Assessment of developmental potential of caprine cloned embryos with ooplasm replenishment under two culture media

The present study was designed to assess the developmental potential of somatic cell nuclear transfer (SCNT) embryos, with and without replenishment of ooplasm into enucleated oocytes, by culturing separately in two culture media. The enucleated oocytes were replenished with exogenous matured ooplasm under replenished nuclear transfer (RNT) method and compared with conventional nuclear transfer (CNT) method without replenishment. The fusion efficiency in RNT group was found to be significantly higher (P < 0.05) than CNT group (59.39 ± 7.36 vs 45.57 ± 3.68%). The completely fused reconstructed oocytes from both groups were cultured separately in research vitro cleave medium (RVCL) and RVCL-Blast medium. The embryonic development of 2 cell, 4 cell, 8-16 cell and 16-32 cell stages in the RNT group was superior to the CNT group regardless of the culture media used (P < 0.05). The embryonic development of the 8-16 cell, 16-32 cell, morula, and blastocyst stages in the RVCL-Blast medium was significantly higher (P < 0.05) than the RVCL alone for both RNT as well as CNT groups. RNT method with RVCL-Blast produced highly significant (P < 0.01) embryonic development for 8-16 cell and 16-32 cell stage when compared to CNT with RVCL. Conclusively, the combination of RNT with RVCL-Blast culture media enabled an overall increase in the embryonic developmental potential.

Developmental potency of pre-implant parthenogenetic goat embryos: effect of activation protocols and culture media

The developmental potency of pre-implant parthenogentic goat embryos were compared under two chemical activation protocols in three different culture media groups. The in vitro matured oocytes were chemically activated by two protocols viz. P1 (CB-CHX-6DMAP) and P2 (Ca-CHX-6DMAP). The activated oocytes under both the protocols were developed in three culture media, viz. modified synthetic oviductal fluid (mSOF), research vitro cleave medium (RVCL), and RVCL-Blast. While comparing the developmental potential of activated oocytes, it was observed that the oocytes activated under P2 protocol pooled over three culture media group producing significantly higher mean cleavage rate (43.2±0.9 vs 40.6±1.5), blastocyst development (16.4±1.1 vs 12.6±0.8), and blastomere count (120.7±4.7 vs 113.2±4.1) as compared to P1 protocol. The comparison of effect of culture media pooled over protocol groups revealed that the mean cleavage rate observed under RVCL-Blast (44.8±1.3) and RVCL (45.3±0.5) were significantly higher (P≤0.01) than mSOF (35.8±1.2). However, the mean blastocyst development observed under RVCL-Blast group (18.8±3.2) was significantly higher than RVCL (14.0±0.8) and mSOF (10.8±0.4). Similarly, the mean blastomere count under RVCL-Blast group (136.0±3.7) was significantly higher (P≤0.01) than RVCL (114.7±1.0) and mSOF (100.2±0.5) groups. The semiquantitative RT PCR analysis showed the expression of pro-apoptotic caspase 3 gene in P1 and anti-apoptotic Mcl-1 gene in P2. This study concludes that the activation protocol P2 and embryo cultured under RVCL-Blast group were optimum for chemical activation and culture medium, respectively.

Protein kinase C as a molecular target for cancer prevention by selenocompounds

Selenium is a very effective cancer-preventive agent, suppressing tumor promotion and early stages of tumor progression. However, the mechanisms by which selenium exerts these cancer-preventive actions are not known. Protein kinase C (PKC) is a receptor for certain tumor promoters and also plays a crucial role in events related to tumor progression. Therefore, it is not only a potential target for the cancer-preventive activity of selenium, but also it has the structural basis for interaction with selenium. Redox-active selenocompounds can inactivate PKC, particularly the Ca(2+)-dependent isozymes, by reacting with the critical cysteine-rich regions present within the catalytic domain while, in some cases, also reacting with the cysteine residues present within the zinc-fingers of the regulatory domain. The selenoprotein thioredoxin reductase (TR), acting through thioredoxin, reverses the inactivation of PKC induced by selenometabolites. Furthermore, TR, through a direct interaction involving its selenosulfur center with the zinc-thiolates of PKC, can reverse the redox modification of this kinase induced by selenometabolites. Thus the selenometabolite-induced toxicity is reversed by a selenoprotein, and therefore an interrelationship exists between these two mechanisms of selenium actions. Moreover, this also explains how a resistance to selenium develops in advanced tumor cells probably due to an overexpression of functional TR. Selenium-induced inactivation of PKC may, at least in part, be responsible for the selenium-induced inhibition of tumor promotion, cell growth, invasion, and metastasis, as well as for the induction of apoptosis.

Interferon beta affects retinal pigment epithelial cell proliferation via protein kinase C pathways

BACKGROUND

The aim of this study is to see the effect of interferon beta (IFN-beta) on cell proliferation and the protein kinase C (PKC) signaling pathway.

METHODS

Proliferation of cultured human retinal pigment epithelium (RPE) cells, with various concentrations of IFN-beta, and with or without 3% fetal calf serum (FCS), was assessed by cell counting. Effects of short (3 h) or prolonged (48 h) exposure of RPE cells to natural human IFN-beta were assessed by (3)H-thymidine uptake. Cytosolic and membranous PKC activity over time in cells treated with IFN-beta and calphostin C was also measured.

RESULTS

IFN-beta inhibited the increased proliferation by FCS in the prolonged-exposure assay. The PKC inhibitor calphostin C also showed an inhibitory effect on RPE cell growth and (3)H-thymidine uptake in the chronic exposure with FCS. Short treatment with IFN-beta had no inhibitory or stimulatory effect on (3)H-thymidine uptake. Cytosolic and membranous PKC activity was strongly upregulated after short IFN-beta exposure but returned to original levels after 1 h. PKC activity was downregulated both in the cytosol and membrane after 24 or 48 h.

CONCLUSION

IFN-beta inhibited RPE proliferation in vitro and the effect is mediated by upregulation of the PKC pathway.

A stress-responsive gene from groundnut, Gdi-15, is homologous to flavonol 3-O-glucosyltransferase involved in anthocyanin biosynthesis

Stress-tolerant crops are expected to express genes not normally expressed in susceptible crops. We have used desiccation stress coupled with high light intensity to identify groundnut as a relatively tolerant crop. Stress-responsive genes (Gdi, Groundnut Desiccation Induced) were cloned by subtractive hybridisation. The sequence of Gdi-15 shows homology to flavonol 3-O-glucosyltransferases, which are involved in anthocyanin biosynthesis. Gdi-15 transcripts increase markedly in response to stress, suggestive of a role in stress tolerance.

Thalidomide: a re-look

Thalidomide was synthesized in 1954 in erstwhile West Germany and marketed as a sedative in over 46 countries until the early 1960s. Owing to serious teratogenic effects, the drug was withdrawn from the market in 1961. A chance observation suggested the utility of thalidomide in erythema nodosum leprosum (ENL). After many controlled and uncontrolled trials were published, the World Health Organization recommended its use in ENL. The Food and Drug Administration, USA approved it for use in ENL in July 1998. Only established and well-defined studies conducted to substantiate the efficacy of thalidomide have been included in this review. Thalidomide is considered the drug of choice for the treatment of ENL, but for other conditions, it is recommended only when resistance to the currently available form of therapy is encountered. Once the anti-inflammatory, immuno-modulatory, anti-TNF-alpha and anti-angiogenic properties of thalidomide were discovered, it was also tried in AIDS and related wasting, apthous ulcers, microsporidiosis and Kaposi's sarcoma. Thalidomide has no clinical place as an immunosuppressant in solid organ transplantation. However, it has a therapeutic role in graft-verus-host-disease. Among the dermatological conditions, thalidomide has been found to be effective in systemic lupus erythematosus, discoid lupus erythematosus, actinic prurigo and prurigo nodularis. Used correctly, it is a safe and effective medicine (except for its teratogenic potential and delayed neuropathy) in a variety of disease conditions.

Protein kinase C signaling and oxidative stress

Oxidative stress is involved in the pathogenesis of various degenerative diseases including cancer. It is now recognized that low levels of oxidants can modify cell-signaling proteins and that these modifications have functional consequences. Identifying the target proteins for redox modification is key to understanding how oxidants mediate pathological processes such as tumor promotion. These proteins are also likely to be important targets for chemopreventive antioxidants, which are known to block signaling induced by oxidants and to induce their own actions. Various antioxidant preventive agents also inhibit PKC-dependent cellular responses. Therefore, PKC is a logical candidate for redox modification by oxidants and antioxidants that may in part determine their cancer-promoting and anticancer activities, respectively. PKCs contain unique structural features that are susceptible to oxidative modification. The N-terminal regulatory domain contains zinc-binding, cysteine-rich motifs that are readily oxidized by peroxide. When oxidized, the autoinhibitory function of the regulatory domain is compromised and, consequently, cellular PKC activity is stimulated. The C-terminal catalytic domain contains several reactive cysteines that are targets for various chemopreventive antioxidants such as selenocompounds, polyphenolic agents such as curcumin, and vitamin E analogues. Modification of these cysteines decreases cellular PKC activity. Thus the two domains of PKC respond differently to two different type of agents: oxidants selectively react with the regulatory domain, stimulate cellular PKC, and signal for tumor promotion and cell growth. In contrast, antioxidant chemopreventive agents react with the catalytic domain, inhibit cellular PKC activity, and thus interfere with the action of tumor promoters.

