, produce an increase in the number of DNA synthesizing cells about 48 hr after the stimulus. Our findings suggest that PGE, a putative mediator of UVL-induced inflammation, may be one of the chemical mediators for the UVL-induced increase in DNA synthesizing cells. Histamine may also contribute to the increase in DNA synthesizing cells following UVL-induced inflammation.

Primary cell culture for biochemical studies of human keratinocytes. A method for production of very large numbers of cells without the necessity of subculturing techniques

Starting with large numbers of small split-thickness explants of human skin, it is possible to grow ample numbers of keratinocytes in vitro as primary cell cultures to permit biochemical studies without the need of subculturing techniques.