The therapeutic role of targeting protein kinase C in solid and hematologic malignancies

The interruption of atypical PKC signaling and Temozolomide combination therapy against glioblastoma

Current treatment options of glioblastoma include chemotherapy and limited surgical resection. Temozolomide (TMZ) is the current therapeutic choice for chemotherapy. Still, it has severe limitations due to the development of resistance that occurs by genetic modification and constitutive activation of several cell signaling pathways. Therefore, it is essential to develop combination therapy of TMZ with other novel compounds to prevent the development of chemo-resistance. In this study, we used two inhibitors; ICA, an inhibitor of PKC-ι and ζ-Stat, an inhibitor of PKC-ζ. T98G and U87MG glioblastoma cells were treated with either ICA or ζ-stat or TMZ monotherapies, as well as TMZ were combined with either ICA or ζ-stat for five consecutive days. Our in vitro results exhibited that ICA when combined with TMZ, significantly decreased the viability of cancerous cells compared with untreated or TMZ or ICA monotherapies. Additionally, glioblastoma cells were remarkably undergoing apoptosis against the combination treatment of TMZ and ICA nucleotide compared with untreated control cells, as suggested by our Annexin-V/PI flow cytometric analysis. Moreover, the combination of TMZ and ICA also decreased the invasion of glioblastoma cell lines by acting on FAK/Paxillin pathway, as evidenced by scratch assay, transwell invasion assay, Western blot and immunoprecipitation analysis. Furthermore, our in vivo data presented that the combination of ICA and TMZ also reduced glioblastoma tumor growth and volume in mice. These data suggest that atypical PKCs, particularly PKC-ι might be an important therapeutic target as adjuvant therapy in the treatment of glioblastoma.

Emerging protein kinase inhibitors for the treatment of multiple myeloma

Introduction: Significant advances have been made during the last two decades in terms of new therapeutic options but also of innovative approaches to diagnosis and management of multiple myeloma (MM). While patient survival has been significantly prolonged, most patients relapse. Including the milestone approval of the first kinase inhibitor imatinib mesylate for CML in 2001, 48 small molecule protein kinase (PK) inhibitors have entered clinical practice until now. However, no PK inhibitor has been approved for MM therapy yet. Areas covered: This review article summarizes up-to-date knowledge on the pathophysiologic role of PKs in MM. Derived small molecules targeting receptor tyrosine kinases (RTKs), the Ras/Raf/MEK/MAPK- pathway, the PI3K/Akt/mTOR- pathway as well as Bruton tyrosine kinase (BTK), Aurora kinases (AURK), and cyclin-dependent kinases (CDKs) are most promising. Preclinical as well as early clinical data focusing on these molecules will be presented and critically reviewed. Expert opinion: Current MM therapy is directed against general vulnerabilities. Novel therapeutic strategies, inhibition of PKs in particular, are directed to target tumor-specific driver aberrations such as genetic abnormalities and microenvironment-driven deregulations. Results of ongoing Precision Medicine trials with PK inhibitors alone or in combination with other agents are eagerly awaited and hold the promise of once more improving MM patient outcome.

Recent advances in colony stimulating factor-1 receptor/c-FMS as an emerging target for various therapeutic implications

Colony stimulating factor-1 (CSF-1) is one of the most common proinflammatory cytokine responsible for various inflammatory disorders. It has a remarkable role in the development and progression of osteoarthritis, cancer and other autoimmune disease conditions. The CSF-1 acts by binding to the receptor, called colony stimulating factor-1 receptor (CSF-1R) also known as c-FMS resulting in the cascade of signalling pathway causing cell proliferation and differentiation. Interleukin-34 (IL-34), recently identified as another ligand for CSF-IR, is a cytokine protein. Both, CSF-1 and IL-34, although two distinct cytokines, follow the similar signalling pathway on binding to the same receptor, CSF-1R. Like CSF-1, IL-34 promotes the differentiation and survival of monocyte, macrophages and osteoclasts. This CSF-1R/c-FMS is over expressed in many cancers and on tumour associated macrophages, consequently, have been exploited as a drug target for promising treatment for cancer and inflammatory diseases. Some CSF-1R/c-FMS inhibitors such as ABT-869, Imatinib, AG013736, JNJ-40346527, PLX3397, DCC-3014 and Ki20227 have been successfully used in these disease conditions. Many c-FMS inhibitors have been the candidates of clinical trials, but suffer from some side effects like cardiotoxicity, vomiting, swollen eyes, diarrhoea, etc. If selectivity of cFMS inhibition is achieved successfully, side effects can be overruled and this approach may become a novel therapy for treatment of various therapeutic interventions. Thus, successful targeting of c-FMS may result in multifunctional therapy. With this background of information, the present review focuses on the recent developments in the area of CSF-1R/c-FMS inhibitors with emphasis on crystal structure, mechanism of action and various therapeutic implications in which c-FMS plays a pivotal role. The review on structure activity relationship of various compounds acting as the inhibitors of c-FMS which gives the selection criteria for the development of novel molecules is also being presented.

Kinase inhibitors as potential agents in the treatment of multiple myeloma

Recent years have witnessed a dramatic increase in the number of therapeutic options available for the treatment of multiple myeloma (MM) - from immunomodulating agents to proteasome inhibitors to histone deacetylase (HDAC) inhibitors and, most recently, monoclonal antibodies. Used in conjunction with autologous hematopoietic stem cell transplantation, these modalities have nearly doubled the disease's five-year survival rate over the last three decades to about 50%. In spite of these advances, MM still is considered incurable as resistance and relapse are common. While small molecule protein kinase inhibitors have made inroads in the therapy of a number of cancers, to date their application to MM has been less than successful. Focusing on MM, this review examines the roles played by a number of kinases in driving the malignant state and the rationale for target development in the design of a number of kinase inhibitors that have demonstrated anti-myeloma activity in both in vitro and in vivo xenograph models, as well as those that have entered clinical trials. Among the targets and their inhibitors examined are receptor and non-receptor tyrosine kinases, cell cycle control kinases, the PI3K/AKT/mTOR pathway kinases, protein kinase C, mitogen-activated protein kinase, glycogen synthase kinase, casein kinase, integrin-linked kinase, sphingosine kinase, and kinases involved in the unfolded protein response.

