Staurosporine, a non-specific PKC inhibitor, induces keratinocyte differentiation and raises intracellular calcium, but Ro31-8220, a specific inhibitor, does not

Identification of Novel Keratinocyte Differentiation Modulating Compounds by High-Throughput Screening

The authors have designed high-throughput screens to identify compounds that promote or inhibit terminal differentiation of primary human epidermal keratinocytes. Eleven known inhibitors of signaling pathways and approximately 4000 compounds of diverse structure were screened using an In-Cell Western system based on immunofluorescent staining of the terminal differentiation marker, involucrin. Staurosporine, a nonspecific protein kinase C inhibitor, and H89, a protein kinase A inhibitor, promoted expression of involucrin. Conversely, U0126, a MEK inhibitor, and SAHA or SBHA, 2 histone deacetylase inhibitors, reduced the expression of involucrin during calcium-induced stratification. In addition, the authors found 1 novel compound that induced keratinocyte differentiation and 2 novel compounds that were inhibitory to calcium-induced differentiation. The differentiation-inducing compound also inhibited growth of a human squamous cell carcinoma line by stimulating both differentiation and apoptosis. Because the compound affected the tumor cells at a lower concentration than primary keratinocytes, it may have potential as an antitumor therapy.

TRAIL resistance in human neuroblastoma SK-N-AS cells is dependent on protein kinase C and involves inhibition of caspase-3 proteolytic processing

Neuroblastoma is the most common solid extracranial cancer form in childhood with an etiology that is mostly unknown. Although tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) has been proposed as a promising future anticancer drug candidate, highly malignant neuroblastoma has been reported to acquire TRAIL resistance by mechanisms that are poorly understood. Here, we show by western blot analysis, and live cell imaging using anchored FRET sensors, that the resistance to TRAIL-induced apoptosis in human neuroblastoma SK-N-AS cells depends on an incomplete processing of procaspase-3, generating an immature and catalytically inactive 21 kDa fragment. We have previously shown that the naturally occurring compound curcumin can sensitize SK-N-AS cells to TRAIL. In the present study, we show that curcumin also has a similar effect on human neuroblastoma SHEP1 cells. Furthermore, we show that curcumin and TRAIL co-treatment induces complete maturation and activation of caspase-3 in both cell lines. The mechanisms behind this effect seem to be dependent on protein kinase C (PKC), since inhibition of PKC using bisindolylmaleimide XI, could also sensitize these cells to TRAIL through a similar effect on caspase-3 activation. Moreover, TRAIL co-treatment with bisindolylmaleimide XI or curcumin resulted in down-regulation of X-linked inhibitor of apoptosis protein. In conclusion, our study shows that PKC can be involved in TRAIL resistance in human neuroblastoma cells by preventing caspase-3 maturation.

Altered calcium handling following the recombinant overexpression of protein kinase C isoforms in HaCaT cells

Both changes in intracellular calcium concentration ([Ca(2+)](i)) and activation of certain protein kinase C (PKC) isoforms play a crucial role in keratinocyte functions. To better understand the interaction between these two signalling pathways we investigated the resting [Ca(2+)](i) and the extracellular ATP-induced changes in [Ca(2+)](i) on HaCaT cell clones overexpressing either the classical alpha or the beta PKC isoform. These PKC isoenzymes were previously shown to decrease (alpha) or increase (beta) cell proliferation and augment (alpha) or suppress (beta) cell differentiation. Keratinocyte clones with decreased proliferation rate were found to have unaltered resting [Ca(2+)](i), but responded with greater calcium transients to the application of 180 mum of ATP. In contrast, clones with increased proliferation rate had elevated resting [Ca(2+)](i) and suppressed calcium responses to ATP. Calcium transients on PKCbeta clones displayed a faster falling phase. Each clone had a distinct purinergic receptor expression pattern, some of which paralleled the altered proliferation rate and calcium handling. Keratinocytes overexpressing PKCbeta revealed decreased P2X1 and increased P2Y1 receptor expression as compared with the control or PKCalpha clones. The expression level of P2X7 was significantly increased in keratinocytes overexpressing PKCalpha. On the other hand neither the P2X2 nor the P2Y2 expression was altered significantly in the cell types investigated. These data indicate that a modified proliferation and differentiation pattern is associated with altered calcium handling in keratinocytes. The observations also suggest that different PKC isoenzymes have different effects on the phosphatidyl-inositol signalling pathway.

Identification and characterization of small-molecule inducers of epidermal keratinocyte differentiation

An essential function of the human epidermis is the maintenance of a protective barrier against the environment. As a consequence, keratinocytes, which make up this layer of the skin, undergo an elaborate process of self-renewal, terminal differentiation, and cell death. Misregulation of these processes can lead to several human diseases, including psoriasis and basal cell and squamous cell carcinomas. To identify novel regulators of keratinocyte differentiation, a cell-based screen of small-molecule libraries was carried out for molecules that induce terminal differentiation of normal human epidermal keratinocytes. One class of molecules was identified, the 2-(3,4,5-trimethoxyphenylamino)-pyrrolo[2,3-d]pyrimidines, which were shown to induce differentiation of epidermal progenitor cells to terminally differentiated keratinocytes. These molecules serve as useful mechanistic probes of the cellular differentiation programs that regulate the formation and homeostasis of the epidermis and may lead to novel therapeutic approaches for the treatment of skin hyperproliferative disorders.

