Immunological quantitation of phospholipid/Ca2+-dependent protein kinase of human mammary carcinoma cells: inverse relationship to estrogen receptors

Protein kinase C inhibitor chelerythrine selectively inhibits proliferation of triple-negative breast cancer cells

Triple-negative breast cancer (TNBC) is a subtype of breast cancer lacking targeted therapy currently. Recent studies imply that protein kinase C may play important roles in TNBC development and could be a specific target. In this study, we evaluated the anti-proliferative activity of PKC inhibitor chelerythrine on a panel of breast cancer cell lines. Chelerythrine selectively inhibited the growth of TNBC cell lines compared to non-TNBC cell lines as demonstrated by in vitro cell proliferation assay and colony formation assay, as well as evidenced by in vivo xenograft assay. The selective anti-proliferative effect of chelerythrine was associated with induction of apoptosis in TNBC cell lines. We further demonstrated that PKN2, one of the PKC subtypes, was highly expressed in TNBC cell lines, and knocking down PKN2 in TNBC cells inhibited colony formation and xenograft growth. This indicates that PKN2 is required for the survival of TNBC cells, and could be the target mediates the selective activity of chelerythrine. Finally, combination of chelerythrine and chemotherapy reagent taxol showed synergistic/additive effect on TNBC cell lines. Our results suggest chelerythrine or other PKC inhibitors may be promising regimens for TNBC tumors.

Resistin, a fat-derived secretory factor, promotes metastasis of MDA-MB-231 human breast cancer cells through ERM activation

Resistin, an adipocyte-secreted factor, is known to be elevated in breast cancer patients. However, the molecular mechanism by which resistin acts is not fully understood. The aim of this study was to investigate whether resistin could stimulate invasion and migration of breast cancer cells. Here, we report that resistin stimulated invasion and migration of breast cancer cells as well as phosphorylation of c-Src. Inhibition of c-Src blocked resistin-induced breast cancer cell invasion. Resistin increased intracellular calcium concentration, and chelation of intracellular calcium blocked resistin-mediated activation of Src. Resistin also induced phosphorylation of protein phosphatase 2A (PP2A). Inhibition of c-Src blocked resistin-mediated PP2A phosphorylation. In addition, resistin increased phosphorylation of PKCα. Inhibition of PP2A enhanced resistin-induced PKCα phosphorylation, demonstrating that PP2A activity is critical for PKCα phosphorylation. Resistin also increased phosphorylation of ezrin, radixin, and moesin (ERM). Additionally, ezrin interacted with PKCα, and resistin promoted co-localization of ezrin and PKCα. Either inhibition of c-Src and PKCα or knock-down of ezrin blocked resistin-induced breast cancer cells invasion. Moreover, resistin increased expression of vimentin, a key molecule for cancer cell invasion. Knock-down of ezrin abrogated resistin-induced vimentin expression. These results suggest that resistin play as a critical regulator of breast cancer metastasis.

The new biology of estrogen-induced apoptosis applied to treat and prevent breast cancer

The successful use of high-dose synthetic estrogens to treat postmenopausal metastatic breast cancer is the first effective 'chemical therapy' proven in clinical trial to treat any cancer. This review documents the clinical use of estrogen for breast cancer treatment or estrogen replacement therapy (ERT) in postmenopausal hysterectomized women, which can either result in breast cancer cell growth or breast cancer regression. This has remained a paradox since the 1950s until the discovery of the new biology of estrogen-induced apoptosis at the end of the 20th century. The key to triggering apoptosis with estrogen is the selection of breast cancer cell populations that are resistant to long-term estrogen deprivation. However, estrogen-independent growth occurs through trial and error. At the cellular level, estrogen-induced apoptosis is dependent upon the presence of the estrogen receptor (ER), which can be blocked by nonsteroidal or steroidal antiestrogens. The shape of an estrogenic ligand programs the conformation of the ER complex, which, in turn, can modulate estrogen-induced apoptosis: class I planar estrogens (e.g., estradiol) trigger apoptosis after 24 h, whereas class II angular estrogens (e.g., bisphenol triphenylethylene) delay the process until after 72 h. This contrasts with paclitaxel, which causes G2 blockade with immediate apoptosis. The process is complete within 24 h. Estrogen-induced apoptosis is modulated by glucocorticoids and cSrc inhibitors, but the target mechanism for estrogen action is genomic and not through a nongenomic pathway. The process is stepwise through the creation of endoplasmic reticulum stress and inflammatory responses, which then initiate an unfolded protein response. This, in turn, initiates apoptosis through the intrinsic pathway (mitochondrial) with the subsequent recruitment of the extrinsic pathway (death receptor) to complete the process. The symmetry of the clinical and laboratory studies now permits the creation of rules for the future clinical application of ERT or phytoestrogen supplements: a 5-year gap is necessary after menopause to permit the selection of estrogen-deprived breast cancer cell populations to cause them to become vulnerable to apoptotic cell death. Earlier treatment with estrogen around menopause encourages growth of ER-positive tumor cells, as the cells are still dependent on estrogen to maintain replication within the expanding population. An awareness of the evidence that the molecular events associated with estrogen-induced apoptosis can be orchestrated in the laboratory in estrogen-deprived breast cancers now supports the clinical findings regarding the treatment of metastatic breast cancer following estrogen deprivation, decreases in mortality following long-term antihormonal adjuvant therapy, and the results of treatment with ERT and ERT plus progestin in the Women's Health Initiative for women over the age of 60. Principles have emerged for understanding and applying physiological estrogen therapy appropriately by targeting the correct patient populations.

Definition of PKC-α, CDK6, and MET as therapeutic targets in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a highly heterogeneous and recurrent subtype of breast cancer that lacks an effective targeted therapy. To identify candidate therapeutic targets, we profiled global gene expression in TNBC and breast tumor-initiating cells with a patient survival dataset. Eight TNBC-related kinases were found to be overexpressed in TNBC cells with stem-like properties. Among them, expression of PKC-α, MET, and CDK6 correlated with poorer survival outcomes. In cases coexpressing two of these three kinases, survival rates were lower than in cases where only one of these kinases was expressed. In functional tests, two-drug combinations targeting these three kinases inhibited TNBC cell proliferation and tumorigenic potential in a cooperative manner. A combination of PKC-α-MET inhibitors also attenuated tumor growth in a cooperative manner in vivo. Our findings define three kinases critical for TNBC growth and offer a preclinical rationale for their candidacy as effective therapeutic targets in treating TNBC.