Protein Kinase C-ζ stimulates colorectal cancer cell carcinogenesis via PKC-ζ/Rac1/Pak1/β-Catenin signaling cascade

Colorectal cancer (CRC) is the second most common cancer in the world and death from CRC accounts for 8% of all cancer deaths both in men and women in the United States. CRC is life-threatening disease due to therapy resistant cancerous cells. The exact mechanisms of cell growth, survival, metastasis and inter & intracellular signaling pathways involved in CRC is still a significant challenge. Hence, investigating the signaling pathways that lead to colon carcinogenesis may give insight into the therapeutic target. In this study, the role of atypical Protein Kinase C (aPKC) on CRC was investigated by using two inhibitors of that protein class: 1) ζ-Stat (8-hydroxynaphthalene-1,3,6-trisulfonic acid) is a specific inhibitor of PKC-ζ and 2) ICA-I 5-amino-1-(2,3-dihydroxy-4-hydroxymethyl)cyclopentyl)-1H-imidazole-4-carboxamide) is a specific inhibitor of PKC-ι. The cell lines tested were CCD18CO normal colon epithelial and LOVO metastatic CRC cells. The inhibition of aPKCs did not bring any significant toxicity on CCD18CO normal colon cell line. Although PKC-ι is an oncogene in many cancers, we found the overexpression of PKC-ζ and its direct association with Rac1. Our findings suggest that the PKC-ζ may be responsible for the abnormal growth, proliferation, and migration of metastatic LOVO colon cancer cells via PKC-ζ/Rac1/Pak1/β-Catenin pathway. These results suggest the possibility of utilizing PKC-ζ inhibitor to block CRC cells growth, proliferation, and metastasis.

Enzastaurin: A lesson in drug development

Enzastaurin is an orally administered drug that was intended for the treatment of solid and haematological cancers. It was initially developed as an isozyme specific inhibitor of protein kinase Cβ (PKCβ), which is involved in both the AKT and MAPK signalling pathways that are active in many cancers. Enzastaurin had shown encouraging preclinical results for the prevention of angiogenesis, inhibition of proliferation and induction of apoptosis as well as showing limited cytotoxicity within phase I clinical trials. However, during its assessment in phase II and III clinical trials the efficacy of enzastaurin was poor both in combination with other drugs and as a single agent. In this review, we will discuss the development of enzastaurin from drug design to clinical testing, exploring target identification, validation and preclinical assessment. Finally, we will consider the clinical evaluation of enzastaurin as an example of the challenges associated with drug development. In particular, we discuss the poor translation of drug efficacy from preclinical animal models, inappropriate end point analysis, limited standards in phase I clinical trials, insufficient use of biomarker analysis and also patient stratification, all of which contributed to the failure to achieve approval of enzastaurin as an anticancer therapeutic.

Enzastaurin inhibits ABCB1-mediated drug efflux independently of effects on protein kinase C signalling and the cellular p53 status

The PKCβ inhibitor enzastaurin was tested in parental neuroblastoma and rhabdomyosarcoma cell lines, their vincristine-resistant sub-lines, primary neuroblastoma cells, ABCB1-transduced, ABCG2-transduced, and p53-depleted cells. Enzastaurin IC₅₀s ranged from 3.3 to 9.5 μM in cell lines and primary cells independently of the ABCB1, ABCG2, or p53 status. Enzastaurin 0.3125 μM interfered with ABCB1-mediated drug transport. PKCα and PKCβ may phosphorylate and activate ABCB1 under the control of p53. However, enzastaurin exerted similar effects on ABCB1 in the presence or absence of functional p53. Also, enzastaurin inhibited PKC signalling only in concentrations ≥ 1.25 μM. The investigated cell lines did not express PKCβ. PKCα depletion reduced PKC signalling but did not affect ABCB1 activity. Intracellular levels of the fluorescent ABCB1 substrate rhodamine 123 rapidly decreased after wash-out of extracellular enzastaurin, and enzastaurin induced ABCB1 ATPase activity resembling the ABCB1 substrate verapamil. Computational docking experiments detected a direct interaction of enzastaurin and ABCB1. These data suggest that enzastaurin directly interferes with ABCB1 function. Enzastaurin further inhibited ABCG2-mediated drug transport but by a different mechanism since it reduced ABCG2 ATPase activity. These findings are important for the further development of therapies combining enzastaurin with ABC transporter substrates.

Intra-lesional injection of the novel PKC activator EBC-46 rapidly ablates tumors in mouse models

Intra-lesional chemotherapy for treatment of cutaneous malignancies has been used for many decades, allowing higher local drug concentrations and less toxicity than systemic agents. Here we describe a novel diterpene ester, EBC-46, and provide preclinical data supporting its use as an intra-lesional treatment. A single injection of EBC-46 caused rapid inflammation and influx of blood, followed by eschar formation and rapid tumor ablation in a range of syngeneic and xenograft models. EBC-46 induced oxidative burst from purified human polymorphonuclear cells, which was prevented by the Protein Kinase C inhibitor bisindolylmaleimide-1. EBC-46 activated a more specific subset of PKC isoforms (PKC-βI, -βII, -α and -γ) compared to the structurally related phorbol 12-myristate 13-acetate (PMA). Although EBC-46 showed threefold less potency for inhibiting cell growth than PMA in vitro, it was more effective for cure of tumors in vivo. No viable tumor cells were evident four hours after injection by ex vivo culture. Pharmacokinetic profiles from treated mice indicated that EBC-46 was retained preferentially within the tumor, and resulted in significantly greater local responses (erythema, oedema) following intra-lesional injection compared with injection into normal skin. The efficacy of EBC-46 was reduced by co-injection with bisindolylmaleimide-1. Loss of vascular integrity following treatment was demonstrated by an increased permeability of endothelial cell monolayers in vitro and by CD31 immunostaining of treated tumors in vivo. Our results demonstrate that a single intra-lesional injection of EBC-46 causes PKC-dependent hemorrhagic necrosis, rapid tumor cell death and ultimate cure of solid tumors in pre-clinical models of cancer.

INPP4B suppresses prostate cancer cell invasion

BACKGROUND

INPP4B and PTEN dual specificity phosphatases are frequently lost during progression of prostate cancer to metastatic disease. We and others have previously shown that loss of INPP4B expression correlates with poor prognosis in multiple malignancies and with metastatic spread in prostate cancer.

