Overexpression of PKCepsilon in R6 fibroblasts causes increased production of active TGFbeta

Protein Kinase C at the Crossroad of Mutations, Cancer, Targeted Therapy and Immune Response

The frequent PKC dysregulations observed in many tumors have made these enzymes natural targets for anticancer applications. Nevertheless, this considerable interest in the development of PKC modulators has not led to the expected therapeutic benefits, likely due to the complex biological activities regulated by PKC isoenzymes, often playing ambiguous and protective functions, further driven by the occurrence of mutations. The structure, regulation and functions of PKCs have been extensively covered in other publications. Herein, we focused on PKC alterations mostly associated with complete functional loss. We also addressed the modest yet encouraging results obtained targeting PKC in selected malignancies and the more frequent negative clinical outcomes. The reported observations advocate the need for more selective molecules and a better understanding of the involved pathways. Furthermore, we underlined the most relevant immune mechanisms controlled by PKC isoforms potentially impacting the immune checkpoint inhibitor blockade-mediated immune recovery. We believe that a comprehensive examination of the molecular features of the tumor microenvironment might improve clinical outcomes by tailoring PKC modulation. This approach can be further supported by the identification of potential response biomarkers, which may indicate patients who may benefit from the manipulation of distinctive PKC isoforms.

Protein kinase C in cancer: The top five unanswered questions

Few kinases have been studied as extensively as protein kinase C (PKC), particularly in the context of cancer. As major cellular targets for the phorbol ester tumor promoters and diacylglycerol (DAG), a second messenger generated by stimulation of membrane receptors, PKC isozymes play major roles in the control of signaling pathways associated with proliferation, migration, invasion, tumorigenesis, and metastasis. However, despite decades of research, fundamental questions remain to be answered or are the subject of intense controversy. Primary among these unresolved issues are the role of PKC isozymes as either tumor promoter or tumor suppressor kinases and the incomplete understanding on isozyme-specific substrates and effectors. The involvement of PKC isozymes in cancer progression needs to be reassessed in the context of specific oncogenic and tumor suppressing alterations. In addition, there are still major hurdles in addressing isozyme-specific function due to the limited specificity of most pharmacological PKC modulators and the lack of validated predictive biomarkers for response, which impacts the translation of these agents to the clinic. In this review we focus on key controversial issues and upcoming challenges, with the expectation that understanding the intricacies of PKC function will help fulfill the yet unsuccessful promise of targeting PKCs for cancer therapeutics.

PKCε Is an Essential Mediator of Prostate Cancer Bone Metastasis

The bone is a preferred site for metastatic homing of prostate cancer cells. Once prostate cancer patients develop skeletal metastases, they eventually succumb to the disease; therefore, it is imperative to identify key molecular drivers of this process. This study examines the involvement of protein kinase C epsilon (PKCε), an oncogenic protein that is abnormally overexpressed in human tumor specimens and cell lines, on prostate cancer cell bone metastasis. PC3-ML cells, a highly invasive prostate cancer PC3 derivative with bone metastatic colonization properties, failed to induce skeletal metastatic foci upon inoculation into nude mice when PKCε expression was silenced using shRNA. Interestingly, while PKCε depletion had only marginal effects on the proliferative, adhesive, and migratory capacities of PC3-ML cells in vitro or in the growth of xenografts upon s.c. inoculation, it caused a significant reduction in cell invasiveness. Notably, PKCε was required for transendothelial cell migration (TEM) as well as for the growth of PC3-ML cells in a bone biomimetic environment. At a mechanistic level, PKCε depletion abrogates the expression of IL1β, a cytokine implicated in skeletal metastasis. Taken together, PKCε is a key factor for driving the formation of bone metastasis by prostate cancer cells and is a potential therapeutic target for advanced stages of the disease.

IMPLICATIONS

This study uncovers an important new function of PKCε in the dissemination of cancer cells to the bone; thus, highlighting the promising potential of this oncogenic kinase as a therapeutic target for skeletal metastasis.