Protein kinase C Beta in the tumor microenvironment promotes mammary tumorigenesis

Protein kinase C beta (PKCβ) expression in breast cancer is associated with a more aggressive tumor phenotype, yet the mechanism for how PKCβ is pro-tumorigenic in this disease is still unclear. Interestingly, while it is known that PKCβ mediates angiogenesis, immunity, fibroblast function and adipogenesis, all components of the mammary tumor microenvironment (TME), no study to date has investigated whether stromal PKCβ is functionally relevant in breast cancer. Herein, we evaluate mouse mammary tumor virus–polyoma middle T-antigen (MMTV–PyMT) induced mammary tumorigenesis in the presence and absence of PKCβ. We utilize two model systems: one where PKCβ is deleted in both the epithelial and stromal compartments to test the global requirement for PKCβ on tumor formation, and second, where PKCβ is deleted only in the stromal compartment to test its role in the TME. MMTV–PyMT mice globally lacking PKCβ live longer and develop smaller tumors with decreased proliferation and decreased macrophage infiltration. Similarly, when PKCβ is null exclusively in the stroma, PyMT-driven B6 cells form smaller tumors with diminished collagen deposition. These experiments reveal for the first time a tumor promoting role for stromal PKCβ in MMTV–PyMT tumorigenesis. In corroboration with these results, PKCβ mRNA (Prkcb) is increased in fibroblasts isolated from MMTV–PyMT tumors. These data were confirmed in a breast cancer patient cohort. Combined these data suggest the continued investigation of PKCβ in the mammary TME is necessary to elucidate how to effectively target this signaling pathway in breast cancer.

Quantification of PKC family genes in sporadic breast cancer by qRT-PCR: evidence that PKCι/λ overexpression is an independent prognostic factor

Drugs targeting protein kinase C (PKC) show promising therapeutic activity. However, little is known about the expression patterns of the 11 PKC genes in human tumors, and the clinical significance of most PKC genes is unknown. We used qRT-PCR assays to quantify mRNA levels of the 11 PKC genes in 458 breast tumors from patients with known clinical/pathological status and long-term outcome. The proportion of tumors in which the expression of the different genes was altered varied widely, from 9.6% for PKN2 to 40.2% for PKCι/λ. In breast tumors, overexpression was the main alteration observed for PKCι/λ (33.4%), PKCδ (29.5%) and PKCζ (9.6%), whereas underexpression was the main alteration observed for PKCα (27.3%), PKCε (11.6%), PKCη (8.7%) and PKN2 (8.1%). Both overexpression and underexpression were observed for PKCβ (underexpression 15.5%, overexpression 13.8%), PKCθ (underexpression 14.8%, overexpression 10.0%) and PKN1 (underexpression 6.6%, overexpression 7.4%). Several links were found between different PKC genes; and also between the expression patterns of PKC genes and several classical pathological and clinical parameters. PKCι/λ alone was found to have prognostic significance (p = 0.043), whereas PKCα showed a trend towards an influence on relapse-free survival (p = 0.052). PKCι/λ retained its prognostic significance in Cox multivariate regression analysis (p = 0.031). These results reveal very complex expression patterns of PKC genes in breast tumors, and suggest that their expression should be considered together when evaluating anti-tumoral drugs. PKCι/λ seems to be the most promising therapeutic target in breast cancer.

PKCα and ERβ Are Associated with Triple-Negative Breast Cancers in African American and Caucasian Patients

Although the incidence of breast cancer in the United States is higher in Caucasian women compared with African American women, African-American patients have more aggressive disease as characterized by a higher percentage of triple-negative breast cancers (TNBCs), high-grade tumors, and a higher mortality rate. PKCα is a biomarker associated with endocrine resistance and poor prognosis and ERβ is emerging as a protective biomarker. Immunohistochemical analysis of ERβ and PKCα expression was performed on 198 formalin-fixed paraffin-embedded primary infiltrating ductal carcinomas from 105 African-American and 93 Caucasian patients. PKCα is positively correlated with TNBC in patients of both races and with high tumor grade in African-American patients. Patients with TNBC express less nuclear ERβ compared with all other subtypes. We find no difference in frequency or intensity of PKCα or ERβ expression between African-American and Caucasian patients. PKCα and ERβ are discussed as potential therapeutic targets for the treatment of patients with TNBC.

Sustained inhibition of PKCα reduces intravasation and lung seeding during mammary tumor metastasis in an in vivo mouse model

Metastasis is the major reason for breast cancer-related deaths. Although there is a host of indirect evidence for a role of PKCα in primary breast cancer growth, its role in the molecular pathways leading to metastasis have not been comprehensively studied. By treating mice with αV5-3, a novel peptide inhibitor selective for PKCα, we were able to determine how PKCα regulates metastasis of mammary cancer cells using a syngeneic and orthotopic model. The primary tumor growth was not affected by αV5-3 treatment. However, the mortality rate was reduced and metastasis in the lung decreased by more than 90% in the αV5-3-treated mice relative to the control-treated mice. αV5-3 treatment reduced intravasation by reducing MMP-9 activities. αV5-3 treatment also reduced lung seeding of tumor cells and decreased cell migration, effects that were accompanied by a reduction in NFκB-activity and cell surface levels of the CXCL12 receptor, CXCR4. αV5-3 treatment caused no apparent toxicity in non-tumor bearing naïve mice. Rather, inhibiting PKCα protected against liver damage and increased the number of immune cells in tumor-bearing mice. Importantly, αV5-3 showed superior efficacy relative to anti-CXCR4 antibody in reducing metastasis, in vivo. Together, these data show that pharmacological inhibition of PKCα effectively reduces mammary cancer metastasis by targeting intravasation and lung seeding steps in the metastatic process and suggest that PKCα-specific inhibitors, such as αV5-3, can be used to study the mechanistic roles of PKCα specifically and may provide a safe and effective treatment for the prevention of lung metastasis of breast cancer patients.

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.

Estradiol-induced regression in T47D:A18/PKCalpha tumors requires the estrogen receptor and interaction with the extracellular matrix

Several breast cancer tumor models respond to estradiol (E(2)) by undergoing apoptosis, a phenomenon known to occur in clinical breast cancer. Before the application of tamoxifen as an endocrine therapy, high-dose E(2) or diethystilbesterol treatment was successfully used, albeit with unfavorable side effects. It is now recognized that such an approach may be a potential endocrine therapy option. We have explored the mechanism of E(2)-induced tumor regression in our T47D:A18/PKCalpha tumor model that exhibits autonomous growth, tamoxifen resistance, and E(2)-induced tumor regression. Fulvestrant, a selective estrogen receptor (ER) down-regulator, prevents T47D:A18/PKCalpha E(2)-induced tumor growth inhibition and regression when given before or after tumor establishment, respectively. Interestingly, E(2)-induced growth inhibition is only observed in vivo or when cells are grown in Matrigel but not in two-dimensional tissue culture, suggesting the requirement of the extracellular matrix. Tumor regression is accompanied by increased expression of the proapoptotic FasL/FasL ligand proteins and down-regulation of the prosurvival Akt pathway. Inhibition of colony formation in Matrigel by E(2) is accompanied by increased expression of FasL and short hairpin RNA knockdown partially reverses colony formation inhibition. Classic estrogen-responsive element-regulated transcription of pS2, PR, transforming growth factor-alpha, C3, and cathepsin D is independent of the inhibitory effects of E(2). A membrane-impermeable E(2)-BSA conjugate is capable of mediating growth inhibition, suggesting the involvement of a plasma membrane ER. We conclude that E(2)-induced T47D:A18/PKCalpha tumor regression requires participation of ER-alpha, the extracellular matrix, FasL/FasL ligand, and Akt pathways, allowing the opportunity to explore new predictive markers and therapeutic targets.

