Further identification of protein kinase C isozymes in mouse epidermis

Molecular mechanisms of mouse skin tumor promotion

Multiple molecular mechanisms are involved in the promotion of skin carcinogenesis. Induction of sustained proliferation and epidermal hyperplasia by direct activation of mitotic signaling pathways or indirectly in response to chronic wounding and/or inflammation, or due to a block in terminal differentiation or resistance to apoptosis is necessary to allow clonal expansion of initiated cells with DNA mutations to form skin tumors. The mitotic pathways include activation of epidermal growth factor receptor and Ras/Raf/mitogen-activated protein kinase signaling. Chronic inflammation results in inflammatory cell secretion of growth factors and cytokines such as tumor necrosis factor-α and interleukins, as well as production of reactive oxygen species, all of which can stimulate proliferation. Persistent activation of these pathways leads to tumor promotion.

Knockdown of PKD1 in normal human epidermal keratinocytes increases mRNA expression of keratin 10 and involucrin: early markers of keratinocyte differentiation

Subconfluent normal human keratinocytes exhibit autonomous (autocrine growth factor driven) proliferation and express the specific markers for keratinocyte proliferation K5 (keratin 5) and K14 (keratin 14). Utilizing this model the effects of PKD1 (Protein kinase D1) knockdown on activation of differentiation was studied. siRNA approach was applied to achieve specific knockdown of PKD1 and the mRNA levels of different keratinocyte markers -- K14 and PCNA (markers of basal proliferating keratinocytes), involucrin and K10 (early differentiation markers) were analyzed. Treatment of cultured keratinocytes with siRNA for PKD1 resulted in reduction of mRNA levels of PKD1, altered cell phenotype and promotion of keratinocyte differentiation, demonstrated by increased expression of involucrin and K10 mRNAs. No significant changes in K14 mRNA expression levels were detected, but the expression of PCNA mRNA was markedly diminished. This study was the first to show that mRNA expression of PKD1 in subconfluent normal human keratinocytes is very low, the PKD1 mRNA levels were more than 8-fold lower than the same ones in hTert keratinocytes. These findings suggest antidifferentiative role of PKD1 in normal human keratinocytes, contrary to the prodiferentiative role of PKD1 in human hTert keratinocytes. We came to the conclusion that there are differences between transduction pathways involving PKD1 in primary human keratinocyte cultures and these in immortalized hTert keratinocytes.

Protein kinase C family: on the crossroads of cell signaling in skin and tumor epithelium

The protein kinase C (PKC) family represents a large group of phospholipid dependent enzymes catalyzing the covalent transfer of phosphate from ATP to serine and threonine residues of proteins. Phosphorylation of the substrate proteins induces a conformational change resulting in modification of their functional properties. The PKC family consists of at least ten members, divided into three subgroups: classical PKCs (alpha, betaI, betaII, gamma), novel PKCs (delta, epsilon, eta, theta), and atypical PKCs (zeta, iota/lambda). The specific cofactor requirements, tissue distribution, and cellular compartmentalization suggest differential functions and fine tuning of specific signaling cascades for each isoform. Thus, specific stimuli can lead to differential responses via isoform specific PKC signaling regulated by their expression, localization, and phosphorylation status in particular biological settings. PKC isoforms are activated by a variety of extracellular signals and, in turn, modify the activities of cellular proteins including receptors, enzymes, cytoskeletal proteins, and transcription factors. Accordingly, the PKC family plays a central role in cellular signal processing. Accumulating data suggest that various PKC isoforms participate in the regulation of cell proliferation, differentiation, survival and death. These findings have enabled identification of abnormalities in PKC isoform function, as they occur in several cancers. Specifically, the initiation of squamous cell carcinoma formation and progression to the malignant phenotype was found to be associated with distinct changes in PKC expression, activation, distribution, and phosphorylation. These studies were recently further extended to transgenic and knockout animals, which allowed a more direct analysis of individual PKC functions. Accordingly, this review is focused on the involvement of PKC in physiology and pathology of the skin.

