Coordinate changes in gene expression which mark the spinous to granular cell transition in epidermis are regulated by protein kinase C.

The protective function of skin depends on successful completion of a tightly regulated multi-step differentiation program, during which the induction of markers for a specific stage in epidermal differentiation is coupled to repression of markers expressed at the preceding stage. We have explored the role of protein kinase C (PKC) in this process using an in vitro model system, in which cultures of primary mouse epidermal keratinocytes are induced to terminally differentiate by raising the Ca2+ concentration in the medium from 0.05 to 0.12 mM. At doses which activate PKC, 12-O-tetradecanoylphorbol-13-acetate (TPA) and 1-oleoyl-2-acetylglycerol block Ca(2+)-mediated induction of the spinous cell markers keratins K1 and K10 at both the protein and mRNA level. TPA and 1-oleoyl-2-acetylglycerol also rapidly repress K1 and K10 mRNA expression when added to differentiating keratinocyte cultures already expressing these markers. The inhibition of K1 mRNA expression by TPA is blocked in cells where PKC has been inactivated with bryostatin. TPA-mediated loss of K1 mRNA is also blocked in cells exposed to cycloheximide or actinomycin D implicating a PKC-induced protein factor in this process. The loss of K1 mRNA in TPA-treated cultures is the result of both a selective destabilization of K1 transcripts and a rapid inhibition of K1 gene transcription. In contrast to the dramatic repression of mRNAs typical for spinous cell differentiation, activation of PKC concurrently enhances expression of mRNAs and proteins for the granular cell markers loricrin and filaggrin. This response does not occur in cells pre-treated with bryostatin to inactivate PKC. Our results suggest that PKC is a fundamental regulator of the coordinate changes in keratinocyte gene expression that occur during the spinous to granular cell transition in epidermis.

The relationship between stress fiber-like structures and nascent myofibrils in cultured cardiac myocytes.

The topographical relationship between stress fiber-like structures (SFLS) and nascent myofibrils was examined in cultured chick cardiac myocytes by immunofluorescence microscopy. Antibodies against muscle-specific light meromyosin (anti-LMM) and desmin were used to distinguish cardiac myocytes from fibroblastic cells. By various combinations of staining with rhodamine-labeled phalloidin, anti-LMM, and antibodies against chick brain myosin and smooth muscle alpha-actinin, we observed the following relationships between transitory SFLS and nascent and mature myofibrils: (a) more SFLS were present in immature than mature myocytes; (b) in immature myocytes a single fluorescent fiber would stain as a SFLS distally and as a striated myofibril proximally, towards the center of the cell; (c) in regions of a myocyte not yet penetrated by the elongating myofibrils, SFLS were abundant; and (d) in regions of a myocyte with numerous mature myofibrils, SFLS had totally disappeared. Spontaneously contracting striated myofibrils with definitive Z-band regions were present long before anti-desmin localized in the I-Z-band region and long before morphologically recognizable structures periodically link Z-bands to the sarcolemma. These results suggest a transient one-on-one relationship between individual SFLS and newly emerging individual nascent myofibrils. Based on these and other relevant data, a complex, multistage molecular model is presented for myofibrillar assembly and maturation. Lastly, it is of considerable theoretical interest to note that mature cardiac myocytes, like mature skeletal myotubes, lack readily detectable stress fibers.

Targeted disruption of mouse EGF receptor: effect of genetic background on mutant phenotype.

Gene targeting was used to create a null allele at the epidermal growth factor receptor locus (Egfr). The phenotype was dependent on genetic background. EGFR deficiency on a CF-1 background resulted in peri-implantation death due to degeneration of the inner cell mass. On a 129/Sv background, homozygous mutants died at mid-gestation due to placental defects; on a CD-1 background, the mutants lived for up to 3 weeks and showed abnormalities in skin, kidney, brain, liver, and gastrointestinal tract. The multiple abnormalities associated with EGFR deficiency indicate that the receptor is involved in a wide range of cellular activities.

Staurosporine induces protein kinase C agonist effects and maturation of normal and neoplastic mouse keratinocytes in vitro.

