The polyether bistratene A activates protein kinase C-delta and induces growth arrest in HL60 cells

Structural Simplification of Natural Products

Natural products (NPs) are important sources of clinical drugs due to their structural diversity and biological prevalidation. However, the structural complexity of NPs leads to synthetic difficulties, unfavorable pharmacokinetic profiles, and poor drug-likeness. Structural simplification by truncating unnecessary substructures is a powerful strategy for overcoming these limitations and improving the efficiency and success rate of NP-based drug development. Herein, we will provide a comprehensive review of the structural simplification of NPs with a focus on design strategies, case studies, and new technologies. In particular, a number of successful examples leading to marketed drugs or drug candidates will be discussed in detail to illustrate how structural simplification is applied in lead optimization of NPs.

Ascidian Toxins with Potential for Drug Development

Ascidians (tunicates) are invertebrate chordates, and prolific producers of a wide variety of biologically active secondary metabolites from cyclic peptides to aromatic alkaloids. Several of these compounds have properties which make them candidates for potential new drugs to treat diseases such as cancer. Many of these natural products are not produced by the ascidians themselves, rather by their associated symbionts. This review will focus mainly on the mechanism of action of important classes of cytotoxic molecules isolated from ascidians. These toxins affect DNA transcription, protein translation, drug efflux pumps, signaling pathways and the cytoskeleton. Two ascidian compounds have already found applications in the treatment of cancer and others are being investigated for their potential in cancer, neurodegenerative and other diseases.

Regulation of Thrombin-Induced Lung Endothelial Cell Barrier Disruption by Protein Kinase C Delta

Protein Kinase C (PKC) plays a significant role in thrombin-induced loss of endothelial cell (EC) barrier integrity; however, the existence of more than 10 isozymes of PKC and tissue-specific isoform expression has limited our understanding of this important second messenger in vascular homeostasis. In this study, we show that PKCδ isoform promotes thrombin-induced loss of human pulmonary artery EC barrier integrity, findings substantiated by PKCδ inhibitory studies (rottlerin), dominant negative PKCδ construct and PKCδ silencing (siRNA). In addition, we identified PKCδ as a signaling mediator upstream of both thrombin-induced MLC phosphorylation and Rho GTPase activation affecting stress fiber formation, cell contraction and loss of EC barrier integrity. Our inhibitor-based studies indicate that thrombin-induced PKCδ activation exerts a positive feedback on Rho GTPase activation and contributes to Rac1 GTPase inhibition. Moreover, PKD (or PKCμ) and CPI-17, two known PKCδ targets, were found to be activated by PKCδ in EC and served as modulators of cytoskeleton rearrangement. These studies clarify the role of PKCδ in EC cytoskeleton regulation, and highlight PKCδ as a therapeutic target in inflammatory lung disorders, characterized by the loss of barrier integrity, such as acute lung injury and sepsis.

Gold-catalyzed diastereoselective synthesis of 2,6-trans-disubstituted tetrahydropyran derivatives: application for the synthesis of the C1–C13 fragment of bistramide A and B

An efficient Au(iii)-catalyzed diastereoselective allylation leading to 2,6-trans-disubstituted tetrahydropyrans and application for the synthesis of the C1–C13 fragment of bistramide A and B is described.

Spiroacetal formation through telescoped cycloaddition and carbon-hydrogen bond functionalization: total synthesis of bistramide A

Spiroacetals can be formed through a one-pot sequence of a hetero-Diels-Alder reaction, an oxidative carbon-hydrogen bond cleavage, and an acid treatment. This convergent approach expedites access to a complex molecular subunit which is present in numerous biologically active structures. The utility of the protocol is demonstrated through its application to a brief synthesis of the actin-binding cytotoxin bistramide A.

Synthesis of a 35-member stereoisomer library of bistramide A: evaluation of effects on actin state, cell cycle and tumor cell growth

Synthesis and preliminary biological evaluation of a 35-member library of bistramide A stereoisomers are reported. All eight stereoisomers of the C1-C13 tetrahydropyran fragment of the molecule were prepared utilizing crotylsilane reagents 9 and 10 in our [4+2]-annulation methodology. In addition, the four isomers of the C14-C18 gamma-amino acid unit were accessed via a Lewis acid mediated crotylation reaction with use of both enantiomers of organosilane 11. The spiroketal subunit of bistramide A was modified at the C39-alcohol to give another point of stereochemical diversification. The fragments were coupled by using a standard peptide coupling protocol to provide 35 stereoisomers of the natural product. These stereochemical analogues were screened for their effects on cellular actin and cytotoxicity against cancer cell lines (UO-31 renal and SF-295 CNS). The results of these assays identified one analogue, 1.21, with enhanced potency relative to the natural product, bistramide A.

Phenolic fraction of tobacco smoke condensate potentiates benzo[a]pyerene diol epoxide-induced cell transformation: role of protein kinase C

In this study we separated weakly acidic phenolic components from other neutral, acidic and basic components of tobacco smoke condensate (TSC) and observed that phenolic fraction of TSC significantly increased the number of colonies of promotion-sensitive JB6 Cl41 cells that showed anchorage-independent growth on soft agar in response to BPDE (an ultimate carcinogen produced by metabolic activation of the PAH benzo[a]pyrene). Anchorage-independent cell growth is indicative of cell transformation resulting in acquisition of tumorigenic potential. In order to understand the underlying mechanism by which TSC phenolic fraction potentiates BPDE-induced tumorigenicity, we examined its effect on the activation of two transcription factors AP-1 and NF-kappaB which are known to be influenced by established tumor promoter TPA. BPDE treatment caused induction of both AP-1 and NF-kappaB activity as determined by luciferase reporter assay and only NF-kappaB induction in response to BPDE was significantly attenuated by TSC phenolic fraction whereas AP-1 induction remains unaltered. Attenuation of NF-kappaB activation by TSC phenolic fraction was associated with significant decrease of intracellular PKC substrate phosphorylation in BPDE treated cells. Non-specific PKC inhibitors staurosporine and bisindolylmaleimide II as well as inhibitors specific to conventional PKCs (Go6976) and PKC-delta (rottlerin) attenuated NF-kappaB activation in BPDE treated cells to a varying degree indicating a possible link between PKC down-regulation and the attenuation of NF-kappaB activity by TSC phenolic fraction. Treatment of cells with PKC inhibitors also potentiated anchorage-independent growth of BPDE treated cells on soft agar. Our data suggest a possible role of PKC down-regulation in potentiation of BPDE-induced tumorogenicity by TSC phenolic fraction.