Tumor promoter benzoyl peroxide induces sulfhydryl oxidation in protein kinase C: its reversibility is related to the cellular resistance to peroxide-induced cytotoxicity

Since tumor promoter benzoyl peroxide (BPO) mimics phorbol esters in some aspects, its effects on protein kinase C (PKC) were previously studied. However, in those studies due to the presence of thiol agents in the PKC preparations, the sensitive reaction of BPO with redox-active cysteine residues in PKC was not observed. In this study, by excluding thiol agents present in the purified PKC preparation, low concentrations of BPO modified PKC, resulting in the loss of both kinase activity and phorbol ester binding (IC50 = 0. 2 to 0.5 microM). This modification, which was not dependent on transition metals, was totally blocked by a variety of thiol agents including GSH, which directly reacted with BPO. Substoichiometric amounts of BPO (0.4 mol/mol of PKC) oxidized two sulfhydryls in PKC and inactivated the enzyme which was readily reversed by dithiothreitol. The regulatory domain having zinc thiolate structures supporting the membrane-inserting region provided the specificity for PKC reaction with BPO, which partitioned into the membrane. Unlike H2O2, BPO did not induce the generation of the Ca2+/lipid-independent activated form of PKC. Other redox-sensitive enzymes such as protein kinase A, phosphorylase kinase, and protein phosphatase 2A required nearly 25- to 100-fold higher concentrations of BPO for inactivation. BPO also inactivated PKC in a variety of cell types. In the JB6 (30 P-) nonpromotable cell line and other normal cell lines, where BPO was more cytotoxic, it readily inactivated PKC due to a slow reversibility of this inactivation by the cell. However, in the JB6 (41 P+) promotable cell line, C3H10T1/2 and B16 melanoma cells, where BPO was less cytotoxic, it did not readily inactivate PKC due to a rapid reversibility of this inactivation by an endogenous mechanism. Nevertheless, BPO inactivated PKC at an equal rate in the homogenates prepared from all these cell types. Inclusion of NADPH reversed this inactivation in the homogenates to a different extent, presumably due to a difference in distribution of a protein disulfide reductase, which reverses this oxidative modification. BPO-induced modification of PKC occurred independent of the cellular status of GSH. However, externally added GSH and cell-impermeable thiol agents prevented the BPO-induced modification of PKC. Since BPO readily partitions into membranes, its reaction with redox-cycling thiols of membrane proteins such as PKC may trigger epigenetic events to prevent cytotoxicity, but favor tumor promotion.

Differential distribution of protein phosphatase 2A in human breast carcinoma cell lines and its relation to estrogen receptor status

Protein phosphatase 2A (PP2A) acts as a growth suppressor and is negatively influenced by oncogenic signals. We determined its activity in various human breast carcinoma (HBC) cell types to understand its relationship to estrogen receptor (ER) expression as well as to the distribution of protein kinase C (PKC), an opposing enzyme. PP2A activity was measured using a preferred substrate, histone H1 phosphorylated by PKC. PP2A activity was higher in both the soluble and nuclear fractions of ER-positive cell lines (MCF-7, T47D and ZR-75-1) than in the ER-negative cell lines (MDA-MB-231, Hs578T and BT-20). PP2A multiple forms (2A0, 2A1, 2A2), separated by DEAE-cellulose chromatography and immunoblot analysis of PP2A catalytic subunit, also showed similar differences in these two HBC cell types. In all cases, PP2A distribution was inversely correlated with the PKC activity profile. Moreover, PP2A activity in MCF-7 cells maintained in estrogen-depleted medium was low. Nonetheless, it was induced by a prolonged treatment with 17beta-estradiol, this induction being blocked by the antiestrogens, tamoxifen and ICI-182,780. Studies in both MCF-7 transfectants stably overexpressing ras and MDA-MB-231 transfectants stably expressing ER, suggested that a low PP2A distribution in ER-negative HBC cell types may be related to tumor progression rather than the loss of ER. Conceivably, the presence of high PP2A along with low PKC in ER-positive HBC cell types may be related to the restricted cell growth associated with the retention of a certain degree of differentiation or hormonal control. Conversely, the presence of low PP2A along with high PKC in ER-negative cell types may be related to hormone-independent enhanced cell growth.

Verapamil inhibits proliferation, migration and protein kinase C activity in human retinal pigment epithelial cells

The effects of three calcium channel blockers, verapamil, diltiazem and nifedipine, were examined on in vitro proliferation and migration of human retinal pigment epithelial cells. Human retinal pigment epithelial cells were seeded in Dulbecco's modified essential medium with 10% fetal bovine serum and different concentrations of the three calcium channel blockers. After 3 days of treatment, cell proliferation was determined by cell counting and by [3H]-thymidine uptake. Cell viability was determined with trypan blue exclusion. For determination of cell migration, retinal pigment epithelial cells were grown to confluence and then growth-inhibited with mitomycin C. After a 3 mm zone was denuded, the cells were treated with different concentrations of the calcium channel antagonists. After 24 hr, the cells that had migrated over the wound edge were counted. To determine the involvement of protein kinase C in the verapamil effect, its activity was measured in both verapamil-treated and untreated cells. Verapamil dose dependently inhibited serum-induced proliferation of retinal pigment epithelial cells, when measured by cell number (IC50 14.6 microM) or [3H]-thymidine incorporation (IC50 11.3 microM). At concentrations of 15 microM and below, there was no effect on cell viability, as determined by morphology and trypan blue exclusion. Diltiazem inhibited cell proliferation at a concentration of 100 microM; however, 100 microM nifedipine had no effect. Verapamil showed a significant inhibition of serum-induced migration in the range of 10 microM to 0.1 microM. The IC50 of the inhibition of retinal pigment epithelial cell proliferation and migration by verapamil is significantly higher than that seen for effects on calcium channel blockage. Eight micromolar verapamil reversibly inhibited total protein kinase-C activity in retinal pigment epithelial cells suggesting the possibility that the drug may act by inhibiting the protein kinase-C pathway. These data suggest the potential of the calcium channel blocker verapamil as a pharmacological modulator of disorders such as proliferative vitreoretinopathy in which there is increased retinal pigment epithelial cell proliferation and migration.

Role of protein kinase C in basal and hydrogen peroxide-stimulated NF-kappa B activation in the murine macrophage J774A.1 cell line

In macrophages, hydrogen peroxide appears to be a physiological activator of the transcription factor, nuclear factor kappa B (NF-kappa B); however, the molecular basis of H2O2-stimulated NF-kappa B activation is not well defined. The observations that NF-kappa B can be activated in cells by phorbol 12-myristate 13-acetate and in vitro by addition of protein kinase C (PKC) are suggestive of a role of PKC in NF-kappa B activation, which was investigated in the J774A.1 murine macrophage cell line. Basal NF-kappa B DNA-binding activity and nuclear localization were decreased by PKC inhibitors. Although PKC activity was modified by H2O2 with a similar time course as H2O2 activation of NF-kappa B, the H2O2-stimulated increase in NF-kappa B DNA binding and translocation to the nucleus was unaffected by PKC inhibitors. Furthermore, PKC down-regulation (through preincubation with phorbol esters) also affected only baseline NF-kappa B DNA binding but not H2O2-stimulated NF-kappa B activation. Buffering of changes in intracellular free calcium concentration also had no effect upon H2O2-stimulated NF-kappa B activation. Thus, classical PKC activity may modulate basal NF-kappa B activity but does not participate in H2O2-stimulated NF-kappa B activation.