Direct Binding to Ceramide Activates Protein Kinase Cζ before the Formation of a Pro-apoptotic Complex with PAR-4 in Differentiating Stem Cells

We have reported that ceramide mediates binding of atypical protein kinase C (PKC) zeta to its inhibitor protein, PAR-4 (prostate apoptosis response-4), thereby inducing apoptosis in differentiating embryonic stem cells. Using a novel method of lipid vesicle-mediated affinity chromatography, we showed here that endogenous ceramide binds directly to the PKCzeta.PAR-4 complex. Ceramide and its analogs activated PKCzeta prior to binding to PAR-4, as determined by increased levels of phosphorylated PKCzeta and glycogen synthase kinase-3beta and emergence of a PAR-4-to-phosphorylated PKCzeta fluorescence resonance energy transfer signal that co-localizes with ceramide. Elevated expression and activation of PKCzeta increased cell survival, whereas expression of PAR-4 promoted apoptosis. This suggests that PKCzeta counteracts apoptosis, unless its ceramide-induced activation is compromised by binding to PAR-4. A luciferase reporter assay showed that ceramide analogs activate nuclear factor (NF)-kappaB unless PAR-4-dependent inhibition of PKCzeta suppresses NF-kappaB activation. Taken together, our results show that direct physical association with ceramide and PAR-4 regulates the activity of PKCzeta. They also indicate that this interaction regulates the activity of glycogen synthase kinase-3beta and NF-kappaB.

Early epidermal destruction with subsequent epidermal hyperplasia is a unique feature of the papilloma-independent squamous cell carcinoma phenotype in PKCepsilon overexpressing transgenic mice

Protein kinase C epsilon (PKCepsilon) overexpressing transgenic (PKCepsilon Tg) mice develop papilloma-independent squamous cell carcinomas (SCC) elicited by 7,12-dimethylbenz[a]anthracene (DMBA) tumor initiation and 12-O-tetradecanoylphorbol-13-acetate (TPA) tumor promotion. We examined whether epidermal cell turnover kinetics was altered during the development of SCC in PKCepsilon Tg mice. Dorsal skin samples were fixed for histological examination. A single application of TPA resulted in extensive infiltration of polymorphonuclear neutrophils (PMNs) into the epidermis at 24 h after TPA treatment in PKCepsilon Tg mice while wild-type (WT) mouse skin showed focal infiltration by PMNs. Complete epidermal necrosis was observed at 48 h in PKCepsilon Tg mice only; at 72 h, epidermal cell regeneration beginning from hair follicles was observed in PKCepsilon Tg mice. Since the first TPA treatment to DMBA-initiated PKCepsilon Tg mouse skin led to epidermal destruction analogous to skin abrasion, we propose the papilloma-independent phenotype may be explained by death of initiated interfollicular cells originally destined to become papillomas. Epidermal destruction did not occur after multiple doses of TPA, presumably reflecting adaptation of epidermis to chronic TPA treatment. Prolonged hyperplasia in the hair follicle may result in the early neoplastic lesions originally described by Jansen et al. (2001) by expanding initiated cells in the hair follicles resulting in the subsequent development of SCC.

Overexpression of skin protein kinase C-alpha in anagen hair follicles during induced growth of mouse hair

A role for protein kinase C (PKC)-alpha has been implicated in the growth of mouse hair. Topical application of PKC activators, hair plucking, allergic contact dermatitis and skin irritation can all enhance growth of mouse hair, and a significant increase in PKC-alpha level in whole mouse skin in mature anagen has been demonstrated in these processes. Overexpression of PKC-alpha in anagen hair follicles has also been reported in natural growth of mouse hair. It is known that overexpression of PKC-alpha is associated with the acceleration of cell growth. Therefore, we postulated that overexpression of PKC-alpha in mature anagen may relate to enhancement of hair growth. The distribution of PKC-alpha in hair follicles during induced growth of mouse hair has not previously been studied. In this study, hair growth in C57BL/6 mice was induced by plucking the telogen hairs on one side of the back. The undepilated contralateral side served as a control. Expression of PKC-alpha in hair follicles during the hair growth cycle induced was evaluated by immunohistochemistry using cryosections and a specific polyclonal anti-PKC-alpha immunoglobulin G (IgG) antibody. No PKC-alpha was detected in telogen hair follicles or in the hair follicles at 1 day post-depilation, when the induced hair cycle was in early anagen. At 4 days after plucking, when the induced hair cycle was in mid-anagen, intense staining for PKC-alpha was found in hair papillae. At 10 and 17 days after depilation, when the induced hair cycle was in mature anagen and early catagen, respectively, all outer root sheath (ORS) cells and outer connective sheaths of hair follicles were stained positive. Because no PKC-alpha was detected in telogen hair follicles in this study, down-regulation of PKC-alpha in early anagen could not be observed. However, consistent with our previous findings, overexpression of PKC-alpha was found in mid-anagen and mature anagen. As overexpression of PKC-alpha has been shown to be associated with acceleration of cell growth, our results support the notion that PKC-alpha may play an important role in growth of hair follicle cells in induced growth of hair. As PKC levels are known to increase in hyperglycaemia, overexpressed PKC-alpha in mature anagen hair follicles may be related to the putative function of the ORS in mobilizing glycogen stores for anagen growth.