Protein Kinase C-Mediated Modulation of Glutamate Transporter Activity in Rat Retina

It has previously been shown that inhibitors of protein kinase C (PKC) attenuate retinal glutamate uptake in situ. The aim of the current study was to determine whether PKCdelta-mediated inhibition differentially reduces the transport of glutamate into retinal Müller cells when compared with retinal neurons. The influence of two different types of PKC inhibitors on the uptake of [3H]D-aspartate was therefore compared in the intact retina, mixed retinal cultures, and Müller cell-enriched retinal cultures. It was found that 25 microM of the pan-isoform PKC inhibitor, chelerythrine, reduced [3H]D-aspartate uptake by 78%, 71%, and 68% in isolated retinas, mixed neuronal/glial cultures, and Müller cell-enriched cultures, respectively. Importantly, 20 microM of the PKCdelta-selective inhibitor rottlerin also reduced the uptake of D-aspartate to similar extents in all three systems, and the reductions were statistically similar to those found for the pan-specific PKC inhibitor. Neither pan-isoform nor PKCdelta-selective activators stimulated glutamate uptake in either culture system or the intact retina. The current results suggest that specific PKC inhibitors are quantitatively similar in reducing the uptake of glutamate into retinal neurons and Müller cells.

Toxins affecting actin filaments and microtubules

Actin and tubulin are the two major proteins of the cytoskeleton in eukaryotic cells and both display a common property to reversibly assemble into long and flexible polymers, actin filaments and microtubules, respectively. These proteins play important roles in a variety of cellular functions and are also involved in numbers of diseases. An emerging number of marine-derived cytotoxins have been found to bind either actin or tublin, resulting in either inhibition or enhancement of polymerization. Thus, these toxins are valuable molecular probes for solving complex mechanisms of biological processes. This chapter describes actin- and tubulin-targeting marine natural products and their modes of action, with reference to their use as research tools and their clinical applications.

Protein kinase Cdelta mediates cyclic adenosine monophosphate-stimulated translocation of sodium taurocholate cotransporting polypeptide and multidrug resistant associated protein 2 in rat hepatocytes

Cyclic adenosine monophosphate (cAMP) stimulates translocation of Na(+)-taurocholate (TC) cotransporting polypeptide (Ntcp) and multidrug resistant associated protein 2 (Mrp2) to the plasma membrane. Because cAMP activates phosphoinositide-3-kinase (PI3K) and protein kinase C (PKC) activation is PI3K-dependent, the aim of the current study was to determine whether cAMP activates conventional and novel PKCs in hepatocytes and whether such activation plays a role in cAMP-stimulated Ntcp and Mrp2 translocation. The effect of cAMP on PKCs, TC uptake, and Ntcp and Mrp2 translocation was studied in isolated rat hepatocytes using a cell-permeable cAMP analog, CPT-cAMP. The activity of PKCs was assessed from membrane translocation of individual PKCs, and phospho-specific antibodies were used to determine PKCdelta phosphorylation. TC uptake was determined from time-dependent uptake of (14)C-TC, and a cell surface biotinylation method was used to determine Ntcp and Mrp2 translocation. CPT-cAMP stimulated nPKCdelta but not cPKCalpha or nPKCepsilon, and induced PI3K-dependent phosphorylation of nPKCdelta at Thr(505). Rottlerin, an inhibitor of nPKCdelta, inhibited cAMP-induced nPKCdelta translocation, TC uptake, and Ntcp and Mrp2 translocation. Bistratene A, an activator of nPKCdelta, stimulated nPKCdelta translocation, TC uptake, and Ntcp and Mrp2 translocation. The effects of cAMP and bistratene A on TC uptake and Ntcp and Mrp2 translocation were not additive.

CONCLUSION

These results suggest that cAMP stimulates Ntcp and Mrp2 translocation, at least in part, by activating nPKCdelta via PI3K-dependent phosphorylation at Thr(505).

TPA-induced multinucleation of a mesenchymal stem cell-like clone is mediated primarily by karyokinesis without cytokinesis, although cell-cell fusion also occurs

The 5F9A cell, which is a mesenchymal stem cell-like clone established from rat bone marrow substrate adherent cells, can differentiate into adipocytes and osteoblasts in vitro under the appropriate conditions. Multinucleated cells could be also induced by 12-O-tetradecanoylphorbol 13-acetate (TPA) in 5F9A cells. This effect was mediated by protein kinase C. Possible mechanisms of multinucleation by TPA were hypothesized to be either karyokinesis without cytokinesis or cell-cell fusion. By observation using time-lapse phase-contrast microscopy, we determined that the multinucleated cells were generated mainly by karyokinesis without cytokinesis. Cell fusion was studied using time-lapse photography, and confocal laser scanning microscopy using two differentially labeled cells. These techniques demonstrated that multinucleated 5F9A cells could be produced by cell fusion, albeit at a low frequency. We conclude that multinucleated 5F9A cells are formed primarily by karyokinesis without cytokinesis, although some cells are also formed by cell-cell fusion.

Total Synthesis of Bistramide A

An asymmetric synthesis of the marine metabolite bistramide A is reported. The synthesis relies on the utility of three different organosilane reagents to construct all principle fragments and 8 of the 11 stereogenic centers of the natural product. [structure: see text].

Nuclear protein kinase C-delta: a possible check-point of cell cycle progression

Protein kinase Cs (PKCs) belong to a serine/threonine kinase family, ubiquitously expressed and claimed to be involved in physiological processes including apoptosis, cell growth and differentiation. The question of the subcellular localization and activity of PKCs remains to be clarified. Here we report that nuclear PKC-delta cooperates to regulate the S-G2/M phase transition of cell cycle, apparently being associated to chromosome condensation and alignment on the metaphase plate.