Protein kinase C isozymes regulate proliferation and high cell density-mediated differentiation in HaCaT keratinocytes

Protein kinase C (PKC) isoforms play pivotal roles in the regulation of differentiation of normal human epidermal keratinocytes (NHEK). In this study, we investigated the participation of the PKC system in the proliferation and high cell density-induced differentiation of the human immortalized keratinocyte line HaCaT. HaCaT keratinocytes possessed a characteristic PKC isoform pattern (PKC alpha, beta, gamma, delta, epsilon, eta, theta, zeta), which altered during proliferation and differentiation. The GF109203X compound, a selective PKC inhibitor, suppressed the expressions of the lat (granular cell) differentiation markers involucrin (INV) and filaggrin (FIL), and the terminal marker keratinocyte-specific transglutaminase-1 (TG), but did not affect the level of the early (spinous cell) marker keratin 10 (K10) and cellular proliferation. Phorbol 12-myristate 13-acetate (PMA), an activator of PKC, inhibited proliferation, elevated intracellular calcium concentration, decreased the expression of K10, and increased the expressions of INV, FIL, and TG. These data indicate that the endogenous activation of PKC regulates the expressions of the late differentiation markers, and that the exogenous activation of PKC by PMA results in the induction of terminal differentiation. Because the cellular effects of PMA were accompanied by differential down-regulations of the sensitive PKC isoforms in proliferating and differentiating cultures, our findings argue for the differential roles of the existing PKC isoforms in the regulation of cellular proliferation and high cell density-induced differentiation of HaCaT cells.

Glycogen synthase kinase-3 regulates formation of long lamellipodia in human keratinocytes

During wound healing, keratinocytes initiate migration from the wound edge by extending lamellipodia into a fibronectin-rich provisional matrix. While lamellipodia-like structures are also found in cultured keratinocytes exposed to epidermal growth factor (EGF), the signaling pathway that regulates the formation of these structures is not defined. In cultured human keratinocytes seeded on fibronectin, we found that protein-serine/threonine kinase inhibitors including staurosporine, induced concentration-dependent formation of extended lamellipodia (E-lams). The formation of E-lams was inhibited by the proteintyrosine kinase inhibitors herbimycin A and genistein and augmented by the protein-tyrosine phosphatase inhibitor sodium orthovanadate. Staurosporine treatment induced relocation of tyrosine phosphorylated phospholipase C-gamma1 (PLC-gamma1) to the tips of lamellipodia where actin assembly was initiated. Consistent with an involvement of PLC-gamma1 in E-lam formation, intracellular free calcium (Ca2+) was elevated during the formation of E-lams and conversely, E-lam formation was blocked by intracellular Ca2+ chelation with BAPTA/AM, but not by extracellular reduction of Ca2+ by EGTA. Notably, glycogen synthase kinase-3alpha/beta (GSK-3alpha/beta) was activated by staurosporine as evidenced by reduced phosphorylation on Ser-21/9. Suppression of GSK-3 activity by LiCl2 or by a specific chemical inhibitor, SB-415286, blocked E-lam formation but without altering cell spreading. Furthermore, GSK-3 inhibitors blocked both staurosporine- and EGF-induced keratinocyte migration in scratch-wounded cultures. We propose that GSK-3 plays a crucial role in the formation of long lamellipodia in human keratinocytes and is potentially a central regulatory molecule in epithelial cell migration during wound healing.

Dual role of protein kinase C on mitogen-activated protein kinase activation and human keratinocyte proliferation

Subconfluent normal human keratinocytes exhibit autonomous (autocrine growth factor driven) proliferation and express the specific markers for keratinocyte proliferation K#5 and K#14. In keratinocyte autocrine culture, the exogenously added epidermal growth factor (EGF) has no effect on cell proliferation and mitogen-activated protein kinase (MAPK) activity. PD98059 inhibits MAPK pathway and autocrine keratinocyte proliferation. Staurosporine and Gö6976 strongly inhibit autonomous keratinocyte proliferation. In contrast, Gö6983 (which does not inhibit PKC micro ) inhibits only 20% of autocrine keratinocyte proliferation. Staurosporine inhibits MAPK activity, whereas Gö6976 and Gö6983 strongly increase it. We have concluded that MAPK, PKC micro and probably PKCalpha take part in autocrine keratinocyte proliferation. The effect of Gö6976 and Gö6983 on MAPK activity could be explained by the inhibition of PKC-dependent MAPK-phosphatase expression. The effect of staurosporine could be explained by its paradoxical action (activation) on protein kinase C (PKC) in keratinocytes.

