Differences in the regulation of intracellular calcium in normal and neoplastic keratinocytes are not caused by ras gene mutations

The development of resistance to terminal differentiation is an early event in epidermal neoplasia. Altered differentiation can be detected in vitro since normal epidermal cells are induced to differentiate in medium with Ca2+ greater than 0.1 mM while neoplastic epidermal cells and keratinocytes transduced with a v-rasHa gene are resistant to Ca2+. In normal epidermal cells, the elevation of extracellular Ca2+ (Cao) from 0.05 to 1.2 mM causes a biphasic intracellular Ca2+ (Cai) response in which a transient (10 min) peak of 4-5-fold over basal values is followed by a sustained (greater than 24 h) 2-fold increase in steady-state Cai. The transient peak in Cai is dependent on a serum component and independent of Cao, while the sustained plateau is directly dependent on Cao. The transient peak responding to a serum factor is lost in normal cells after 24 h in 1.2 mM Ca2+, a time when these cells are differentiating. Two neoplastic keratinocyte cell lines, SP-1 and 308, which produce benign tumors in vivo, also have a biphasic Cai response to an increase in Cao. In these cells, the transient peak is also serum dependent and amplified to 10-fold over basal values. However, the plateau value is not sustained and returns to basal values by 8 h, independent of Cao. Furthermore, 308 cells remain sensitive to the serum-induced Cai transient after 24 h in 1.2 mM Ca2+. To determine whether the activating c-rasHa mutation in 308 and SP-1 cells was responsible for the altered Cai regulation, a v-rasHa gene was introduced into normal keratinocytes by a defective retrovirus. This also produces the papilloma phenotype in vivo. Recipient cells were resistant to Ca(2+)-induced terminal differentiation although they did not proliferate in 1.2 mM Ca2+. The Cai profile in response to 1.2 mM Ca2+ was identical in normal and v-rasHa keratinocytes, and these cells lost the serum-induced transient Cai peak after 24 h. Thus, the activation of the c-rasHa gene in 308 or SP-1 cells is probably not solely responsible for the altered Cai response in neoplastic cell lines. Sustained physiological elevation of Cai may be relevant to the loss of proliferative potential in both normal and v-rasHa keratinocytes in 1.2 mM Ca2+. In addition, v-rasHa-mediated or activated c-rasHa-mediated changes in a complementary pathway may contribute to the block in terminal differentiation in neoplastic cells.     

Changes in inositol phosphate metabolism are associated with terminal differentiation and neoplasia in mouse keratinocytes

Cultured murine keratinocytes respond to specific Ca2+ levels in medium (Ca0) by expressing markers of terminal differentiation. A Ca0 of 0.05 mM selects for a basal cell phenotype, whereas spinous cell characteristics occur in 0.12 mM Ca2+ and cornified envelopes develop in 1.0 mM Ca2+. An increase in inositol phosphate (InsP) metabolism is associated with higher Ca2+ in the medium. The magnitude of Ca(2+)-stimulated InsP turnover is Ca0-dependent, whereas Ca0 of 0.05, 0.12 or 1.4 mM resulted in a graded, sustained (greater than 24 h) increase in InsPs. Diacylglycerol (DAG) levels similarly increased in a graded manner. The major inositol trisphosphate (InsP3) to accumulate was Ins-1,3,4-P3 while Ins-1,4,5-P3 increased transiently. Neoplastic keratinocyte cell lines, 308 and SP-1, which produce benign tumors and have a mutated c-rasHa gene, do not express markers of differentiation in response to Ca2+. Basal InsP and DAG are 2- and 5-fold higher respectively in the neoplastic cells relative to normal keratinocytes. However, the metabolic profiles of InsPs were similar in normal and neoplastic cells. In neoplastic cells, InsP metabolism was stimulated even further following a Ca2+ increase, and this was graded to the Ca0. The unusual, sustained Ca(2+)-graded InsP response in normal cells is consistent with the turnover of InsP contributing to the signals controlling expression of markers of differentiation. Very high InsP turnover and DAG levels, as in neoplastic cells, may be inhibitory to marker expression.     

Development of an in vitro model to study carcinogen-induced neoplastic progression of initiated mouse epidermal cells.