Involvement of protein kinase C delta in the alteration of mitochondrial mass in human cells under oxidative stress

Alteration of mitochondrial mass of human 143B osteosarcoma cells upon exposure to hydrogen peroxide (H(2)O(2)) was investigated. We found that mitochondrial mass and the intracellular level of H(2)O(2) were increased by exogenous H(2)O(2), which was accompanied with up-regulation of functional PKCdelta. To investigate the role of PKCdelta in H(2)O(2)-induced increase of mitochondrial mass, we treated 143B cells with PKCdelta activator, bistratene A, and PKCdelta inhibitor, rottlerin, respectively. The results show that bistratene A caused an increase of mitochondrial mass and that the H(2)O(2)-induced increase of mitochondrial mass was completely suppressed by rottlerin. Furthermore, we found that activation of PKCdelta by bistratene A increased the intracellular levels of H(2)O(2) and MnSOD protein expression. By contrast, suppression of PKCdelta by rottlerin decreased the intracellular levels of H(2)O(2) and MnSOD protein expression. Moreover, we noted that MnSOD expression was highly correlated with the expression of p53, which was controlled by PKCdelta. Finally, we demonstrated that PKCdelta was overexpressed in skin fibroblasts of patients with MERRF syndrome. Taken together, we conclude that PKCdelta is involved in the regulation of mitochondrial mass and intracellular H(2)O(2) in human cells and may play a key role in the overproliferation of mitochondria in the affected tissues of patients with mitochondrial diseases such as MERRF syndrome.

1Alpha,25-dihydroxyvitamin D3-mediated stimulation of steroid sulphatase activity in myeloid leukaemic cell lines requires VDRnuc-mediated activation of the RAS/RAF/ERK-MAP kinase signalling pathway

1Alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) stimulates the activity of steroid sulphatase (STS) in myeloid cells [Hughes et al., 2001, 2005]. This was attenuated by inhibitors of phospholipase D (PLD) (n-butanol, 2,3-diphosphoglyceric acid, C(2)-ceramide) and phosphatidate phosphohydrolase (PAP) (propranolol and chlorpromazine), but was unaffected by inhibitors of phospholipase C. The 1alpha,25(OH)(2)D(3)-induced STS activity was also attenuated by inhibitors of protein kinase Calpha and protein kinase Cdelta (Go 6976, HBDDE and rottlerin), but not by an inhibitor of protein kinase Cbeta (LY379196). Additionally, 1alpha,25(OH)(2)D(3)-induced STS activity was attenuated by inhibitors of RAS (manumycin A), RAF (GW5074), MEK (PD098059 and U1026) and JNK (SP600125), but not p38 (PD169316). 1alpha,25(OH)(2)D(3) produced a rapid and long lasting stimulation of the ERK-MAP kinase signalling cascade in HL60 myeloid leukaemic cells. This 'non-genomic' effect of 1alpha,25(OH)(2)D(3) blocked by pharmacological antagonists of nuclear vitamin D receptors (VDR(nuc)) and does not appear to require hetero-dimerisation with the retinoid-X receptor (RXR). Inhibitors of the Src tyrosine kinase (PP1), RAS (manumycin A), RAS-RAF interactions (sulindac sulphide and RAS inhibitory peptide), RAF (GW5074 or chloroquine), and protein kinase Calpha (HBDDE) abrogated the 1alpha,25(OH)(2)D(3)-stimulated increase in ERK-MAP kinase activity. Taken together, these results show that 1alpha,25(OH)(2)D(3)/VDR(nuc) activation of the RAS/RAF/ERK-MAP kinase signalling pathway plays an important role in augmenting STS activity in human myeloid leukaemic cell lines.

Retinoid-mediated stimulation of steroid sulfatase activity in myeloid leukemic cell lines requires RARalpha and RXR and involves the phosphoinositide 3-kinase and ERK-MAP kinase pathways

All-trans retinoic acid and 9-cis-retinoic acid stimulate the activity of steroid sulfatase in HL60 acute myeloid leukemia cells in a concentration- and time-dependent manner. Neither of these 'natural retinoids' augmented steroid sulfatase activity in a HL60 sub-line that expresses a dominant-negative retinoic acid receptor alpha (RARalpha). Experiments with synthetic RAR and RXR agonists and antagonists suggest that RARalpha/RXR heterodimers play a role in the retinoid-stimulated increase in steroid sulfatase activity. The retinoid-driven increase in steroid sulfatase activity was attenuated by inhibition of phospholipase D (PLD), but not by inhibitors of phospholipase C. Experiments with inhibitors of protein kinase C (PKC) show that PKCalpha and PKCdelta play an important role in modulating the retinoid-stimulation of steroid sulfatase activity in HL60 cells. Furthermore, we show that pharmacological inhibition of the RAF-1 and ERK MAP kinases blocked the retinoid-stimulated increase in steroid sulfatase activity in HL60 cells and, by contrast, inhibition of the p38-MAP kinase or JNK-MAP kinase had no effect. Pharmacological inhibitors of the phosphatidylinositol 3-kinase, Akt, and PDK-1 also abrogated the retinoid-stimulated increase in steroid sulfatase activity in HL60 cells. These results show that crosstalk between the retinoid-stimulated genomic and non-genomic pathways is necessary to increase steroid sulfatase activity in HL60 cells.

Protein kinase C delta activates topoisomerase IIalpha to induce apoptotic cell death in response to DNA damage

DNA topoisomerase II is an essential nuclear enzyme that modulates DNA processes by altering the topological state of double-stranded DNA. This enzyme is required for chromosome condensation and segregation; however, the regulatory mechanism of its activation is largely unknown. Here we demonstrate that topoisomerase IIalpha is activated in response to genotoxic stress. Concomitant with the activation, the expression of topoisomerase IIalpha is increased following DNA damage. The results also demonstrate that the proapoptotic kinase protein kinase C delta (PKCdelta) interacts with topoisomerase IIalpha. This association is in an S-phase-specific manner and is required for stabilization and catalytic activation of topoisomerase IIalpha in response to DNA damage. Conversely, inhibition of PKCdelta activity attenuates DNA damage-induced activation of topoisomerase IIalpha. Finally, aberrant activation of topoisomerase IIalpha by PKCdelta is associated with induction of apoptosis upon exposure to genotoxic agents. These findings indicate that PKCdelta regulates topoisomerase IIalpha and thereby cell fate in the genotoxic stress response.

Activation of protein kinase C delta induces growth arrest in NPA thyroid cancer cells through extracellular signal-regulated kinase mitogen-activated protein kinase

Protein kinase C (PKC) is a family of serine-threonine kinases that regulate many cell processes. To study the role of PKCdelta in thyroid cancer cells, we used a replication-deficient adenovirus (PKCdeltaAdV), to tightly control PKCdelta expression. In NPA cells, activation of wild-type (WT) PKCdelta with phorbol 12-myristate 13-acetate (PMA) induced an arrest in cell growth at G(1) phase, which was itself inhibited by the PKCdelta inhibitor rottlerin. Furthermore, overexpression of a dominant negative PKCdelta did not induce G(1) arrest. These findings strongly suggested that PKCdelta induced cell growth arrest in NPA cells. We investigated the mechanism of G1 arrest by examining G(1)-related proteins and mitogen-activated protein kinase (MAPK) by Western blotting. After activation of WTPKCdelta with PMA, cyclin E expression and retinoblastoma protein (Rb) phosphorylation decreased; the expression of p27(Kip1) increased and the phosphorylation of extracellular signal-regulated kinase (ERK) MAPK decreased. These results indicated that the activation of PKCdelta induced cell growth arrest in NPA cells, through an ERK MAPK-p27(Kip1)-cyclin E-pRb pathway. PKCdelta may therefore be an effective molecular target for novel therapy in thyroid cancer.