RIP4 is an ankyrin repeat-containing kinase essential for keratinocyte differentiation

The epidermis is a stratified, continually renewing epithelium dependent on a balance among cell proliferation, differentiation, and death for homeostasis. In normal epidermis, a mitotically active basal layer gives rise to terminally differentiating keratinocytes that migrate outward and are ultimately sloughed from the skin surface as enucleated squames. Although many proteins are known to function in maintaining epidermal homeostasis, the molecular coordination of these events is poorly understood. RIP4 is a novel RIP (receptor-interacting protein) family kinase with ankyrin repeats cloned from a keratinocyte cDNA library. RIP4 deficiency in mice results in perinatal lethality associated with abnormal epidermal differentiation. The phenotype of RIP4(-/-) mice in part resembles that of mice lacking IKKalpha, a component of a complex that regulates NF-kappaB. Despite the similar keratinocyte defects in RIP4- and IKKalpha-deficient mice, these kinases function in distinct pathways. RIP4 functions cell autonomously within the keratinocyte lineage. Unlike IKKalpha, RIP4-deficient skin fails to fully differentiate when grafted onto a normal host. Instead, abnormal hair follicle development and epidermal dysplasia, indicative of progression into a more pathologic state, are observed. Thus, RIP4 is a critical component of a novel pathway that controls keratinocyte differentiation.

Procyanidin B-2, extracted from apples, promotes hair growth: a laboratory study

BACKGROUND

We have previously reported that several selective protein kinase C (PKC) inhibitors, including procyanidin B-2, promote hair epithelial cell growth and stimulate anagen induction.

OBJECTIVES

We discuss the hypothesis that the hair-growing activity of procyanidin B-2 is related to its downregulation or inhibition of translocation of PKC isozymes in hair epithelial cells.

METHODS

We examined the effect of procyanidin B-2 on the expression of PKC isozymes in cultured murine hair epithelial cells as well as PKC isozyme localization in murine dorsal skin at different stages in the hair cycle.

RESULTS

We observed that procyanidin B-2 reduces the expression of PKC-alpha, -betaI, -betaII and -eta in cultured murine hair epithelial cells and also inhibits the translocation of these isozymes to the particulate fraction of hair epithelial cells. Our immunohistochemical analyses demonstrated that PKC-alpha, -betaI, -betaII and -eta are specifically expressed in the outer root sheaths of both anagen and telogen hair follicles. The hair matrix at the anagen stage showed no positive staining for these PKC isozymes. Moderate to intense staining for PKC-betaI and -betaII in the epidermis and hair follicles was observed in a telogen-specific manner; however, expression of PKC-alpha and -eta during the telogen stage was not conspicuous. Gö 6976, an inhibitor of calcium-dependent (conventional) PKC, proved to promote hair epithelial cell growth.

CONCLUSIONS

These results suggest that PKC isozymes, especially PKC-betaI and -betaII, play an important role in hair cycle progression and that the hair-growing mechanisms of procyanidin B-2 are at least partially related to its downregulation of PKC isozymes or its inhibition of translocation of PKC isozymes to the particulate fraction of hair epithelial cells.