PKCtheta promotes c-Rel-driven mammary tumorigenesis in mice and humans by repressing estrogen receptor alpha synthesis

The vast majority of primary human breast cancer tissues display aberrant nuclear NF-kappaB c-Rel expression. A causal role for c-Rel in mammary tumorigenesis has been demonstrated using a c-Rel transgenic mouse model; however, tumors developed with a long latency, suggesting a second event is needed to trigger tumorigenesis. Here we show that c-Rel activity in the mammary gland is repressed by estrogen receptor alpha (ERalpha) signaling, and we identify an epigenetic mechanism in breast cancer mediated by activation of what we believe is a novel PKCtheta-Akt pathway that leads to downregulation of ERalpha synthesis and derepression of c-Rel. ERalpha levels were lower in c-Rel-induced mammary tumors compared with normal mammary gland tissue. PKCtheta induced c-Rel activity and target gene expression and promoted growth of c-Rel- and c-RelxCK2alpha-driven mouse mammary tumor-derived cell lines. RNA expression levels of PKCtheta and c-Rel target genes were inversely correlated with ERalpha levels in human breast cancer specimens. PKCtheta activated Akt, thereby inactivating forkhead box O protein 3a (FOXO3a) and leading to decreased synthesis of its target genes, ERalpha and p27(Kip1). Thus we have shown that activation of PKCtheta inhibits the FOXO3a/ERalpha/p27(Kip1) axis that normally maintains an epithelial cell phenotype and induces c-Rel target genes, thereby promoting proliferation, survival, and more invasive breast cancer.

Protein kinase C isoform expression as a predictor of disease outcome on endocrine therapy in breast cancer

BACKGROUND

Although in vitro breast cancer models have demonstrated a role for protein kinase C (PKC) alpha and delta isoforms in endocrine insensitivity and resistance respectively, there is currently little clinical evidence to support these observations.

AIMS

To define the pattern of PKC alpha and delta expression using breast cancer cell lines, with and without endocrine resistance, and also breast cancer samples, where expression can be correlated with clinicopathological and endocrine therapy outcome data.

METHODS

PKC isoform expression was examined in tamoxifen responsive, oestrogen receptor positive (ER(+)), ER(+) acquired tamoxifen resistant (TAM-R) and oestrogen receptor negative (ER(-)) cell lines by western blotting and immunocytochemical analysis. PKC isoform expression was then examined by immunohistochemistry in archival breast cancer specimens from primary breast cancer patients with known clinical outcome in relation to endocrine response and survival on therapy.

RESULTS

ER(+) breast cancer cell lines expressed considerable PKC-delta but barely detectable levels of PKC-alpha, whereas ER(-) cell lines expressed PKC-alpha but little PKC-delta. ER(+) acquired TAM-R cell lines expressed substantial levels of both PKC-alpha and delta. In clinical samples, high PKC-delta expression correlated to endocrine responsiveness whereas PKC-alpha expression correlated to ER negativity. PKC-delta was an independent predictor of duration of response to therapy. Patients showing a PKC-delta(+)/PKC-alpha(-) phenotype had a six times longer endocrine response than patients with the PKC-delta(+)/ PKC-alpha(+) phenotype (equating to tamoxifen resistance in vitro).

CONCLUSIONS

Levels of PKC-alpha and delta expression appear to be indicative of response to anti-oestrogen therapy and could be useful in predicting a patient's suitability for endocrine therapy.

Targeting the protein kinase C family: are we there yet?

Protein kinase C (PKC) comprises a family of serine/threonine kinases that are involved in the transduction of signals for cell proliferation, differentiation, apoptosis and angiogenesis. Unsurprisingly, disruption of PKC regulation is implicated in tumorigenesis and drug resistance. PKC function is complex in this context owing to the differing roles of individual isozymes within the cell and across tumour types. Therapeutically targeting PKC isozymes is not new; however, with many of the early PKC inhibitor cytotoxic drug combinations being discarded at the phase II level, and recent phase III studies in non-small-cell lung cancer proving negative, what's going wrong?

Protein kinase C beta enhances growth and expression of cyclin D1 in human breast cancer cells

Although alterations in the expressions of protein kinase C (PKC) have been implicated in breast carcinogenesis, the roles of specific isoforms in this process remain elusive. In the present study, we examined the specific roles of PKCbeta1 and beta2 in growth control in human breast cancer cell lines. The PKCbeta-specific inhibitor LY379196 significantly inhibited growth of the breast cancer cell lines MCF-7, MDA-MB-231, and BT474, but not the normal mammary epithelial cell line MCF-10F. Treatment of MCF-7 cells with LY379196 caused an increase in the fraction of cells in the G(1) phase of the cell cycle. To explore the roles of PKCbeta1 and beta2, we used cDNA expression vectors that encode wild-type and constitutively activated or dominant negative mutants of these two proteins. When compared with vector controls, derivatives of MCF-7 cells that stably overexpress wild-type PKCbeta1 or PKCbeta2 displayed a slight increase in growth rate; derivatives that stably express the constitutively active mutants of PKCbeta1 or PKCbeta2 displayed a marked increase in growth rate; and derivatives that stably express a dominant negative mutant of PKCbeta1 or beta2 displayed inhibition of growth. The derivatives of MCF-7 cells that stably express the constitutively activated mutants of PKCbeta1 or beta2 were more resistant to growth inhibition by LY379196 than the vector control MCF-7 cells. Immunoblot analysis indicated that MCF-7 cells that stably overexpress wild-type or constitutively activated mutants of PKCbeta1 or beta2 had higher cellular levels of cyclin D1 than vector control cells, whereas cells that express a dominant negative mutant had decreased levels of cyclin D1. The derivatives that stably express the constitutively activated mutants of PKCbeta1 or beta2 also displayed increased cyclin D1 promoter activity in transient transfection luciferase reporter assays, and this induction of activity requires activator protein 1. Constitutively activated PKCbeta1 and beta2 also enhanced the transcription of c-fos in transient transfection luciferase reporter assays. Thus, PKCbeta1 and beta2 may play important positive roles in the growth of at least a subset of human breast cancers. Therefore, inhibitors of these isoforms may be useful in breast cancer chemoprevention or therapy.