Structure of Bistramide A−Actin Complex at a 1.35 Å Resolution

Bistramide A is a highly potent antiproliferative marine natural product from Lissoclinum bistratum. We have previously established actin as the primary cellular receptor of bistramide A. We report herein the X-ray structure of bistramide A bound to monomeric actin at a resolution of 1.35 A. The most notable aspect of the bistramide A-actin structure is an extensive hydrogen-bonding network established upon a deep penetration of the central segment of bistramide A into the actin-binding cleft between subdomains 1 and 3. The structure presents the first insight into the observed ability of bistramide A to modulate G-actin polymerization. The structural information combined with our ability to chemically modify the bistramide framework provides the basis for rational development of a series of new synthetic analogues as useful probes for studying actin cytoskeleton and as potential therapeutic leads.

The proapoptotic tumor suppressor protein kinase C-delta is lost in human squamous cell carcinomas

Protein kinase C (PKC)-delta is proapoptotic in human keratinocytes, and is downregulated or inactivated in keratinocytes expressing the activated Ha-ras oncogene, making it a candidate tumor suppressor gene for squamous cell carcinoma (SCC). We evaluated the significance of PKC-delta loss in transformed human keratinocytes using tumorigenic HaCaT Ras II-4 cells that have significantly reduced PKC-delta levels. Re-expression of PKC-delta by retrovirus transduction caused an increase in apoptosis and growth inhibition in culture. The growth inhibition induced by PKC-delta could be partially reversed by Bcl-x(L) expression, indicating that apoptosis was in part responsible for PKC-delta-induced growth inhibition. PKC-delta re-expression suppressed the tumorigenicity of HaCaT Ras II-4 cells in nude mice (P<0.05), and the small tumors that did form contained elevated levels of activated caspase-3, indicating increased apoptosis. In addition, we found that 29% (12/42) of human Bowen's disease (squamous carcinoma in situ) or SCC cases had absent or reduced PKC-delta when compared to the surrounding normal epidermis. These results indicate that PKC-delta inhibits transformed keratinocyte growth by inducing apoptosis, and that PKC-delta may function as a tumor suppressor in human SCCs where its loss in cells harboring activated ras could provide a growth advantage by conferring resistance to apoptosis.

Actin is the primary cellular receptor of bistramide A

Bistramide A (1) is a marine natural product with broad, potent antiproliferative effects. Bistramide A has been reported to selectively activate protein kinase C (PKC) delta, leading to the view that PKCdelta is the principal mediator of antiproliferative activity of this natural product. Contrary to this observation, we established that bistramide A binds PKCdelta with low affinity, does not activate this kinase in vitro and does not translocate GFP-PKCdelta. Furthermore, we identified actin as the cellular receptor of bistramide A. We report that bistramide A disrupts the actin cytoskeleton, inhibits actin polymerization, depolymerizes filamentous F-actin in vitro and binds directly to monomeric G-actin in a 1:1 ratio with a Kd of 7 nM. We also constructed a fully synthetic9 bistramide A-based affinity matrix and isolated actin as a specific bistramide A-binding protein. This activity provides a molecular explanation for the potent antiproliferative effects of bistramide A, identifying it as a new biochemical tool for studies of the actin cytoskeleton and as a potential lead for development of a new class of antitumor agents.

Rottlerin inhibits stimulated enzymatic secretion and several intracellular signaling transduction pathways in pancreatic acinar cells by a non-PKC-delta-dependent mechanism

Protein kinase C-delta (PKC-delta) becomes activated in pancreatic acini in response to cholecystokinin (CCK) and plays a pivotal role in the exocrine pancreatic secretion. Rottlerin, a polyphenolic compound, has been widely used as a potent and specific PKC-delta inhibitor. However, some recent studies showed that rottlerin was not effective in inhibiting PKCdelta activity in vitro and that may display unspecific effects. The aims of this work were to investigate the specificity of rottlerin as an inhibitor of PKC-delta activity in intact cells and to elucidate the biochemical causes of its unspecificity. Preincubation of pancreatic acini with rottlerin (6 microM) inhibited CCK-stimulated translocation, tyrosine phosphorylation (TyrP) and activation of PKC-delta in pancreatic acini in a time-dependent manner. Rottlerin inhibited amylase secretion stimulated by both PKC-dependent pathways (CCK, bombesin, carbachol, TPA) and also by PKC-independent pathways (secretin, VIP, cAMP analogue). CCK-stimulation of MAPK activation and p125(FAK) TyrP which are mediated by PKC-dependent and -independent pathways were also inhibited by rottlerin. Moreover, rottlerin rapidly depleted ATP content in pancreatic acini in a similar way as the mitochondrial uncouplers CCCP and FCCP. All studied inhibitory effects of rottlerin in pancreatic acini were mimicked by FCCP (agonists-stimulated amylase secretion, p125(FAK) TyrP, MAPK activation and PKC-delta TyrP and translocation). Finally, rottlerin as well as FCCP display a potent inhibitory effect on the activation of other PKC isoforms present in pancreatic acini. Our results suggest that rottlerin effects in pancreatic acini are not due to a specific PKC-delta blockade, but likely due to its negative effect on acini energy resulting in ATP depletion. Therefore, to study the role of PKC-delta in cellular processes using rottlerin it is essential to keep in mind that may deplete ATP levels and inhibit different PKC isoforms. Our results give reasons for a more careful choice of rottlerin for PKC-delta investigation.