RESULTS

We demonstrate that de novo expression of INPP4B in highly invasive human prostate carcinoma PC-3 cells suppresses their invasion both in vitro and in vivo. Using global gene expression analysis, we found that INPP4B regulates a number of genes associated with cell adhesion, the extracellular matrix, and the cytoskeleton. Importantly, de novo expressed INPP4B suppressed the proinflammatory chemokine IL-8 and induced PAK6. These genes were regulated in a reciprocal manner following downregulation of INPP4B in the independently derived INPP4B-positive LNCaP prostate cancer cell line. Inhibition of PI3K/Akt pathway, which is highly active in both PC-3 and LNCaP cells, did not reproduce INPP4B mediated suppression of IL-8 mRNA expression in either cell type. In contrast, inhibition of PKC signaling phenocopied INPP4B-mediated inhibitory effect on IL-8 in either prostate cancer cell line. In PC-3 cells, INPP4B overexpression caused a decline in the level of metastases associated BIRC5 protein, phosphorylation of PKC, and expression of the common PKC and IL-8 downstream target, COX-2. Reciprocally, COX-2 expression was increased in LNCaP cells following depletion of endogenous INPP4B.

CONCLUSION

Taken together, we discovered that INPP4B is a novel suppressor of oncogenic PKC signaling, further emphasizing the role of INPP4B in maintaining normal physiology of the prostate epithelium and suppressing metastatic potential of prostate tumors.

INPP4B suppresses prostate cancer cell invasion

Background

INPP4B and PTEN dual specificity phosphatases are frequently lost during progression of prostate cancer to metastatic disease. We and others have previously shown that loss of INPP4B expression correlates with poor prognosis in multiple malignancies and with metastatic spread in prostate cancer.

Results

We demonstrate that de novo expression of INPP4B in highly invasive human prostate carcinoma PC-3 cells suppresses their invasion both in vitro and in vivo. Using global gene expression analysis, we found that INPP4B regulates a number of genes associated with cell adhesion, the extracellular matrix, and the cytoskeleton. Importantly, de novo expressed INPP4B suppressed the proinflammatory chemokine IL-8 and induced PAK6. These genes were regulated in a reciprocal manner following downregulation of INPP4B in the independently derived INPP4B-positive LNCaP prostate cancer cell line. Inhibition of PI3K/Akt pathway, which is highly active in both PC-3 and LNCaP cells, did not reproduce INPP4B mediated suppression of IL-8 mRNA expression in either cell type. In contrast, inhibition of PKC signaling phenocopied INPP4B-mediated inhibitory effect on IL-8 in either prostate cancer cell line. In PC-3 cells, INPP4B overexpression caused a decline in the level of metastases associated BIRC5 protein, phosphorylation of PKC, and expression of the common PKC and IL-8 downstream target, COX-2. Reciprocally, COX-2 expression was increased in LNCaP cells following depletion of endogenous INPP4B.

Conclusion

Taken together, we discovered that INPP4B is a novel suppressor of oncogenic PKC signaling, further emphasizing the role of INPP4B in maintaining normal physiology of the prostate epithelium and suppressing metastatic potential of prostate tumors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12964-014-0061-y) contains supplementary material, which is available to authorized users.

Targeting phospho-MARCKS overcomes drug-resistance and induces antitumor activity in preclinical models of multiple myeloma

Multiple myeloma (MM) is incurable in virtually all patients due to the presence of innate and emergent drug-resistance. To identify potential drug resistance mechanisms in MM we used iTRAQ (isobaric tags for relative and absolute quantitation) mass spectrometry to compare protein expression profiles of drug-resistant (RPMI 8226-R5) and sensitive (RPMI 8226-S) isogenic cell lines. We identified selective overexpression of myristoylated alanine-rich C-kinase substrate (MARCKS) in drug-resistant R5 cells. MARCKS overexpression was also observed in several drug-resistant human myeloma cell lines (HMCLs) and in drug-resistant primary MM samples. Functionally, inhibition of MARCKS phosphorylation by enzastaurin or knockdown of the gene by RNAi significantly enhanced the sensitivity of resistant HMCLs and primary MM samples to bortezomib and to other anti-myeloma drugs, providing evidence that MARCKS can modulate drug response. Mechanistically, pMARCKS (phosphorylated form of MARCKS) was found to function as an E2F-1 cofactor to regulate SKP2 transcription. pMARCKS promoted cell-cycle progression by facilitating SKP2 expression, suppressing p27(Kip1) and potentially counteracting drug-induced cell-cycle arrest by promoting Cyclin E/CDK2 activity. Importantly, MARCKS knockdown in combination with bortezomib treatment overcame bortezomib resistance, significantly inhibited tumor growth and prolonged host survival in a MM xenograft model. These data provide a rationale for therapeutic targeting of pMARCKS to improve the outcome of patients with refractory/relapsed MM.

The serine-threonine kinase p90RSK is a new target of enzastaurin in follicular lymphoma cells

BACKGROUND AND PURPOSE

Follicular lymphoma is the second most common non-Hodgkin's lymphoma and, despite the introduction of rituximab for its treatment, this disease is still considered incurable. Besides genetic alterations involving Bcl-2, Bcl-6 or c-Myc, follicular lymphoma cells often display altered B-cell receptor signalling pathways including overactive PKC and PI3K/Akt systems.

EXPERIMENTAL APPROACH

The effect of enzastaurin, an inhibitor of PKC, was evaluated both in vitro on follicular lymphoma cell lines and in vivo on a xenograft murine model. Using pharmacological inhibitors and siRNA transfection, we determined the different signalling pathways after enzastaurin treatment.

KEY RESULTS

Enzastaurin inhibited the serine-threonine kinase p90RSK which has downstream effects on GSK3β. Bad and p70S6K. These signalling proteins control follicular lymphoma cell survival and apoptosis; which accounted for the inhibition by enzastaurin of cell survival and its induction of apoptosis of follicular lymphoma cell lines in vitro. Importantly, these results were replicated in vivo where enzastaurin inhibited the growth of follicular lymphoma xenografts in mice.

CONCLUSIONS AND IMPLICATIONS

The targeting of p90RSK by enzastaurin represents a new therapeutic option for the treatment of follicular lymphoma.

AKT as a therapeutic target in multiple myeloma

INTRODUCTION

Multiple myeloma remains an incurable malignancy with poor survival. Novel therapeutic approaches capable of improving outcomes in patients with multiple myeloma are urgently required. AKT is a central node in the phosphatidylinositol-3-kinase/AKT/mammalian target of rapamycin signaling pathway with high expression in advanced and resistant multiple myeloma. AKT contributes to multiple oncogenic functions in multiple myeloma which may be exploited therapeutically. Promising preclinical data has lent support for pursuing further development of AKT inhibitors in multiple myeloma. Lead drugs are now entering the clinic.