Selenocompounds induce a redox modulation of protein kinase C in the cell, compartmentally independent from cytosolic glutathione: its role in inhibition of tumor promotion

Since selenite and other redox-active selenocompounds can modify protein kinase C (PKC) in the test tube, we have determined whether or not this redox regulation occurs inside the cell despite having high concentrations of GSH and the role of this regulation in the inhibition of tumor promotion. By using phorbol ester-promoted JB6 epidermal cell transformation assay, the concentrations of selenite, selenocystine, and selenodiglutathione which are optimal for chemopreventive activity were determined. At such concentrations (0.5 to 2 microM) in the cells treated with these agents, only a slight but transient decrease in PKC activity was observed when measured with a low (5 microM), but not with a high (100 microM) concentration of ATP. However, when the cells were serum starved or pretreated with 2-deoxyglucose, there was a pronounced but transient inactivation of PKC when assayed with both low and high concentrations of ATP. The inactivation was reversed in the cell by an endogenous mechanism or by treatment with thiol agents in the test tube. In spite of a substantial (90%) depletion of GSH in the cells by pretreatment with buthionine sulfoximine, there was no further increase in the redox modification of PKC by selenite as well as no change in the inhibitory effect of selenite on the phorbol ester-stimulated induction of ornithine decarboxylase, which is an intermediate marker related to cell transformation. While GSH is known to influence certain actions of selenium, it may not be required to mediate the effects of selenite tested in this study. The water-soluble cytosolic GSH did not interfere with the redox modification of PKC probably due to the shielding of the cysteine-rich region of the enzyme by a weak hydrophobic association with the membrane. Due to the presence of cofactors in the crude cell extracts, PKC was more sensitive to selenite than in the purified form and was inactivated by low concentrations of selenite (IC50 = 0.05 microM). This modification was reversed by thiol agents as well as by NADPH. A protein disulfide reductase, which can regenerate PKC, was present in the homogenate. Conceivably, selenite and other selenocompounds induce a redox modification of cellular PKC, compartmentally independent from the cytosolic GSH, but intimately connected to a NADPH-dependent reductase system, to mediate, at least in part, some of the cancer-preventive actions.

Cancer-preventive selenocompounds induce a specific redox modification of cysteine-rich regions in Ca(2+)-dependent isoenzymes of protein kinase C

Since protein kinase C (PKC) serves as a receptor for phorbol ester type tumor promoters and oxidants and has unique redox-active cysteine-rich regions, we have determined whether various chemopreventive selenocompounds could affect this enzyme. At lower concentrations, selenite decreased the kinase activity (IC50 = 0.5 microM), while at higher concentrations it decreased phorbol ester binding. However, when the catalytic and regulatory domains of PKC were separated by proteolysis, the catalytic domain retained its sensitivity to selenite, while the regulatory domain lost its sensitivity. Cysteine residues were quantitated in PKC modified with selenite by using 5,5'-dithiobis(2-nitrobenzoic acid) and also by using 2-nitro-5-thiosulfobenzoic acid after sulfitolysis. At lower concentrations, selenite induced a modification of four cysteine residues resulting in the formation of two disulfides, while at higher concentrations it induced a modification of seven to eight cysteine residues resulting in the formation of three to four disulfides. Contrary to selenite, selenocystine and selenodiglutathione (GSSeSG) readily inactivated the kinase activity, but not the phorbol ester binding. These two agents induced a two-stage modification of PKC; a limited modification at low concentrations leads to a loss of affinity for ATP, while an excessive modification at high concentrations leads to a loss of Vmax. Selenocystine and GSSeSG were 100,000-fold more potent than GSSG in inactivating PKC. The isoenzymes alpha, beta, and gamma exhibited an identical susceptibility to these selenocompounds. These results suggested that the cysteine residues present within the catalytic domain of these isoenzymes, although apart in the sequence, may be clustered in the tertiary structure to react with selenite, as well as may be in close proximity to some of the cysteines in the regulatory domain. Selenite did not affect protein kinase A, whereas GSSeSG and selenocystine inactivated the catalytic subunit after dissociation from the regulatory subunit at concentrations 100- and 800-fold, respectively, higher than that required for PKC inactivation. All three selenocompounds did not affect the activities of phosphorylase kinase and protein phosphatase 2A. Taken together, these results suggest that the accessible redox-active cysteine residues present in the PKC catalytic domain can react with certain specificity with redox-active selenocompounds such as selenite, selenocystine, and GSSeSG relative to other protein kinases tested.

Hypericin inhibits choroidal endothelial cell proliferation and cord formation in vitro

PURPOSE

To evaluate the effect of hypericin on bovine choroidal endothelial cell proliferation and cord formation and on protein kinase C activity.

METHODS

The effect of hypericin (0.1-5 microM) on bovine choroidal endothelial cell proliferation was determined by cell number counting and a 3H-thymidine uptake assay in media containing 1, 5 or 10% serum. For the cord formation assay, bovine choroidal endothelial cells were seeded on basement membrane matrix, and the lengths of the capillary-like structures (cords) formed were quantified by image analysis. The effect of hypericin on cord formation was evaluated in the presence of serum or vascular endothelial growth factor. The effect of hypericin on protein kinase C activity was also measured in the presence or absence of light.

RESULTS

Hypericin inhibited bovine choroidal endothelial cell proliferation in a dose-dependent manner in the presence of light but not in the dark. Serum dose-dependently masked the inhibition of DNA synthesis by hypericin. Cord formation by bovine choroidal endothelial cells was stimulated by serum or vascular endothelial growth factor and inhibited by hypericin in the presence of light. Protein kinase C activity was completely inhibited by hypericin in the presence of light but only mildly inhibited in the absence of light.

CONCLUSIONS

Hypericin inhibits bovine choroidal endothelial cell proliferation and cord formation and choroidal endothelial cell protein kinase C activity. These results suggest that hypericin should be further investigated in animal models for its potential to inhibit subretinal neovascularization.

Inhibition of cellular growth and induction of apoptosis in pituitary adenoma cell lines by the protein kinase C inhibitor hypericin: potential therapeutic application

Protein kinase C (PKC) is an enzyme involved in the regulation of cellular growth, proliferation, and differentiation in a number of tissues including the anterior pituitary, in which it is also believed to play a role in hormone secretion. Protein kinase C activity and expression have been found to be greater in adenomatous pituitary cells than in normal human and rat pituitary cells and higher in invasive pituitary tumor cells than in noninvasive ones. Inhibition of PKC activity has been shown in a variety of tumor cells to inhibit growth in a dose-related fashion. The purpose of the current study was to determine whether hypericin, a potent inhibitor of PKC activity that may be administered clinically, alters the growth and proliferation in established pituitary adenoma lines and to determine if inhibition of PKC activity induces apoptosis, as reported in some other tumor cell types. Two established pituitary adenoma cell lines, AtT-20 and GH4C1, were treated with hypericin in tissue culture for defined periods following passage. Inhibition of growth was found to be dose dependent in all three cell lines in low micromolar concentrations of hypericin, as determined by viable cell counts, methylthiotetrazole assay, and [3H]thymidine uptake studies. Concentrations of hypericin as low as 100 nM also induced apoptosis in these established lines, whereas treatment of normal human fibroblasts with a concentration of 10 microM failed to induce apoptosis. The potential use of hypericin in the therapy of pituitary adenomas warrants additional in vitro investigations with the aim of later moving toward therapeutic trials in selected patients in whom surgical or medical therapy has failed.

Tamoxifen modulates protein kinase C via oxidative stress in estrogen receptor-negative breast cancer cells

Nonsteroidal agent tamoxifen (Tam), a therapeutic/chemopreventive agent for breast cancer, inhibits protein kinase C (PKC), which is considered to be one of its extra-estrogen receptor sites of action. This drug is required at higher (>100 microM) concentrations to inhibit PKC in the test tube, whereas it is required at lower (1-10 microM) concentrations to induce inhibition of cell growth in estrogen receptor-negative cell types. To identify additional mechanisms of action of Tam on PKC and cell growth, studies with MDA-MB-231, an estrogen receptor-negative breast carcinoma cell type, have been carried out. Upon treatment with 5-20 microM Tam, a cytosol to membrane translocation of PKC occurred within 30 min, which was then followed by a down-regulation of the enzyme within 2 h. A transient generation of Ca2+/lipid-independent activated form of PKC was observed during this period. Rapidly growing cells require nearly 2-3-fold lower concentrations (2-5 microM) of Tam than do confluent cells to induce changes in PKC. Furthermore, phorbol ester binding observed with intact cells also decreased in Tam-treated cells only under the conditions PKC was inactivated. Unlike phorbol esters, Tam did not directly support the membrane association of PKC. The release of arachidonic acid correlated with the PKC membrane translocation. Studies carried out with [3H]Tam revealed that Tam partitioned into the membrane, and there was no appreciable covalent association of [3H]Tam with cellular proteins within this limited time period (2 h). Various antioxidants (vitamin E, vitamin C, beta-carotene, catalase, and superoxide dismutase) inhibited all these cellular effects of Tam. Moreover, vitamin E strikingly blocked Tam-induced growth inhibition. To determine whether oxymetabolites of Tam can affect PKC permanently, OH-Tam was tested with purified PKC. In contrast to Tam, which reversibly inhibited PKC, OH-Tam permanently inactivated the enzyme by modifying the catalytic domain at lower concentrations. The vicinal thiols present within this domain were found to be required to induce this inactivation. This effect was partially blocked by various antioxidants. This is the first report showing the role of oxidative stress in mediating the actions of Tam. Taken together these results suggest that Tam, by initially partitioning into the membranes, induces a generation of transmembrane signals and an oxidative stress to elicit the membrane association of PKC, followed by an irreversible activation, and subsequent down-regulation of this enzyme, which, in part, may lead to cell growth inhibition.