Protein Kinase C alpha is a marker for antiestrogen resistance and is involved in the growth of tamoxifen resistant human breast cancer cells

Development of resistance to antiestrogen treatment in breast cancer patients is a serious therapeutic problem. The molecular mechanisms contributing to resistance are currently unclear; however it is known that increased activation of growth signaling pathways is involved. Protein Kinase C alpha (PKCalpha) is associated with a diverse range of cancers and is previously shown to be overexpressed in three out of four antiestrogen resistant breast cancer cell lines. In this study we investigated whether PKCalpha contributes to antiestrogen resistant growth. A panel of nine resistant cell lines was investigated, all of which displayed elevated levels of PKCalpha expression relative to parental MCF-7 cells. Stable PKCalpha overexpression in MCF-7 cells significantly reduced sensitivity to the antiestrogens, tamoxifen and ICI 182,780. Two resistant cell lines were chosen for further studies: tamoxifen resistant MCF-7/TAM(R)-1 (TAM(R)-1) and ICI 182,780 resistant MCF-7/182(R)-6 (182(R)-6). Treatment with the PKCalpha inhibitor Ro-32-0432 resulted in a preferential growth inhibition of these two cell lines relative to MCF-7 cells. Moreover, transient down-regulation of PKCalpha resulted in a 30-40% growth inhibition of TAM(R)-1 and 182(R)-6, while MCF-7 remained unaffected. Stable PKCalpha knock-down in TAM(R)-1 using small hairpin RNA, resulted in a tamoxifen sensitive "MCF-7-like" growth phenotype, while the same approach in 182(R)-6 cells did not alter their sensitivity to ICI 182,780. These results demonstrate a functional contribution of PKCalpha to tamoxifen resistant growth. Furthermore, our data suggest the potential for PKCalpha as a marker for antiestrogen resistance and as a promising therapeutic target in the treatment of tamoxifen resistant breast cancer.

Impact of PKCdelta on estrogen receptor localization and activity in breast cancer cells

Regulation of estrogen receptor (ER) function in breast cancer cells is a complex process involving different signalling mechanisms. One signal transduction component that appears to influence ER signalling is protein kinase C (PKC). PKCdelta is a particular isoenzyme of the novel PKC subfamily that plays a role in growth control, differentiation and apoptosis. The aim of the present study was to investigate the impact of PKCdelta on the regulation of the transcriptional activity of the human ERalpha. By using 12-O-tetradecanoylphorbol-13-acetate (TPA), Bryostatin1 and Rottlerin, we show that active PKCdelta is a proproliferative factor in estrogen-dependent breast cancer cells. Furthermore, activation of PKCdelta by TPA resulted in activation and nuclear translocation of ERalpha and in an increase of ER-dependent reporter gene expression. Transfection and expression of the regulatory domain RDdelta of PKCdelta, which is inhibitory to PKCdelta, inhibited the TPA-induced ERalpha activation and translocation. ERalpha was not phosphorylated by PKCdelta; however, glycogen synthase kinase-3 (GSK3) was identified as a substrate of PKCdelta. The expression of RDdelta resulted in a decrease of TPA-induced GSK3 phosphorylation and translocation into the nucleus. We suggest that GSK3 plays a role in the PKCdelta-related nuclear translocation of ERalpha.

Upregulation of PKC-delta contributes to antiestrogen resistance in mammary tumor cells

Acquired resistance to tamoxifen (Tam) in breast cancer patients is a serious therapeutic problem. We have previously reported that protein kinase C-delta (PKC-delta) plays a major role in estrogen (E2)-mediated cell proliferation. To determine if PKC-delta is one of the major alternate signaling pathways that supports cell growth in the presence of Tam, we determined the levels of PKC isoforms in four different models of antiestrogen-resistant cells. Three out of four antiestrogen resistance cell lines (Tam/MCF-7, ICI/MCF-7 and HER-2/MCF-7) expressed significantly high levels of both total and activated PKC-delta levels compared to sensitive cells. Estrogen receptor (ER) alpha content and function are maintained in all the antiestrogen-resistant cell lines. Overexpressing active PKC-delta in Tam-sensitive MCF-7 cells (PKC-delta/MCF-7) led to Tam resistance both in vitro and in vivo. Inhibition of PKC-delta by rottlerin (a relatively specific inhibitor of PKC-delta) or siRNA significantly inhibited estrogen- and Tam-induced growth in antiestrogen-resistant cells. PKC-delta levels are significantly higher in Tam-resistant tumors compared to Tam-sensitive tumors in xenograft model (P<0.05). Taken together, these data suggest that PKC-delta plays a major role in antiestrogen resistance in breast tumor cells and thus provides a new target for treatment.

Protein kinase C alpha expression in breast and ovarian cancer

In recent years research has focused on the development of specific, targeted drugs to treat cancer. One approach has been to block intracellular signaling proteins, such as protein kinase C alpha (PKC-alpha). To help support the rationale for clinical studies of a PKC-alpha-targeted therapy in breast and ovarian cancers, we reviewed publications studying PKC-alpha expression in these tumors. Since these investigations were mostly performed in cell lines, we supplemented this review with some preliminary findings from studies examining PKC-alpha expression in tumor tissue biopsies obtained from patients with breast and ovarian cancer. Based on the reviewed publications using representative cell lines and our preliminary findings on tumor tissue of patients with breast cancer, we infer that PKC-alpha levels may especially be increased in breast cancer patients with low or negative estrogen receptor (ER) levels. Thus, clinical studies determining efficacy of selective or specific inhibitors of PKC-alpha should include determination of ER status in order to help answer whether blocking PKC-alpha in patients with low or absent ER can result in clinical benefit.

In vivo bone metastases, osteoclastogenic ability, and phenotypic characterization of human breast cancer cells

Mouse bone marrow cells cultured with human breast cancer MCF-7 cell-conditioned media showed osteoclastogenesis with an increment of bone resorption, although conditioned media from an adriamycin-selected MCF-7 clone (MCF-7ADR) had no effect. Consistently, MCF-7 cells induced 5-fold more in vivo experimental osteolytic bone metastases, with no soft tissue lesions, compared to MCF-7ADR cells. Paracrine factors stimulating (interleukin (IL)-6, IL-1beta, tumor necrosis factor-alpha (TNF-alpha)) or inhibiting (IL-12, IL-18, granulocyte macrophage-colony stimulating factor (GM-CSF)) osteoclastogenesis were significantly increased in MCF-7ADR relative to MCF-7 cells, suggesting that the inhibitory cytokines could selectively overwhelm the effects of the stimulatory ones. Treatment of osteoblast primary cultures with MCF-7-conditioned medium induced a selective upregulation of IL-6 expression, suggesting an indirect stimulation of osteoclastogenesis via the osteoblasts. MCF-7 and MCF-7ADR showed no difference in proliferation rate. However, a higher ability to migrate and invade gelatin and matrigel was observed in MCF-7ADR. Enhanced invasiveness might result from increased metalloproteinase (MMP) activity and cytoskeleton rearrangement. MCF-7ADR cells expressed higher levels of c-Src, focal adhesion kinase (FAK), and protein tyrosine kinase 2 (PYK2) involved in cell adhesion and motility. MCF-7 and MCF-7ADR expressed high and faint levels of functional estrogen receptor alpha (ERalpha), respectively. MCF-7ADR also showed significantly higher levels of the protein kinase C (PKC) alpha and beta2 and a selective activation of PKC compared to MCF-7, where the most abundant isoforms were beta1 and delta. Heat shock protein 27 (Hsp27) was more abundant in MCF-7 cells, but failed to translocate to the nucleus in response to heat shock. In conclusion, we have demonstrated that despite the fact that MCF-7ADR cells showed a more invasive phenotype relative to MCF-7, they have low potential to induce osteolytic bone lesions and stimulate osteoclastogenesis and osteoclast activity. Therefore, we believe that reduced aggressiveness of breast carcinomas could correlate with a greater osteolytic activity featuring their bone metastases.