Protein kinase C and cancer: what we know and what we do not

Since their discovery in the late 1970s, protein kinase C (PKC) isozymes represent one of the most extensively studied signaling kinases. PKCs signal through multiple pathways and control the expression of genes relevant for cell cycle progression, tumorigenesis and metastatic dissemination. Despite the vast amount of information concerning the mechanisms that control PKC activation and function in cellular models, the relevance of individual PKC isozymes in the progression of human cancer is still a matter of controversy. Although the expression of PKC isozymes is altered in multiple cancer types, the causal relationship between such changes and the initiation and progression of the disease remains poorly defined. Animal models developed in the last years helped to better understand the involvement of individual PKCs in various cancer types and in the context of specific oncogenic alterations. Unraveling the enormous complexity in the mechanisms by which PKC isozymes have an impact on tumorigenesis and metastasis is key for reassessing their potential as pharmacological targets for cancer treatment.

The Multifunctional Protein Kinase C-ε in Cancer Development and Progression

The protein kinase C (PKC) family proteins are important signal transducers and have long been the focus of cancer research. PKCɛ, a member of this family, is overexpressed in most solid tumors and plays critical roles in different processes that lead to cancer development. Studies using cell lines and animal models demonstrated the transforming potential of PKCɛ. While earlier research established the survival functions of PKCɛ, recent studies revealed its role in cell migration, invasion and cancer metastasis. PKCɛ has also been implicated in epithelial to mesenchymal transition (EMT), which may be the underlying mechanism by which it contributes to cell motility. In addition, PKCɛ affects cell-extracellular matrix (ECM) interactions by direct regulation of the cytoskeletal elements. Recent studies have also linked PKCɛ signaling to cancer stem cell functioning. This review focuses on the role of PKCɛ in different processes that lead to cancer development and progression. We also discussed current literatures on the pursuit of PKCɛ as a target for cancer therapy.

Protein Kinase C-ε Promotes EMT in Breast Cancer

Protein kinase C (PKC), a family of serine/threonine kinases, plays critical roles in signal transduction and cell regulation. PKCε, a member of the novel PKC family, is known to be a transforming oncogene and a tumor biomarker for aggressive breast cancers. In this study, we examined the involvement of PKCε in epithelial to mesenchymal transition (EMT), the process that leads the way to metastasis. Overexpression of PKCε was sufficient to induce a mesenchymal phenotype in non-tumorigenic mammary epithelial MCF-10 A cells. This was accompanied by a decrease in the epithelial markers, such as E-cadherin, zonula occludens (ZO)-1, and claudin-1, and an increase in mesenchymal marker vimentin. Transforming growth factor β (TGFβ) induced Snail expression and mesenchymal morphology in MCF-10 A cells, and these effects were partially reversed by the PKCε knockdown. PKCε also mediated cell migration and anoikis resistance, which are hallmarks of EMT. Thus, our study demonstrates that PKCε is an important mediator of EMT in breast cancer.

Transgenic overexpression of PKCε in the mouse prostate induces preneoplastic lesions

It is well established that protein kinase C (PKC) isozymes play distinctive roles in mitogenic and survival signaling as well as in cancer progression. PKCε, the product of the PRKCE gene, is up-regulated in various types of cancers including prostate, lung and breast cancer. To address a potential role for PKCs in prostate cancer progression we generated three mouse transgenic lines expressing PKCα, PKCδ, or PKCε in the prostate epithelium under the control of the rat probasin (PB) promoter. Whereas PB-PKCε and PB-PKCδ mice did not show any evident phenotype, PB-PKCε mice developed prostate hyperplasia as well as prostate intraepithelial neoplasia (PIN) that displayed enhanced phospho-Akt, phospho-S6, and phospho-Stat3 levels, as well as enhanced resistance to apoptotic stimuli. PKCε overexpression was insufficient to drive neoplastic changes in the mouse prostate. Notably, overexpression of PKCε by adenoviral means in normal immortalized RWPE-1 prostate cells confers a growth advantage and hyperactivation of Erk and Akt. Our results argue for a causal link between PKCε overexpression and prostate cancer development.