Induction of cyclooxygenase-2 by staurosporine through the activation of nuclear factor for IL-6 (NF-IL6) and activator protein 2 (AP2) in an osteoblast-like cell line

The induction of cyclooxygenase-2 (COX-2) plays a crucial role in many physiological and pathological processes. The expression of the COX-2 gene is regulated by many extracellular stimuli, including growth factors, cytokines, and tumor promoters. Staurosporine, a potential anti-tumor drug, was found recently to up-regulate the expression of the COX-2 gene in the mouse osteoblast-like cell line MC3T3-E1. The ability of staurosporine to induce the expression of the COX-2 gene was investigated using luciferase reporters controlled by various COX-2 core promoter regions. Two cis-acting sites for activator protein 2 (AP2) and nuclear factor for IL-6 (NF-IL6), respectively, were identified as responsible for the staurosporine-mediated COX-2 up-regulation. Mutational analysis further verified that both NF-IL6 and AP2 are involved in this process. Further studies showed the stimulatory effect of staurosporine on luciferase activity to be both time- and concentration-dependent. Luciferase activity could be induced at as low as 5 nM staurosporine and reached a maximum at around 20 nM. At 50 nM, the stimulatory effect of staurosporine on luciferase activity reached a maximum at about 8 hr and fell rapidly following 10 hr of incubation. Interestingly, a selective protein kinase C inhibitor, 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl) maleimide (GF109203X), failed to stimulate luciferase activity under the same conditions. This finding implies that staurosporine-mediated COX-2 gene expression is specific and independent of protein kinase C activity.

Altered response of a human squamous cell carcinoma cell line to 1, 25-dihydroxyvitamin D(3) after transfer of a normal chromosome 11

Previous work in our laboratory using functional assays for tumorigenicity identified a tumor suppressor element on human chromosome 11q for the cutaneous squamous cell carcinoma cell line A388.6TG.c2. In this report, we screened a variety of agents for differential effects on A388.6TG.c2 compared to a growth-suppressed chromosome 11 microcell hybrid of A388.6TG.c2. One of the agents, 1, 25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3); calcitriol), exerted a growth-altering effect on A388.6TG.c2, which formed rounded cell clusters across the surface of the raft by Day 6 of treatment. In contrast, full-length chromosome 11 hybrids of A388.6TG.c2, as well as two other squamous cell carcinoma cell lines (FaDu and A431), when treated with 1,25(OH)(2)D(3), failed to demonstrate this cell-clumping phenotype. To pursue the hypothesis that the growth suppressor element is involved in altering the response to 1, 25(OH)(2)D(3), we tested microcell hybrids carrying t(X;11) chromosomes lacking large portions of 11q. Although these hybrids, like the parent A388.6TG.c2 cells, demonstrated extensive growth in organotypic cultures, they failed to form cell clusters with 1, 25(OH)(2)D(3) treatment. These results suggest that the chromosome 11 element that alters the response to 1,25(OH)(2)D(3) is distinct from the growth-suppressing element. An examination of differentiation marker expression revealed identical patterns of basal and suprabasal markers for A388.6TG.c2 and for a chromosome 11 hybrid with or without treatment with 1,25(OH)(2)D(3). Finally, characterization of candidate tumor suppressor gene PPP2R1B, which encodes for a subunit of protein phosphatase 2A (PP2A), showed seemingly insignificant alterations by cDNA sequence analysis. Collectively, the data suggest that human chromosome 11 contains two different tumor suppressor elements that may account for the two areas of loss of heterozygosity observed on the long arm of this chromosome.

Divergent responses of ras-transfected and non-ras-transfected human keratinocytes to extracellular calcium

Raising extracellular calcium (Cao) induces terminal differentiation in cultured epidermal keratinocytes. The introduction of the ras oncogene into keratinocytes results in resistance to Cao-mediated differentiation. To understand the signaling mechanism involved, we examined the Cao-induced formation of inositol triphosphate (IP3) and changes in intracellular Ca2+ (Cai) concentration in non-ras-transfected and ras-transfected HaCaT lines of human keratinocytes. When switched from 0.05- to 1.5-mM Cao medium, the non-ras HaCaT line showed a rapid twofold increase in IP3 formation, whereas the IP3 level in the ras-transfected I-7 line was slightly affected. G-protein-coupled activation of phospholipase was intact in both lines, as evidenced by the generation of similar amounts of IP3 in response to addition of bradykinin or guanosine 5'-[γ-thio]-triphosphate. Addition of 1.0 mM Cao evoked similar Cai responses in both non-ras- and ras-transfected cells: a transient elevation, followed by a sustained lower plateau. However, the two lines differed in their later responses: after being maintained in 1.0 mM Ca2+ for 24 h, the Cai level was significantly lower in ras-transfected cells than in non-ras-transfected HaCaT cells. The Cao-induced increase in Cai in both lines was inhibited by the Ca2+ entry blocker SK&F 96365 or depolarization in high K+ bathing solution, demonstrating its dependence of calcium influx. The results suggest fundamental differences in the early signal that are generated in response to an increase in Cao in ras-transfected keratinocytes, with the absence of a Cao-induced rise in IP3-a signaling pathway defect that may play a role in the differentiation block the cells exhibit. In addition, the inability of ras-transfected cells to sustain a prolonged Cai plateau may also contribute to their inability to differentiate in response to the Cao signal.Key words: cell differentiation, intracellular Ca2+, IP3, keratinocytes, ras transfection, signal transduction.