PEP005, a selective small-molecule activator of protein kinase C, has potent antileukemic activity mediated via the delta isoform of PKC

Ingenol 3-angelate (PEP005) is a selective small molecule activator of protein kinase C (PKC) extracted from the plant Euphorbia peplus, whose sap has been used as a traditional medicine for the treatment of skin conditions including warts and cancer. We report here that PEP005 also has potent antileukemic effects, inducing apoptosis in myeloid leukemia cell lines and primary acute myeloid leukemia (AML) cells at nanomolar concentrations. Of importance, PEP005 did not induce apoptosis in normal CD34+ cord blood myeloblasts at up to 2-log concentrations higher than those required to induce cell death in primary AML cells. The effects of PEP005 were PKC dependent, and PEP005 efficacy correlated with expression of PKC-delta. The delta isoform of PKC plays a key role in apoptosis and is therefore a rational potential target for antileukemic therapies. Transfection of KG1a leukemia cells, which did not express PKC-delta or respond to PEP005, with enhanced green fluorescent protein (EGFP)-PKC-delta restored sensitivity to induction of apoptosis by PEP005. Our data therefore suggest that activation of PKC-delta provides a novel approach for treatment of acute myeloid leukemia and that screening for PKC-delta expression may identify patients for potential responsiveness to PEP005. (Blood. 2005;106:1362-1368)

Protein kinase C delta is not activated by caspase-3 and its inhibition is sufficient to induce apoptosis in the colon cancer line, COLO 205

Activation of protein kinase C delta (PKCdelta) is believed to be pro-apoptotic. PKCdelta is reported to be reduced in colon cancers. Using a colon cancer cell line, COLO 205, we have examined the roles of PKCdelta in apoptosis and of caspase-3 in the activation and inhibition of PKCdelta. PKCdelta activation with bistratene A and its inhibition with rottlerin induced apoptosis. Effects of PKC activators and inhibitors were additive, suggesting that PKCdelta down-regulation was responsible for the effects on apoptosis. Different apoptotic pathways induced PKCdelta cleavage, but the fragment produced was inactive in kinase assays. Caspase-3 inhibition did not block DNA fragmentation or PKCdelta proteolysis despite blocking intracellular caspase-3 activity. Calpain inhibition with calpeptin did not prevent TPA-induced PKCdelta cleavage. We conclude that in colonocytes, inhibition of PKCdelta is sufficient to lead to caspase-3-independent apoptosis. Caspase-3 does not cleave PKCdelta to an active form, nor does caspase-3 inhibition block apoptosis.

PKCdelta and MAPK mediate G(1) arrest induced by PMA in SKBR-3 breast cancer cells

The effects of activating endogenous protein kinase C (PKC) on cell proliferation and the cell cycle were investigated by treating the breast cancer cell line SKBR-3 with phorbol 12-myristate 13 acetate (PMA). This inhibited cell growth in a concentration-dependent manner, causing a marked arrest of cells in G(1). Pre-treatment with GF109203X completely blocked the antiproliferative effect of PMA, and pre-treatment with the PKCdelta inhibitor rottlerin partially blocked it. Infecting SKBR-3 cells with an adenovirus vector containing wild-type PKCdelta, WTPKCdeltaAdV, had similar effects on PMA. Infecting the cells with a dominant-negative PKCdeltaAdV construct blocked the growth inhibition induced by PMA. Downstream of PKC, PMA treatment inhibited extracellular signal-regulated kinase mitogen-activated protein kinase phosphorylation, up-regulated c-jun NH(2)-terminal kinase phosphorylation, and inhibited retinoblastoma (Rb) phosphorylation. These results strongly implicated PKC (mainly PKCdelta) in the G(1) arrest induced by PMA and suggested PKC as a target for breast cancer treatment.

Role of protein kinase C delta in reactivation of Kaposi's sarcoma-associated herpesvirus

TPA (12-O-tetradecanoylphorbol-13-acetate), a well-known activator of protein kinase C (PKC), can experimentally induce reactivation of Kaposi's sarcoma-associated herpesvirus (KSHV) in certain latently infected cells. We selectively blocked the activity of PKC isoforms by using GF 109203X or rottlerin and demonstrated that this inhibition largely decreased lytic KSHV reactivation by TPA. Translocation of the PKCdelta isoform was evident shortly after TPA stimulation. Overexpression of the dominant-negative PKCdelta mutant supported an essential role for the PKCdelta isoform in virus reactivation, yet overexpression of PKCdelta alone was not sufficient to induce lytic reactivation of KSHV, suggesting that additional signaling molecules participate in this pathway.

Protein kinase C delta blocks immediate-early gene expression in senescent cells by inactivating serum response factor

Fibroblasts lose the ability to replicate in response to growth factors and become unable to express growth-associated immediate-early genes, including c-fos and egr-1, as they become senescent. The serum response factor (SRF), a major transcriptional activator of immediate-early gene promoters, loses the ability to bind to the serum response element (SRE) and becomes hyperphosphorylated in senescent cells. We identify protein kinase C delta (PKC delta) as the kinase responsible for inactivation of SRF both in vitro and endogenously in senescent cells. This is due to a higher level of PKC delta activity as cells age, production of the PKC delta catalytic fragment, and its nuclear localization in senescent but not in low-passage-number cells. The phosphorylation of T160 of SRF by PKC delta in vitro and in vivo led to loss of SRF DNA binding activity. Both the PKC delta inhibitor rottlerin and ectopic expression of a dominant negative form of PKC delta independently restored SRE-dependent transcription and immediate-early gene expression in senescent cells. Modulation of PKC delta activity in vivo with rottlerin or bistratene A altered senescent- and young-cell morphology, respectively. These observations support the idea that the coordinate transcriptional inhibition of several growth-associated genes by PKC delta contributes to the senescent phenotype.

Synthesis of Bistramide A

We have developed an efficient and highly stereocontrolled synthesis of bistramide A, a selective activator of protein kinase C isotype delta. Our synthetic strategy featured a novel bidirectional approach for spiroketal construction based on the ring-opening/cross-metathesis sequence employing a highly strained cyclopropenone acetal. The synthesis afforded the final target with the longest linear sequence of 15 steps and provided unambiguous structural determination of bistramide A, including assignment of the previously unknown C(37) stereochemistry.

The enigmatic protein kinase Cdelta: complex roles in cell proliferation and survival

Protein kinase Cdelta (PKCdelta) has been implicated both as a tumor suppressor and a positive regulator of cell cycle progression. PKCdelta has also been reported to positively and negatively regulate apoptotic programs. This has led to conflicting hypotheses on the role of PKCdelta in the control of cell proliferation and survival. Surprisingly, PKCdelta mice develop normally and are fertile, indicating that PKCdelta is not critical for normal cell proliferation during development. However, PKCdelta may play important roles in neoplastic cell proliferation. In this review, we have summarized the apparent multifunctional properties of this enigmatic protein with regard to its role in the regulation of cell cycle progression and cell survival. It is proposed that PKCdelta has both tumor suppressor and proliferation capabilities that can be recruited as a backup kinase for both gatekeeper tumor suppression and as an activator of the Ras/Raf/MEK/MAP kinase signaling pathway in cell proliferation.