Regulation of prostate cancer cell survival by protein kinase Cepsilon involves bad phosphorylation and modulation of the TNFalpha/JNK pathway

Protein kinase Cepsilon (PKCepsilon), a diacyglycerol- and phorbol ester-responsive serine-threonine kinase, has been implicated in mitogenic and survival control, and it is markedly overexpressed in human tumors, including in prostate cancer. Although prostate cancer cells undergo apoptosis in response to phorbol ester stimulation via PKCdelta-mediated release of death factors, the involvement of PKCepsilon in this response is not known. PKCepsilon depletion by RNAi or expression of a dominant negative kinase-dead PKCepsilon mutant potentiated the apoptotic response of PMA and sensitized LNCaP cells to the death receptor ligand TNFalpha. On the other hand, overexpression of PKCepsilon by adenoviral means protected LNCaP cells against apoptotic stimuli. Interestingly, PKCepsilon RNAi depletion significantly enhanced the release of TNFalpha in response to PMA and greatly potentiated JNK activation by this cytokine. Further mechanistic analysis revealed that PMA fails to promote phosphorylation of Bad in Ser(112) in PKCepsilon-depleted LNCaP cells, whereas PKCepsilon overexpression greatly enhanced Bad phosphorylation. This effect was independent of Akt, ERK, or p90Rsk, well established kinases for Ser(112) in Bad. Moreover, expression of a S112A-Bad mutant potentiated PMA-induced apoptosis. Finally, we found that upon activation PKCepsilon accumulated in mitochondrial fractions in LNCaP cells and that Bad was a substrate of PKCepsilon in vitro. Our results established that PKCepsilon modulates survival in prostate cancer cells via multiple pathways.

Molecular mechanisms of protein kinase C-induced apoptosis in prostate cancer cells

Protein kinase C (PKC) isozymes, a family of serine-threonine kinases, are important regulators of cell proliferation and malignant transformation. Phorbol esters, the prototype PKC activators, cause PKC translocation to the plasma membrane in prostate cancer cells, and trigger an apoptotic response. Studies in recent years have determined that each member of the PKC family exerts different effects on apoptotic or survival pathways. PKCdelta, one of the novel PKCs, is a key player of the apoptotic response via the activation of the p38 MAPK pathway. Studies using RNAi revealed that depletion of PKCdelta totally abolishes the apoptotic effect of the phorbol ester PMA. Activation of the classical PKCalpha promotes the dephosphorylation and inactivation of the survival kinase Akt. Studies have assigned a pro-survival role to PKCepsilon, but the function of this PKC isozyme remains controversial. Recently, it has been determined that the PKC apoptotic effect in androgen-dependent prostate cancer cells is mediated by the autocrine secretion of death factors. PKCdelta stimulates the release of TNFalpha from the plasma membrane, and blockade of TNFalpha secretion or TNFalpha receptors abrogates the apoptotic response of PMA. Molecular analysis indicates the requirement of the extrinsic apoptotic cascade via the activation of death receptors and caspase-8. Dissecting the pathways downstream of PKC isozymes represents a major challenge to understanding the molecular basis of phorbol ester-induced apoptosis.

Hydrogen peroxide-induced cellular apoptosis is mediated by TGF-beta2 signaling pathway in cultured human lens epithelial cells