Staurosporine is a potent but nonselective inhibitor of protein kinase C (PKC) and blocks responses to 12-O-tetradecanoylphorbol-13-acetate (TPA) in several cell types in vitro. In cultured primary mouse keratinocytes, however, staurosporine fails to inhibit TPA-mediated keratinocyte maturation and itself elicits responses that are similar to TPA (T. Sako et al., Cancer Res., 48: 4646-4650, 1988). After exposure to 10 nM staurosporine for 24 h, essentially all keratinocytes undergo morphological differentiation, whereas 160 nM TPA induces this response in about 50% of epidermal cells. These concentrations of staurosporine and TPA cause a 4-5-fold induction of epidermal transglutaminase activity and cornified envelopes, both markers of the terminal stage of keratinocyte differentiation. Staurosporine, but not TPA, also induces morphological and biochemical maturation in 2 neoplastic mouse keratinocyte cell lines, 308 and SP-1. The ability of staurosporine to elicit the same responses as TPA suggested that it may be functioning paradoxically as a PKC agonist in intact keratinocytes. In support of this hypothesis, staurosporine induces ornithine decarboxylase activity, inhibits 125I-labeled epidermal growth factor binding, and induces expression of c-fos mRNA. Down-regulation of PKC by pretreatment of primary keratinocytes with 60 nM bryostatin partially blocks staurosporine-mediated induction of cornified envelopes and inhibition of 125I-labeled epidermal growth factor binding, implicating PKC in these responses. The ability of staurosporine to mimic and/or enhance certain responses to TPA suggests that this agent is acting as a functional PKC agonist in cultured keratinocytes.

Effects of phorbol 12-myristate 13-acetate on the differentiation program of embryonic chick skeletal myoblasts.

The effects of phorbol 12-myristate 13-acetate (PMA) on three aspects of myogenesis have been analyzed: (a) fusion of mononucleated myogenic cells to form myotubes; (b) synthesis and accumulation of two muscle-specific proteins; and (c) DNA synthesis. Using autoradiography combined with immunofluorescent localization of muscle-specific light meromyosin and the muscle-specific intermediate filament protein desmin, we have found that embryonic chick myogenic cells cultured in the presence of PMA (50 nM) initiate the synthesis of both desmin and muscle-specific light meromyosin and, by these criteria, partially differentiate. These cells differ from normal definitive postmitotic myoblasts, however, since they (a) do not fuse; (b) do not assemble normal myofibrils; and (c) incorporate [3H]thymidine. PMA does not appear to induce DNA synthesis in postmitotic myoblasts, but it apparently permits cells to initiate expression of muscle-specific proteins while preventing complete withdrawal from the cell cycle. Inhibition of fusion by PMA has been reported, but continued incorporation of [3H]thymidine in nuclei of cells expressing muscle-specific proteins is a previously undescribed effect of PMA. This effect is not achieved by 4-alpha-phorbol-12, 13-didecanoate, a nonpromoting phorbol ester, and may be relevant to the action of PMA as a tumor promoter.

Alterations in murine keratinocyte differentiation induced by activated rasHa genes are mediated by protein kinase C-alpha.

Primary mouse keratinocytes expressing the v-rasHa oncogene (v-rasHa keratinocytes) produce squamous papillomas when grafted onto nude mice and respond abnormally to signals for terminal differentiation both in vivo and in vitro. Since protein kinase C (PKC) activators and v-rasHa induce similar phenotypic changes in cultured keratinocytes, and cellular diacylglycerol levels are constitutively elevated in ras-transformed keratinocytes, we tested whether PKC is a downstream target for oncogenic ras in this cell type. Ca(2+)-dependent PKC activity was increased in lysates from cultured v-rasHa keratinocytes when compared to control cells; in contrast, Ca(2+)-independent activity decreased. Similar to PKC activators, v-rasHa blocked Ca(2+)-mediated expression of the early epidermal differentiation markers keratins K1 and K10 while inducing aberrant expression of K8. Pretreatment of v-rasHa keratinocytes with bryostatin to block PKC function restored Ca(2+)-mediated expression of K1 and K10 and blocked abnormal expression of K8, suggesting that these responses are mediated by the PKC pathway. Furthermore, expression of K1 is restored at bryostatin doses which specifically down-modulate PKC-alpha, the only Ca(2+)-dependent PKC isozyme detected in cultured keratinocytes. In contrast to the inhibition of K1 and K10, Ca(2+)-induced expression of the late epidermal differentiation markers loricrin, filaggrin, and keratinocyte transglutaminase was accelerated by v-rasHa, as previously reported in normal keratinocytes treated with PKC activators. Pretreatment of v-rasHa keratinocytes with bryostatin blocked expression of late markers in these cells, and this response was correlated with down-regulation of PKC-alpha. The results of this study suggest that oncogenic ras alters keratinocyte differentiation by altering the function of the PKC signaling pathway, and that PKC-alpha is the specific isozyme involved in down-modulating expression of keratins K1 and K10 and up-regulating expression of loricrin, filaggrin, and keratinocyte transglutaminase.