Regulatory role of diacylglycerol kinase gamma in macrophage differentiation of leukemia cells

Although nine diacylglycerol kinase (DGK) isozymes have been identified, our knowledge of their individual functions is still limited. Here we report that the levels of DGKgamma mRNA/protein in human leukemia HL-60 and U937 cells were rapidly and markedly decreased upon cellular differentiation into macrophages. In contrast, the enzyme expression remained almost unchanged in granulocytic differentiation pathway. Interestingly, the overexpression of wild-type or constitutively active DGKgamma, but not its kinase-dead mutant, markedly inhibited phorbol ester-induced cell attachment and nonspecific esterase activity, which are hallmarks of macrophage differentiation. We noted in this case that no effects were observed for the corresponding constructs of a closely related isozyme, DGKalpha. Prior to the cell attachment, phorbol ester induced translocation of DGKgamma from the cytoplasm to the cell periphery, resulting in its co-localization with F-actin together with protein kinase Cdelta. The results suggest that DGKgamma negatively regulates macrophage differentiation through its catalytic action operating on the cytoskeleton.

Opposing effects of butyrate and bile acids on apoptosis of human colon adenoma cells: differential activation of PKC and MAP kinases

Butyrate, produced in the colon by fermentation of dietary fibre, induces apoptosis in colon adenoma and cancer cell lines, which may contribute to protection against colorectal cancer. However, butyrate is present in the colon along with other dietary factors, including unconjugated bile acids, which are tumour promoters. We have shown previously that the proapoptotic effects of butyrate on AA/C1 human adenoma cells were reduced in the presence of bile acids. To determine the cellular basis of this interaction, we examined the effects of butyrate and the secondary bile acid ursodeoxycholic acid (UDCA) on signalling pathways known to regulate apoptosis using AA/C1 cells. Butyrate activated PKC-delta and p38 MAP (mitogen-activated protein) kinase, whereas UDCA activated PKC-alpha and p42/44 MAP kinase. Butyrate treatment also resulted in the caspase-3-mediated proteolysis of PKC-delta. Butyrate-induced apoptosis was reduced by inhibitors of PKC-delta (Rottlerin), p38 MAP kinase (SB202190) and caspase 3 (DEVD-fmk), whereas the proliferative/survival effects of UDCA were blocked by inhibitors of PKC-alpha (Gö6976) and MEK 1 (PD98059). The effects of butyrate and bile acids are therefore mediated by the differential activation of signalling pathways that are known to regulate apoptosis.

Biomic study of human myeloid leukemia cells differentiation to macrophages using DNA array, proteomic, and bioinformatic analytical methods

A biomic approach by integrating three independent methods, DNA microarray, proteomics and bioinformatics, is used to study the differentiation of human myeloid leukemia cell line HL-60 into macrophages when induced by 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Analysis of gene expression changes at the RNA level using cDNA against an array of 6033 human genes showed that 5950 (98.6%) of the genes were expressed in the HL-60 cells. A total of 624 genes (10.5%) were found to be regulated during HL-60 cell differentiation. Most of these genes have not been previously associated with HL-60 cells and include genes encoded for secreted proteins as well as genes involved in cell adhesion, signaling transduction, and metabolism. Protein analysis using two-dimensional gel electrophoresis showed a total of 682 distinct protein spots; 136 spots (19.9%) exhibited quantitative changes between HL-60 control and macrophages. These differentially expressed proteins were identified by mass spectrometry. We developed a bioinformatics program, the Bulk Gene Search System (BGSS, http://www.sinica.edu.tw:8900/perl/genequery.pl) to search for the functions of genes and proteins identified by cDNA microarrays and proteomics. The identified regulated proteins and genes were classified into seven groups according to subcellular locations and functions. This powerful holistic biomic approach using cDNA microarray, proteomics coupled to bioinformatics can provide in-depth information on the impact and importance of the regulated genes and proteins for HL-60 differentiation.

Mechanism of 17-beta-estradiol-induced Erk1/2 activation in breast cancer cells. A role for HER2 AND PKC-delta

Activation of mitogen-activated protein kinase (Erk/MAPK) is a critical signal transduction event for estrogen (E(2))-mediated cell proliferation. Recent studies from our group and others have shown that persistent activation of Erk plays a major role in cell migration and tumor progression. The signaling mechanism(s) responsible for persistent Erk activation are not fully characterized, however. In this study, we have shown that E(2) induces a slow but persistent activation of Erk in MCF-7 breast carcinoma cells. The E(2)-induced Erk activation is dependent on new protein synthesis, suggesting that E(2)-induced growth factors play a major role in Erk activation. When MCF-7 cells were treated with E(2) in the presence of an anti-HER-2 monoclonal antibody (herceptin), 60-70% of E(2)-induced Erk activation is blocked. In addition, when untreated MCF-7 cells were exposed to conditioned medium from E(2)-treated cells, Erk activity was significantly enhanced. Furthermore Erk activity was blocked by an antibody against HER-2 or by heregulin (HRG) depletion from the conditioned medium through immunoprecipitation. In contrast, epidermal growth factor receptor (Ab528) antibody only blocked 10-20% of E(2)-induced Erk activation, suggesting that E(2)-induced Erk activation is predominantly mediated through the secretion of HRG and activation of HER-2 by an autoctine/paracrine mechanism. Inhibition of PKC-delta-mediated signaling by a dominant negative mutant or the relatively specific PKC-delta inhibitor rottlerin blocked most of the E(2)-induced Erk activation but had no effect on TGF alpha-induced Erk activation. By contrast inhibition of Ras, by inhibition of farnesyl transferase (Ftase-1) or dominant negative (N17)-Ras, significantly inhibited both E(2)- and TGF alpha-induced Erk activation. This evaluation of downstream signaling revealed that E(2)-induced Erk activation is mediated by a HRG/HER-2/PKC-delta/Ras pathway that could be crucial for E(2)-dependent growth-promoting effects in early stages of tumor progression.