Role of protein kinase C isoforms in phorbol ester-induced vascular endothelial growth factor expression in human glioblastoma cells

Aberrant expression of the potent angiogenic cytokine, vascular endothelial growth factor (VEGF), has been demonstrated to be associated with most human solid tumors. Both transcriptional and post-transcriptional mechanisms have been shown to modulate VEGF expression in a multitude of cell types. Here we report that when protein kinase C (PKC) pathways were activated in human glioblastoma U373 cells by phorbol 12-myristate 13-acetate (PMA), VEGF mRNA expression was up-regulated via a post-transcriptional mRNA stabilization mechanism. PMA treatment exhibited no increase in VEGF-specific transcriptional activation as determined by run-off transcription assays and VEGF promoter-luciferase reporter assays. However, PMA increased VEGF mRNA half-life from 0.8 to 3.6 h which was blocked by PKC inhibitors but not by protein kinase A or cyclic nucleotide-dependent protein kinase inhibitors. When U373 cells were transfected with antisense oligonucleotide sequences to the translation start sites of PKC-alpha, -beta, -gamma, -delta, -epsilon, or -zeta isoforms, both PKC-alpha and -zeta antisense oligonucleotides showed substantial inhibition of PMA-induced VEGF mRNA. In addition, overexpression of PKC-zeta resulted in a strong constitutive up-regulation of VEGF mRNA expression. This study demonstrates for the first time that specific PKC isoforms regulate VEGF mRNA expression through post-transcriptional mechanisms.

Cytoplasmic elongation and rupture in megakaryoblastic leukemia cells via activation of adhesion and motility by staurosporine on fibronectin-bound substratum

Human megakaryoblastic leukemia Meg-01 cells were attached to fibronectin (FN)-coated substratum, on which remarkable spreading and cytoplasmic elongation was induced by treatment with a protein kinase inhibitor, staurosporine (stp). This effect was inhibited by RGDS and was also not seen on FN-lacking substratum. The extended cytoplasm had swollen terminals and nodes, which contained GpIIb and beta-thromboglobulin, occasionally included alpha granules, and tended to form particles (2-5 microm) after rupture of the narrowed cytoplasm. Among other protein kinase modulators tested, only K252a promoted the elongation, while calphostin, herbimycin, TPA, and calyculin suppressed it. The cells began to migrate soon after addition of stp, with attachment to the substratum held at some sites during the migration. This tethered movement seemed to cause the cytoplasmic elongation and the rupture into particles. The elongation was retarded by pretreating the cells with cytochalasin A and Clostridium C3 toxin but not with demecolcine. Actin microfilaments in the stp-treated Meg-01 cells accumulated in the filopodia and periphery of the extended cytoplasm, in which vinculin was colocalized as adhesion plaques. The microtubules were longitudinally oriented through the cytoplasmic extension and showed no ring profile in the nodes and particles. Thus, stp in the presence of FN appears to stimulate reorganization of actin-based cytoskeleton and formation of focal contacts in Meg-01 cells. This leads to the activation of cell adhesion and motility, and then cytoplasmic elongation and rupture into particles.

1,25 dihydroxyvitamin D3 enhances the calcium response of keratinocytes

The steroid hormone 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) regulates cell proliferation and differentiation. Intracellular calcium (Cai) concentrations play a crucial role in these events. From our previous studies, we have demonstrated a calcium receptor (CaR) in keratinocytes which appears to regulate the initial release of Cai from intracellular stores in response to extracellular calcium (Cao) and so is likely to participate in the differentiation process. In this study, we determined whether the ability of 1,25(OH)2D3 to enhance Ca++ -induced differentiation was mediated at least in part through changes in the CaR. Keratinocytes were grown in keratinocyte growth medium (KGM) with 0.03 mM, 0.1 mM, or 1.2 mM Ca and treated with 10(-8) M 1,25(OH)2D3 till harvest after 5, 7, 14, and 21 days. CaR mRNA levels were quantitated by polymerase chain reaction. The results were compared to the ability of 1,25(OH)2D3 to enhance calcium-stimulated increases in Cai. In cells grown in 0.03 mM Ca, the CaR mRNA levels decreased with time. 1,25(OH)2D3 stimulated the levels at 5 days and prevented the falloff over the subsequent 16 days. On the other hand, in cells grown in 0.1 or 1.2 mM Ca, the message levels remained high, and 1,25(OH)2D3 had no further effect. To study the functional relationship, we harvested cells after 5 and 7 days in culture following a 24 h treatment with 1,25(OH)2D3 or vehicle to measure the Cai response to 2 mM Cao. The preconfluent cells grown in 0.03 mM Ca showed a nearly twofold increase in the Cai response to Cao when pretreated with 1,25(OH)2D3, whereas the confluent cells and those grown in 1.2 mM Ca showed no enhancement by 1,25(OH)2D3. Studies with 45Ca influx into keratinocytes revealed that 1,25(OH)2D3 enhanced the influx in preconfluent and confluent cells when grown in KGM containing 0.03 mM Ca but not in cells grown in 1.2 mM calcium. We conclude that 1,25(OH)2D3 maintains the CaR mRNA levels in cells grown in 0.03 mM Ca, thus maintaining their responsiveness to Cao and so ensuring their ability to differentiate in response to the calcium signal.