AREAS COVERED

The rationale for AKT inhibition in multiple myeloma, pharmacological subtypes of AKT inhibitors in development, available results of clinical studies of AKT inhibitors and suitable drug partners for further development in combination with AKT inhibition in multiple myeloma are discussed.

EXPERT OPINION

AKT inhibitors are a welcome addition to the armamentarium against multiple myeloma and promising clinical activity is being reported from ongoing trials in combination with established and/or novel treatment approaches. AKT inhibitors may be set to improve patient outcomes when used in combination with synergistic drug partners.

Combination treatments with the PKC inhibitor, enzastaurin, enhance the cytotoxicity of the anti-mesothelin immunotoxin, SS1P

Activated protein kinase C (PKC) contributes to tumor survival and proliferation, provoking the development of inhibitory agents as potential cancer therapeutics. Immunotoxins are antibody-based recombinant proteins that employ antibody fragments for cancer targeting and bacterial toxins as the cytotoxic agent. Pseudomonas exotoxin-based immunotoxins act via the ADP-ribosylation of EF2 leading to the enzymatic inhibition of protein synthesis. Combining PKC inhibitors with the immunotoxin SS1P, targeted to surface mesothelin, was undertaken to explore possible therapeutic strategies. Enzastaurin but not two other PKC inhibitors combined with SS1P to produce synergistic cell death via apoptosis. Mechanistic insights of the synergistic killing centered on the complete loss of the prosurvival Bcl2 protein, Mcl-1, the loss of AKT and the activation of caspase 3/7. Synergy was most evident when cells exhibited resistance to the immunotoxin alone. Further, because PKC inhibition by itself was not sufficient to enhance SS1P action, enzastaurin must target other kinases that are involved in the immunotoxin pathway.

Protein kinase C, an elusive therapeutic target?

Protein kinase C (PKC) has been a tantalizing target for drug discovery ever since it was first identified as the receptor for the tumour promoter phorbol ester in 1982. Although initial therapeutic efforts focused on cancer, additional indications--including diabetic complications, heart failure, myocardial infarction, pain and bipolar disorder--were targeted as researchers developed a better understanding of the roles of eight conventional and novel PKC isozymes in health and disease. Unfortunately, both academic and pharmaceutical efforts have yet to result in the approval of a single new drug that specifically targets PKC. Why does PKC remain an elusive drug target? This Review provides a short account of some of the efforts, challenges and opportunities in developing PKC modulators to address unmet clinical needs.

HDAC inhibitors augmented cell migration and metastasis through induction of PKCs leading to identification of low toxicity modalities for combination cancer therapy

PURPOSE

Histone deacetylase inhibitors (HDACi) are actively explored as new-generation epigenetic drugs but have low efficacy in cancer monotherapy. To reveal new mechanism for combination therapy, we show that HDACi induce cell death but simultaneously activate tumor-progressive genes to ruin therapeutic efficacy. Combined treatments to target tumorigenesis and HDACi-activated metastasis with low toxic modalities could develop new strategies for long-term cancer therapy.

EXPERIMENTAL DESIGN

Because metastasis is the major cause of cancer mortality, we measured cell migration activity and profiled metastasis-related gene expressions in HDACi-treated cancer cells. We developed low toxic combination modalities targeting tumorigenesis and HDACi-activated metastasis for preclinical therapies in mice.

RESULTS

We showed that cell migration activity was dramatically and dose dependently enhanced by various classes of HDACi treatments in 13 of 30 examined human breast, gastric, liver, and lung cancer cell lines. Tumor metastasis was also enhanced in HDACi-treated mice. HDACi treatments activated multiple PKCs and downstream substrates along with upregulated proapoptotic p21. For targeting tumorigenesis and metastasis with immediate clinical impact, we showed that new modalities of HDACi combined drugs with PKC inhibitory agent, curcumin or tamoxifen, not only suppressed HDACi-activated tumor progressive proteins and cell migration in vitro but also inhibited tumor growth and metastasis in vivo.

CONCLUSION

Treatments of different structural classes of HDACi simultaneously induced cell death and promoted cell migration and metastasis in multiple cancer cell types. Suppression of HDACi-induced PKCs leads to development of low toxic and long-term therapeutic strategies to potentially treat cancer as a chronic disease.

M-CSF induces monocyte survival by activating NF-κB p65 phosphorylation at Ser276 via protein kinase C

Macrophage colony-stimulating factor (M-CSF) promotes mononuclear phagocyte survival and proliferation. The transcription factor Nuclear Factor-kappaB (NF-κB) is a key regulator of genes involved in M-CSF-induced mononuclear phagocyte survival and this study focused at identifying the mechanism of NF-κB transcriptional activation. Here, we demonstrate that M-CSF stimulated NF-κB transcriptional activity in human monocyte-derived macrophages (MDMs) and the murine macrophage cell line RAW 264.7. The general protein kinase C (PKC) inhibitor Ro-31-8220, the conventional PKCα/β inhibitor Gö-6976, overexpression of dominant negative PKCα constructs and PKCα siRNA reduced NF-κB activity in response to M-CSF. Interestingly, Ro-31-8220 reduced Ser276 phosphorylation of NF-κBp65 leading to decreased M-CSF-induced monocyte survival. In this report, we identify conventional PKCs, including PKCα as important upstream kinases for M-CSF-induced NF-κB transcriptional activation, NF-κB-regulated gene expression, NF-κB p65 Ser276 phosphorylation, and macrophage survival. Lastly, we find that NF-κB p65 Ser276 plays an important role in basal and M-CSF-stimulated NF-κB activation in human mononuclear phagocytes.