A p53-independent G1 cell cycle checkpoint induced by the suppression of protein kinase C alpha and theta isoforms

The protein kinase C (PKC) family consists of multiple isoforms that are involved in the regulation of diverse cellular responses. Suppression of PKC induces growth arrest in various types of cells. However, the underlying molecular mechanisms have not been thoroughly investigated. In this report, we demonstrated that the concurrent inhibition, rather than separate inhibition, of phorbol ester-dependent PKC alpha and theta isoforms is crucial for the induction of G1 cell cycle arrest and that this negative cell cycle regulation is via p53-independent mechanisms. PKC suppression-mediated growth arrest is associated with the induction of cell cycle inhibitor p21WAF1/CIP1 and the occurrence of hypophosphorylated Rb. The G1 checkpoint induced by the suppression of PKC occurs not only in murine Swiss3T3 but also in p53-deficient cells and human lung cancer cells containing mutated p53. Luciferase and nuclear run-off assays demonstrated that p21WAF1/CIP1 is, in part, transcriptionally regulated in response to the suppression of PKC alpha and theta. However, the stability of p21 mRNA is also augmented after the addition of PKC alpha and theta antisense oligonucleotides, indicating the involvement of post-transcriptional mechanisms in p21WAF1/CIP1 expression. These data suggest the existence of a cell cycle checkpoint pathway regulated by PKC alpha and theta isoforms. Furthermore, our findings support the notion that G1 checkpoint control can be restored in tumor cells containing abnormal p53, by targeting the PKC-regulated p21WAF1/CIP1 induction.

Elevated protein kinase C alpha expression may be predictive of tamoxifen treatment failure

We previously reported that stable transfection of protein kinase C alpha (PKCalpha) into T47D human breast cancer cells results in tamoxifen (TAM)-resistant tumour growth. Relevance of PKCalpha expression in clinical specimens was determined by comparing PKCalpha expression in tumours from patients exhibiting disease recurrence with patients remaining disease-free following TAM treatment. Our results suggest that PKCalpha expression may predict TAM treatment failure.

Adaptor proteins in protein kinase C-mediated signal transduction

Spatial and temporal organization of signal transduction is essential in determining the speed and precision by which signaling events occur. Adaptor proteins are key to organizing signaling enzymes near their select substrates and away from others in order to optimize precision and speed of response. Here, we describe the role of adaptor proteins in determining the specific function of individual protein kinase C (PKC) isozymes. These isozyme-selective proteins were called collectively RACKs (receptors for activated C-kinase). The role of RACKs in PKC-mediated signaling was determined using isozyme-specific inhibitors and activators of the binding of each isozyme to its respective RACK. In addition to anchoring activated PKC isozymes, RACKs anchor other signaling enzymes. RACK1, the anchoring protein for activated betaIIPKC, binds for example, Src tyrosine kinase, integrin, and phosphodiesterase. RACK2, the epsilonPKC-specific RACK, is a coated-vesicle protein and thus is involved in vesicular release and cell-cell communication. Therefore, RACKs are not only adaptors for PKC, but also serve as adaptor proteins for several other signaling enzymes. Because at least some of the proteins that bind to RACKs, including PKC itself, regulate cell growth, modulating their interactions with RACKs may help elucidate signaling pathways leading to carcinogenesis and could result in the identification of novel therapeutic targets.

Protein Kinase C Alpha Expression Is Inversely Related to ER Status in Endometrial Carcinoma: Possible Role in AP-1-Mediated Proliferation of ER-Negative Endometrial Cancer

OBJECTIVE

Tamoxifen is the most widely used antiestrogen to treat all stages of estrogen-receptor (ER)-positive breast cancers. However, tamoxifen acts as a partial estrogen in the uterus and is known to increase the risk of endometrial cancer by two- to threefold. Recent evidence indicates that there is a connection between tamoxifen resistance and activation of the activator protein-1 (AP-1) pathway. We have previously reported a possible role for overexpression of protein kinase C alpha (PKCalpha), an upstream activator of the AP-1 pathway, in hormone-independent breast cancer and antiestrogen-stimulated endometrial tumors. We hypothesize that alterations of the PKC isozyme profile of endometrial carcinomas are similar to that of hormone-independent breast cancer and determine whether specific PKC isozyme alterations correlated with known clinicopathological features of endometrial cancer.

METHODS

The PKC isozyme profile of endometrial carcinomas from 42 patients who were not previously exposed to antiestrogens was examined by Western blot. The relationship between PKC isozyme expression and key prognostic factors for endometrial carcinoma including hormone receptor status, tumor grade, stage, size, and depth of myometrial invasion was examined using the Spearman's rho correlation coefficient.

RESULTS

As previously found in breast cancers, PKCalpha and estrogen receptor alpha (ERalpha) expression are inversely related (r(s) = -0.35, P = 0.046). We report significant inverse correlations among ER/progesterone receptor (PR) expression and tumor grade (r(s) = -0.49, P = 0.001 and r(s) = -0.44, P = 0.004, respectively), ER, and depth of myometrial invasion (r(s) = -0.40, P = 0.009). There were no other significant correlations between PKC isozyme expression and other key prognostic factors examined.

CONCLUSION

This study indicates that, similar to what was previously observed in breast cancer, PKCalpha and ER expression is inversely related in endometrial cancer. PKCalpha expression may be a useful prognostic indicator in endometrial cancers. A model is offered which describes the putative role of PKCalpha overexpression in activation of the AP-1 pathway and increased proliferation of ER negative endometrial cancers.

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.