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.

PMA and staurosporine affect expression of the PCK gene in LLC-PK1-F+ cells

The addition of phorbol 12-myristate 13-acetate (PMA) to renal LLC-PK1-F+ cells caused a rapid decrease in the level of phosphoenolpyruvate carboxykinase (PCK) mRNA and reversed the stimulatory effects of exposure to acidic medium (pH 6.9, 10 mM HCO-3) or cAMP. In contrast, prolonged treatment with PMA increased the levels of PCK mRNA. The two effects correlated with the membrane translocation and downregulation of the alpha-isozyme of protein kinase C and were blocked by pretreatment with specific inhibitors of protein kinase C. The rapid decrease in PCK mRNA caused by PMA occurred with a half-life (t1/2 = 1 h) that is significantly faster than that measured during recovery from acid medium or following inhibition of transcription (t1/2 = 4 h). The effect of PMA was reversed by staurosporine, which apparently acts by inhibiting a signaling pathway other than protein kinase C. Staurosporine had no effect on the half-life of the PCK mRNA, but it stimulated the activity of a chloramphenicol acetyltransferase gene that was driven by the initial 490 base pairs of the PCK promoter and transiently transfected into LLC-PK1-F+ cells. This effect was additive to that of cAMP, and neither stimulation was reversed by PMA. The stimulatory effect of staurosporine was mapped to the cAMP response element (CRE-1) and P3(II) element of the PCK promoter. The data indicate that, in LLC-PK1-F+ cells, activation of protein kinase C decreases the stability of the PCK mRNA, whereas transcription of the PCK gene may be suppressed by a kinase that is inhibited by staurosporine.

Staurosporine-induced versus spontaneous squamous metaplasia in pre- and postmenopausal breast tissue

Breast cancers from pre- vs. postmenopausal women display unique characteristics that may be related to differences in epithelial differentiation between these two populations. In addition to lobular development, lactational changes, and involution, breast epithelium can undergo metaplastic alterations, often in association with carcinoma. Because protein kinase C (PKC) regulates differentiation and proliferation in many cell types, we asked whether modulation of PKC activity could define biochemical differences in breast epithelium from pre- vs. postmenopausal women. Organ cultures of normal human breast were treated with PKC agonists and antagonists. Epithelial differentiation was evaluated based on morphologic criteria and the expression of cell-type specific proteins. Staurosporine, a nonspecific but extremely potent inhibitor of PKC, induced squamous metaplasia in eight of eight cases within 2 weeks of treatment. Other inhibitors of PKC, such as calphostin C and tamoxifen, had no effect on epithelial differentiation. Long-term treatment with phorbol esters also did not induce squamous metaplasia. However, stimulation of cAMP levels by forskolin and isobutyl-methyl-xanthene (IMX) rapidly induced squamous metaplasia, as has been previously reported. Surprisingly, squamous metaplasia occurred in 10 of 12 cultures derived from postmenopausal women in the absence of exogenous agents. Untreated cultures derived from premenopausal women never developed this type of epithelium (0 of 11). Therefore, breast epithelium from pre- and postmenopausal women responded differently to in vitro culture. Forskolin/IMX or staurosporine can reproduce these conditions, acting independent of menopausal status. Because staurosporine's action was unique among PKC inhibitors, staurosporine may induce squamous metaplasia of breast epithelium by a PKC-independent mechanism.