Autocrine transforming growth factor alpha is dispensible for v-rasHa-induced epidermal neoplasia: potential involvement of alternate epidermal growth factor receptor ligands.

Autocrine epidermal growth factor receptor activation by transforming growth factor alpha (TGF alpha) has been implicated in growth stimulation during epithelial neoplasia. Using keratinocytes isolated from mice with genetic defects in TGF alpha expression, we tested whether TGF alpha is required for transformation by the v-rasHa oncogene. Introduction of v-rasHa into primary epidermal cultures using a retroviral vector stimulated growth of both control (TGF alpha +/+, BALB/c) and TGF alpha-deficient (TGF alpha -/-, wa-1) keratinocytes. Moreover, v-rasHa elicited characteristic changes in marker expression (keratin 1 was suppressed; keratin 8 was induced), previously shown to be associated with epidermal growth factor (EGF) receptor activation, in both TGF alpha +/+ and TGF alpha -/- keratinocytes. v-rasHa markedly increased secreted (> 10-fold) and cell-associated (2-3-fold) TGF alpha levels in keratinocytes from TGF alpha +/+ and BALB/c mice, but not TGF alpha -/- or wa-1 mice. Based on Northern blot analysis, v-rasHa induced striking up-regulation of transcripts encoding the additional EGF family members amphiregulin, heparin-binding EGF-like growth factor, and betacellulin in cultured keratinocytes from all four mouse strains. Interestingly, in addition to the normal 4.5-kilobase TGF alpha transcript, wa-1 keratinocytes expressed two additional TGF alpha transcripts, 4.7 and 5.2 kilobases long. All three transcripts were up-regulated in response to v-rasHa, as well as exogenous TGF alpha or keratinocyte growth factor treatment, and were also detected in RNA isolated from wa-1 brain and skin. In vivo, v-rasHa keratinocytes from control as well as TGF alpha-deficient mice produced squamous tumors when grafted onto nude mice, and these lesions expressed high levels of amphiregulin, heparin-binding EGF-like growth factor, and betacellulin mRNA, regardless of their TGF alpha status. These findings indicate that TGF alpha is not essential for epidermal neoplasia induced by the v-rasHa oncogene and suggest that another EGF family member(s) may contribute to autocrine growth stimulation of ras-transformed keratinocytes.

Activation of human squamous cell carcinoma ornithine decarboxylase activity by guanosine triphosphate.

Previous studies have demonstrated the presence in mouse epidermal tumors of a structurally and functionally altered ornithine decarboxylase (ODC). In this report, the enzymatic properties of ODC from normal human skin and squamous cell carcinomas are examined. Some tumors contained a more heat stable ODC than the enzyme found in normal skin. GTP stimulated enzyme activity in four of seven tumor extracts tested but had no effect on normal skin ODC. Kinetic analyses indicated that GTP either lowered the apparent Km of tumor ODC for L-ornithine, increased the Vmax, or had both effects, depending on the tumor examined. Gel filtration chromatography of crude tumor extracts indicated the existence of multiple molecular weight forms of ODC, some of which can be activated by GTP and some of which are unaffected by GTP. Some tumors contain both a GTP-activatable and -nonactivatable form of the enzyme. Immunolocalization studies demonstrated the presence within squamous cell carcinomas of cells with a constitutively high level of immunoreactive ODC, a situation never observed in normal skin tissue. These results suggest that some human squamous cell carcinomas contain a functionally altered ODC that may be aberrantly regulated.

Targeted disruption of the epidermal growth factor receptor impairs growth of squamous papillomas expressing the v-ras(Ha) oncogene but does not block in vitro keratinocyte responses to oncogenic ras.