Overexpression of protein kinase C-alpha in the epidermis of transgenic mice results in striking alterations in phorbol ester-induced inflammation and COX-2, MIP-2 and TNF-alpha expression but not tumor promotion

Protein kinase Calpha (PKCalpha) is one of six PKC isoforms expressed in keratinocytes of mouse epidermis. To gain an understanding of the role of epidermal PKCalpha, we have localized its expression to specific cells of normal mouse skin and examined the effect of keratin 5 (K5) promoter directed expression of PKCalpha in transgenic mice. In normal mouse skin, PKCalpha was extensively expressed in the outer root sheath (ORS) keratinocytes of the anagen hair follicle and weakly expressed in keratinocytes of interfollicular epidermis. K5-targeted expression of PKCalpha to epidermal basal keratinocytes and follicular ORS keratinocytes resulted in a tenfold increase in epidermal PKCalpha. K5-PKCalpha mice exhibited no abnormalities in keratinocyte growth and differentiation in the epidermis. However, a single topical treatment with the PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in a striking inflammatory response characterized by edema and extensive epidermal infiltration of neutrophils that formed intraepidermal microabscesses in the epidermis. Compared to TPA-treated wild-type mice, the epidermis of TPA-treated K5-PKCalpha mice displayed increased expression of cyclooxygenase-2 (COX-2), the neutrophil chemotactic factor macrophage inflammatory protein-2 (MIP-2) mRNA and the proinflammatory cytokine TNFalpha mRNA but not IL-6 or IL-1alpha mRNA. To determine if K5-PKCalpha mice display an altered response to TPA-promotion, 7, 12-dimethylbenz[a]anthracene-initiated K5-PKCalpha mice and wild-type mice were promoted with TPA. No differences in papilloma incidence or multiplicity were observed between K5-PKCalpha mice and wild-type littermates. These results demonstrate that the overexpression of PKCalpha in epidermis increases the expression of specific proinflammatory mediators and induces cutaneous inflammation but has little to no effect on epidermal differentiation, proliferation or TPA tumor promotion.

Induction of hair growth by skin irritants and its relation to skin protein kinase C isoforms

Induction of hair growth by skin irritants and its relation to skin protein kinase C (PKC) isoforms were evaluated. Dorsal skin of BALB/c mice was shaved and anthralin (0.1% in corn oil) was applied on one side of the shaved backs in 20 mice daily for 5 days. The corresponding opposite side treated with corn oil served as a control. In another 20 mice, sodium dodecyl sulphate (SDS, 10% in water) was applied on one side of the shaved backs for 5 days by the same procedure as above and the corresponding opposite side treated with water served as control. Visible acceleration of hair growth on anthralin-treated skin was observed as early as 14 days after the application of anthralin and significant hair growth was observed at about 17-20 days. Enhancement of hair growth on SDS-treated skin was observed at about 3 weeks from the beginning of the treatment. None of the mice showed remarkable hair growth on the control side in either group. Mouse skin PKC isoforms levels detected by Western blot showed a similar pattern in both treatment groups. PKC alpha was downregulated initially, and was then elevated from 3 days after anthralin treatment and 14 days after SDS application. PKC beta was unchanged initially, decreased at 8 and 14 days after anthralin and SDS treatment, respectively, and reverted to the control level at 25 days after anthralin treatment, when it was still lower than the control in SDS-treated skin. PKC delta was also unchanged at first, but was elevated from 3 days after anthralin treatment and 14 days after SDS application. These results suggest that involvement of PKC may be a general phenomenon in irritant-induced hair growth in mice. Considering the stimulatory effect of PKC alpha and inhibitory effect of PKC delta on cell growth, we postulate that PKC alpha may be responsible for enhancement of hair growth while PKC delta may inhibit hair growth to keep the hair growth in balance.