The objective of this study was to investigate the signaling characteristics of transforming growth factor-beta2 (TGF-beta2) and the Smads (Caenorhabditis elegans, Sma; Drosophila mothers against dpp, Mad) signal pathway of cellular apoptosis induced by hydrogen peroxide with human lens epithelial cells (HLECs). HLECs were starved for 24 h before exposure to 0.1 mumol/ml of hydrogen peroxide in the presence and in the absence of 0.01 mug/ml of AF-302-NA, a monoclonal anti-TGF-beta2 neutralization antibody. Non-stimulated cells served as controls. Cell apoptosis was examined by in situ immunocytochemistry using terminal deoxynucleotidyl transferase dUTP-mediated biotin nick end labeling (TUNEL) and by flow cytometry (FCM) using Annexin V-FITC apoptosis detection. Gene expression was assessed using the reverse transcription-polymerase chain reaction (RT-PCR). Smad-4 localization was observed by immunocytochemistry. Hydrogen peroxide induced the accumulation of Smad-4 in the nucleus of HLECs, and upregulated the expression of TGF-beta receptors (TbetaRs) mRNA in HLECs, as well as upregulated the expression of the apoptotic gene bax, which leads HLECs to apoptosis. AF-302-NA decreased cellular apoptosis induced by hydrogen peroxide in HLECs and inhibited the translocation of Smad-4 from the cytoplasm to the cell nucleus. Moreover, AF-302-NA upregulated the expression of TbetaRs mRNA and downregulated the expression of bax mRNA in HLECs incubated with hydrogen peroxide. Our study demonstrated that the TGF-beta2 signal pathway participated in the apoptotic signal transfer and might be an initiator of cellular apoptosis of HLECs after incubation with hydrogen peroxide. Interruption of the TGF-beta2 signal pathway could partially protect HLECs from apoptosis induced by incubation with hydrogen peroxide.

PKC-mediated secretion of death factors in LNCaP prostate cancer cells is regulated by androgens

Activation of PKCdelta in androgen-dependent LNCaP prostate cancer cells leads to apoptosis via the activation of p38 MAPK and JNK cascades. We have recently shown that treatment of LNCaP cells with phorbol 12-myristate 13-acetate (PMA) leads to a PKCdelta-mediated autocrine release of death factors, including the cytokines TNFalpha and TRAIL, and that conditioned medium (CM) collected from PMA-treated LNCaP cells promotes the activation of the extrinsic apoptotic cascade. Interfering with this autocrine loop either at the level of factor release or death receptor activation/signaling markedly impaired the PMA apoptotic response. In the present study we show that this PKCdelta-dependent autocrine mechanism is greatly influenced by androgens. Indeed, upon androgen depletion, which down-regulates PKCdelta expression, TNFalpha and TRAIL mRNA induction and release by PMA are significantly diminished, resulting in a reduced apoptogenic activity of the CM and an impaired ability of the CM to activate p38 MAPK and JNK. These effects can be rescued by addition of the synthetic androgen R1881. Furthermore, RNAi depletion of the androgen-receptor (AR) from LNCaP cells equally impaired PMA responses, suggesting that PKC-mediated induction of death factor secretion and apoptosis in LNCaP prostate cancer cells are highly sensitive to hormonal control.

Protein kinase Cepsilon makes the life and death decision

Cancer is caused by dysregulation in cellular signaling systems that control cell proliferation, differentiation and cell death. Protein kinase C (PKC), a family of serine/threonine kinases, plays an important role in the growth factor signal transduction pathway. PKCepsilon, however, is the only PKCepsilon isozyme that has been considered as an oncogene. It can contribute to malignancy by enhancing cell proliferation or by inhibiting cell death. This review focuses on how PKCepsilon collaborates with other signaling pathways, such as Ras/Raf/ERK and Akt, to regulate cell survival and cell death. We have also discussed how PKCepsilon mediates its antiapoptotic signal by altering the level or function of pro- and antiapoptotic Bcl-2 family members.