Stable transfection of protein kinase C alpha cDNA in hormone-dependent breast cancer cell lines

An inverse relationship between protein kinase C (PKC) activity and oestrogen receptor (ER) expression in human breast cell lines and tumours has been firmly established over the past 10 years. To determine whether specific alterations in PKC expression accompany hormone-independence, we examined the expression of PKC isozymes in the hormone-independent human breast cancer cell clones MCF-7 5C and T47D:C42 compared with their hormone-dependent counterparts, MCF-7 A4, MCF-7 WS8 and T47D:A18 respectively. Both hormone-independent cell clones exhibit elevated PKC alpha expression and increased basal AP-1 activity compared with the hormone-dependent cell clones. To determine whether PKC alpha overexpression is sufficient to mediate the hormone-independent phenotype, we stably transfected an expression plasmid containing PKC alpha cDNA to the T47D:A18 and MCF-7 A4 cell lines. This is the first report of PKC alpha transfection in T47D cells. In contrast to MCF-7 cells, T47D has the propensity to lose the ER and more readily forms tamoxifen-stimulated tumours in athymic mice. We find that in T47D:A18/PKC alpha clones, there is concomitant up-regulation of PKC beta I and delta, whereas in the MCF-7 A4/PKC alpha transfectants PKC epsilon is up-regulated. In T47D:A18, but not in MCF-7 A4, PKC alpha stable transfection is accompanied by down-regulation of ER function whilst basal AP-1 activity is elevated. Our results suggest PKC alpha overexpression may play a role in growth signalling during the shift from hormone dependent to hormone-independent breast cancers.

The alpha isoform of protein kinase C mediates phorbol ester-induced growth inhibition and p21cip1 induction in HC11 mammary epithelial cells

To clarify the roles of specific isoforms of PKC in regulating growth and cell cycle progression of the HC11 mammary epithelial cell line, we investigated the effects of activating endogenous PKC isoforms with the phorbol ester tumor promoter TPA, and also the effects of TPA on genetically engineered cells containing increased levels of individual PKC isoforms. We found that TPA treatment of HC11 cells induced a transient cell cycle arrest in G0/G1. Western blot analyses of the TPA treated cells provided evidence that the endogenous PKC alpha present in these cells mediated these effects. Indeed, derivatives of the HC11 cell line that inducibly overexpress an exogenous PKC alpha or ectopic PKC beta 1 exhibited more marked growth inhibition by TPA than control cells. Immunohistochemical staining of cells following treatment with TPA revealed selective translocation of PKC alpha into the nucleus, whereas PKC beta 1 remained in the cytoplasm. The transient arrest of HC11 cells following treatment with TPA was associated with marked induction of both p21cip1 mRNA and protein. This induction was exaggerated in the derivatives that overexpressed either PKC alpha or PKC beta 1. Therefore, in mouse mammary epithelial cells activation of the endogenous PKC alpha can transiently arrest cells in G0/G1 which may be due, at least in part, to induction of the transcription of p21cip1.

Regulation of protein kinase C delta by estrogen in the MCF-7 human breast cancer cell line

We have previously shown that estrogen up-regulates expression of protein kinase C (PKC) delta in the rat and rabbit corpus luteum as well as in luteinized rat granulosa primary cell cultures. To determine whether a similar regulation of the PKC delta isoform by estrogen occurred in another estrogen responsive system, we investigated the estrogen receptor positive MCF-7 human breast cancer cells. In a characterization of PKC isoforms in MCF-7 cells we determined that PKC delta was the predominant PKC isoform. However in contrast to the effect of estrogen on PKC delta expression in ovarian cells, estrogen treatment of MCF-7 cells resulted in a significant decrease in PKC delta protein and mRNA expression in a time and dose dependent manner. Treatment of MCF-7 cells with 10(-10)-10(-8) M estrogen for 7 days down-regulated specifically PKC delta mRNA and protein while expression of other PKC isoforms was unchanged. The opposite regulation of PKC delta expression in ovarian and breast cancer cells prompted us to evaluate the type of estrogen receptor present in both cell types. Results showed that luteinized rat granulosa cells expressed predominantly estrogen receptor beta while the MCF-7 cells expressed predominantly estrogen receptor alpha and barely detectable levels of estrogen receptor beta. These results suggest that the differential ability of estrogen to regulate PKC beta expression could potentially be a result of differential signaling through the two estrogen receptor subtypes.

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

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

Antiestrogen stimulated human endometrial cancer growth: laboratory and clinical considerations

The new antiestrogen toremifene (TOR) is currently on the market for the treatment of advanced breast cancer in postmenopausal women. TOR is known to exhibit a similar efficacy profile as tamoxifen (TAM) in the treatment of advanced breast cancer and there are studies to suggest that the beneficial side effects of TAM on bone and blood lipids are also achieved with TOR. However, the data concerning the action of TOR on the endometrium is sorely lacking. In light of the estrogenic effect of TAM on the uterus and the 2-3-fold increased incidence in endometrial carcinoma detected in patients receiving TAM therapy, it is imperative to investigate the effect of TOR on endometrial carcinoma. We compared the actions of TAM and TOR on the EnCa101 human endometrial tumor model and find that both antiestrogens have similar growth stimulatory effects. To investigate a potential mechanism of antiestrogen-stimulated endometrial tumor growth, we have examined known activators of the AP-1 signal transduction pathway, the protein kinase C (PKC) family of isozymes, in the EnCa101 human endometrial tumor model. We find that increased PKC isozyme expression correlates with hormone-independent breast cancer as well as antiestrogen-stimulated endometrial cancer.

Methylation-mediated regulation of the glutathione S-transferase P1 gene in human breast cancer cells

Understanding the mechanisms that regulate the human pi class GST (GSTP1) gene expression in breast cancer cells is of particular importance to the study of breast cancer biology. In cultured human breast cancer cell lines, GSTP1 is exclusively expressed in estrogen receptor-negative (ER-) cells but is undetectable in receptor-positive (ER+) cells. Previously, we examined transiently transfected GSTP1 promoter activities, in vitro GSTP1 promoter-DNA interactions, and GSTP1 mRNA stability. These studies indicated that transiently transfected GSTP1 promoter elements and GSTP1 mRNA stability could only partially explain cell line-specific expression of endogenous GSTP1. In the present study, we examined whether the methylation status of the GSTP1 CpG island plays an important role in GSTP1 regulation. Southern blot analysis revealed that the GSTP1 CpG island is hypermethlyated in ER+, GSTP1 non-expressing cell lines but is undermethylated in ER-, GSTP1 expressing cell lines. Moreover, partial demethylation of the GSTP1 CpG island by treatment with 5-aza-2'-deoxycytidine resulted in de novo gene expression in ER+ cell lines, as detected by RT-PCR, Northern blot and Western blot analyses. Our data strongly indicate that methylation status of the promoter contributes significantly to the levels of GSTP1 expressed in ER- and ER+ breast cancer cell lines.