Initiation and promotion in mouse skin carcinogenesis produce multiple benign tumors, squamous papillomas, but only a few squamous cell carcinomas. The spontaneous conversion from the benign to the malignant phenotype occurs over many months and in stages, but induced malignant conversion can be accomplished more rapidly by exposure of papilloma-bearing mice to mutagens or by transfection of papilloma cell lines with specific oncogenes. The analysis of genetic targets responsible for carcinogen-induced neoplastic progression would be facilitated by the development of in vitro models where the process is rapid, focal, and quantitative. To this end, primary newborn mouse keratinocytes were initiated in vitro by the introduction of the v-rasHa oncogene via a defective retrovirus. Recipient cells produce squamous papillomas and have a high proliferation rate in culture medium with 0.05 mM Ca2+, but fail to grow in medium with 0.5 mM Ca2+ which is permissive for growth of malignant keratinocytes. When v-rasHa-keratinocytes were exposed to mutagens in vitro, proliferative foci emerged after culture in 0.5 mM Ca2+ for 4 weeks. These foci stained intensely red with rhodamine stain, could be easily quantitated, and readily incorporated bromodeoxyuridine. Dose-response studies with several mutagens indicated that the number of foci increased with concentration to the point where excessive cytotoxicity developed. Mutagens varied in potency for producing foci in the following order: cis-diamminedichloroplatinum greater than or equal to benzo(a)pyrene diolexpoxide I greater than N-methyl-N'-nitro-N-nitrosoguanidine greater than or equal to 4-nitroquinoline-N-oxide greater than N-acetoxy-acetyl- aminofluorene. The tumor promoter 12-O-tetradecanoylphorbol-13-acetate was inactive in the assay. A subset of cell lines derived from foci produced malignant tumors in vivo, while others were not tumorigenic. Analysis of DNA from cell lines and tumors revealed that most tumorigenic cell lines maintained the v-rasHa genome, whereas the viral sequences were deleted in nontumorigenic cell lines. Immunohistochemical analysis indicated that proliferative foci and quiescent v-rasHa keratinocytes expressed keratin 8, a marker of v-rasHa expression in cultured keratinocytes. Cells in foci, but not v-rasHa control cells, expressed keratin 13, a marker which is strongly associated with the malignant progression of skin tumors in vivo. This in vitro assay provides a quantitative model to study chemically induced focal neoplastic progression at the cellular level and to identify agents which may be selective for enhancing malignant conversion.     

Early signals for keratinocyte differentiation: role of Ca2+-mediated inositol lipid metabolism in normal and neoplastic epidermal cells

Differentiation of cultured keratinocytes is regulated by theCa²⁺ concentration of the culture medium. Below 0.1 mM Ca²⁺,a monolayer of basal cells is formed which fully differentiatesin response to a rise in medium Ca²⁺. A role for protein kinaseC in this differentiation program has been suggested becausephorbol esters induce epidermal differentiation in cells grownin reduced Ca²⁺ medium, and exogenously added phospholipaseC (which increases cellular diacylglycerol) mimics phorbol esteraction. These findings suggested that the external Ca²⁺ signalmay lead to protein kinase C activation via stimulation of cellularphospholipase C activity. The effect of the external Ca²⁺ signalon phospholipase C was studied in cultures prelabeled with [³H]inositol. Within 2 min after addition of Ca²⁺ to 1 mM, an increasein inositol phosphates was measured. This correlated with adecrease in radiolabeled phosphoinositides, suggesting thatthese were the source of the increased inositol phosphates.After 3 h in 1 mM Ca²⁺ medium, each of the inositol phosphatesremained increased to 130–140% of control levels. Inositolphosphate metabolism in neoplastic epidermal cells was quantitativelysimilar to normal cells in response to the Ca²⁺ signal. Stimulationof phosphatidylinositol (PIP) metabolism appears to be mediatedby a rise in intracellular free Ca²⁺ because Ca²⁺ ionophoresA23187 and ionomycin also cause a similar rise in inositol phosphatelevels. Phorbol esters did not increase PIP turnover but insteadstimulated phosphatidylcholine metabolism. The induction ofepidermal differentiation by phorbol esters was enhanced byionomycin, suggesting that both protein kinase C activation,elevation of intracellular calcium and PIP turnover were importantcomponents of the signal for epidermal differentiation. Theseresults demonstrate that the second messenger system for Ca²⁺-mediated keratinocyte differentiation may be through a directeffect on phospholipase C activity.     

Galanin stimulates Ca2+ mobilization, inositol phosphate accumulation, and clonal growth in small cell lung cancer cells.