A phase I safety study of enzastaurin plus bortezomib in the treatment of relapsed or refractory multiple myeloma

The purpose of this study was to assess the safety and identify the recommended doses of enzastaurin and bortezomib in combination for future Phase II studies in patients with relapsed or refractory multiple myeloma. Three dose levels (DLs) of oral enzastaurin and intravenous bortezomib were used according to a conventional "3 + 3" design. A loading dose of enzastaurin (250 mg twice/day [BID]) on Day 1 was followed by enzastaurin 125 mg BID for 1 week, after which bortezomib was added (Cycle 1, 28 days, 1.0 mg/m(2) : Days 8, 11, 15, and 18; seven subsequent 21-day cycles, 1.3 mg/m(2) : Days 1, 4, 8, and 11). Twenty-three patients received treatment; all patients received prior systemic therapy. Most patients received ≥3 regimens; 17 patients were bortezomib-refractory. A median of four treatment cycles (range 1-24) was completed. No dose-limiting toxicities were observed; thus, DL 3 was the recommended Phase II dose. The most common drug-related Grade 3/4 toxicities were thrombocytopenia (n = 6) and anemia (n = 2). No patients died on therapy. One patient (DL 1) achieved a very good partial response; three patients (DLs 2 and 3), a partial response; nine patients, stable disease; and four patients, progressive disease. The recommended Phase II doses in patients with relapsed or refractory multiple myeloma are as follows: enzastaurin loading dose of 375 mg three times/day on Day 1 followed by 250 mg BID, with bortezomib 1.3 mg/m(2) on Days 1, 4, 8, and 11 of a 21-day cycle. The combination was well-tolerated and demonstrated some antimyeloma activity.

Enzastaurin hydrochloride for lymphoma: reassessing the results of clinical trials in light of recent advances in the biology of B-cell malignancies

INTRODUCTION

The B-cell receptor (BCR) is critical for the development and persistence of B-cell non-Hodgkin lymphoma (B-NHL). Protein kinase C-beta (PKC-?) has been identified as one of the key signaling hubs downstream of the BCR and constitutes a valuable target in B-NHL. As a potent PKC-? inhibitor, enzastaurin is currently being tested in Phase II/III trials.

AREAS COVERED

This review summarizes the latest results and ongoing clinical trials with enzastaurin in light of basic scientific advances in the understanding of various lymphoid cancers, including diffuse large B-cell lymphoma (DLBCL), mantle cell lymphoma (MCL), follicular lymphoma (FL), chronic lymphocytic leukemia (CLL) and Waldenstr?m's macroglobulinemia (WM).

EXPERT OPINION

While its continued clinical development is uncertain, enzastaurin should be regarded as a stepping stone for the development of future therapies; indeed, the recent research has provided valuable insight into the possible molecular mechanisms that explain its limited clinical activity especially in the treatment of DLBCL and MCL. It should be noted that there is still some interest in enzastaurin, in combination, for the treatment of WM.

Submaximal inhibition of protein kinase C restores ADP-induced dense granule secretion in platelets in the presence of Ca2+

Protein kinase C (PKC) is a family of serine/threonine kinases that play isoform-specific inhibitory and stimulatory roles in platelet activation. We show here that the pan-PKC inhibitor Ro31-8220 can be used to dissect these events following platelet activation by ADP. Submaximal concentrations of Ro31-8220 potentiated aggregation and dense granule secretion to ADP in plasma anticoagulated with citrate, in d-Phe-Pro-Arg-chloromethyl ketone-anticoagulated plasma, which has physiological levels of Ca²⁺, and in washed platelets. Potentiation was retained on inhibition of cyclooxygenase and was associated with an increase in intracellular Ca²⁺. Potentiation of aggregation and secretion was abolished by a maximally effective concentration of Ro31-8220, consistent with a critical role of PKC in secretion. ADP-induced secretion was potentiated in the presence of an inhibitor of PKCβ but not in the presence of available inhibitors of other PKC isoforms in human and mouse platelets. ADP-induced secretion was also potentiated in mouse platelets deficient in PKCϵ but not PKCθ. These results demonstrate that partial blockade of PKC potentiates aggregation and dense granule secretion by ADP in association with increased Ca²⁺. This provides a molecular explanation for the inability of ADP to induce secretion in plasma in the presence of physiological Ca²⁺ concentrations, and it reveals a novel role for PKC in inhibiting platelet activation by ADP in vivo. These results also demonstrate isoform-specific inhibitory effects of PKC in platelets.

Novel PKCs activate ERK through PKD1 in MCF-7 cells

PKCs can have opposite effects on ERK phosphorylation. Novel (n)PKCs can inhibit ERK by phosphorylation of Raf-1, classical and atypical PKCs can activate ERK by removing an inhibitory protein from Raf-1. The aim of this work was to clarify how PMA-activated PKCs lead to ERK activation in MCF-7 cells expressing mainly nPKCs. Using chemical inhibitors and antibodies against PKCs, delivered into cells by the Chariot transfection system, we found that nPKCs activate ERK through transphosphorylation of PKD1, the blockage of which prevented PMA-stimulated ERK activation. We conclude that the nPKCs/PKD1 cascade is determinant for ERK activation by PMA in MCF-7 cells.

Phorbol ester phorbol-12-myristate-13-acetate induces epithelial to mesenchymal transition in human prostate cancer ARCaPE cells

BACKGROUND

We have reported that human prostate cancer ARCaP(E) cells undertake epithelial to mesenchymal transition (EMT) when stimulated by certain soluble factors, and that EMT is regulated by surface receptor-elicited signaling pathways through protein phosphorylation. It is known that phorbol ester phorbol-12-myristate-13-acetate (PMA), a potent antagonist to both conventional and novel protein kinase C (PKC) isoenzymes, induces cancer cell scattering.

METHODS

To assess the effect of PMA on EMT, ARCaP(E) cells were treated with PMA and were assayed for EMT-related morphologic and behavioral changes. Specific inhibitors were used to investigate the PMA-induced EMT.

RESULTS

PMA at 100 nM induced EMT in a time-dependent manner, resulting in a complete change from epithelial to mesenchymal stromal morphology. Concurrently, PMA inhibited expression of epithelial marker E-cadherin and increased the level of stromal marker protein vimentin, while the treated cells showed increased migratory and invasive capacities. Using specific inhibitors, we confirmed that the effect of PMA was mediated by PKC, while isoenzymes of the novel PKC subfamily were implicated as the main mediator. Finally, we determined that the EMT was dependent on newly synthesized proteins, because inhibitors for gene transcription and protein translation could both inhibit the initiation of EMT.

CONCLUSIONS

Although PMA is well known for its effects on cell migration and tumor formation, this work is the first to define PMA as an EMT inducer in prostate cancer cells. Further investigation in this experimental model may reveal important regulatory mechanisms and additional molecular changes underlying EMT.