The potential role of skin protein kinase C isoforms alpha and delta in mouse hair growth induced by diphencyprone-allergic contact dermatitis

The levels of protein kinase C (PKC) isoforms alpha and delta in mouse hair growth induced by diphencyprone (DPCP)-allergic contact dermatitis (ACD) were studied. BALB/c mice were sensitized by 2% DPCP in acetone on one side of their shaved backs and rechallenged with 0.1% DPCP on the same side weekly for 2 weeks. The opposite side treated with acetone served as a control. Before each elicitation, mice were shaved again in order to observe the hair growth that followed. Enhancement of hair growth on DPCP treated skin was observed in 94% of mice after first elicitation and significant hair growth was shown in all mice after second elicitation. No remarkable hair growth was seen on the control side. Western immunoblot analysis revealed that the level of skin PKC alpha on the DPCP treated side was decreased at 2 and 4 days after sensitization and returned to the control level after first elicitation. At 5 days after the second elicitation, a higher level of PKC alpha was detected. The level of PKC delta remained at the control level and increased at 5 days after second elicitation. These results suggest that: 1) In the first week after sensitization, PKC alpha was down-regulated. This down-regulation may play a role in DPCP-ACD induced hair growth; 2) after the elicitation, PKC alpha was over-expressed and this over-expression was roughly correlated with the enhancement of mouse hair growth, suggesting that over-expression of PKC alpha may also play a part in the proliferation of hair follicle cells; and 3) overexpression of PKC delta after second elicitation may have an inhibitory effect on hair growth that keeps hair growth in balance.

Evidence of nuclear PKC/MAP-kinase cascade in guinea pig model of epidermal hyperproliferation

In order to delineate the biochemical events in the nuclear compartment of an in vivo proliferating epidermis, we produced a model of hyperproliferative epidermis by topical application of docosahexaenoic acid (22:6n-3) on guinea pig skin. Employing this model we demonstrated: (i) that protein kinase C (PKC)-a and atypical PKC-zeta are the two major PKC isozymes in the normal epidermal nuclear membrane, in contrast to PKC-alpha and PKC-beta in the epidermal plasma membrane; (ii) that topical application of docosahexaenoic acid induced epidermal hyperproliferation and enhanced total nuclear PKC, particularly nuclear PKC-alpha and the atypical PKC-zeta isozymes. The increase in the nuclear PKC isozymes paralleled a marked increase in the expression of nuclear mitogen-activated protein-kinase. These data suggest that epidermal hyperproliferative activity is accompanied by the upregulation of nuclear PKC/mitogen-activated protein-kinase signaling pathway.

Protein kinase C isoform levels in normal and sodium dodecyl sulphate-irritated mouse skin

Protein kinase C (PKC) comprises a family of related phospholipid-dependent serine/threonine protein kinases. PKC is important in signal transduction, regulating cell proliferation and differentiation. Recently, it has also been suggested that PKC may play a part in the pathogenesis of contact dermatitis. However, the expression of PKC isoforms in the skin of mice with irritant contact dermatitis (ICD) has not been examined. In this study, ICD was induced in mouse skin by applying 5%, 10% and 20% sodium dodecyl sulphate (SDS) in Finn chambers on the backs of mice and fixing with surgical dressings for 24 h. Depending upon the SDS concentration, mild to strong skin irritant reactions were observed 24 h after removal of the irritant patches. The intensity of the reactions increased with the increasing concentration of SDS. PKC isoforms alpha, beta, gamma and delta were all detected in normal mouse skin by Western immunoblotting. The specificity of the PKC isoforms detected was identified further by competitive Western immunoblotting. Compared with normal mouse skin treated with double-distilled water, the levels of PKC isoforms alpha, beta, gamma and delta in the SDS-irritated mouse skin was decreased by 24.8-75.8%. These results suggest that, in SDS-ICD, mouse skin PKC isoforms alpha, beta, gamma and delta are down-regulated. The significance of this decrease is under further investigation.

Dietary lipid varying in corn and coconut oil influences protein kinase C in phorbol ester-treated mouse skin