Addition of the neuropeptide galanin to small cell lung cancer (SCLC) cells loaded with the fluorescent Ca2+ indicator fura-2-tetraacetoxymethylester causes a rapid and transient increase in the intracellular concentration of Ca2+ ([Ca2+]i) followed by homologous desensitization. Galanin increased [Ca2+]i in a concentration-dependent fashion with half-maximum effect (EC50) at 20-22 nM in H69 and H510 SCLC cells. Galanin mobilized Ca2+ from intracellular stores since its effects on [Ca2+]i were not blocked by chelation of extracellular Ca2+. Pretreatment with pertussis toxin (200 ng/ml for 4 h) did not prevent galanin-induced Ca2+ mobilization. In contrast, direct activation of protein kinase C with phorbol esters attenuated the Ca2+ response induced by galanin. The effects of galanin could be dissociated from changes in membrane potential: galanin did not increase membrane potential in SCLC cells loaded with bis(1,3-diethyltiobarbiturate)-trimethineoxonol and induced Ca2+ mobilization in depolarized SCLC cells, i.e., in cells suspended in a solution containing 145 mM K+ instead of Na+. Galanin also caused an increase in the formation of inositol phosphates in a time- and dose-dependent manner (EC50 10 nM). A rapid increase in the inositol trisphosphate fraction was followed by a slower increase in the inositol monophosphate fraction. Galanin stimulated clonal growth of both H69 and H510 cells in semisolid (agarose-containing) medium. This growth-promoting effect was sharply dependent on galanin concentration (EC50 20 nM) and markedly inhibited by [Arg6,D-Trp7,9,MePhe8]substance P, a recently identified broad spectrum neuropeptide antagonist. The results show for the first time that galanin receptors are coupled to inositol phosphate and [Ca2+]i responses in SCLC cells and, in particular, that this neuropeptide can act as a direct growth factor for these human cancer cells.     

Inhibition of phorbol ester-induced phenotypic differentiation of HL-60 cells by 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, a protein kinase inhibitor

To identify the possible role of calcium ions in cell differentiation, we studied the extracellular Ca2+ requirement and the effect of Ca2+/phospholipid-dependent protein kinase (protein kinase C) inhibitor on proliferation and differentiation of human promyelocytic leukemic HL-60 cells. HL-60 cells grew equally well in 0.1 and 1.0 mM Ca2+ media. The addition of 12-O-tetradecanoyl-phorbol-13-acetate (TPA), 1,25-dihydroxyvitamin D3, and all-trans-beta-retinoic acid inhibited the cell growth and induced mature macrophage and granulocyte phenotypes in 1.0 mM Ca2+ medium. 1,25-Dihydroxyvitamin D3 and all-trans-beta-retinoic acid induced HL-60 differentiation to the same degree in 0.1 mM Ca2+ and 1.0 mM Ca2+ media. However, TPA failed to induce HL-60 differentiation or to inhibit proliferation in a 0.1 mM Ca2+ medium. The decrease of extracellular Ca2+ from 1.0 to 0.1 mM caused a significant drop in the intracellular Ca2+ level in undifferentiated and TPA-treated HL-60 cells, although no rapid change in cytosolic Ca2+ was detected in response to TPA addition. 1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine (H-7), a protein kinase C inhibitor, inhibited proliferation of HL-60 cells in a dose-dependent manner. In contrast, H-7 selectively restored the proliferation of TPA-treated HL-60 cells and inhibited TPA-induced phenotypic differentiation. However, the same concentrations of 1-(5-isoquinolinylsulfonyl)-2,3-dimethylpiperazin and N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide, analogues of H-7 that inhibit protein kinase C more weakly, had no effect on the proliferation or differentiation induction. H-7 also suppressed 1,25-dihydroxyvitamin D3- and all-trans-beta-retinoic acid-induced phenotypic changes of HL-60 cells but did not eliminate the growth inhibition by these inducers. These results demonstrate the Ca2+ requirement and the protein kinase C involvement in phorbol ester-induced phenotypic differentiation of HL-60 cells.     

Association of resistance to terminal differentiation with initiation of carcinogenesis in adult mouse epidermal cells