Oral enzastaurin in prostate cancer: a two-cohort phase II trial in patients with PSA progression in the non-metastatic castrate state and following docetaxel-based chemotherapy for castrate metastatic disease

PURPOSE

Enzastaurin is an oral serine/threonine kinase inhibitor of the beta isoform of protein kinase C that may have therapeutic activity in prostate cancer. We explored the efficacy of enzastaurin on two cohorts of patients with prostate cancer progression in the castrate state.

PATIENTS AND METHODS

A two-cohort phase II trial was conducted, with both groups participating simultaneously. Cohort 1 consisted of patients with non-metastatic castrate prostate-specific antigen progressive disease. Cohort 2 consisted of patients with castrate metastatic disease with progression following docetaxel-based chemotherapy. Patients in both cohorts received 500 mg/day enzastaurin.

RESULTS

Therapy was well tolerated in both cohorts. One complete response was observed in Cohort 1, with limited activity in the majority of patients. In Cohort 2, no objective responses were seen and the median progression-free survival (11 weeks [90% confidence interval: 7.6, 11.7]) did not differ from the historical control.

CONCLUSIONS

Enzastaurin as a single agent has limited activity in castrate progressive prostate cancer. Evaluation in combination with docetaxel is ongoing.

Differential subcellular expression of protein kinase C betaII in breast cancer: correlation with breast cancer subtypes

Protein kinase C betaII (PKCβII) represents a novel potential target for anticancer therapies in breast cancer. In order to identify patient subgroups which might benefit from PKC-targeting therapies, we investigated the expression of PKCβII in human breast cancer cell lines and in a tissue microarray (TMA). We first screened breast cancer cell line representatives of breast cancer subtypes for PKCβII expression at the mRNA and at the protein levels. We analyzed a TMA comprising of tumors from 438 patients with a median followup of 15.4 years for PKCβII expression by immunohistochemistry along with other prognostic factors in breast cancer. Among a panel of human breast cancer cell lines, only MDA-MB-436, a triple negative basal cell line, showed overexpression for PKCβII both at the mRNA and at the protein levels. In breast cancer patients, cytoplasmic expression of PKCβII correlated positively with human epidermal growth factor receptor-2 (HER-2; P = 0.01) and Ki-67 (P = 0.016), while nuclear PKCβII correlated positively with estrogen receptor (ER; P = 0.016). The positive correlation of CK5/6 with cytoplasmic PKCβII (P = 0.033) lost statistical significance after adjusting for multiple comparisons (P = 0.198). Cytoplasmic PKCβII did not correlate with cyclooxygenase (COX-2; P = 0.925) and vascular endothelial growth factor (P = 1). There was no significant association between PKCβII staining and overall survival. Cytoplasmic PKCβII correlates with HER-2 and Ki-67, while nuclear PKCβII correlates with ER in breast cancer. Our study suggests the necessity for assessing the subcellular localization of PKCβII in breast cancer subtypes when evaluating the possible effectiveness of PKCβII-targeting agents.

Protein kinase Cbeta modulates ligand-induced cell surface death receptor accumulation: a mechanistic basis for enzastaurin-death ligand synergy

Although treatment with the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) is known to protect a subset of cells from induction of apoptosis by death ligands such as Fas ligand and tumor necrosis factor-alpha-related apoptosis-inducing ligand, the mechanism of this protection is unknown. This study demonstrated that protection in short term apoptosis assays and long term proliferation assays was maximal when Jurkat or HL-60 human leukemia cells were treated with 2-5 nm PMA. Immunoblotting demonstrated that multiple PKC isoforms, including PKCalpha, PKCbeta, PKCepsilon, and PKC, translocated from the cytosol to a membrane-bound fraction at these PMA concentrations. When the ability of short hairpin RNA (shRNA) constructs that specifically down-regulated each of these isoforms was examined, PKCbeta shRNA uniquely reversed PMA-induced protection against cell death. The PKCbeta-selective small molecule inhibitor enzastaurin had a similar effect. Although mass spectrometry suggested that Fas is phosphorylated on a number of serines and threonines, mutation of these sites individually or collectively had no effect on Fas-mediated death signaling or PMA protection. Further experiments demonstrated that PMA diminished ligand-induced cell surface accumulation of Fas and DR5, and PKCbeta shRNA or enzastaurin reversed this effect. Moreover, enzastaurin sensitized a variety of human tumor cell lines and clinical acute myelogenous leukemia isolates, which express abundant PKCbeta, to tumor necrosis factor-alpha related apoptosis-inducing ligand-induced death in the absence of PMA. Collectively, these results identify a specific PKC isoform that modulates death receptor-mediated cytotoxicity as well as a small molecule inhibitor that mitigates the inhibitory effects of PKC activation on ligand-induced death receptor trafficking and cell death.

Emerging therapies for multiple myeloma

Multiple myeloma (MM) is a clonal plasma cell malignancy clinically characterized by osteolytic lesions, immunodeficiency, and renal disease. There are an estimated 750,000 people diagnosed with MM worldwide, with a median overall survival of 3 - 5 years. Besides chromosomal aberrations, translocations, and mutations in essential growth and tumor-suppressor genes, accumulating data strongly highlight the pathophysiologic role of the bone marrow (BM) microenvironment in MM pathogenesis. Based on this knowledge, several novel agents have been identified, and treatment options in MM have fundamentally changed during the last decade. Thalidomide, bortezomib, and lenalidomide have been incorporated into conventional cytotoxic and transplantation regimens, first in relapsed and refractory and now also in newly diagnosed MM. Despite these significant advances, there remains an urgent need for more efficacious and tolerable drugs. Indeed, a plethora of preclinical agents awaits translation from the bench to the bedside. This article reviews the scientific rationale of new therapy regimens and newly identified therapeutic agents - small molecules as well as therapeutic antibodies - that hold promise to further improve outcome in MM.

Synthesis and Cytotoxicity of Novel 3-amido-4-indolylmaleimide Derivatives

A series of novel 3-amido-4-indolylmaleimides have been synthesised from succinimide in five steps sequence consisting of bromination, indole addition, azide substitution, reduction and selective acylation. Cytotoxicity was evaluated for the products against cervical cancer Hela cell lines and human hepatocellular cancer SMMC 7721 cell line by standard MTT assay in vitro. Some of these compounds showed moderate cytotoxic potencies.