Enhancement of radiosensitivity in human malignant glioma cells by hypericin in vitro

Hypericin, an antidepressant and antiviral agent being evaluated in phase I and II trials for patients with HIV infection, is known to be a potent protein kinase C inhibitor. We have investigated its effects on cellular response to radiation via a tetrazolium-formazan cell growth rate assay using 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide and clonogenic assay in three human glioblastoma cell lines, U87-MG, A-172, and T98G, and a low-passage malignant glioma culture, 93-492. At a concentration of 5 microM, hypericin inhibited these cells slightly but caused significant radiosensitization (e.g., the cell survival rate after the radiation treatment was 50.2 and 26.0% in cells treated with 6 Gy and 6 Gy plus 5 microM hypericin in U87-MG cells, respectively; P = 0.0285). Hypericin also enhanced the radiosensitivity significantly in the low-passage glioma 93-492 cells. These findings suggest that hypericin represents a potential new agent in combination with radiation therapy of malignant gliomas.

Vitamin E succinate inhibits proliferation and migration of retinal pigment epithelial cells in vitro: therapeutic implication for proliferative vitreoretinopathy

BACKGROUND

Retinal pigment epithelial (RPE) cells play an important role in proliferative vitreoretinopathy (PVR). Vitamin E succinate is an ester form of a potent biological antioxidant, vitamin E, and has unique effects on various cells. We examined the effect of vitamin E succinate on proliferation and migration of cultured bovine RPE cells, since these are critical steps in the development of PVR.

METHODS

Bovine RPE cells were cultured in minimal essential medium (MEM) containing 10% fetal calf serum (MEM-10). Cells were incubated with MEM-10 containing 25 microM vitamin E, vitamin E succinate, butylated hydroxytoluene (BHT) or d-mannitol. Cell proliferation was assessed by counting cell numbers on days 2, 4 and 6. 3H-Thymidine uptake was also examined in RPE cells incubated with various forms of vitamin E-- vitamin E, vitamin E succinate, Trolox, gamma-tocopherol, vitamin E acetate, vitamin E phosphate, vitamin E nicotinate--or antioxidants-- BHT or d-mannitol (25 microM each). RPE cell migration was studied as follows: A small area (5 x 15 mm) of confluent cultured RPE cells was denuded using a straight razor blade and incubation was continued for 20 h with MEM-10 containing vitamin E, vitamin E succinate, gamma-tocopherol or BHT. The number of cells that migrated into the denuded area from the wound edge in each microscopic field (x20) was counted and expressed as a percentage of control (MEM-10 alone).

RESULTS

The antioxidants, vitamin E and BHT, stimulated RPE cell proliferation and 3H-thymidine incorporation compared with the control, while vitamin E succinate significantly inhibited both proliferation and 3H-thymidine uptake (IC50, 23 microM). Other forms of vitamin E or d-mannitol had no effect. Neither vitamin E nor BHT had a significant effect on RPE cell migration (108.2% and 112.6% of control, respectively), but vitamin E succinate inhibited migration (58.3%). Cell viability, assessed by the trypan blue dye exclusion test, was not impaired by a 3-day incubation with 50 microM of vitamin E succinate.

CONCLUSIONS

An ester form of a physiological antioxidant, vitamin E succinate, inhibits RPE cell proliferation and migration without causing cellular toxicity. These findings suggest its therapeutic potential for the pharmacological treatment of PVR.

Hypericin inhibits cell growth and induces apoptosis in retinal pigment epithelial cells: possible involvement of protein kinase C

Proliferative vitreoretinopathy (PVR) is characterized by the proliferation and migration of retinal pigment epithelial (RPE) cells in the vitreous cavity. The drug hypericin, which is already in clinical use as an antidepressant, has shown promise as an antiviral and antineoplastic agent. To investigate the therapeutic potential of hypericin in PVR, we incubated RPE cells in standard medium with various serum concentrations containing 0.5 to 5 microM hypericin. In some experiments we studied the effects of hypericin in conjunction with the RPE growth stimulating cytokine tumor necrosis factor alpha (TNF-alpha). Dose-dependent inhibition of RPE cell proliferation with IC50 values of 0.7 microM and 3.3 microM in 1% and 5% serum respectively, was found. Even in conjunction with TNF-alpha, hypericin inhibited RPE proliferation with an IC50 value of 1.5 microM. The drug inhibited PKC activity in cells treated with a 2.5 microM dose by 72% after 30 min and by 100% after 180 min. Finally, hypericin induced RPE cells to undergo apoptotic cell death, as shown by the presence of DNA laddering. These results suggest that hypericin may have potential as a therapeutic drug for PVR and that its antiproliferative and apoptotic effects on RPE cells in vitro are in part mediated by PKC.

Induction of intercellular adhesion molecule-1 by tumor necrosis factor-alpha through the 55-kDa receptor is dependent on protein kinase C in human retinal pigment epithelial cells

PURPOSE

To determine second messenger signaling pathways associated with tumor necrosis factor-alpha (TNF)-mediated induction of intercellular adhesion molecule (ICAM)-1 expression on human retinal pigment epithelial (HRPE) cells, a cell type known to express only the 55-kDa TNF receptor (TNFR p55).

METHODS

SV 40-immortalized HRPE (SVRPE) cells were exposed to TNF with and without pretreatment with the protein kinase C (PKC) inhibitor calphostin C or the protein kinase A (PKA) inhibitor H8. SV40-immortalized HRPE cells also were treated with the PKC activator phorbol 12-myristate 13-acetate (PMA) or with the PKA activators forskolin plus 3-isobutyl-1-methyl-xanthine or dibutyryl cyclic adenosine monophosphate (cAMP) alone. Membrane fractions from untreated and treated SVRPE cells were assayed for PKC activity, and whole cell lysates were assayed for cAMP accumulation and PKA activity. Flow cytometry was performed on SVRPE cells using a monoclonal antibody specific to ICAM-1.

RESULTS

Activation of TNFR p55 on SVRPE cells with TNF resulted in a rapid increase of PKC activity at 1 minute, with a subsequent downregulation to baseline. There was no increase in intracellular cAMP accumulation or PKA activity within the first 10 minutes; however, both increased within 30 minutes and returned to baseline within 1 hour. SV40-immortalized HRPE cells treated with TNF for 1 hour showed maximal induction of ICAM-1 expression at 18 hours. ICAM-1 induction by TNF treatment was inhibited by calphostin C pretreatment and not by H8 pretreatment. Protein kinase C activation with PMA for 3 hours was sufficient to induce ICAM-1 on SVRPE cells at 18 hours, whereas treatment with the PKA activators forskolin or dibutyryl cAMP did not induce ICAM-1 expression.

CONCLUSIONS

Tumor necrosis factor sequentially activates the PKC and PKA pathways in SVRPE cells by way of the TNFR p55. The PKC pathway in necessary for TNF-mediated ICAM-1 upregulation, and specific activation of the PKC pathway with PMA is sufficient to induce ICAM-1 on these cells. SV40-immortalized HRPE cells may serve as a model in which to study further the functional signaling pathways associated with TNFR p55.

Protein kinase C and growth regulation of pituitary adenomas

The present study was undertaken to explore the role of the Protein Kinase C (PKC) signal transduction system in growth regulation of pituitary adenomas. Primary tumor cultures were plated from fresh surgical tumor specimens. The PKC inhibitors Staurosporine and Tamoxifen were added at varying dosages to the cell cultures. Measurements of cell proliferation were performed by [3H]-thymidine uptake and the [3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H tetrazolium bromide] (MTT) assay. After a 48 h treatment period, both [3H]-thymidine uptake and absorbance on the MTT assay decreased in a dose-related manner in both the staurosporine and tamoxifen-treated cultures (IC50 of 10 nM and 30 microM respectively). Direct measurement of PKC activity using an in vitro assay revealed very high activity (range of 1465-5708 pmol/min/mg protein; within the range previously published for malignant glioma specimens) in 12 frozen specimens of pituitary adenomas (9 nonfunctional adenomas, 1 prolactinoma, 1 gonadotrophin-secreting and 1 corticotroph-secreting adenoma). In contrast, PKC activity measured in normal adenohypophysis was comparatively very low. These data indicate that pituitary adenoma cells display high PKC activity and are sensitive to growth inhibition by PKC inhibitors. These data suggest a role for the PKC system in regulating pituitary tumor growth, which may have implications for future therapy of these tumors.