Primary adult mouse epidermal cells were maintained as a monolayer culture with a high proliferation rate in fibroblast-conditioned medium with low Ca2+ concentration (less than 0.1 mM). Terminal differentiation of the cultures was induced by raising the Ca2+ level in the medium above 0.1 mM. Treatment of adult mouse epidermal cells either in vivo or in vitro with 7,12-dimethylbenz(a)anthracene or N-methyl-N'-nitro-N-nitrosoguanidine yielded colonies which were resistant to terminal differentiation induced by Ca2+. The number of resistant colonies was dependent upon the dose of each carcinogen used whether exposure was in vivo or in vitro. Cultures derived from skin initiated in vivo with 7,12-dimethylbenz(a)anthracene, a strong initiator, resulted in more colonies than were derived from skin initiated with N-methyl-N'-nitro-N-nitrosoguanidine, a moderately potent initiator. Two mouse strains, BALB/c and SENCAR, which differ in sensitivity to skin carcinogenesis, yielded similar numbers of Ca2+-resistant colonies following carcinogen exposure. However, colonies developed spontaneously from untreated SENCAR cells (the sensitive strain), but not from BALB/c cells (the resistant strain). These results support the concept that cells resistant to terminal differentiation are initiated cells. The results also suggest that initiation may occur spontaneously in SENCAR skin, a finding consistent with the reported occurrence of tumors in mice of this strain receiving promoters without exogenous initiator.     

Defects in TGF-beta signaling overcome senescence of mouse keratinocytes expressing v-Ha-ras

Previous studies have shown that TGFbeta1 expression is upregulated in mouse keratinocytes infected with a v-rasHa retrovirus, although the functional significance of this has not been clear. Here we show that v-rasHa retrovirus transduced primary mouse keratinocytes undergo hyperproliferation followed by a TGFbeta1 dependent G1 growth arrest and senescence. The growth arrest is accompanied by a 15-fold increase in total TGFbeta1 secreted and a fourfold increase in secreted active TGFbeta1. When cultured in the presence of a neutralizing antibody to TGFbeta1, the senescence response is suppressed. Levels of the TGFbeta1 target p15ink4b increase during senescence as does association of this kinase inhibitor with cyclinD/cdk4 complexes. However, p16ink4a, p53 and p19ARF expression also increase during senescence. Genetic analysis shows that TGFbeta1 null and dominant negative TbetaBRII expressing v-rasHa keratinocytes resist the G1 growth arrest and do not senescence. This resistance is associated with low expression of p15ink4b and p16ink4a, constitutive Rb phosphorylation and high levels of cdk4 and cdk2 kinase activity. In contrast, inactivation of TGFbetabeta1 secretion or response does not block the induction of p53 and p19ARF, but the level of p21waf1, a p53 target gene, is reduced in cyclin D/cdk4 and cyclin E/cdk2 complexes. Thus, although multiple senescence pathways are activated in response to a ras oncogene, inactivation of TGFbeta1 secretion or response is sufficient to block the senescence program. Since v-rasHa transduced TGFbeta1-/- keratinocytes form squamous cell carcinomas following skin grafting, these results suggest that in mouse keratinocytes, defects in TGFbeta1 signaling accelerate malignant progression by overcoming oncogene induced replicative senescence.     

Phosphoinositide metabolism in a polyoma-bk-virus-transformed pancreatic islet cell line: Evidence for constitutively activated phospholipase C

We have characterized the phosphoinositide metabolism in a polyoma-BK-virus-transformed rat pancreatic islet cell line which has highly malignant characteristics, expresses viral T-antigen and has lost insulin-secreting capacity. After incorporation with [³H]linositol to isotopic equilibrium, all inositol metabolites were analyzed. When comared with normal pancreatic islets, increased levels of inositol 1,4,5-trisphosphate (Ins- 1,4,5-P₃), inositol 1,3,4-trisphosphates and inositol tetrakisphosphate (Ins-P₄), and decreased levels of phosphatidylinositol monophosphate (PIP) and phosphatidylinositol bisphosphate (PIP₂) were found. The Ins-1,4,5-P₃/PIP₂ ratio increased, whereas the PIP₂/PIP ratio was not altered after the transformation. In the pancreatic islet cell line there was a stable accumulation of inositol phosphates at 3.3 mM glucose. Glucose, KCL, cholecystokinin (CCK) and carbachol with and without LiCL were all without effect on the accumulation of inositol phosphates. Somatostatin inhibited the accumulation of inositol phosphates but a Ca²+-free/EDTA solution did not. Preincubation with cholera toxin or pertussis toxin inhibited the accumulation of inositol phosphates at 3.3 mM glucose except for Ins-P₄, whereas no effect was obsewed on the phosphoinositides. NaF stimulated the accumulation of inositol phosphates, with a concomitant decrease in the phosphoinositides, whereas neomycin was without effect on the inositol phosphates. In normal pancreatic islets, pertussis toxin inhibited the CCK- induced increase in Ins- 1,4,5-P₃, whereas no effect was seen at 3.3 mM glucose. Finally, pertussis toxin inhibited the CCK- induced increase in the Ins- 1,4,5-P₃/PIP₂ ratio in normal pancreatic islets. The same inhibition was also found in the pancreatic islet cell line at 3.3 mM glucose. We conclude that in the transformed pancreatic islet cell line the phosphoinositide hydrolysis is constitutively activated at the level of phospholipase C, with a substantial loss of regulatory control.     