Breast cancer cell invasiveness: correlation with protein kinase C activity and differential regulation by phorbol ester in estrogen receptor-positive and -negative cells

Increased protein kinase C (PKC) activity in malignant breast tissue and in most aggressive breast cancer cell lines has suggested a possible role of PKC in breast carcinogenesis and tumor progression. We have investigated here the involvement of PKC in the in vitro invasiveness and motility of several breast cancer cell lines. Modulation of PKC activity by treatment with a phorbol ester (TPA), drastically increased the invasiveness of 2 estrogen receptor-positive (ER+) lines (MCF7 and ZR 75.1), whereas it markedly decreased the invasiveness of 2 ER- cell lines (MDA-MB-231 and MDA-MB-435). A PKC inhibitor (H7) reversed the TPA effects in MCF7 cells, whereas it mimicked TPA action in MDA-MB-231 cells. All of these effects of TPA also were observed to a similar extent for cell chemotaxis, and they were not dependent on protein neo-synthesis. In parallel, short TPA treatment induced cell spreading and microtubule organization in MCF7 cells and inverse morphological changes in MDA-MB-231 cells. In ER+ cells, constitutive PKC activity and PKCalpha expression were very low as compared to ER- cells, and this correlated with the invasive potential of the cells. The opposed effects of TPA in ER+ and ER- cells could be due to the abnormal TPA regulation of PKCalpha observed in ER- cells.

Retinoic acid induced growth arrest of human breast carcinoma cells requires protein kinase C alpha expression and activity

Retinoic acid inhibits proliferation of hormone-dependent, but not hormone-independent breast cancer cells. Retinoic acid-induced changes in cellular proliferation and differentiation are associated with disturbances in growth factor signaling and frequently with changes in protein kinase C expression. PKC delta, epsilon, and zeta are expressed in both hormone-dependent (T-47D) and hormone-independent (MDA-MB-231) cell lines. Retinoic acid arrested T-47D proliferation, induced PKC alpha expression and concomitantly repressed PKC zeta expression. The changes in PKC alpha and PKC zeta reflect retinoic acid-induced changes in mRNA. In contrast, retinoic acid had no effect on growth, or PKC expression in MDA-MB-231 cells. Growth arrest and the induction of PKC alpha, but not the reduction in PKC zeta, resulted from selective activation of RAR alpha. In total, these results support an important role for PKC alpha in mediating the anti-proliferative action of retinoids on human breast carcinoma cells.

Elevated levels of ERK2 in human breast carcinoma MCF-7 cells transfected with protein kinase C alpha

We investigated the effect of elevated levels of protein kinase C alpha (PKC alpha) on cell proliferation in human breast carcinoma cells (MCF-7). MCF-7 cells transfected with either the pSV2M(2)6 vector without the insert (MCF-7/Vector) or containing a full length cDNA encoding PKC alpha (MCF-7/PKC alpha) were compared. MCF-7/PKC alpha cells were found to have an increased proliferative rate with a doubling time of 15 h as compared to 42 h for MCF-7/Vector cells. Flow cytometry illustrated a greater percentage of MCF-7/PKC alpha cells in the S phase of the cell cycle. Western and Northern blot analyses demonstrated an increase in extracellular regulated protein kinase 2 (ERK2) gene expression in MCF-7/PKC alpha cells but no alteration of this gene expression in MCF-7/Vector cells. These results suggested that the elevated level of ERK2 which is also known as mitogen activated protein kinase is probably involved in the increase in MCF-7/PKC alpha cell proliferation.

Dual regulation of the epidermal growth factor family of growth factors in breast cancer by sex steroids and protein kinase C

There has been increased interest in the last few years in seeking a better understanding of the local regulation of polypeptide growth factors by systemic hormones, such as sex steroids and by polypeptide hormones. Growth factors and systemic hormones play pivotal roles in hormone-regulated cancers such as breast cancer. In this review, we discuss the regulation of members of the epidermal growth factor (EGF) family by sex steroids and by regulators of the polypeptide hormone signal transduction enzyme termed protein kinase C (PKC). Regulation of the EGF family of genes will be discussed as a model system to evaluate interactions between these two important types of regulatory pathways in breast cancer.

Effects of 12-O-tetradecanoylphorbol-13-acetate on estrogen receptor activity in MCF-7 cells

The effects of long term treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA) on estrogen receptor (ER) expression in the human breast cancer cell line, MCF-7, were studied. This study demonstrates that treatment of cells with the phorbol ester blocked estrogen receptor activity. Treatment of cells with 100 nM TPA resulted in an 80% decrease in the level of ER protein and a parallel decrease in ER mRNA and binding capacity. Following removal of TPA from the medium, the level of ER protein and mRNA returned to control values; however, the receptor failed to bind estradiol. These cells also failed to induce progesterone receptor in response to estradiol. In addition, TPA treatment blocked transcription from an estrogen response element in transient transfection assays and inhibited ER binding to its response element in a DNA mobility shift assay. The estrogen receptor in treated cells was recognized by two monoclonal anti-ER antibodies and was not quantitatively different from ER in control cells. RNase protection analysis failed to detect any qualitative changes in the ER mRNA transcript. Mixing experiments suggest that TPA induces/activates a factor which interacts with the ER to block binding of estradiol. The effects of TPA on ER levels and binding capacity were concentration-dependent. Low concentrations of TPA inhibited estradiol binding without a decrease in the level of protein, whereas higher concentrations were required to decrease the level of ER protein. The effects of TPA appear to be mediated by activation of protein kinase C since the protein kinase C inhibitors, H-7 and bryostatin, block the effects of TPA on estradiol induction of progesterone receptor. TPA treatment had no effect on the level or binding capacity of the glucocorticoid receptor, indicating that the effects are not universal for steroid receptors. These data demonstrate that activation of the protein kinase C signal transduction pathway modulates the estrogen receptor pathway. The long term effect of protein kinase C activation is to inhibit estrogen receptor function through induction/activation of a factor which interacts with the receptor.

Phorbol esters induce death in MCF-7 breast cancer cells with altered expression of protein kinase C isoforms. Role for p53-independent induction of gadd-45 in initiating death

Protein kinase C (PKC) modulates growth, differentiation and apoptosis in a cell-specific fashion. Overexpression of PKC-alpha in MCF-7 breast cancer cells (MCF-7-PKC-alpha cell) leads to expression of a more transformed phenotype. The response of MCF-7 and MCF-7-PKC-alpha cells to phorbol esters (TPA) was examined. TPA-treated MCF-7 cells demonstrated a modest cytostatic response associated with a G1 arrest that was accompanied by Cip1 expression and retinoblastoma hypophosphorylation. While p53 was detected in MCF-7 cells, evidence for TPA-induced stimulation of p53 transcriptional activity was not evident. In contrast, TPA treatment induced death of MCF-7-PKC-alpha cells. Bryostatin 1, another PKC activator, exerted modest cytostatic effects on MCF-7 cells while producing a cytotoxic response at low doses in MCF-7-PKC-alpha cells that waned at higher concentrations. TPA-treated MCF-7-PKC-alpha cells accumulated in G2/M, did not express p53, displayed decreased Cip1 expression, and demonstrated a reduction in retinoblastoma hypophosphorylation. TPA-treated MCF-7-PKC-alpha cells expressed gadd-45 which occurred before the onset of apoptosis. Thus, alterations in the PKC pathway can modulate the decision of a breast cancer cell to undergo death or differentiation. In addition, these data show that PKC activation can induce expression of gadd45 in a p53-independent fashion.