Protein kinase C mediates phosphorylation of the regulatory light chain of myosin-II during mitosis

Phosphorylation of the regulatory light chain (RLC) of myosin-II is cell cycle dependent. Early in mitosis the RLC is phosphorylated predominantly on Ser-1/2, while during cytokinesis the primary site of phosphorylation is Ser-19 (Yamakita et al., 1994). To identify candidate kinases likely to mediate the mitotic phosphorylation on Ser-1/2, we assayed RLC kinase activity in mitotic cell extracts and measured apparent steady-state kinetic constants using purified enzymes. The mitotic RLC kinase is distinct from cdc2 kinase, protein kinase A and protein kinase G, as activators or inhibitors specific for these kinases do not affect the mitotic kinase activity. The activity of the mitotic RLC kinase is enhanced by the addition of Ca2+ and DAG and/or phorbol esters, characteristics of a conventional protein kinase C (PKC). Moreover, the PKC inhibitors, Gö6983 and Gö6976, significantly attenuate the phosphorylation of the RLC in mitotic extracts. Apparent steady-state kinetic studies indicate that several PKC isoforms display high specificity for myosin-II. These results suggest that current models describing Ser-1/2 phosphorylation during mitosis need to be re-evaluated.

Diacylglycerol (DAG)-lactones, a new class of protein kinase C (PKC) agonists, induce apoptosis in LNCaP prostate cancer cells by selective activation of PKCalpha

Phorbol esters, the archetypical (PKC) activators, induce apoptosis in androgen-sensitive LNCaP prostate cancer cells. In this study we evaluate the effect of a novel class of PKC ligands, the diacylglycerol (DAG)-lactones, as inducers of apoptosis in LNCaP cells. These unique ligands were designed using novel pharmacophore- and receptor-guided approaches to achieve highly potent DAG surrogates. Two of these compounds, HK434 and HK654, induced apoptosis in LNCaP cells with much higher potency than oleoyl-acetyl-glycerol or phorbol 12,13-dibutyrate. Moreover, different PKC isozymes were found to mediate the apoptotic effect of phorbol 12-myristate 13-acetate (PMA) and HK654 in LNCaP cells. Using PKC inhibitors and dominant negative PKC isoforms, we found that both PKCalpha and PKCdelta mediated the apoptotic effect of PMA, whereas only PKCalpha was involved in the effect of the DAG-lactone. The PKCalpha selectivity of HK654 in LNCaP cells contrasts with similar potencies in vitro for binding and activation of PKCalpha and PKCdelta. Consistent with the differences in isoform dependence in intact cells, PMA and HK654 show marked differences in their abilities to translocate PKC isozymes. Both PMA and HK654 induce a marked redistribution of PKCalpha to the plasma membrane. On the other hand, unlike PMA, HK654 translocates PKCdelta predominantly to the nuclear membrane. Thus, DAG-lactones have a unique profile of activation of PKC isozymes for inducing apoptosis in LNCaP cells and represent the first example of a selective activator of a classical PKC in cellular models. An attractive hypothesis is that selective activation of PKC isozymes by pharmacological agents in cells can be achieved by differential intracellular targeting of each PKC.

Possible involvement of PKC isoforms in signalling placental apoptosis in intrauterine growth retardation

Apoptosis contributes to uteroplacental dsyfunction in intrauterine growth retardation (IUGR). Specific protein kinase C (PKC) isoforms regulate apoptosis in other cell types. PKC isoforms thought to be anti-apoptotic include the conventional PKC isoforms (cPKC-alpha, -beta I and -beta II), whereas the novel PKC isoforms nPKC-delta and nPKC-epsilon may be apoptotic. Dexamethasone administration during the last third of pregnancy in the rat leads to IUGR. We used the dexamethasone model to test the hypothesis that adverse changes in fetal growth might be associated with a modified placental PKC isoform profile. Dexamethasone administered from day 15 of gestation via subcutaneous infusion (osmotic minipump; 100 or 200 microg dexamethasone/kg maternal body wt. per day) induced a dose-dependent decline in placental and fetal body weights at day 21 of gestation. Placental protein expression of all three cPKC isoforms was downregulated by maternal dexamethasone exposure, whereas placental nPKC-epsilon protein expression and activity was significantly upregulated in a dose-dependent manner. These data indicate that IUGR induced by excessive glucocorticoid exposure late in pregnancy leads to changes in the placental PKC isoform profile consistent with the concept that members of the PKC family participate in apoptosis signalling in the placenta.

Involvement of protein kinase Cdelta in contact-dependent inhibition of growth in human and murine fibroblasts

There is evidence that protein kinase C delta (PKCdelta) is a tumor suppressor, although its physiological role has not been elucidated so far. Since important anti-proliferative signals are mediated by cell-cell contacts we studied whether PKCdelta is involved in contact-dependent inhibition of growth in human (FH109) and murine (NIH3T3) fibroblasts. Cell-cell contacts were imitated by the addition of glutardialdehyde-fixed cells to sparsely seeded fibroblasts. Downregulation of the PKC isoforms alpha, delta, epsilon, and mu after prolonged treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA, 0.1 microM) resulted in a significant release from contact-inhibition in FH109 cells. Bryostatin 1 selectively prevented TPA-induced PKCdelta-downregulation and reversed TPA-induced release from contact-inhibition arguing for a role of PKCdelta in contact-inhibition. In accordance, the PKCdelta specific inhibitor Rottlerin (1 microM) totally abolished contact-inhibition. Interestingly, immunofluorescence revealed a rapid translocation of PKCdelta to the nucleus when cultures reached confluence with a peak in early-mid G1 phase. Nuclear translocation of PKCdelta in response to cell-cell contacts could also be demonstrated after subcellular fractionation by Western blotting and by measuring PKCdelta-activity after immunoprecipitation. Transient transfection of NIH3T3 cells with a dominant negative mutant of PKCdelta induced a transformed phenotype. We conclude that PKCdelta is involved in contact-dependent inhibition of growth.