Upregulation of calcium-sensing receptor and mitogen-activated protein kinase signalling in the regulation of growth and differentiation in colon carcinoma

In the present study, we demonstrate that Ca2+-induced growth inhibition and induction of differentiation in a line of human colon carcinoma cells (CBS) is dependent on mitogen-activated protein (MAP) kinase signaling and is associated with upregulation of extracellular calcium-sensing receptor (CaSR) expression. When CBS cells were grown in Ca2+-free medium and then switched to medium supplemented with 1.4 mM Ca2+, proliferation was reduced and morphologic features of differentiation were expressed. E-cadherin, which was minimally expressed in nonsupplemented medium, was rapidly induced in response to Ca2+ stimulation. Sustained activation of the extracellular signal-regulated kinase (ERK) occurred in Ca2+-supplemented medium. When an inhibitor of ERK activation (10 microM U0126) was included in the Ca2+-supplemented culture medium, ERK-activation did not occur. Concomitantly, E-cadherin was not induced, cell proliferation remained high and differentiation was not observed. The same level of Ca2+ supplementation that induced MAP kinase activation also stimulated CaSR upregulation in CBS cells. A clonal isolate of the CBS line that did not upregulate CaSR expression in response to extracellular Ca2+ was isolated from the parent cells. This isolate failed to produce E-cadherin or undergo growth inhibition/induction of differentiation when exposed to Ca2+ in the culture medium. However, ERK-activation occurred as efficiently in this isolate as in parent CBS cells or in a cloned isolate that underwent growth reduction and differentiation in response to Ca2+ stimulation. Together, these data indicate that CaSR upregulation and MAP kinase signalling are both intermediates in the control of colon carcinoma cell growth and differentiation. They appear to function, at least in part, independently of one another.     

Effect of extracellular calcium concentration on the metabolism of polycyclic aromatic hydrocarbons by cultured mouse keratinocytes

Cultures of adult mouse epidermal keratinocytes (MEKs) were utilized to determine whether the metabolism and metabolic activation of polycyclic aromatic hydrocarbons varied as a function of extracellular calcium (Ca2+) concentration. MEKs grown in low Ca2+-containing medium (0.05-0.10 mM) maintain basal cell morphology and proliferate while increasing the Ca2+ concentration in the medium to 1.2-1.4 mM signals the cells to undergo terminal differentiation. Relative to cultures of undifferentiated MEKs (low Ca2+), cultures of differentiated MEKs that had been switched to high Ca2+ medium 48 h prior to treatment with benzo(a)-pyrene [B(a)P] and 7,12-dimethylbenz(a)anthracene (DMBA) exhibited more rapid overall metabolism of both hydrocarbons. The greatest differences in the metabolism of B(a)P and DMBA between the two types of cultures occurred after a 3-6-h lag period. In addition, the levels of DNA-adducts formed from B(a)P and DMBA after a 24-h exposure to the hydrocarbon were 4- and 3-fold higher respectively, in cultures of differentiated MEKs (high Ca2+). Higher levels of mutagenesis and cytotoxicity were also observed in cocultures of Chinese hamster lung V-79 cells and MEKs that had been switched to high Ca2+-containing medium. In cocultures treated with the hydrocarbons at the time of Ca2+ shift, several hours elapsed before differences in mutagenesis were apparent between high and low Ca2+-containing cultures. This lag period was eliminated if the MEKs were switched to high Ca2+ medium 24 h prior to exposure to DMBA. Based on the present data, we propose that the expression and inducibility of certain enzyme activities involved in the metabolism of B(a)P and DMBA by cultured MEKs is regulated by the extracellular Ca2+ concentration and possibly the Ca2+-induced differentiation of MEKs.     