Effect of linoleic acid hydroperoxide on endothelial cell calcium homeostasis and phospholipid hydrolysis

The relationship between intracellular free calcium ion concentrations ([Ca2+]i) and release of arachidonic acid from membrane phospholipids following peroxidation was examined in rabbit aortic endothelial cells treated with linoleic acid hydroperoxide (LOOH). LOOH (0.1-0.4) mumol/10(6) cells) caused a rapid and dose-dependent transient increase in [Ca2+]i in the presence of extracellular Ca2+ that remained elevated over baseline for 15 to 30 s. In the absence of extracellular Ca2+, LOOH also evoked a transient increase in [Ca2+]i of lesser magnitude which immediately returned to basal (or below basal) levels. In this regard, the rise in intracellular Ca2+ after LOOH or vasopressin (AVP) treatments involved, at least in part, related intracellular pools that in each case was followed by influx of extracellular Ca2+. The intracellular membrane sources known to be affected by vasopressin were not directly involved. Most notably, the LOOH evoked rise in [Ca2+]i was not associated with release of IP3, suggesting that the source of intracellular Ca2+ is not IP3-sensitive pools. However, pretreatment with LOOH strongly inhibited the rise in [Ca2+]i upon subsequent addition of AVP or LOOH and the extent of such inhibition was dependent on the availability of free intracellular Ca2+ and presence of extracellular Ca2+. These findings suggest that reuptake of Ca2+ into intracellular membrane pools is reduced in the presence of LOOH and/or the availability of Ca2+ from agonist-sensitive sites is inhibited by LOOH. An increase in free 20:4 levels was found after LOOH treatment that was only partly prevented using intracellular Ca2+ chelators which maintained [Ca2+]i at basal levels after LOOH treatment. These findings suggest that LOOH induction of phospholipid hydrolysis proceeds following small transients in [Ca2+]i that are considerably less than that evoked by agents such as AVP, approximating basal Ca2+ concentrations. Inhibition of LOOH-induced lipid peroxidation by vitamin E also prevented the rise in [Ca2+]i and 20:4 release indicating that phospholipid hydrolysis is dependent, at least in part, on membrane lipid peroxidation. Inhibition of protein kinase C (PKC) completely blocked LOOH-induced release of 20:4 but had little effect on the LOOH-induced rise in [Ca2+]i, suggesting an indirect relationship between LOOH-induced membrane Ca2+ signalling events, with intervention via PKC-mediated induction of phospholipid hydrolysis. A rapid and progressive translocation of PKC to the membrane fraction was evident after LOOH addition over the time course corresponding to the maximal release of 20:4 which was also inhibited by vitamin E.(ABSTRACT TRUNCATED AT 400 WORDS)

Migration of retinal pigment epithelium cells in vitro is regulated by protein kinase C

The migration of retinal pigment epithelial (RPE) cells is an important step in various pathologic conditions, including subretinal neovascularization (SRN) and proliferative vitreoretinopathy (PVR). Therefore, elucidation of the mechanism of RPE migration may be useful in devising effective treatment for these disorders. Since protein kinase C (PKC) has been shown to regulate the migration of other cell types, we studied the effects of PKC agonists and antagonists on RPE migration. We used an in vitro wound healing model in which a small area of a confluent monolayer of bovine RPE cells was denuded with a razor blade. The cultures were subsequently incubated with agents known to stimulate [phorbol 12-myristate 13-acetate (PMA)] or inhibit (calphostin C, staurosporine) PKC. After 20 hr, migration was measured as the number of cells that had entered the denuded area. We also measured the translocation of PKC from the cytosol to the membrane in order to determine the activation or inhibition of PKC by PMA and calphostin C in the cells. The phorbol ester PMA stimulated migration by 41%, and calphostin C and staurosporine inhibited migration by 38% and 31%, respectively, in a medium supplemented with 10% serum. To determine the requirement for serum in this modulation, we also measured the effects of PMA and calphostin C on RPE migration in serum-free medium. Under these conditions, basal migration was greatly decreased, but PMA stimulated migration by 177% and calphostin C inhibited migration by 93%. Since PKC modulation is known to induce the proliferation of cells, we also tested the effects of these agents on growth-inhibited migration by pretreating the cells with the antiproliferative drug mitomycin C. We found that modulation of PKC under these conditions equally affected growth-inhibited and growth-dependent migration. Therefore, based on the increase in RPE migration induced by a PKC agonist, and the decrease in migration caused by PKC antagonists, it is suggested that PKC-mediated signal transduction plays a crucial role in RPE cell migration. This knowledge may be useful in devising effective treatments for SRN and PVR.

Thrombin induced cytoskeletal change in cultured bovine corneal endothelial cells mediated via protein kinase C pathway

We studied the participation of the protein kinase C pathway in thrombin-induced cytoskeletal alterations in confluent cultured bovine corneal endothelial (BCE) cells. Cultured BCE cells were exposed to alpha-thrombin (0.1-10 U/ml for 15-60 min) and the distribution of F-actin and vinculin plaques was examined using immunofluorescent staining and electron microscopy. Phorbol 12-myristate 13-acetate (PMA, 10 nM for 15 min), the broad spectrum protein kinase inhibitors staurosporine (10 nM) and H-7 (10 nM), and highly specific PKC inhibitor calphostin C (10 nM) were used to evaluate the role of PKC/phosphorylation in this phenomenon. HA-1004 (10 nM) was used as a negative control for these inhibitors. In a parallel experiment, PKC activity of cytosol and membrane of BCE cells was also evaluated. In control samples, F-actin was distributed mainly at the periphery of cells, where it formed dense peripheral bundles; vinculin plaques were also present at the cell boundary. Exposure of BCE cells to thrombin changed the distribution of F-actin and vinculin into a diffuse pattern; a similar alteration was also induced by incubation with PMA. These phenomena were blocked by incubation with H-7, staurosporine and calphostin C. Both cytosolic and membrane PKC activity was increased after 5 to 30 min exposure of alpha-thrombin and returned to the control level after 1 h. alpha-Thrombin induces alteration in the cytoskeleton of BCE cells, and this message is transduced at least in part by PKC dependent pathways. PKC/phosphorylation may thus play an important role in physiological processes that involve alterations of the cytoskeleton.

Tobacco smoke tumor promoters, catechol and hydroquinone, induce oxidative regulation of protein kinase C and influence invasion and metastasis of lung carcinoma cells

Cigarette smoke polyphenolic agents (catechol and hydroquinone) that generate oxidants have been shown to be tumor promoters. Furthermore, oxidants can influence protein kinase C (PKC)-mediated signal transduction. Since terpenoid tumor promoters, phorbol esters, increase invasion and metastasis by activating PKC, we have determined whether polyphenolic agents present in the cigarette smoke condensate (CSC) could also influence these events. Hydroquinone (50 microM), catechol (500 microM), or CSC (50 micrograms/ml) induced an initial cytosol-to-membrane translocation of PKC in LL/2 lung carcinoma cells, followed by a later down-regulation of the enzyme. LL/2 cells treated with these CSC-related agents for a limited time (45 min) and exhibiting high membrane-associated PKC activity, when injected into mice through the tail vein, produced an increase in metastatic nodules in the lungs after 20 days. However, cells treated with CSC-related agents for a prolonged period did not exhibit an increase in metastasis. Agents that decrease the rate of production of reactive oxygen species, such as catalase either alone or in combination with superoxide dismutase, and a cell-permeable iron-chelator, o-phenanthroline, inhibited CSC-mediated membrane association of PKC and metastasis. Prior treatment of CSC with tyrosinase to modify polyphenols resulted in a partial loss of CSC stimulation of metastasis. Furthermore, a cell-permeable Ca2+ chelator and diverse PKC inhibitors, such as calphostin C, hypericin, chelerythrine, and bisindolylmaleimide, inhibited CSC-enhanced metastasis. CSC increased in vitro tumor cell adhesion to endothelial monolayers and to reconstituted basement membrane (Matrigel) and also enhanced the invasion through Matrigel coated on the polycarbonate filters in Transwells. All these CSC effects were found to be temporary and were blocked by the above mentioned antioxidant systems and PKC inhibitors. Thus, these results suggest that the oxidants generated by autooxidation of polyphenolic agents present in tobacco smoke increase tumor cell invasion and metastasis, at least in part by activation of Ca2+/PKC signal transduction. Conceivably, cigarette smoke constituents not only promote tumorigenesis but also may increase the spread of cancer in the body.

Transforming growth factor-beta regulates human retinal pigment epithelial cell phagocytosis by influencing a protein kinase C-dependent pathway

BACKGROUND

Transforming growth factor-beta (TGF-beta) plays an important role in the pathogenesis of many ocular diseases, including proliferative vitreoretinopathy. We examined the effect of TGF-beta on the phagocytosis of rod outer segments by retinal pigment epithelium (RPE), which is a major function of RPE, and investigated the dependence of this effect on the protein kinase C (PKC) pathway.

METHODS

Phagocytotic uptake of fluoresceinated bovine rod outer segments was determined by flow cytometry. RPE cells were treated with TGF-beta 1 or TGF-beta 2 and their effects on phagocytosis were examined. The effects of various PKC inhibitors (calphostin C, staurosporine, and extended exposure to phorbol 12-myristate 13-acetate, PMA) and a stimulator (brief exposure to PMA) on RPE phagocytosis was evaluated.

RESULTS

Both TGF-beta 1 and TGF-beta 2 up-regulated RPE phagocytosis and PMA abolished the up-regulating effect of TGF-beta. In contrast, PKC inhibition by staurosporine and calphostin C resulted in increased phagocytosis. A combination of TGF-beta and PKC inhibitor treatment did not produced any additive effect on phagocytosis.