We have assessed the role of epidermal growth factor receptor (EGFR) signaling in biological responses to the v-ras(Ha) oncogene using primary keratinocytes from Egfr -/- mice and wild-type littermates. On the basis of several criteria, Egfr -/- keratinocytes were unresponsive to either acute or chronic exposure to several EGFR ligands but were stimulated to proliferate in response to several other mitogens. Although conditioned medium from primary keratinocytes transduced with v-ras(Ha) retrovirus (v-ras(Ha) keratinocytes) was a potent mitogen for wild-type but not Egfr -/- keratinocytes, v-ras(Ha) transduction of primary keratinocytes of either genotype resulted in a strong mitogenic response, arguing against an obligatory role for EGFR activation in v-ras(Ha)-mediated stimulation of keratinocyte proliferation. Infection with high-titer v-ras(Ha) retrovirus altered the keratin expression pattern in keratinocytes of both genotypes, suppressing differentiation-specific keratins K1 and K10 while activating aberrant expression of K8 and K18. In wild-type but not Egfr -/- cultures, K1 and K10 were also suppressed following infection at lower retroviral titers, presumably as a result of paracrine EGFR activation on uninfected cells present in these cultures. Squamous papillomas produced by grafting Egfr -/- v-ras(Ha) keratinocytes onto nude mice were only 21% of the size of wild-type v-ras(Ha) tumors, and a striking redistribution of S-phase cells was detected by immunostaining for bromodeoxyuridine. In Egfr -/- v-ras(Ha) papillomas, the fraction of total labeled nuclei detected in suprabasal layers was increased from 19 to 39%. In contrast, the basal layer labeling index of Egfr -/- papillomas was reduced to 34%, compared to 43% in wild-type tumors. Our results indicate that, although autocrine EGFR signaling is not required for keratinocyte responses to oncogenic ras in culture or benign tumor formation in nude mouse grafts, disruption of this pathway impairs growth of v-ras(Ha) papillomas by a mechanism that may involve alterations in keratinocyte cell cycle progression and/or migration in vivo.

Differentiation of mouse keratinocytes is accompanied by PKC-dependent changes in AP-1 proteins.

The conversion of cultured basal keratinocytes to the spinous and granular cell phenotypes seen in the skin can be stimulated by raising the levels of extracellular calcium. Here we show that AP-1 DNA binding activity is very low in primary cultures of basal keratinocytes, but that this activity is induced 24-48 h after increasing the concentration of extracellular calcium from 0.05 to 0.12 mM. As such, the induction of AP-1 DNA binding activity correlates with events occurring during the terminal stages of keratinocyte differentiation. Calcium-induced AP-1 DNA binding complexes consist of Fra-1, Fra-2, c-Jun, JunB and JunD and are independent of c-Fos, since the induction of DNA binding activity and the composition of the AP-1 binding complexes are identical in differentiating keratinocytes derived from c-fos null and wild type mice. The formation of calcium-induced AP-1 binding complexes is regulated by protein kinase C (PKC) and requires a functional PKCalpha isozyme, as determined through pharmacological down-modulation of specific PKC isozymes in differentiating keratinocytes. Moreover, PKC activation is required for the increased expression of Fra-2, JunB and JunD in the nucleus of differentiating cells in vitro. This observation provides a link between the obligate activation of PKC during keratinocyte differentiation and the nuclear response required to alter gene expression. In vivo expression patterns suggest that the predominant AP-1 heterodimer in the granular layer consists of Fra-2 and JunB while a JunD and Fra-1 complex predominates the spinous layer of mouse epidermis. These findings suggest distinct functions for different AP-1 proteins in the regulation of events related to keratinocyte maturation.

Protein kinase C regulates keratinocyte transglutaminase (TGK) gene expression in cultured primary mouse epidermal keratinocytes induced to terminally differentiate by calcium.