Effect of staurosporine on transcription factor NF-kappaB in human keratinocytes

Activation of the transcription factor NF-kappaB is known to be important in the regulated expression of a large number of pro-inflammatory genes including interleukin-8 (IL-8). Previously, we showed that the protein kinase inhibitor staurosporine potentiates IL-1-stimulated IL-8 production in human keratinocytes. Moreover, recent studies by other investigators demonstrated that staurosporine treatment alone results in a concentration-dependent increase in IL-8 mRNA and protein production. Therefore, in order to understand the mechanism underlying this observation, the effect of staurosporine on the activation of NF-kappaB was investigated. Electrophoretic mobility shift assays using an oligonucleotide containing the NF-kappaB consensus motif demonstrated that staurosporine treatment resulted in the activation of NF-kappaB by 15 min post-treatment. The ability of staurosporine to activate NF-kappaB was investigated further, using luciferase reporters under the control of the HIV-LTR or IL-8 core promoter transfected into human U937 cells. Stimulation with staurosporine resulted in a concentration-dependent induction of luciferase activity. In contrast, the very selective protein kinase C inhibitor 3-[8-[(dimethylamino)methyl]-6,7,8,9-tetrahydropyrido-[1,2-a]indol -10-yl]-4-(1-methyl-3-indolyl)-1H-pyrrole-2,5-dione hydrochloride (Ro32-0432) did not stimulate the activation of NF-kappaB, as measured in the luciferase reporter assay. The mechanism underlying NF-kappaB activation does not appear to involve the classical activation pathways in that staurosporine does not induce the disappearance of IkappaB family members. In conclusion, staurosporine appears to stimulate the activation of NF-kappaB in at least two cell types, and this effect appears to be independent of protein kinase C.

Up-regulation of p21WAF1 by phorbol ester and calcium in human keratinocytes through a protein kinase C-dependent pathway

Terminal differentiation in a variety of cell types has been associated with p53-independent up-regulation of p21WAF1 p21WAF1 mRNA and protein are expressed at low levels in normal human skin, but overexpression of p21WAF1 has been observed in differentiating keratinocytes in involved psoriatic epidermis and in human squamous cell carcinoma. In this study we investigated by immunohistochemistry and Western blotting whether calcium and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, well characterized differentiation signals, induce p21WAF1 in cultured normal human keratinocytes and whether induction of p21WAF1 in this system depends on protein kinase C activation or functional p53. Phorbol ester induced p21WAF1 expression, which was maximal at 4 to 8 h with reduction back to baseline by 24 to 48 h. In contrast, increasing the extracellular Ca2+ concentration from 70 micromol/L to 1.5 mmol/L resulted in upregulation of p21WAF1 expression with a slower time course, with peak induction at 18 to 24 h. No parallel increase in p53 expression was observed in normal human keratinocytes. Up-regulation of p21WAF1 was also observed in response to phorbol ester in HaCaT cells, which carry homozygous and inactivating mutations for p53. Induction of p21WAF1 by phorbol ester and Ca2+ was inhibited by the specific protein kinase C inhibitor Ro 31-8220. The results demonstrate a differential time course of p21WAF1 protein up-regulation in response to phorbol ester and Ca2+, signals that result in keratinocyte differentiation, and suggest that induction of p21WAF1 in differentiating human keratinocytes occurs through protein kinase C-dependent and p53-independent mechanisms.

The regulating effects of protein kinase C on the tone of guinea-pig trachea and human lobus bronchi

The regulating effects of protein kinase C (PKC) on the tone of guinea-pig trachea and human lobus bronchi were investigated by measuring the tone of isolated tracheal and bronchial strips. The effects of PKC on the tone of guinea-pig tracheal and human lobus bronchi were observed and compared. The results showed that: (1) PKC activator PMA induced concentration-dependent relaxation in guinea-pig isolated trachea strips. This relaxation was completely ablated by the pretreatment with 5 x 10(-6) mol/L Ro31-8220 which is a PKC-specific inhibitor, but was not affected by the removal of epithelium (EP), or by the pretreatment with propranolol (beta-receptor blocker) or atropin (M-receptor blocker); (2) PMA led to concentration-dependent contraction in human lobus bronchi. This contractile response was completely depressed by the pretreatment with 5 x 10(-6) mol/L Ro31-8220 and was partly inhibited by 1 x 10(-5) mol/L isoptin (Ca(2+)-antagonist), but was not significantly affected by propranolol or atropin. It is concluded that PKC is involved in the regulation of airway smooth muscle tone. The regulating effects may vary in different animals.

Mitogenic signaling of insulin-like growth factor I in MCF-7 human breast cancer cells requires phosphatidylinositol 3-kinase and is independent of mitogen-activated protein kinase

Addition of insulin-like growth factor I (IGF-I) to quiescent breast tumor-derived MCF-7 cells causes stimulation of cyclin D1 synthesis, hyperphosphorylation of the retinoblastoma protein pRb, DNA synthesis, and cell division. All of these effects are independent of the mitogen-activated protein kinase (MAPK) pathway since none of them is blocked by PD098059, the specific inhibitor of the MAPK activating kinase MEK1. This observation is consistent with the finding that the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), a strong inducer of MAPK activity in MCF-7 cells, effectively inhibits proliferation. The anti-proliferative effect of TPA in these cells may be accounted for, at least in part, by the MAPK-dependent stimulation of the synthesis of p21(WAF1/CIP1), an inhibitor of cyclin/cyclin-dependent kinase complexes. In contrast, all of the observed stimulatory effects of IGF-I on cell cycle progression, cyclin D1 synthesis, and pRb hyperphosphorylation were blocked by the specific phosphatidylinositol 3-kinase inhibitor LY294002, suggesting that phosphatidylinositol 3-kinase activity but not MAPK activity is required for transduction of the mitogenic IGF-I signal in MCF-7 cells.