CONCLUSION

We concluded that TGF-beta up-regulates human RPE phagocytosis, but that this effect is counteracted by PKC activation. It is possible that this TGF-beta-induced effect is due, in part, to a negative modulation of the PKC-dependent pathway.

Hypericin: a potential antiglioma therapy

Hypericin, a polycyclic aromatic dione isolated from plants, is presently being clinically evaluated as an antiviral agent in the treatment of human immunodeficiency virus (HIV) infection. In addition, it is known to be a potent protein kinase C inhibitor. To evaluate its potential as an inhibitor of glioma growth, an established (U87) and low-passage glioma line (93-492) were treated with hypericin in tissue culture for a period of 48 hours after passage. Hypericin inhibited the glioma growth in a dose-related manner, with a marked inhibition of growth in the low-micromolar concentration range (e.g., in line U87 and low-passage line 93-492, a concentration of hypericin of 10 mumol/L produced 62 and 76% decreases in [3H]thymidine uptake, respectively). Because the reported inhibitory effects of protein kinase C are enhanced by visible light, [3H]thymidine uptake was measured in both the presence and the absence of visible light. In glioma line A172, the presence of light slightly increased the inhibitory effect of hypericin. Moreover, an apoptosis (i.e., programmed cell death) assay was performed to determine whether the treatment of glioma cells with hypericin was cytostatic or cytocidal. Cells were harvested, and purified deoxyribonucleic acid (DNA) was analyzed by agarose gel electrophoresis. DNA from cells treated with hypericin for 48 hours exhibited a classical "ladder" pattern of oligonucleosome-sized fragments characteristic of apoptosis. These data suggest that the proven safe drug hypericin may have potential as an antiglioma agent; we suggest clinical trials.

Collagen gel contraction induced by retinal pigment epithelial cells and choroidal fibroblasts involves the protein kinase C pathway

Contraction of intraocular fibrous membranes is an important feature in the pathogenesis of retinal detachment in proliferative vitreoretinopathy (PVR). Collagen gel contraction is a useful in vitro model of membrane contraction in PVR. We studied the role of protein kinase C (PKC) in collagen gel contraction induced by bovine choroidal fibroblasts and retinal pigment epithelial (RPE) cells. Collagen gels embedded with the cells were formed in culture dishes and gel contraction was evaluated. The PKC stimulator, phorbol 12-myristate 13-acetate (PMA), and the protein phosphatase 1 and 2A inhibitor, okadaic acid (OA), were used to evaluate the role of the PKC-mediated phosphorylation system in this gel contraction. Fifteen min incubation with PMA stimulated gel contraction, but 180 min incubation had no effect. Choroidal fibroblast- but not RPE cell-induced gel contraction was stimulated by OA. These effects were inhibited by the broad spectrum protein kinase inhibitor staurosporine and the specific PKC antagonist calphostin C. Transforming growth factor-beta (TGF-beta)1 and TGF-beta 2, which are known to be present in eyes with PVR, were evaluated to determine their effect on gel contraction. Both TGF-beta 1 and 2 had a stimulatory effect on contraction of gels seeded with choroidal fibroblasts and RPE cells, but staurosporine and calphostin C inhibited this TGF-beta-induced gel contraction. These results indicate that activation of PKC/protein phosphorylation is an important factor in gel contraction caused by choroidal fibroblasts and RPE cells, and that TGF-beta-induced gel contraction is mediated at least in part via the PKC pathway.

Nitric oxide and nitric oxide-generating agents induce a reversible inactivation of protein kinase C activity and phorbol ester binding

Since S-nitrosylation of protein thiols is one of the cellular regulatory mechanisms induced by nitric oxide (NO), and since protein kinase C (PKC) has critical thiol residues which influence its kinase activity, we have determined whether NO could regulate this enzyme. Initial studies were carried out with purified PKC and the NO-generating agent S-nitrosocysteine. This agent decreased phosphotransferase activity of PKC in a Ca(2+)- and oxygen-dependent manner with an IC50 of 75 microM. Phorbol ester binding was affected partially only at higher concentrations (> 100 microM) of S-nitrosocysteine. This inactivation of PKC was blocked by the NO scavenger oxyhemoglobin or reversed by dithiothreitol. It is likely that NO initially induced an S-nitrosylation of vicinal thiols, which were then oxidized to form an intramolecular disulfide. Other NO-generating agents such as S-nitroso-N-acetylpenicillamine and sodium nitroprusside, as well as authentic NO gas, induced similar types of PKC modifications. In intact B16 melanoma cells treated with S-nitrosocysteine a rapid decrease in PKC activity in both cytosol and membrane was observed. Unlike in experiments with purified PKC, in intact cells treated with S-nitrosocysteine the phorbol ester binding also decreased to a rate equal to that of PKC activity. These modifications were readily reversed by treating the homogenates with dithiothreitol in test tubes or by removing the NO-generating source from intact cells. To determine whether the limited amounts of NO generated within the intact cells could induce this type of PKC modification, the macrophage cell line IC-21 was treated with lipopolysacharide and Ca2+ ionophore A23187 to induce the NO production. With an increase in generation of NO (3-12-h period) in these cells, a parallel and irreversible decrease in PKC activity and phorbol ester binding was observed. A specific inhibitor for NO synthase, NG-monomethyl-L-arginine, inhibited both the production of NO and PKC inactivation. In experiments using purified enzyme or intact cells there was no decrease in cAMP-dependent protein kinase activity. Conceivably, NO production for limited time induces a reversible inactivation of PKC due to the formation of a disulfide bridge(s), whereas the chronic production of NO could induce irreversible inactivation of PKC. The reversible or irreversible inactivations of PKC may in part influence NO-mediated cytoprotective or cytotoxic actions, respectively.

Retinoids inhibit the oxidative modification of protein kinase C induced by oxidant tumor promoters

Recently we reported that oxidant tumor promoters can induce the oxidative modification of protein kinase C (PKC) resulting in either activation or inactivation of the kinase (R. Gopalakrishna and W. B. Anderson, Arch. Biochem. Biophys. 285, 382-387, 1991). Since retinoids previously have been shown to antagonize the actions of tumor promoters, studies were carried out to determine if retinoids can inhibit the oxidative modification of PKC induced by tumor promoters. Prior treatment of B16 melanoma cells or C6 glioma cells with all-trans-retinoic acid (0.1 microM) for a short time period (15 to 60 min) followed by subsequent treatment with oxidants such as hydrogen peroxide resulted in a 30 to 70% decrease in the oxidative modification of PKC. This resulted in a decrease in oxidant-induced conversion of PKC from a Ca2+/lipid-dependent form (peak A) to a Ca2+/lipid-independent form (peak B). This retinoid-mediated protection also was observed with the reversible oxidative modification of PKC induced by m-periodate treatment of intact cells. To understand whether this protection offered by retinoids was caused by a direct influence of retinoids on PKC, experiments were carried out using the purified enzyme. The results of experiments using isolated PKC suggested that retinoids can act directly to protect the regulatory domain of PKC from oxidative modification induced by oxidants. However, high (1-10 microM) concentrations of retinoids are necessary to elicit this protection of isolated PKC. In contrast, in experiments with intact cells, only low (submicromolar) concentrations of retinoids are required to protect PKC from oxidation. The differences noted in the retinoid concentrations required to protect PKC from oxidant modification in the test tube versus in the intact cell may be due to increased retention of retinoids in the cell membrane by partitioning, or to other indirect actions of retinoids in the intact cells to decrease cellular oxidations. These results suggest that some of the anti-tumor promoter actions of retinoids may be mediated, in part, by inhibiting the oxidative modification of protein kinase C induced by oxidant tumor promoters.

Nonphorbol tumor promoters okadaic acid and calyculin-A induce membrane translocation of protein kinase C

The cell-permeable inhibitors of type 1 and 2A protein phosphatases, okadaic acid and calyculin-A, induced a redistribution of protein kinase C (PKC) activity and immunoreactivity (40 to 60%) from cytosol to membrane in some cell types. Calyculin-A was 100-fold more potent than okadaic acid and required only 5 to 10 nM concentrations to induce this PKC translocation. The concentration of these agents required to induce the redistribution of PKC correlated with the potency of these agents to inhibit both type 1 and 2A protein phosphatases. There was a lag period of 15 to 30 min before the onset of PKC translocation, as this process might have been induced by indirect cellular events triggered by inhibitions of protein phosphatases (1 and 2A). Taken together these results suggest that although the okadaic acid class of tumor promoters and phorbol ester-related agents bind to two different cellular receptors having counteracting enzymic activities, they share a common mechanism of action, namely the induction of cytosol to membrane translocation of PKC.