Phorbol ester-induced apoptosis in prostate cancer cells via autocrine activation of the extrinsic apoptotic cascade: a key role for protein kinase C delta

It is well established that activation of protein kinase C (PKC) by phorbol esters promotes apoptosis in androgen-dependent prostate cancer cells. However, there is limited information regarding the cellular mechanisms involved in this effect. In this report we identified a novel autocrine pro-apoptotic loop triggered by PKCdelta activation in prostate cancer cells that is mediated by death receptor ligands. The apoptotic effect of phorbol 12-myristate 13-acetate in LNCaP cells was impaired by inhibition or depletion of tumor necrosis factor alpha-converting enzyme, the enzyme responsible for tumor necrosis factor alpha (TNFalpha) shedding. Moreover, the apoptogenic effect of conditioned medium collected after phorbol 12-myristate 13-acetate treatment could be inhibited by blocking antibodies against TNFalpha and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), but not FasL, as well as by RNA interference depletion of TNFalpha and TRAIL receptors. Moreover, depletion or inhibition of death receptor downstream effectors, including caspase-8, FADD, p38 MAPK, and JNK, significantly reduced the apoptogenic effect of the conditioned medium. PKCdelta played a major role in this autocrine loop, both in the secretion of autocrine factors as well as a downstream effector. Taken together, our results demonstrate that activation of PKCdelta in prostate cancer cells causes apoptosis via the release of death receptor ligands and the activation of the extrinsic apoptotic cascade.

Overexpression of protein kinase C-{epsilon} in the mouse epidermis leads to a spontaneous myeloproliferative-like disease

Protein kinase C (PKC)-epsilon, a Ca(2+)-independent, phospholipid-dependent serine/threonine kinase, is among the PKC isoforms expressed in mouse epidermis. We reported that FVB/N transgenic mouse lines that overexpress (8- or 18-fold) PKC-epsilon protein in basal epidermal cells and cells of the hair follicle develop papilloma-independent squamous cell carcinoma (SCC) elicited by 7,12-dimethylbenz(a)anthracene initiation and 12-O-tetradecanoylphorbol-13-acetate-promotion or by repeated ultraviolet radiation exposures. The susceptibility to the development of SCC was proportional to the level of expression of the PKC-epsilon transgene. We now report that PKC-epsilon FVB/N transgenic mice (line 215) that overexpress in epidermis approximately 18-fold PKC-epsilon protein more than their wild-type littermates spontaneously develop a myeloproliferative-like disease (MPD) in 100% of PKC-epsilon transgenic mice. The MPD was characterized by an excess of neutrophils and eosinophils, resulting in invasion of almost all vital organs of the mouse by 6 months of age. On gross examination these mice present with splenomegaly, hepatomegaly, and severe lymphadenopathy. Examination of the bone marrow revealed almost complete effacement by neutrophils, eosinophils, and their precursors. Furthermore, the spleen and lymph nodes were enlarged and exhibited marked extramedullary hematopoiesis. Complete pathological analysis of the second PKC-epsilon transgenic mouse (line 224) that expresses approximately eightfold PKC-epsilon protein more than their wild-type littermates revealed no remarkable findings in any of the affected organs as seen in line 215. However, peripheral blood analyses of PKC-epsilon transgenic mice indicated significant increases of neutrophils in the circulating blood in both PKC-epsilon transgenic lines. To determine whether there was an imbalance of cytokines in PKC-epsilon transgenic mice (line 215), resulting in aberrant myelopoiesis, we analyzed 17 cytokines in the peripheral blood. This analysis indicated that interleukin-5, interleukin-6, and granulocyte-colony stimulating factor were up-regulated as a function of age. The transgene PKC-epsilon was not detected in any of the affected organs (bone marrow, liver, spleen, lung) We suggest that overexpression of PKC-epsilon in the epidermis may lead to the induction of specific cytokines that may, in a paracrine mechanism, perturb normal hematopoiesis in bone marrow resulting in a granulocytic skew toward that of neutrophils and eosinophils. The susceptibility of PKC-epsilon transgenic mice to the induction of SCC and the spontaneous development of MPD are unrelated.