Neuroprotective versus tumorigenic protein kinase C activators

Protein kinase C (PKC) activators possess potent neurotrophic and neuroprotective activity, thus indicating potential applications in treating neurodegenerative diseases, stroke and traumatic brain injury. Although some activators, such as bryostatin and gnidimacrin, have been tested as antitumor agents, others, such as phorbol esters, are potent tumor promoters. All PKC activators downregulate PKC at high concentrations and long application times. However, tumorigenic activators downregulate certain PKC isozymes, especially PKCdelta, more strongly. Tumorigenic activators possess unique structural features that could account for this difference. At concentrations that minimize PKC downregulation, PKC activators can improve long-term memory, reduce beta-amyloid levels, induce synaptogenesis, promote neuronal repair and inhibit cell proliferation. Intermittent, low concentrations of structurally specific, non-tumorigenic PKC activators, therefore, could offer therapeutic benefit for a variety of neurologic disorders.

ARC (NSC 188491) has identical activity to Sangivamycin (NSC 65346) including inhibition of both P-TEFb and PKC

Background

The nucleoside analog, ARC (NSC 188491) is a recently characterized transcriptional inhibitor that selectively kills cancer cells and has the ability to perturb angiogenesis in vitro. In this study, the mechanism of action of ARC was further investigated by comparing in vitro and in vivo activity with other anti-neoplastic purines.

Methods

Structure-based homology searches were used to identify those compounds with similarity to ARC. Comparator compounds were then evaluated alongside ARC in the context of viability, cell cycle and apoptosis assays to establish any similarities. Following this, biological overlap was explored in detail using gene-expression analysis and kinase inhibition assays.

Results

Results demonstrated that sangivamycin, an extensively characterized pro-apoptotic nucleoside isolated from Streptomyces, had identical activity to ARC in terms of 1) cytotoxicity assays, 2) ability to induce a G₂/M block, 3) inhibitory effects on RNA/DNA/protein synthesis, 4) transcriptomic response to treatment, 5) inhibition of protein kinase C, 6) inhibition of positive transcription elongation factor b (P-TEFb), 7) inhibition of VEGF secretion, and 8) activity within hollow fiber assays. Extending ARC activity to PKC inhibition provides a molecular basis for ARC cancer selectivity and anti-angiogenic effects. Furthermore, functional overlap between ARC and sangivamycin suggests that development of ARC may benefit from a retrospective of previous sangivamycin clinical trials. However, ARC was found to be inactive in several xenograft models, likely a consequence of rapid serum clearance.

Conclusion

Overall, these data expand on the biological properties of ARC but suggest additional studies are required before it can be considered a clinical trials candidate.

Activation of protein kinase C{eta} by type I interferons

Type I interferons (IFNs) are cytokines with diverse biological properties, including antiviral, growth inhibitory, and immunomodulatory effects. Although several signaling pathways are activated during engagement of the type I IFN receptor and participate in the induction of IFN responses, the mechanisms of generation of specific signals for distinct biological effects remain to be elucidated. We provide evidence that a novel member of the protein kinase C (PKC) family of proteins is rapidly phosphorylated and activated during engagement of the type I IFN receptor. In contrast to other members of the PKC family that are also regulated by IFN receptors, PKCeta does not regulate IFN-inducible transcription of interferon-stimulated genes or generation of antiviral responses. However, its function promotes cell cycle arrest and is essential for the generation of the suppressive effects of IFNalpha on normal and leukemic human myeloid (colony-forming unit-granulocyte macrophage) bone marrow progenitors. Altogether, our studies establish PKCeta as a unique element in IFN signaling that plays a key and essential role in the generation of the regulatory effects of type I IFNs on normal and leukemic hematopoiesis.

Bone marrow microenvironment and the identification of new targets for myeloma therapy

The development of multiple myeloma (MM) is a complex multi-step process involving both early and late genetic changes in the tumor cell as well as selective supportive conditions by the bone marrow (BM) microenvironment. Indeed, it is now well established that MM cell-induced disruption of the BM homeostasis between the highly organized cellular and extracellular compartments supports MM cell proliferation, survival, migration and drug resistance through activation of various signaling (for example, PI3K/Akt, JAK/Stat-, Raf/MEK/MAPK-, NFkappaB- and Wnt-) pathways. Based on our enhanced understanding of the functional importance of the MM BM microenvironment and its inter-relation with the MM cell resulting in homing, seeding, proliferation and survival, new molecular targets have been identified and derived treatment regimens in MM have already changed fundamentally during recent years. These agents include thalidomide, its immunomodulatory derivative lenalidomide and the proteasome inhibitor bortezomib, which mediate tumor cytotoxicity in the BM milieu. Ongoing studies are further delineating MM pathogenesis in the BM to enhance cytotoxicity, avoid drug resistance and improve patient outcome.

Protein kinase C inhibitors: rationale for use and potential in the treatment of bipolar disorder

Bipolar disorder is one of the most severely debilitating of all medical illnesses. For a large number of patients, outcomes are quite poor. The illness results in tremendous suffering for patients and their families and commonly impairs functioning and workplace productivity. Risks of increased morbidity and mortality, unfortunately, are frequent occurrences as well. Until recently, little has been known about the specific molecular and cellular underpinnings of bipolar disorder. Such knowledge is crucial for the prospect of developing specific targeted therapies that are more effective and that have a more rapid onset of action than currently available treatments. Exciting recent data suggest that regulation of certain signalling pathways may be involved in the aetiology of bipolar disorder and that these pathways may be profitably targeted to treat the disorder. In particular, mania is associated with overactive protein kinase C (PKC) intracellular signalling, and recent genome-wide association studies of bipolar disorder have implicated an enzyme that reduces the activation of PKC. Importantly, the current mainstays in the treatment of mania, lithium (a monovalent cation) and valproate (a small fatty acid) indirectly inhibit PKC. In addition, recent clinical studies with the relatively selective PKC inhibitor tamoxifen add support to the relevance of the PKC target in bipolar disorder. Overall, a growing body of work both on a preclinical and clinical level indicates that PKC signalling may play an important role in the pathophysiology and treatment of bipolar disorder. The development of CNS-penetrant PKC inhibitors may have considerable benefit for this devastating illness.