During the final stage of epidermal differentiation, activation of keratinocyte transglutaminase results in covalent crosslinking of a variety of proteins to form highly protective cornified cell envelopes. We have studied the regulation of keratinocyte transglutaminase (TGK) gene expression in murine epidermal keratinocytes induced to terminally differentiate in vitro by increasing the level of extracellular Ca++ or treatment with the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate (TPA). Raising extracellular Ca++ induces squamous differentiation of cultured keratinocytes and elicits a concentration-dependent increase in expression of TGK mRNA; keratinocytes grown for 24 h in 0.12 mM Ca++ medium express approximately 12 times as much TGK mRNA as basal cells (grown in 0.05 mM Ca++ medium), whereas cultures exposed to 1.4 mM Ca++ express approximately 17 times as much. TPA induces squamous differentiation and TGK mRNA even in basal keratinocyte cultures grown in 0.05 mM Ca++ medium, suggesting that expression of this differentiation marker is regulated by the PKC signaling pathway. Induction of TGK mRNA in response to TPA treatment is transient, reaching a peak at 6-8 h and returning to baseline by 24 h. In contrast, elevation of TGK mRNA levels in response to Ca++ persists for at least 24 h. The increased abundance of TGK mRNA reflects increased transcription of the TGK gene, based on nuclear run-on analysis of Ca(++)- and TPA-treated keratinocytes. Induction of TGK mRNA by either TPA or Ca++ is blocked in the presence of cycloheximide, suggesting that a PKC-dependent protein factor is required for TGK gene expression in response to both stimuli. Furthermore, the accumulation of TGK mRNA in keratinocytes treated with TPA or Ca++ is blocked in cells treated with the PKC inhibitor GF 109203X or bryostatin. These results suggest that the induction of TGK gene expression by Ca++ is dependent on PKC, providing further support for the hypothesis that PKC plays a central role in regulating the late stages of epidermal differentiation.

Differentiation of cultured human epidermal keratinocytes at high cell densities is mediated by endogenous activation of the protein kinase C signaling pathway.

Normal human epidermal keratinocytes (NHEK) grown in serum-free medium on a plastic substrate spontaneously differentiate at high cell densities in vitro. Because protein kinase C (PKC) regulates murine keratinocyte differentiation triggered by a variety of stimuli, we examined the role of this signaling pathway in density-dependent activation of NHEK differentiation. Relative to subconfluent cultures, confluent NHEK expressed markedly higher levels of multiple differentiation markers assayed by immunoblotting, including keratin 1, loricrin, filaggrin, involucrin, TGK, and SPR-1. Expression of several of these markers continued to increase for several days after cells reached confluency. The total level of several PKC isoforms was not substantially altered in NHEK harvested at different cell densities, based on immunoblotting; however, subcellular fractionation revealed that PKCalpha underwent a redistribution to the particulate fraction in confluent and postconfluent NHEK cultures, suggesting that this isozyme was activated under these conditions and may be involved in triggering the terminal differentiation program. Supporting this concept, inhibition of PKC function using bryostatin 1 or GF 109203X blocked the induction of keratinocyte differentiation markers at high cell densities. These data suggest that endogenous activation of PKC is responsible for cell density-mediated stimulation of NHEK differentiation, establishing a critical role for this pathway in regulating human as well as murine keratinocyte differentiation.

Staurosporine induces a sequential program of mouse keratinocyte terminal differentiation through activation of PKC isozymes.

Staurosporine (stsp) induces assembly of cornified envelopes in mouse keratinocyte cultures. To clarify whether this effect is the consequence of a coordinated differentiation program similar to that observed in epidermis, we assessed the expression of multiple differentiation-specific markers in stsp-treated keratinocytes. In medium containing 0.05 mM Ca2+, in which the basal cell phenotype is normally maintained, stsp induced dose-dependent increases in keratin 1, epidermal and keratinocyte transglutaminases, SPR-1, loricrin, and profilaggrin mRNA. Based on nuclear run-on analysis, stsp-mediated marker expression was found to be due at least in part to increased transcription. Since protein kinase C (PKC) activation is required for keratinocyte differentiation, we tested whether stsp influenced this signaling pathway. Stsp induced the translocation of multiple PKC isoforms from the cytosol to membrane and/or cytoskeletal fractions, inducing isozyme downregulation within 24 h. Moreover, AP-1 DNA binding activity was elevated in stsp-treated keratinocytes, consistent with the notion that this agent influences keratinocyte-specific gene expression via the PKC pathway. Stsp-mediated marker expression was inhibited by the PKC inhibitor GF 109203X. In cells pre-treated with bryostatin 1 to selectively down-modulate specific PKC isoforms, stsp-induced loricrin, filaggrin, and SPR-1 expression was suppressed when PKC alpha, epsilon, and/or delta were downregulated, suggesting that these isozymes may be necessary for marker expression in response to this agent. Thus, in addition to its effects on cornified envelope assembly, stsp induces a coordinate program of differentiation-specific keratinocyte gene expression that is mediated at least in part by the PKC signaling pathway.