Protein kinase C epsilon is an endogenous photosensitizer that enhances ultraviolet radiation-induced cutaneous damage and development of squamous cell carcinomas

Chronic exposure to UV radiation (UVR), especially in the UVA (315-400 nm) and UVB (280-315 nm) spectrum of sunlight, is the major risk factor for the development of nonmelanoma skin cancer. UVR is a complete carcinogen, which both initiates and promotes carcinogenesis. We found that protein kinase C epsilon (PKCepsilon), a member of the phospholipid-dependent threonine/serine kinase family, is an endogenous photosensitizer, the overexpression of which in the epidermis increases the susceptibility of mice to UVR-induced cutaneous damage and development of squamous cell carcinoma. The PKCepsilon transgenic mouse (FVB/N) lines 224 and 215 overexpressed 8- and 18-fold PKCepsilon protein, respectively, over endogenous levels in basal epidermal cells. UVR exposure (1 kJ/m(2) three times weekly) induced irreparable skin damage in high PKCepsilon-overexpressing mouse line 215. However, the PKCepsilon transgenic mouse line 224, when exposed to UVR (2 kJ/m(2) three times weekly), exhibited minimum cutaneous damage but increased squamous cell carcinoma multiplicity by 3-fold and decreased tumor latency by 12 weeks. UVR exposure of PKCepsilon transgenic mice compared with wild-type littermates (1) elevated the levels of neither cyclobutane pyrimidine dimer nor pyrimidine (6-4) pyrimidone dimer, (2) reduced the appearance of sunburn cells, (3) induced extensive hyperplasia and increased the levels of mouse skin tumor promoter marker ornithine decarboxylase, and (4) elevated the levels of tumor necrosis factor alpha (TNFalpha) and other growth stimulatory cytokines, granulocyte colony-stimulating factor, and granulocyte macrophage colony-stimulating factor. The role of TNFalpha in UVR-induced cutaneous damage was evaluated using PKCepsilon transgenic mice deficient in TNFalpha. UVR treatment three times weekly for 13 weeks at 2 kJ/m(2) induced severe cutaneous damage in PKCepsilon transgenic mice (line 215), which was partially prevented in PKCepsilon-transgenic TNFalpha-knockout mice. Taken together, the results indicate that PKCepsilon signals UVR-induced TNFalpha release that is linked, at least in part, to the photosensitivity of PKCepsilon transgenic mice.

Regulation of tumor invasion and metastasis in protein kinase C epsilon-transformed NIH3T3 fibroblasts

Protein kinase C epsilon is an oncogenic, actin nucleating protein that coordinately regulates changes in cell growth and shape. Cells constitutively expressing PKCepsilon spontaneously acquire a polarized morphology and extend long cellular membrane protrusions. Here we report that the regulatory C1 domain of PKCepsilon contains an actin binding site that is essential for the formation of elongate invadopodial-like structures, increased pericellular metalloproteinase activity, in vitro invasion of a Matrigel barrier, and the invasion and metastasis of tumors grown in vivo by PKCepsilon-transformed NIH3T3 fibroblasts in nude mice. While removing this small actin binding motif caused a dramatic reversion of tumor invasion, the deletion mutant of PKCepsilon remained oncogenic and tumorigenic in this experimental system. We propose that PKCepsilon directly interacts with actin to stimulate polymerization and the extension of membrane protrusions that transformed NIH3T3 cells use in vivo to penetrate and degrade surrounding tissue boundaries.

Targeting protein kinase C: new therapeutic opportunities against high-grade malignant gliomas?

A large body of evidence suggests that the abnormal phenotype of neoplastic astrocytes, including their excessive proliferation rate and high propensity to invade surrounding tissues, results from mutations in critical genes involved in key cellular events. These genetic alterations can affect cell-surface-associated receptors, elements of signaling pathways, or components of the cell cycle clock, conferring a gain or a loss of relevant metabolic functions of the cells. The understanding of such phenomena may allow the development of more efficacious forms of cancer treatment. Examples are therapies specifically directed against overexpressed epidermal growth factor receptor, hyperactive Ras, excessively stimulated Raf-1, overproduced ornithine decarboxylase, or aberrantly activated cyclin-dependent kinases. The applicability of some of these approaches is now being assessed in patients suffering from primary malignant central nervous system tumors that are not amenable to current therapeutic modalities. Another potentially useful therapeutic strategy against such tumors involves the inhibition of hyperactive or overexpressed protein kinase C (PKC). This strategy is justified by the decrease in cell proliferation and invasion following inhibition of the activity of this enzyme observed in preclinical glioma models. Thus, interference with PKC activity may represent a novel form of experimental cancer treatment that may simultaneously restrain the hyperproliferative state and the invasive capacity of high-grade malignant gliomas without inducing the expected toxicity of classical cytotoxic agents. Of note, the experimental use of PKC-inhibiting agents in patients with refractory high-grade malignant gliomas has indeed led to some clinical responses. The present paper reviews the current status of the biochemistry and molecular biology of PKC, as well as the possibilities for developing novel anti-PKC-based therapies for central nervous system malignancies.