Intracellular calcium oscillations in cell populations of ras-transfected I-7 subline of human HaCaT keratinocytes

We have observed oscillations of intracellular Ca2+ (Ca[i]) concentration in populations of ras-transfected HaCaT keratinocytes of I-7 subline. In postconfluent monolayers of I-7 keratinocytes, an increase in extracellular Ca2+ (Ca[o]) concentration to 0.25-0.5 mM induced sinusoidal Ca(i) oscillations, which persisted longer than 1 h with amplitudes of 50-150 nM and periods of 5-10 min. Thapsigargin, which depletes internal Ca2+ stores, did not prevent Ca(o)-induced Ca(i) oscillations, and it also induced Ca(i) oscillations in the ras-transfected I-7 line. Removal of extracellular Ca2+ or addition of Ca2+-entry blocker La3+ or SK&F 96365 inhibited Ca(i) oscillations, suggesting that Ca(i) oscillations in ras-transfected HaCaT keratinocytes were dependent on Ca2+ influx across the plasma membrane. Because the Ca(o)-induced Ca(i) oscillations have been observed only in ras-transfected I-7 subline and not in its nontransfected parental HaCaT line, this may provide a partial explanation for the divergent responses of ras-transfected and nontransfected keratinocytes to Ca(o) signal for control of growth and differentiation.

Selected isozymes of PKC contribute to augmented growth of fetal and neonatal bovine PA adventitial fibroblasts

We sought to determine which isozymes of protein kinase C (PKC) contribute to the increased proliferation of immature bovine pulmonary artery (PA) adventitial fibroblasts. Seven were identified in lysates of neonatal PA fibroblasts by Western blot: three Ca2+ dependent (alpha, beta I, and beta II) and four Ca2+ independent (delta, epsilon, zeta, and mu). Four isozymes (gamma, eta, theta, and iota) were not detected in fibroblasts isolated at any developmental stage. Of the seven detected isozymes, only PKC-alpha and -beta II protein levels were higher in fetal and neonatal cells compared with adult fibroblasts. Their role in the enhanced growth of immature fibroblasts was then evaluated. The isozyme nonselective PKC inhibitor Ro-31-8220 was first compared with GF-109203X, a structural analog of Ro-31-8220 with relative specificity for the Ca(2+)-dependent isozymes of PKC. GF-109203X selectively inhibited the growth of immature cells and was nearly as potent as Ro-31-8220. Go-6976, a more specific inhibitor of the Ca(2+)-dependent isozymes, mimicked the antiproliferative effect of GF-109203X. PKC downregulation with 1 microM phorbol 12-myristate 13-acetate had the same selective antiproliferative effect on immature fibroblasts as GF-109203X and Go-6976. The protein levels of PKC-alpha and -beta II, but not of PKC-beta I, were completely degraded in response to phorbol 12-myristate 13-acetate pretreatment. These results suggest that PKC-alpha and -beta II are important in the augmented growth of immature bovine PA adventitial fibroblasts.

Staurosporine, but not Ro 31-8220, induces interleukin 2 production and synergizes with interleukin 1alpha in EL4 thymoma cells

Protein kinase C (PKC) has been implicated in interleukin 1 (IL1) signal transduction in a number of cellular systems, either as a key event in IL1 action or as a negative regulator. Here we have examined the effects of two PKC inhibitors, staurosporine and the more selective agent Ro 31-8220, on IL1 responses in the murine thymoma line EL4.NOB-1. A 1 h pulse of staurosporine was found to strongly potentiate the induction of IL2 by IL1alpha in these cells. In contrast, neither a pulse nor prolonged incubation with Ro 31-8220 affected the response to IL1alpha. Both agents blocked the response to PMA, however. A 1 h pulse of staurosporine was also found to induce IL2 production on its own, activate the transcription factor nuclear factor kappaB (NFkappaB) and increase the expression of a NFkappaB-linked reporter gene. It synergized with IL1alpha in all of these responses. Ro 31-8220 was again without effect, although both staurosporine and Ro 31-8220 blocked the activation of NFkappaB by PMA. Finally, staurosporine caused the translocation of PKC-alpha and -epsilon, and to a lesser extent PKC-beta, but not PKC-θ or -zeta, from the cytosol to the membrane, although a similar effect was observed with Ro 31-8220. The results suggest that PKC is not involved in IL1alpha signalling in EL4 cells. Furthermore, the potentiating effect of staurosporine on IL1alpha action does not involve PKC inhibition, and is likely to be at the level of NFkappaB activation.