Stimulation of protein kinase C-dependent and -independent signaling pathways by bistratene A in intestinal epithelial cells

The marine toxin bistratene A (BisA) potently induces cytostasis and differentiation in a variety of systems. Evidence that BisA is a selective activator of protein kinase C (PKC) delta implicates PKC delta signaling in the negative growth-regulatory effects of this agent. The current study further investigates the signaling pathways activated by BisA by comparing its effects with those of the PKC agonist phorbol 12-myristate 13-acetate (PMA) in the IEC-18 intestinal crypt cell line. Both BisA and PMA induced cell cycle arrest in these cells, albeit with different kinetics. While BisA produced sustained cell cycle arrest in G(0)/G(1) and G(2)/M, the effects of PMA were transient and involved mainly a G(0)/G(1) blockade. BisA also produced apoptosis in a proportion of the population, an effect not seen with PMA. Both agents induced membrane translocation/activation of PKC, with BisA translocating only PKC delta and PMA translocating PKC alpha, delta, and epsilon in these cells. Notably, while depletion of PKC alpha, delta, and epsilon abrogated the cell cycle-specific effects of PMA in IEC-18 cells, the absence of these PKC isozymes failed to inhibit BisA-induced G(0)/G(1) and G(2)/M arrest or apoptosis. The cell cycle inhibitory and apoptotic effects of BisA, therefore, appear to be PKC-independent in IEC-18 cells. On the other hand, BisA and PMA both promoted PKC-dependent activation of Erk 1 and 2 in this system. Thus, intestinal epithelial cells respond to BisA through activation of at least two signaling pathways: a PKC delta-dependent pathway, which leads to activation of mitogen-activated protein kinase and possibly cytostasis in the appropriate context, and a PKC-independent pathway, which induces both cell cycle arrest in G(0)/G(1) and G(2)/M and apoptosis through as yet unknown mechanisms.

Modulation of protein kinase C in antitumor treatment

PKC isoenzymes were found to be involved in proliferation, antitumor drug resistance and apoptosis. Therefore, it has been tried to exploit PKC as a target for antitumor treatment. PKC alpha activity was found to be elevated, for example, in breast cancers and malignant gliomas, whereas it seems to be underexpressed in many colon cancers. So it can be expected that inhibition of PKC activity will not show similar antitumor activity in all tumors. In some tumors it seems to be essential to inhibit PKC to reduce growth. However, for inhibition of tumor proliferation it may be an advantage to induce apoptosis. In this case an activation of PKC delta should be achieved. The situation is complicated by the facts that bryostatin leads to the activation of PKC and later to a downmodulation and that the PKC inhibitors available to date are not specific for one PKC isoenzyme. For these reasons, PKC modulation led to many contradicting results. Despite these problems, PKC modulators such as miltefosine, bryostatin, safingol, CGP41251 and UCN-01 are used in the clinic or are in clinical evaluation. The question is whether PKC is the major or the only target of these compounds, because they also interfere with other targets. PKC may also be involved in apoptosis. Oncogenes and growth factors can induce cell proliferation and cell survival, however, they can also induce apoptosis, depending on the cell type or conditions in which the cells or grown. PKC participates in these signalling pathways and cross-talks. Induction of apoptosis is also dependent on many additional factors, such as p53, bcl-2, mdm2, etc. Therefore, there are also many contradicting results on PKC modulation of apoptosis. Similar controversial data have been reported about MDR1-mediated multidrug resistance. At present it seems that PKC inhibition alone without direct interaction with PGP will not lead to successful reversal of PGP-mediated drug efflux. One possibility to improve chemotherapy would be to combine established antitumor drugs with modulators of PKC. However, here also very contrasting results were obtained. Many indicate that inhibition, others, that activation of PKC enhances the antiproliferative activity of anticancer drugs. The problem is that the exact functions of the different PKC isoenzymes are not clear at present. So further investigations into the role of PKC isoenzymes in the complex and interacting signalling pathways are essential. It is a major challenge in the future to reveal whether modulation of PKC can be used for the improvement of cancer therapy.

Phorbol ester-induced expression of airway squamous cell differentiation marker, SPRR1B, is regulated by protein kinase Cdelta /Ras/MEKK1/MKK1-dependent/AP-1 signal transduction pathway

The transcriptional induction of SPRR1B by phorbol 12-myristate 13-acetate (PMA) is mainly mediated by the first -152-base pair 5'-flanking region containing two functional AP-1 sites. In this study, we have analyzed the signaling pathways that mediate the induction in tracheobronchial epithelial cells. PKC inhibitor ablated PMA-stimulated expression of endogenous SPRR1B and reporter gene expression driven by SPRR1B promoter. PKC activator promoted the transcription. The dominant negative protein kinase Cdelta (dn-PKCdelta) and rottlerin (PKCdelta inhibitor) completely suppressed PMA-stimulated promoter activity. dn-Ras or dn-MEKK1 inhibited PMA-stimulated promoter activity, while their corresponding constitutively active mutants augmented it. dn-c-Raf-1 did not have any effect on reporter gene expression. Since MEKK1 activates multiple parallel pathways, we examined involvement of JNK/SAPK, p38, and MKK1 in promoter regulation. Co-expression of the dominant negative forms of MKK4, MKK7, JNK/SAPK, MKK3, MKK6, or p38alpha did not suppress PMA-stimulated reporter gene expression. However, MKK1 inhibitors UO126 and PD98095 suppressed gene expression. Consistent with this, expression of dn-MKK1 strongly suppressed PMA-stimulated promoter activity, while the constitutively active MKK1 augmented it. However, MKK1-mediated induction of SPRR1B probably does not depend on extracellular signal-regulated kinases 1 and 2, suggesting the requirement of another kinase(s). dn-c-Jun mutants abolished PMA-stimulated expression supporting an important role for AP-1 proteins in SPRR1B expression. Together, these results suggest that a PKCdelta/Ras/MEKK1/MKK1-dependent/AP-1 pathway regulates the PMA-inducible expression of the SPRR1B in tracheobronchial epithelial cells.

PKC-delta is an apoptotic lamin kinase

Protein kinase C-delta is activated during apoptosis, following proteolytic cleavage by caspase 3. Furthermore, overexpression of the catalytic kinase fragment of PKC-delta induces the nuclear phenotype associated with apoptosis, though the molecular basis of this effect has not been determined. In these studies we have examined the role of PKC-delta in the disassembly of the nuclear lamina at apoptosis. The nuclear lamina is disassembled during mitosis and apoptosis and mitotic disassembly involves hyperphosphorylation of lamin proteins by mitotic lamin kinases. During apoptosis, lamin proteins are degraded by caspase 6 and the contribution made by phosphorylation has not been proven. We show here that protein kinase C-delta co-localized with lamin B during apoptosis and activation of PKC-delta by caspase 3 was concomitant with lamin B phosphorylation and proteolysis. Inhibition of PKC-delta delayed lamin proteolysis, even in the presence of active caspase 6, whilst inhibitors of mitotic lamin kinases were without effect. In addition recombinant human PKC-delta was able to phosphorylate lamin B in vitro suggesting that its actions are direct and not via an intermediary kinase. We propose that PKC-delta is an apoptotic lamin kinase and that efficient lamina disassembly at apoptosis requires both lamin hyperphosphorylation and caspase mediated proteolysis.