Biochemical and morphogenic effects of the interaction between protein kinase C-epsilon and actin in vitro and in cultured NIH3T3 cells

Protein kinase C-epsilon coordinately regulates changes in cell growth and shape. Cells overproducing protein kinase C-epsilon spontaneously acquire a polarized morphology and extend long cellular membrane protrusions that are reminiscent of the morphology observed in ras-transformed fibroblasts. Here we report that the regulatory C1 domain contains an actin binding hexapeptide motif that is essential for the morphogenic effects of protein kinase C-epsilon in cultured NIH3T3 murine fibroblasts. The extension of elongate processes by protein kinase C-epsilon transformed fibroblasts appeared to be driven by a kinase-independent mechanism that required organized networks of both actin and microtubules. Flow cytometry of phalloidin-stained cells demonstrated that protein kinase C-epsilon significantly increased the cellular content of polymerized actin in NIH3T3 cells. Studies with a cell-free system suggest that protein kinase C-epsilon inhibits the in vitro disassembly of actin filaments, is capable of desequestering actin monomers from physiologically relevant concentrations of thymosin beta4, and increases the rate of actin filament elongation by decreasing the critical concentration of actin. Based on these and other observations, it is proposed that protein kinase C-epsilon may function as a terminal downstream effector in at least one of the signaling pathways that mitogens engage to initiate outgrowth of cellular protrusions.

The role of mitogen-activated protein kinase and protein kinase C in fibronectin production in human vascular smooth muscle cells

BACKGROUND

After evaluating various growth factors, cytokines, and extracellular matrix (ECM) proteins, we found that the most potent agonists of smooth muscle cell (SMC) fibronectin (Fn) production were transforming growth factor-beta (TGF-beta) and epidermal growth factor (EGF). To determine the possible signaling pathways involved in the production of this matrix protein, we investigated the role of the intracellular proteins, protein kinase C (PKC) and mitogen-activated protein kinase (MAP-K), in TGF-beta- and EGF-induced human vascular SMC Fn production.

MATERIALS AND METHODS

After stimulation of human SMCs with TGF-beta (10 ng/ml) and EGF (100 ng/ml), Fn in the cell medium was assayed by immunoblotting using a specific antibody. PKC was activated by brief stimulation of SMC with phorbol 12,13-dibutyrate (PDBu) and inhibited by downregulation with PDBu or the inhibitor, GF109203X. MAP-K was inhibited with PD098059.

RESULTS

PKC activation increased basal and synergistically enhanced TGF-beta- and EGF-induced Fn production. However, inhibition of PKC by downregulation and GF109203X did not diminish Fn production by TGF-beta and EGF. Surprisingly, these two methods of inhibition slightly increased basal and agonist-induced Fn production. The MAP-K kinase inhibitor, PD098059, produced an almost complete inhibition of EGF and a partial inhibition of TGF-beta-induced Fn production.

CONCLUSIONS

Activation of PKC stimulates Fn production; however, neither TGF-beta nor EGF produce Fn through a PKC-dependent pathway. EGF and TGF-beta both stimulate Fn production at least in part through the intracellular signaling protein MAP-K. Understanding the signaling pathways involved in extracellular matrix protein production will allow the design of specific inhibitors of intimal hyperplasia.