An earlier study indicated that increased levels of corn oil in the diet resulted in decreased tumor yield after promotion by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate in Sencar mouse epidermis (J Leyton, ML Lee, M Locniskar, MA Belury, TJ Slaga, et al. Cancer Res 51, 907-915, 1991). In the present study we investigated whether corn oil diets could alter the subcellular distribution and activity of protein kinase C (PKC), which is part of an important signaling pathway in carcinogenesis. We used three 15% (wt/wt) fat semipurified diets containing three ratios of corn oil to coconut oil: 1.0%:14.0% (Diet L), 7.9%:7.1% (Diet M), and 15.0%:0.0% (Diet H). The translocation to the membrane fraction of epidermal PKC by 12-O-tetradecanoylphorbol-13-acetate was decreased as the corn oil content of the diet was increased, and this correlates with the decrease in tumor yield. The translocation to the membrane fraction of specific isoforms of PKC was affected by increased dietary corn oil: the largest decreases were in cytosolic PKC-alpha and -beta, and the smallest change was in PKC-epsilon. The other isoforms, PKC-delta and -zeta, were unaffected. The major constituent of corn oil is linoleic acid, which did not affect the binding of phorbol ester to PKC, which suggests that inhibition of such binding was not responsible for the effects of increased dietary corn oil. Products of linoleic acid metabolism, i.e., arachidonic acid and 13-hydroxyoctadecadienoic acid, also did not affect the binding of phorbol ester to PKC. Thus the results of these studies suggest that the subcellular distributions of PKC and its isoforms can be modulated by dietary lipids.

Upregulation of nuclear PKC and MAP-kinase during hyperproliferation of guinea pig epidermis: modulation by 13-(S)-hydroxyoctadecadienoic acid (13-HODE)

13-(S)-Hydroxyoctadecadienoic acid (13-HODE), the lipoxygenase metabolite of linoleic acid, has been shown to reverse the epidermal hyperproliferation induced by topical application of docosahexaenoic acid (DNA, 22:6 n-3) on guinea pig skin. Our initial studies demonstrated that 13-HODE exerts a selective inhibition of the membrane-bound PKC-beta activity in the hyperproliferative skin. To delineate the antiproliferative effects of 13-HODE, we investigated the nuclear events associated with this process. Our data demonstrated that the major PKC isozymes in the epidermal nuclear fraction are alpha and zeta. Epidermal hyperproliferation induced by DHA caused an increase in nuclear total PKC and atypical PKC activities, and this was accompanied by an increase in the two nuclear isozymes, alpha and zeta (P < 0.05). This increase was reversed after topical application of 13-HODE. Similarly, 13-HODE suppressed elevated nuclear MAP-kinase. Taken together, these data suggest that nuclear signalling events in the epidermis involve PKC-MAP-kinase pathway.

Carbachol stimulates c-fos expression and proliferation in oligodendrocyte progenitors

To determine if muscarinic receptor-activation plays a role in oligodendrocyte development, the effect of carbachol a stable acetylcholine analog, on gene expression and proliferation was investigated. Using Northern blot analysis we showed that carbachol caused a time and concentration-dependent increase in c-fos mRNA. This effect was blocked by atropine, a non-selective muscarinic antagonist. In addition, the muscarinic-stimulated c-fos increase was inhibited by 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine (H-7), a potent inhibitor of protein kinase C (PKC), but not by N-2-(p-bromocinnamylamino)-ethyl-5-isoquinoline-sulfonamide (H-89), a potent inhibitor of protein kinase A, suggesting the involvement of PKC in mediating the response. Down-regulation of PKC by overnight pre-treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) blocked only the phorbol ester-stimulated c-fos accumulation while no effect was observed in the carbachol-induced response. These results suggested that carbachol stimulated an H-7 sensitive PKC pathway which may be different than that activated by TPA. Further evidence for two separate mechanisms of proto-oncogene induction was provided by the additive effect of carbachol and TPA. Induction of c-fos mRNA by carbachol was dependent on both influx of extracellular Ca2+ and release from intracellular stores, as both EDTA and BAPTA blocked the response. Since activation of muscarinic receptors can affect cell division in other cellular systems, the effect of carbachol on [3H]thymidine and bromodeoxyuridine incorporation into oligodendrocyte DNA was measured. Carbachol stimulated DNA synthesis in oligodendrocyte progenitors. This effect was mediated by muscarinic receptors as [3H]thymidine incorporation was prevented or significantly reduced by the addition of atropine. In conclusion, the present findings suggest that, the neurotransmitter, acetylcholine may act as a trophic factor in developing oligodendrocytes, regulating their growth and development in the central nervous system.