Role of oncogenes and tumor suppressor genes in multistage carcinogenesis.

The introduction of the techniques of molecular biology as tools to study skin carcinogenesis has provided more precise localization of biochemical pathways that regulate the tumor phenotype. This approach has identified genetic changes that are characteristic of each of the specific stages of squamous cancer pathogenesis: initiation, exogenous promotion, premalignant progression, and malignant conversion. Initiation can result from mutations in a single gene, and the Harvey allele of the ras gene family has been identified as a frequent site for initiating mutations. Heterozygous activating mutations in c-rasHa are dominant, and affected keratinocytes hyperproliferate and are resistant to signals for terminal differentiation. An important pathway impacted by c-rasHa activation is the protein kinase C (PKC) pathway, a major regulator of keratinocyte differentiation. Increased activity of PKC alpha and suppression of PKC delta by tyrosine phosphorylation contribute to the phenotypic consequences of rasHa gene activation in keratinocytes. Tumor promoters disturb epidermal homeostasis and cause selective clonal expansion of initiated cells to produce multiple benign squamous papillomas. Resistance to differentiation and enhanced growth rate of initiated cells impart a growth advantage when the epidermis is exposed to promoters. The frequency of premalignant progression varies among papillomas, and subpopulations at high risk for progression have been identified. These high-risk papillomas overexpress the alpha 6 beta 4 integrin and are deficient in transforming growth factor beta 1 and beta 2 peptides, two changes associated with a very high proliferation rate in this subset of tumors. The introduction of an oncogenic rasHa gene into epidermal cells derived from transgenic mice with a null mutation in the TGF beta 1 gene have an accelerated rate of malignant progression when examined in vivo. Thus members of the TGF beta gene family contribute a tumor-suppressor function in carcinogenesis. Accelerated malignant progression is also found with v-rasHa transduced keratinocytes from skin of mice with a null mutation in the p53 gene. The similarities in risk for malignant conversion by initiated keratinocytes from TG beta 1 and p53 null geneotypes suggest that a common, growth-related pathway may underly the tumor-suppressive functions of these proteins in the skin carcinogenesis model.

Staurosporine induces a complete program of terminal differentiation in neoplastic mouse keratinocytes via activation of protein kinase C.

Staurosporine (stsp) is a kinase inhibitor which induces cornified envelope assembly and terminal differentiation in normal and neoplastic mouse keratinocytes. In the tumorigenic cell lines 308 and SP-1 experiments were performed to determine if this effect was due only to activation of transglutaminases (TGases) already residing within the cell, or whether stsp was capable of inducing a full program of differentiation. Assessment of keratinocyte differentiation-specific protein expression in neoplastic cells revealed that expression of the suprabasal marker SPR-1 and the granular markers loricrin and filaggrin were induced by stsp. Protein expression was controlled by changes in mRNA expression, determined by Northern blotting. Transcripts for the TGase isoforms TGK and TGE were also induced by stsp in SP-1 cells, whereas only TGK expression was increased in 308 cells, which appear not to express TGE. Protein kinase C (PKC) activation is required for differentiation in normal mouse keratinocytes. To determine if stsp induces differentiation in neoplastic cells by regulation of this signaling pathway cells were treated with the specific PKC inhibitor GF 109203X or with different concentrations of bryostatin 1 to down-regulate specific isoforms of PKC prior to and during treatment with stsp. Stsp-induced protein cross-linking and marker expression were inhibited by GF 109203X, suggesting paradoxical activation of PKC by stsp. PKC alpha, epsilon and delta, but not PKC eta and zeta, were down-regulated by treating both cell types with bryostatin; pre-treatment of cells with bryostatin inhibited stsp-induced protein cross-linking and marker expression, suggesting a necessity for the alpha, delta and/or epsilon isoforms in stsp-induced differentiation.

Selective effects of phorbol 12-myristate 13-acetate on myofibrils and 10-nm filaments.