Targeting PKC: a novel role for beta-catenin in ER stress and apoptotic signaling

Targeting protein kinase C (PKC) isoforms by the small molecule inhibitor enzastaurin has shown promising preclinical activity in a wide range of tumor cells. We further delineated its mechanism of action in multiple myeloma (MM) cells and found a novel role of beta-catenin in regulating growth and survival of tumor cells. Specifically, inhibition of PKC leads to rapid accumulation of beta-catenin by preventing the phosphorylation required for its proteasomal degradation. Microarray analysis and small-interfering RNA (siRNA)-mediated gene silencing in MM cells revealed that accumulated beta-catenin activates early endoplasmic reticulum stress signaling via eIF2alpha, C/EBP-homologous protein (CHOP), and p21, leading to immediate growth inhibition. Furthermore, accumulated beta-catenin contributes to enzastaurin-induced cell death. Sequential knockdown of beta-catenin, c-Jun, and p73, as well as overexpression of beta-catenin or p73 confirmed that accumulated beta-catenin triggers c-Jun-dependent induction of p73, thereby conferring MM cell apoptosis. Our data reveal a novel role of beta-catenin in endoplasmic reticulum (ER) stress-mediated growth inhibition and a new proapoptotic mechanism triggered by beta-catenin on inhibition of PKC isoforms. Moreover, we identify p73 as a potential novel therapeutic target in MM. Based on these and previous data, enzastaurin is currently under clinical investigation in a variety of hematologic malignancies, including MM.

Enzastaurin

BACKGROUND

Enzastaurin is a novel antineoplastic and antiangiogenic agent that acts through inhibition of protein kinase C (PKC).

OBJECTIVE

This review summarizes the scientific rationale and current clinical evidence for the use of enzastaurin in oncology.

METHODS

We performed a systematic review of the literature using the keywords protein kinase C-beta and enzastaurin in order to characterize the therapeutic target PKC-beta. We then reviewed the in-vitro, Phase I, and Phase II data for enzastaurin with a focus on hematologic malignancies.

RESULTS/CONCLUSIONS

After preliminary Phase I trials established a favorable toxicity profile, enzastaurin has been studied in completed and ongoing Phase II and III studies in solid and hematologic malignancies, including B-cell lymphomas where the rationale for its use is most promising.

Exploitation of protein kinase C: a useful target for cancer therapy

Protein kinase C is a family of serine/threonine kinases. The PKC family is made up of at least 12 isozymes, which have a role in cell proliferation, differentiation, angiogenesis, and apoptosis. Activation of PKC isozyme is dependent on tyrosine-kinase receptors and G-protein-coupled receptors. PKC isozymes regulate multiple signaling pathways including PI3-K/Akt, MAPK, and GSK-3beta. PKC isozymes have variable roles in tumor biology which in part depend on the cell type and intracellular localization. PKC isozymes are commonly dysregulated in the cancer of the prostate, breast, colon, pancreatic, liver, and kidney. Currently, several classes of PKC inhibitors are being evaluated in clinical trials and several challenges in targeting PKC isozymes have been recently identified. In conclusion, PKC remains a promising target for cancer prevention and therapy.

Wealth of opportunity - the C1 domain as a target for drug development

The diacylglycerol-responsive C1 domains of protein kinase C and of the related classes of signaling proteins represent highly attractive targets for drug development. The signaling functions that are regulated by C1 domains are central to cellular control, thereby impacting many pathological conditions. Our understanding of the diacylglycerol signaling pathways provides great confidence in the utility of intervention in these pathways for treatment of cancer and other conditions. Multiple compounds directed at these signaling proteins, including compounds directed at the C1 domains, are currently in clinical trials, providing strong validation for these targets. Extensive understanding of the structure and function of C1 domains, coupled with detailed insights into the molecular details of ligand - C1 domain interactions, provides a solid basis for rational and semi-rational drug design. Finally, the complexity of the factors contributing to ligand - C1 domain interactions affords abundant opportunities for manipulation of selectivity; indeed, substantially selective compounds have already been identified.

Protein kinase C isozymes as potential therapeutic targets in immune disorders

BACKGROUND

Members of the protein kinase C (PKC) family are key signalling mediators in immune responses, and pharmacological inhibition of PKCs may be useful for treating immune-mediated diseases.

OBJECTIVE

To review and discuss the insights gained so far into various PKC isozymes and the therapeutic potential and challenges of developing PKC inhibitors for immune disorder therapy.

METHODS

A literature review of the role of PKCs in immune cell signalling and recent studies describing immune functions associated with PKC isozyme deficiency in relevant mouse disease models, followed by specific case studies of current and potential therapeutic strategies targeting PKCs.

RESULTS/CONCLUSION

There is vast amount of data supporting PKC isozymes as attractive drug targets for certain immune disorders. Although the development of specific PKC isozyme inhibitors has been challenging, some progress has been made. It remains to be seen if broad-scale or isozyme-selective inhibition of PKC will have clinical efficacy.

Neurochemical characterization of neuronal populations expressing protein kinase C gamma isoform in the spinal cord and gracile nucleus of the rat

Protein kinase C gamma (PKCgamma) is widely distributed throughout the CNS and is thought to play a role in long term hyper-excitability in nociceptive neurones. Here, we provide the first report of PKCgamma cells in the dorsal column nuclei of the adult rat. Retrograde labeling of PKCgamma cells from the thalamus with choleragenoid revealed that 25% of the PKCgamma positive gracile cells projected to the thalamus. Further, we have characterized the distribution of PKCgamma within gracile nucleus in terms of colocalization with various neurotransmitter receptors or enzymes and calcium binding proteins, and compared this with PKCgamma colocalization in cells of laminae I-III of the spinal cord. We show that approximately 90% of the PKCgamma cells in the gracile nucleus and 60% in the dorsal horn were immuno-positive for the AMPA receptor subunit glutamate 2/3 (GluR2/3). Little coexpression was seen with neurokinin 1 receptor, nitric oxide synthase (NOS) and the AMPA receptor subunit GluR1, markers of distinct neuronal subpopulations. In the spinal cord, a quarter of PKCgamma cells expressed calbindin, but very few cells did so in the gracile nucleus. Electrical stimulation at c-fiber strength of the normal or injured sciatic nerve was used to induce c-fos as a marker of postsynaptic activation in the spinal cord and gracile nucleus. Quantitative analysis of the number of PKCgamma positive gracile cells that expressed also c-fos increased from none to 24% after injury, indicating an alteration in the sensory activation pattern in these neurones after injury. C-fos was not induced in inner lamina II following c-fiber electrical stimulation of the intact or axotomized sciatic nerve, indicating no such plasticity at the spinal cord level. As dorsal column nuclei cells may contribute to allodynia after peripheral nerve injury, pharmacological modulation of PKCgamma activity may therefore be a possible way to ameliorate neuropathic pain after peripheral nerve injury.