Arachidonic acid-induced down-regulation of protein kinase C delta in beta-cells

We have previously identified expression of multiple protein kinase C (PKC) isoforms in insulinoma-derived beta-cells and whole islets. Both PKC delta and PKC alpha appear to be the more abundantly expressed isoforms. In this report we studied the effects of arachidonic acid (AA) on the subcellular distribution of PKC alpha and PKC delta. AA has been reported to activate both PKC alpha and PKC delta and it is thought to be an important second messenger in beta-cells. Here we report that AA interacted with and altered beta-cell pools of PKC delta preferentially over PKC alpha. AA (100 microM) over the course of 45 min reduced cytosolic levels of PKC delta (to 40 +/- 15%, compared to time zero control) leaving membrane- and cytoskeleton-associated levels near control levels. Analysis of whole cell homogenates showed a slight down-regulation of PKC delta indicating proteolysis. The down-regulation of cytosolic PKC delta appeared to be isoform specific since cytosolic PKC alpha remained at control levels over the time course. The response was dose-dependent and negligible at concentrations below 30 microM and occurred, at least partially, in the cytosolic compartment of the cell. Indomethacin also down-regulated cytosolic PKC delta preferentially over PKC alpha possibly through accumulation of AA. These findings suggest that cytosolic PKC delta may be a downstream target of this beta-cell second messenger.

In vitro activation of mouse skin protein kinase C by fatty acids and their hydroxylated metabolites

To understand how dietary fatty acids differentially modulate mouse skin tumorigenesis, the ability of specific fatty acids and their derivatives to activate murine epidermal protein kinase C (PKC) in vitro was investigated. Total PKC from untreated female SSIN mouse skin was partially purified and incubated with specific fatty acids at concentrations up to 300 microM in the presence of Ca2+ and phosphatidylserine. The cis-unsaturated fatty acids tested, ranging from 16:1 to 22:6, stimulated PKC activity in a similar dose-dependent manner with an approximate threefold maximum increase over control. Neither the number of cis-double bonds nor the chainlength of these fatty acids affected their relative ability to activate PKC. trans-Fatty acids, with the exception of linoelaidic acid (t,t-18:2n-6), exhibited about half of the potency of their corresponding cis-isomers in stimulating PKC at the plateau concentration (200 microM) or lower. Substitutions close to the double bond on cis-fatty acids abolished their ability to activate PKC. The hydroxylated metabolites of arachidonic acid (20:4n-6) and linoleic acid (c,c-18:2n-6), i.e., the hydroxyeicosatetraenoic acids (HETE) and hydroxyoctadecadienoic acids (HODE), also activated mouse skin PKC in vitro, but only about half as effectively as did the respective parent fatty acids. The results suggest that both hydroxyl substitution and trans-configuration of HETE and HODE are responsible for their reduced ability to activate PKC. Overall the data suggests that the reduced skin tumor yield observed in mice fed diets high in c,c-18:2n-6 is not likely to be due to differences in the ability of c,c-18:2n-6 or 20:4n-6, or their metabolites, to activate PKC.

KP-10, a novel protein kinase C substrate in intact mouse epidermal cells, is phosphorylated by novel protein kinase C eta and/or zeta

Recently this group found an endogenous substrate protein for Ca(2+)-independent novel protein kinase C (nPKC), i.e. KP-10 (pI 4.7/25,500 M(r)), in primary cultured mouse epidermal cells [Nishikawa, K. et al. (1992) Cell. Signal. 4, 757-776]. In the present study, the nPKC isozymes which phosphorylate KP-10 in these cells were determined. Western blot analysis revealed that PKC alpha, eta and zeta were present in epidermal cell 105,000 g supernatants and that the content of PKC zeta was much higher than those of PKC alpha and eta. Neither PKC beta, delta nor epsilon was detected in the 105,000 g supernatants. Phosphatidylserine and phorbol 12-myristate 13-acetate (PMA)-dependent KP-10 phosphorylating activity was immunoprecipitated by anti-PKC eta and zeta antibodies, but not by antiPKC alpha antibody. These results suggest that PKC eta and/or zeta phosphorylate KP-10 and play pivotal roles in intracellular signal pathways in intact epidermal cells.