Protein kinase C mediates up-regulation of urokinase and its receptor in the migrating keratinocytes of wounded cultures, but urokinase is not required for movement across a substratum in vitro

Both in cell culture and in vivo, keratinocytes that are migrating in response to a wound express enhanced levels of both urokinase-type plasminogen activator (uPA) and the uPA cell surface receptor (uPA-R). To explore the mechanism of this up-regulation, keratinocyte cultures were treated proir to wounding with a variety of metabolic and growth factor inhibitors in order to evaluate their effect on uPA and uPA-R expression. Actinomycin D and cycloheximide inhibited the up-regulation of both uPA and uPA-R, as determined by immunohistochemistry, indicating that RNA and protein syntheses are required for their induction in migrating keratinocytes. Neither removal of protein growth factors from the medium nor addition of inhibitory antibodies to a number of growth factors depressed uPA or uPA-R induction; these findings suggest that a variety of exogenous or endogenous growth factors [i.e., basic fibroblast growth factor (bFGF), epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), amphiregulin, and tumor necrosis factor-alpha (TNF-alpha) do not have a critical role in the induction of uPA or uPA-R. In contrast, when protein kinase C (PKC) was either down-regulated with bryostatin 5 or inhibited with Ro31-8220 or staurosporine, the expression of both uPA and uPA-R was greatly decreased in migrating keratinocytes. Furthermore, pharmacologic activation of PKC enhanced uPA levels in non-wounded cultures. These data suggest that the enhanced expression of uPA and uPA-R in migrating keratinocytes is mediated by selective activation of PKC in these cells, perhaps secondary to alterations in the cytoskeleton induced by wounding. To test the requirement for uPA during keratinocyte migration in vitro, the extent of migration was quantified in the presence and absence of a variety of inhibitors in the wounded culture model. Migration was not altered by actinomycin D, cycloheximide, any of the above growth factor inhibitors, anti-uPA antibodies, a variety of inhibitors of uPA or plasmin enzymatic activity, or exogenous uPA. The independence of keratinocyte migration in vitro from uPA was further suggested by experiments which combined the phagokinetic assay of migration and the zymographic assay for pericellular uPA activity; no relationship was observed between pericellular uPA activity and the motility of individual cells.

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.

Transcription factor regulation of epidermal keratinocyte gene expression

The epidermis is a tissue that undergoes a very complex and tightly controlled differentiation program. The elaboration of this program is generally flawless, resulting in the production of an effective protective barrier for the organism. Many of the genes expressed during keratinocyte differentiation are expressed in a coordinate manner; this suggests that common regulatory models may emerge. The simplest model envisions a 'common regulatory element' that is possessed by all genes that are regulated together (e.g., involucrin and transglutaminase type 1). Studies to date, however, have not identified any such elements and, if anything, the available studies suggest that appropriate expression of each gene is achieved using sometime subtly and sometime grossly different mechanisms. Recent studies indicate that a variety of transcription factors (AP1, AP2, POU domain. Sp1, STAT factors) are expressed in the epidermis and, in many cases, multiple members of several families are present (e.g., AP1 and POU domain factors). The simultaneous expression of multiple members of a single transcription factor family provides numerous opportunities for complex regulation. Some studies suggest that specific members of these families interact with specific keratinocyte genes. The best studied of these families in epidermis is the AP1 family of factors. All of the known AP1 factors are expressed in epidermis [52] and each is expressed in a specific spatial pattern that suggests the potential to regulate multiple genes. It will be important to determine the role of each of these members in regulating keratinocyte gene expression. Finally, information is beginning to emerge regarding signal transduction in keratinocytes. Some of the early events in signal transduction have been identified (e.g., PLC and PKC activation, etc.) and some of the molecular targets of these pathways (e.g., AP1 transcription factors) are beginning to be identified. Eventually we can expect to elucidation of all of the steps between the interaction of the stimulating agent with its receptor and the activation of target gene expression.

1,25-Dihydroxyvitamin D3 upregulates the phosphatidylinositol signaling pathway in human keratinocytes by increasing phospholipase C levels

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) induces the differentiation of normal human keratinocytes, in part by increasing their basal intracellular calcium levels (Cai) over a period of hours. Agonists such as ATP acting through membrane receptors cause an immediate but transient increase in Cai accompanied by an increase in inositol trisphosphate (IP3). Treatment of keratinocytes for 24 h with 1 nM 1,25(OH)2D3 resulted in a two- to four-fold potentiation of the Cai response of these cells to ATP. This potentiation was inhibitable with cycloheximide, unaccompanied by a change in total intracellular calcium pools, but associated with an increase in basal IP3 levels and ATP-stimulated IP3 production. Treatment with 1,25(OH)2D3 raised the protein and mRNA levels of phospholipase C isoenzymes, particularly phospholipase C-beta 1 in a dose-dependent manner. These studies indicate that 1,25(OH)2D3 modulates the keratinocyte signal transduction pathway by induction of phospholipase isoenzymes, a previously undescribed action for this hormone.