Effects of altered Ca2+ and Mg2+ concentrations on proliferation and functional differentiation of the clonal insulin-producing cells RINm5F

Effects of calcium and magnesium on proliferation and functional differentiation were investigated in the clonal insulin-producing rat cell line RINm5F. Variations of the extracellular concentrations of Ca2+ and Mg2+ had only minor effects on the cellular contents of these elements. Even when the medium was depleted of extracellular Ca2+ by the addition of EGTA, the Ca/Mg ratio corresponded to approximately 0.1. In a Mg2+-deficient medium a lowering of the extracellular calcium to 0.13 mM reduced the amount of insulin in the media but increased that in the cells. Irrespective of the concentration of Mg2+, depletion of Ca2+ was associated with low media levels of insulin and a reduced proliferation rate. In addition, the cells became smaller in size and showed an increased ability to exclude trypan blue. a rise of the magnesium concentration to 6.30 mM was accompanied by a decrease of the cellular content of insulin despite reduced amounts of hormone in the medium. This finding might be due to suppression of insulin biosynthesis, suggesting that the RINm5F cells have a markedly different metabolism and/or sensitivity to extracellular magnesium than non-tumorigenic beta-cells.     

Growth modulation of mouse keratinocytes by transforming growth factors

The effects of exogenously added transforming growth factor (TGF alpha and TGF beta on the growth of BALB/MK cells were examined. TGF alpha supplanted the epidermal growth factor (EGF) requirement in these cells. In contrast, TGF beta reversibly inhibited the growth of BALB/MK cells by abrogating the stimulatory actions of EGF or TGF alpha. The inhibitory effects of TGF beta appeared to be mediated by events distal to EGF ligand-receptor interactions. Growth inhibition of BALB/MK cells by TGF beta did not result in the induction of differentiation. This finding is different from the growth inhibition of these cells induced by elevated calcium levels (1.5 mM) which was tightly coupled to terminal differentiation. The BALB/MK cells were found to express TGF alpha mRNA, as well as TGF beta mRNA and protein. In addition, TGF alpha, as well as EGF, enhanced TGF alpha gene expression. These studies suggest a role for endogenous TGFs in regulating BALB/MK proliferation. TGF alpha provides a positive growth signal, while TGF beta is a potent inhibitor of growth even in the presence of such positive modulators as TGF alpha and EGF.     

Production of transforming growth factors by simian sarcoma virus-transformed cells

Cellular transformation of normal rat kidney (NRK) cells by simian sarcoma virus (SSV) results in a complete loss of the cellular requirement of externally added polypeptide growth factors for proliferation. Moreover, SSV-transformed NRK cells have a strongly reduced ability to bind both external platelet-derived growth factor and epidermal growth factor, when compared to nontransformed NRK cells. Analysis of serum-free medium conditioned by SSV-transformed NRK cells shows that this cell line secretes both types alpha and beta transforming growth factor (TGF). The level of TGF alpha production (300 ng/liter conditioned medium) by SSV-transformed NRK is among the highest described to date. Since addition of TGF alpha and beta in combination is sufficient to induce phenotypic transformation of NRK cells, it is concluded that although expression of the sis oncogene is essential for transformation, expression of additional genes may be required for the phenotypic alterations accompanying complete cellular transformation.     

Modulation of catalase activities in murine epidermal cells as a function of differentiation and exposure to 12-O-tetradecanoylphorbol-13-acetate

A procedure was developed for the per cell estimation of catalase activities in suspensions and cultures of murine epidermal keratinocytes (MEKs). Per cell catalase activity in MEKs cultured in low Ca2+ medium was relatively constant during the proliferation phase of culturing, but increased approximately 100% within 24 h of cessation of cell division. 12-O-Tetradecanoylphorbol-13-acetate (TPA) treatment of proliferating MEKs cultured in low Ca2+ medium resulted in (i) an initial suppression of proliferation, (ii) the accelerated detachment and differentiation of detached MEKs and (iii) a suppression of catalase induction in the detached population. Induction of MEK differentiation by raising the medium Ca2+ concentration resulted in rapid inhibition of cell division and approximately 200% increases in per cell catalase activities. Addition of TPA immediately prior to Ca2+ shift completely suppressed the Ca2(+)-dependent increases in activity. However, the addition of TPA 48 h after the induction of differentiation by Ca2+ shift had no effects on the elevated, pre-existing catalase activities. Per cell catalase activities varied in vivo with the stage of MEK differentiation. Specifically, the lowest and highest per cell activities (approximately 4-fold difference) were measured in enriched basal cell and spinous cell populations respectively. Catalase activity in the more differentiated MEKs was reduced approximately 33% within 24 h of topical treatment of dorsal skin with a promoting dose of TPA. However, catalase activity in enriched basal cell preparations was unaffected. Collectively, these studies demonstrate that per cell catalase activities increase as MEKs differentiate, and that TPA suppresses the increases in catalase activities that normally occur during differentiation.