Irreversible oxidative inactivation of protein kinase C by photosensitive inhibitor calphostin C

Isolated protein kinase C (PKC) was irreversibly inactivated by a brief (min) incubation with calphostin C in the presence of light. This inactivation required Ca2+ either in a millimolar range in the absence of lipid activators or in a submicromolar range in the presence of lipid activators. In addition, an oxygen atmosphere was required suggesting the involvement of oxidation(s) in this inactivation process. Furthermore, PKC inactivation might involve a site-specific oxidative modification of the enzyme at the Ca(2+)-induced hydrophobic region. Physical quenchers of singlet oxygen such as lycopene, beta-carotene, and alpha-tocopherol all reduced the calphostin C-induced inactivation of PKC. In intact cells treated with calphostin C, the inactivation of PKC was rapid in the membrane fraction compared to cytosol. This intracellular PKC inactivation was also found to be irreversible. Therefore, calphostin C can bring prolonged effects for several hours in cells treated for a short time. Taken together these results suggest that the calphostin C-mediated inactivation of PKC involves a site-specific and a 'cage' type oxidative modification of PKC.

Rapid filtration assays for protein kinase C activity and phorbol ester binding using multiwell plates with fitted filtration discs

In the conventional approach protein kinase activity and phorbol ester binding associated with protein kinase C (PKC) are measured by initially incubating samples in either test tubes or multiwell plates, followed by filtration of the terminated reaction mixture using either a manifold filtration device or a cell harvester. Here we report a method in which both the incubations and filtrations necessary for the determination of either protein kinase activity or phorbol ester binding are carried out in the same multiwell plate with fitted filtration discs made of polyvinylidene difluoride (Durapore membrane). Due to the very low binding of protein to these filters, there is no interference caused by these filters during the incubation period of the assays. The drawback with these filters compared to commonly used cellulose acetate membrane filters is that they retain less of the phosphate acceptor substrate histone H1 (only 15%) if filtered and washed with standard 5% trichloroacetic acid. However, this can be overcome by increasing the trichloroacetic acid concentration to 25% during filtration. For phorbol ester binding determinations, the samples are incubated with [3H]phorbol 12,13-dibutyrate in the microwells, the ligand bound PKC is adsorbed onto DEAE-Sephadex beads, and the beads then are filtered and washed in the same microwells. Furthermore, this multiwell filtration approach can also be adopted to previously described cytosolic phorbol ester receptor assays, which have the broader conditions for optimal binding to receptors. Durapore membrane filters are found to work well for punching into scintillation vials and there is complete recovery of the radioactivity retained with the filters. In the protein kinase assay the background radioactivity is very low (< 200 cpm) and in the phorbol ester binding assay the nonspecific binding is less than 1%. Thus, these low background values result in at least a fourfold increase in sensitivity for these assays. Since the incubations and filtrations are carried out in the same well without any transfer of the sample, the coefficient of variation in multiple determinations is found to be low. Furthermore, this method is rapid and more convenient for analyzing a larger number of samples than conventional methods which use test tubes, and it is less expensive to set up compared to the automated methods that use a cell harvester.

Unusually large mastoid antrum ('mega antrum')

Patients who present with a unilateral non-tender bony swelling in the mastoid region without any clinical evidence of middle ear infection could be diagnosed as having a fibrous or bony lesion affecting the temporal bone. In such cases, if there is radiological evidence of large lucent area in the mastoid antrum without any bony dehiscence one should keep in mind in the differential diagnosis a mega antrum in addition to congenital cholesteatoma and eosinophilic granuloma. A large lytic lesion in the mastoid segment of the temporal bone with an intact tympanic membrane therefore presents a diagnostic dilemma. A case of an unusually large mastoid antrum in an young adult with no middle ear suppuration and a cosmetically unacceptable swelling behind the ear is presented.

Reversible oxidative activation and inactivation of protein kinase C by the mitogen/tumor promoter periodate

The oxidant mitogen/tumor promoter, periodate, was used to selectively modify either the regulatory domain or the catalytic domain of protein kinase C (PKC) to induce oxidative activation or inactivation of PKC, respectively. Periodate, at micromolar concentrations, modified the regulatory domain of PKC as determined by the loss of ability to stimulate kinase activity by Ca2+/phospholipid, and also by the loss of phorbol ester binding. This modification resulted in an increase in Ca2+/phospholipid-independent kinase activity (oxidative activation). However, at higher concentrations (greater than 100 microM) periodate also modified the catalytic domain, resulting in complete inactivation of PKC. The oxidative modification induced by low periodate concentrations (less than 0.5 mM) was completely reversed by a brief treatment with 2 mM dithiothreitol. In this aspect, the modification induced by periodate was different from that of the previously reported irreversible modification of PKC induced by H2O2. However, the inactivation of PKC induced by periodate at concentrations greater than 1 mM was not reversed by dithiothreitol. Among the phospholipids and ligands of the regulatory domain tested, only phosphatidylserine protected the regulatory domain from oxidative modification. In the presence of phosphatidylserine, the catalytic site was selectively modified by periodate, resulting in formation of a form of PKC that exhibited phorbol ester binding but not kinase activity. Both reversible and irreversible oxidative activation and inactivation of PKC also were observed in intact cells treated with periodate. Taken together these results suggest that periodate, by virtue of having a tetrahedral structure, binds to the phosphate-binding regions present within the phosphatidylserine-binding site of the regulatory domain and the ATP-binding site of the catalytic domain, and modifies the vicinal thiols present within these sites. This results in the formation of intramolecular disulfide bridge(s) within the regulatory domain or catalytic domain leading to either reversible activation or inactivation of PKC, respectively. Thus, oxidant mitogen/tumor promoters such as periodate may be able to bypass normal transmembrane signalling systems to directly activate pathways involved in cellular regulation.

Ca2+- and phospholipid-independent activation of protein kinase C by selective oxidative modification of the regulatory domain

The susceptibility of purified protein kinase C to oxidative inactivation by H2O2 was found to be increased by Ca2+ either alone at a high (5 mM) concentration or at a low (approximately 50 microM) concentration along with phosphatidylserine and diacylglycerol and by tumor-promoting phorbol esters even in the absence of Ca2+. This suggested that the membrane-bound and/or catalytically active form of protein kinase C is relatively more susceptible to oxidative inactivation. Although both the regulatory and catalytic domains of protein kinase C were susceptible to oxidative inactivation, a selective modification of the regulatory domain was obtained under mild oxidative conditions by protecting the catalytic site with ATP/Mg2+. Under these conditions there was a loss of both phorbol ester binding and Ca2+/phospholipid-stimulated kinase activity. However, this modified form of enzyme exhibited an increase in Ca2+/phospholipid-independent kinase activity. This suggests that selective oxidative modification of the regulatory domain may negate the requirement for Ca2+ and lipids for activation. Treatment of intact C6 glioma or B16 melanoma cells with H2O2 resulted in a time- and temperature-dependent decrease in Ca2+/phospholipid-dependent protein kinase C activity along with a concomitant transient increase in an oxidatively modified isoform of protein kinase C that exhibited activity in the absence of Ca2+ and phospholipids. Since protein kinase C can initially be activated by mild oxidative modification and subsequently inactivated by further oxidation, this dual activation-inactivation of protein kinase C in response to H2O2 suggests an effective on/off signal mechanism to influence cellular events.

High metalloproteinase inhibitor content of human cirrhosis and its possible conference of metastasis resistance

Human cirrhotic livers exhibited a strong resistance to metastasis and demonstrated high levels of both soluble and 3 M NaCl extractable metalloproteinase inhibitor(s) directed against tumor type I and type IV collagenases. This inhibitory activity was detected in human cirrhosis from diverse causes, including alpha-1-antitrypsin deficiency, alcoholic liver disease, postviral hepatitis, and biliary cirrhosis, but was nearly undetectable in normal liver. The inhibitory activity was able to be purified by carboxymethyl cellulose chromatography and gel filtration, had a molecular weight of approximately 40,000, and was relatively heat stable. Such metalloproteinase inhibitory activity was also present in conditioned media of human myofibroblast primary cultures obtained from cirrhotic liver explants but absent in conditioned media of primary normal liver explants. Purified fractions of the metalloproteinase inhibitor activity obtained from both myofibroblast conditioned media and extracts of cirrhotic liver blocked invasion but not attachment of MCF-7 cells on human amnion in vitro. The results further support the role of myofibroblasts in inhibiting tumor invasion and metastasis and provide one possible mechanism by which cirrhotic livers resist metastasis.