Phorbol 12-myristate 13-acetate (PMA) has a prompt and selective catabolic effect on striated myofibrils in postmitotic myotubes. Fluorescein-labeled antibodies against light meromyosin were used to follow the effects of PMA on the muscle-specific myosin in myofibrils. The response of actin filaments was monitored by decoration with heavy meromyosin. The response of the two types of 10-nm filaments in myotubes was followed by fluorescein- and rhodamine-labeled antibodies to the fibroblastic and muscle-specific filament proteins, respectively. Within 2-3 days, PMA induced dismantling of virtually every striated myofibril in every myotube in the culture. These myotubes bound little or no anti-light meromyosin, and tests to detect the alpha-actin filaments of the myofibrils with heavy meromyosin were negative. In contrast, the nonmuscle actin in the subsarcolemmal microfilaments persisted in PMA-treated myotubes and was decorated with heavy meromyosin. The sarcoplasmic reticulum, mitochondria, and Golgi bodies appeared normal. Myotubes depleted of myofibrils by PMA displayed large numbers of muscle-specific 10-nm filaments. This preferential degradation of the myosin and actin of the myofibrils were reversible. These myotubes formed a normal complement of myofibrils 24-48 hr after removal of PMA. When, after 3 days in PMA, the cultures were treated for an additional 3-8 days, a transitory subpopulation of PMA-resistant myotubes appeared.

Activation of the epidermal growth factor receptor signal transduction pathway stimulates tyrosine phosphorylation of protein kinase C delta.

The expression of an oncogenic rasHa gene in epidermal keratinocytes stimulates the tyrosine phosphorylation of protein kinase C delta and inhibits its enzymatic activity (Denning, M. F., Dlugosz, A. A., Howett, M. K., and Yuspa, S. H. (1993) J. Biol. Chem. 268, 26079-26081). Keratinocytes expressing an activated rasHa gene secrete transforming growth factor alpha (TGFalpha) and have an altered response to differentiation signals involving protein kinase C (PKC). Because the neoplastic phenotype of v-rasHa expressing keratinocytes can be partially mimicked in vitro by chronic treatment with TGF alpha and the G protein activator aluminum fluoride (AlF4-), we determined if TGF alpha or AlF4- could induce tyrosine phosphorylation of PKCdelta. Treatment of primary keratinocyte cultures for 4 days with TGFalpha induced tyrosine phosphorylation of PKCdelta, whereas AlF4- only slightly stimulated PKCdelta tyrosine phosphorylation. The PKCdelta that was tyrosine-phosphorylated in response to TGFalpha had reduced activity compared with the nontyrosine-phosphorylated PKCdelta. Treatment of keratinocytes expressing a normal epidermal growth factor receptor (EGFR) with TGFalpha or epidermal growth factor for 5 min induced PKCdelta tyrosine phosphorylation. This acute epidermal growth factor treatment did not induce tyrosine phosphorylation of PKCdelta in keratinocytes isolated from waved-2 mice that have a defective epidermal growth factor receptor. In addition, the level of PKCdelta tyrosine phosphorylation in v-rasHa-transduced keratinocytes from EGFR null mice was substantially lower than in v-rasHa transduced wild type cells, suggesting that activation of the EGFR is important for PKC delta tyrosine phosphorylation in ras transformation. However, purified EGFR did not phosphorylate recombinant PKC delta in vitro, whereas members of the Src family (c-Src, c-Fyn) and membrane preparations from keratinocytes did. Furthermore, clearing c-Src or c-Fyn from keratinocyte membrane lysates decreased PKCdelta tyrosine phosphorylation, and c-Src and c-Fyn isolated from keratinocytes treated with TGFalpha had increased kinase activity. Acute or chronic treatment with TGFalpha did not induce significant PKCdelta translocation in contrast to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, which induced both translocation and tyrosine phosphorylation of PKCdelta. This suggests that TGFalpha-induced tyrosine phosphorylation of PKC delta results from the activation of a tyrosine kinase rather than physical association of PKCdelta with a membrane-anchored tyrosine kinase. Taken together, these results indicate that PKCdelta activity is inhibited by tyrosine phosphorylation in response to EGFR-mediated signaling and activation of a member of the Src kinase family may be the proximal tyrosine kinase acting on PKCdelta in keratinocytes.