Tumor promoter-induced membrane-bound protein kinase C regulates hematogenous metastasis

A strong correlation was found between the basal levels of membrane-bound protein kinase C and the ability of B16 melanoma cell sublines (F10, F1, and BL6) to metastasize to the lung after intravenous injection. By treating with tumor-promoting phorbol esters for 1 hr, the low-metastasizing F1 cells exhibited both translocation of protein kinase C from cytosol to plasma membrane and an increase in metastasis to a level comparable to the (untreated) highly metastatic subline F10. Prolonged treatment of melanoma sublines with phorbol 12-myristate 13-acetate for 24 hr resulted in both inactivation of protein kinase C activity and loss of their metastasizing capabilities. Under conditions that induced only the activation of protein kinase C but not its membrane association, no increase in metastasis occurred, suggesting that activation of protein kinase C alone is insufficient to promote metastasis and that its membrane association is also necessary. Exposure of B16 melanoma sublines to phorbol esters for 1 hr had (i) no effect on the growth and morphology of these cells in vivo and in vitro and (ii) a short-term effect (approximately equal to 5 hr) on membrane association of protein kinase C. Nonetheless, in this period, the membrane-bound protein kinase C, probably by influencing cell-surface and cell-attachment properties, increased the retention of circulating melanoma cells in the lung, which eventually led to an increased number of metastatic nodules. The membrane-bound protein kinase C activity also correlated with metastatic ability in rapidly growing cells, growth-arrested cells, and cells growing in a low-Ca2+ medium. The results strongly suggest that the membrane-bound protein kinase C influences hematogenous metastasis of tumor cells and show that tumor promoters like phorbol esters have an additional role in promoting hematogenous spread of cancer in the body.

Isolation of a myofibroblast growth factor from human breast carcinoma cell lines

Conditioned media of a series of well-established human breast carcinoma cell lines were screened for mitogenic activity on human myofibroblasts. Whereas all carcinoma lines derived from desmoplastic ductal breast carcinoma primaries exhibited moderate to high levels of mitogenic activity, the single line derived from a non-desmoplastic (medullary) carcinoma exhibited low activity. Levels of mitogenic activity were independent of estrogen receptor status and estrogen/antiestrogen treatment. Fractionation of the conditioned media revealed a cationic, hydrophobic mitogenic factor of M.W. 25,000. The factor did not stimulate the growth of endothelial or carcinoma cells nor the growth of NRK fibroblasts in soft agar.

Susceptibility of protein kinase C to oxidative inactivation: loss of both phosphotransferase activity and phorbol diester binding

Exposure of protein kinase C to low concentrations of either N-chlorosuccinimide or H2O2 resulted in rapid and parallel loss of phosphotransferase activity and phorbol ester binding. This oxidative inactivation of protein kinase C also occurred in intact cells exposed to a low concentration of H2O2. With H2O2 treatment the rate of inactivation of protein kinase C in the cytosol of MCF-7 cells was rather slower than that which occurred in the cytosol of PYS cells. However, in both cell types, the oxidative inactivation of membrane-associated protein kinase C occurred rapidly in comparison to the enzyme in the cytosol. Prior treatment of cells with phorbol ester to induce membrane association (stabilization) of protein kinase C, followed by exposure to H2O2, resulted in increased inactivation of protein kinase C, suggesting that membrane association of protein kinase C increases its susceptibility to oxidative inactivation.

An experimental model for studying the desmoplastic response to tumor invasion

BL6 melanoma cells injected subcutaneously in 18-month-old syngeneic C57BL/6 mice elicit a marked fibrotic response highly similar in myofibroblast composition and Type V collagen content to that characterizing the desmoplastic response of human carcinomas. This host response can be quantitated in vivo by measuring both hydroxyproline content (total collagen) and incorporation of intraperitoneally injected [14C] proline into collagenase-sensitive protein (new collagen synthesis). 70% inhibition of the response can be achieved with daily L-3,4-dehydroproline. The response can be similarly quantitated in vitro in explants of desmoplastic tumor tissue. The model allows for subsequent investigations of the effects of the desmoplastic response on tumor invasion and metastasis.

Increased invasion and spontaneous metastasis of BL6 melanoma with inhibition of the desmoplastic response in C57 BL/6 mice

BL6 melanoma cells injected s.c. in 18-month C57 BL/6 mice elicit a markedly fibrotic response similar in myofibroblast and collagen composition to that characterizing the desmoplastic response of human breast carcinoma. This host response can be quantitated by measuring hydroxyproline (total collagen) and incorporation of i.p.-injected [14C]proline into collagenase-sensitive protein (new collagen synthesis). Inhibition (70%) of the desmoplastic response can be achieved by daily injections of L-3,4-dehydroproline. Inhibiting the response in this manner promotes local invasion of tumor and increases the incidence of spontaneous pulmonary metastasis. 10(5) BL6 melanoma cells produce tumor nodules with a mean diameter of 1.5 +/- 0.5 cm and mean collagen content of 36 +/- 15 mg/g wet tissue at 4 weeks and 10% incidence of pulmonary metastasis at 7 weeks. L-3,4-dehydroproline produces nodules with a mean diameter of 2.3 +/- 0.5 cm and mean collagen content of 12 +/- 2 mg/g with a 40% incidence of metastasis. L-3,4-dehydroproline exerts a selective effect on myofibroblast collagen synthesis in vitro and no effect on [3H]thymidine uptake, doubling time, and viability of BL6 cells and myofibroblasts. Furthermore, this drug exerts no effect on BL6 invasion and metastasis in 6-week C57 BL/6 mice, hosts which exhibit a negligible desmoplastic response.

Factors influencing chelator-stable, detergent-extractable, phorbol diester-induced membrane association of protein kinase C. Differences between Ca2+-induced and phorbol ester-stabilized membrane bindings of protein kinase C

One of the early events associated with the treatment of cells by tumor promotor phorbol esters is the tight association of protein kinase C to the plasma membrane. To better understand the factors that regulate this process, phorbol ester-induced membrane binding of protein kinase C was studied using homogenates, as well as isolated membranes and purified enzyme. Addition of 12-O-tetradecanoylphorbol 13-acetate (TPA) to the homogenates of parietal yolk sac cells and NIH 3T3 cells in the presence of Ca2+ resulted in plasma membrane binding of protein kinase C which subsequently remained bound to the membrane independent of Ca2+. Although protein kinase C was activated by TPA in the absence of Ca2+ and by diolein in the presence of Ca2+, both these agents when added to homogenates under these respective conditions had no effect on membrane association of protein kinase C. However, under these conditions relatively weak binding of protein kinase C was found if purified protein kinase C was used with isolated membranes. Binding studies using purified protein kinase C and washed membranes showed that the binding of the TPA-kinase complex to membranes required phospholipids and reached saturation at 0.1 unit (24 ng of protein kinase C)/mg of parietal yolk sac cell membrane protein. Phorbol ester treatment of cells in media with and without Ca2+ showed that the TPA-induced increase in membrane-associated protein kinase C was regulated by Ca2+ levels even in intact cells. TPA-stabilized membrane binding of protein kinase C differs in several aspects from the previously reported Ca2+-induced reversible binding. TPA-stabilized binding of protein kinase C to isolated membranes is temperature dependent, relatively high in the plasma membrane-enriched fraction, saturable at physiological levels of protein kinase C, requires the presence of both membrane protein(s) and phospholipids, and further requires the addition of phospholipid micelles. In contrast, Ca2+-induced reversible binding is more rapid, not appreciably influenced by temperature, not selective for a particular subcellular fraction, not saturable with physiological amounts of protein kinase C, exhibits trypsin-insensitive membrane binding sites, and requires membrane phospholipids but not added phospholipid micelles.

Hydrophobic association of calpains with subcellular organelles. Compartmentalization of calpains and the endogenous inhibitor calpastatin in tissues

Calpains I and II isolated from diverse tissues possess both Ca2+-independent, and Ca2+-dependent accessible hydrophobic regions. Possible subcellular organelle association of calpains involving these hydrophobic regions was studied. By homogenizing rat tissues directly in Ca2+ (50 microM), about 30-60% of the cytosolic calpain I and II activity reversibly associated with isolated subcellular fractions (microsomal greater than plasma membrane greater than nuclear). After binding to the particulate fraction, calpain II converted to a calpain I-like form exhibiting stronger Ca2+-independent binding to phenyl-Sepharose and a lower Ca2+ requirement for optimal activity. However, it retained its DEAE-cellulose chromatographic pattern, and precipitated with monospecific anti-calpain II antibodies. Although purified calpastatin (endogenous inhibitor) is known to form a Ca2+-dependent complex with calpains, it was not able to reverse the binding of calpains to the particulate fraction upon short incubation. It was, however, effective in blocking calpain binding when the isolated cytosolic fraction or a mixture of purified calpain and calpastatin was preincubated in the presence of Ca2+, and then added to the particulate fraction. Extraction of tissues under controlled conditions revealed that in fact calpains are already loosely associated with subcellular organelles even in the absence of Ca2+. This is the reason why in the crude homogenates with the addition of Ca2+, calpains strongly bind to the particulate fraction without interference by cytosolic calpastatin. Although calpastatin by complexing initially to calpain can prevent the association of this protease with subcellular organelles, it cannot dissociate calpains already bound to these subcellular fractions. By prior Ca2+-independent association with the hydrophobic proteins present in the subcellular fractions, calpains overcome the 3- to 30-fold inhibitory excess of calpastatin in tissues.