MCF-7 breast cancer cells transfected with protein kinase C-alpha exhibit altered expression of other protein kinase C isoforms and display a more aggressive neoplastic phenotype

Increased protein kinase C (PKC) activity in malignant breast tissue and positive correlations between PKC activity and expression of a more aggressive phenotype in breast cancer cell lines suggest a role for this signal transduction pathway in the pathogenesis and/or progression of breast cancer. To examine the role of PKC in the progression of breast cancer, human MCF-7 breast cancer cells were transfected with PKC-alpha, and a group of heterogenous cells stably overexpressing PKC-alpha were isolated (MCF-7-PKC-alpha). MCF-7-PKC-alpha cells expressed fivefold higher levels of PKC-alpha as compared to parental or vector-transfected MCF-7 cells. MCF-7-PKC-alpha cells also displayed a substantial increase in endogenous expression of PKC-beta and decreases in expression of the novel delta- and eta-PKC isoforms. MCF-7-PKC-alpha cells displayed an enhanced proliferative rate, anchorage-independent growth, dramatic morphologic alterations including loss of an epithelioid appearance, and increased tumorigenicity in nude mice. MCF-7-PKC-alpha cells exhibited a significant reduction in estrogen receptor expression and decreases in estrogen-dependent gene expression. These findings suggest that the PKC pathway may modulate progression of breast cancer to a more aggressive neoplastic process.

Regulation of protein kinase C and role in cancer biology

Protein kinase C (PKC) is a family of closely related lipid-dependent and diacyglycerol-activated isoenzymes known to play an important role in the signal transduction pathways involved in hormone release, mitogenesis and tumor promotion. Reversible activation of PKC by the second messengers diacylglycerol and calcium is an established model for the short term regulation of PKC in the immediate events of signal transduction. PKC can also be modulated long term by changes in the levels of activators or inhibitors for a prolonged period or by changes in the levels of functional PKC isoenzymes in the cell during development or in response to hormones and/or differentiation factors. Indeed, studies have indicated that the sustained activation or inhibition of PKC activity in vivo may play a critical role in regulation of long term cellular events such as proliferation, differentiation and tumorigenesis. In addition, these regulatory events are important in colon cancer, where a decrease in PKC activators and activity suggests PKC acts as an anti-oncogene, in breast cancer, where an increase in PKC activity suggests an oncogenic role for PKC, and in multidrug resistance (MDR) and metastasis where an increase in PKC activity correlates with increased resistance and metastatic potential. These studies highlight the importance and significance of regulation of PKC activity in vivo.

The protein bcl-2 alpha does not require membrane attachment, but two conserved domains to suppress apoptosis

Bcl-2 is a mitochondrial- and perinuclear-associated protein that prolongs the lifespan of a variety of cell types by interfering with programmed cell death (apoptosis). Bcl-2 seems to function in an antioxidant pathway, and it is believed that membrane attachment mediated by a COOH-terminal hydrophobic tail is required for its full activity. To identify critical regions in bcl-2 alpha for subcellular localization, activity, and/or interaction with other proteins, we created, by site-directed mutagenesis, various deletion, truncation, and point mutations. We show here that membrane attachment is not required for the survival activity of bcl-2 alpha. A truncation mutant of bcl-2 alpha lacking the last 33 amino acids (T3.1) including the hydrophobic COOH terminus shows full activity in blocking apoptosis of nerve growth factor-deprived sympathetic neurons or TNF-alpha-treated L929 fibroblasts. Confocal microscopy reveals that the T3 mutant departs into the extremities of neurites in neurons and filopodias in fibroblasts. Consistently, T3 is predominantly detected in the soluble fraction by Western blotting, and is not inserted into microsomes after in vitro transcription/translation. We further provide evidence for motifs (S-N and S-II) at the NH2 and COOH terminus of bcl-2, which are crucial for its activity.

Design, synthesis and investigation of mechanisms of action of novel protein kinase C inhibitors: perylenequinonoid pigments

A series of perylenequinonoid pigments (PQPs) and related compounds were synthesized and screened for the inhibition of protein kinase C (PKC), a key enzyme involved in cellular differentiation and proliferation, and a potential target for anticancer and antiviral chemotherapeutic drugs. This study has established PQPs as efficient PKC inhibitors, and elucidated aspects of the light-enhanced action mode of the PKC inhibitors. Comparative studies between natural and synthetic PQPs led to the recognition of the effect of certain structural features of PQPs on PKC inhibition, including the skeleton of the 3,10-dihydroxy-4,9-perylenequinonoid chromophore and the configuration of the two side chains at positions 1 and 12. Calphostin C was identified as a superior PKC inhibitor of the POP class, and with the latter as a representative structure, we investigated the mechanism of PKC inhibition by PQPs via electron paramagnetic resonance spectroscopy in conjunction with the spin-trapping technique, absorption and fluorescence spectroscopy, photochemical and photobiological studies, and enzyme methodology. Multiple modes of action are suggested for PKC inhibition, comprising the following steps: (1) the binding of PQPs to the PKC regulatory domain via complexation; (2) the photobonding between mercapto groups of PKC cysteine residues and the PQP quinonoid moiety; and (3) the PQP-sensitized photodamage of PKC via Type I and/or Type II photosensitization.

Hypoxia-induced accumulation of erythropoietin mRNA in isolated hepatocytes is inhibited by protein kinase C

To define the role of protein kinase C (PKC) in oxygen-dependent production of erythropoietin (EPO) in the liver, we have determined EPO messenger ribonucleic acid (mRNA) expression in primary cultures of juvenile rat hepatocytes incubated at different oxygen tensions in the absence and presence of phorbol esters, vasopressin, and structurally different kinase inhibitors. Upon reduction of oxygen concentrations from 40% to 3% EPO mRNA in cultured hepatocytes increased markedly within 1.25 h, reached maximal values after 2.5 h and remained elevated for up to 72 h. Treatment of hepatocytes during 1.25-5 h of hypoxic exposure with phorbol 12-myristate-13 acetate (PMA) attenuated hypoxia-induced EPO mRNA levels dose-dependently by a maximum of approximately 50%. This inhibitory effect of PMA disappeared upon treatment for more than 5 h and was completely lost after incubation for 9 and 18 h in the presence of 10(-6) M and 10(-7) M PMA, respectively. Phorbol 12,13-dibutyrate and vasopressin also inhibited EPO mRNA accumulation, whereas 4 alpha-phorbol 12,13-didecanoate was ineffective. Western blot analysis of PKC isozymes revealed the presence of PKC alpha, beta II, delta, epsilon and zeta and provided no evidence that the PMA-induced inhibition of EPO expression was associated with depletion of any of these isozymes. Conversely, PMA-induced inhibition of EPO mRNA accumulation was paralleled by translocation of PKC alpha from cytosol to membranes and the time- and dose-dependent attenuation of the inhibitory effect of PMA on EPO mRNA levels was paralleled by down-regulation of PKC alpha. A dose-dependent inhibition of EPO mRNA formation, independent of effects on total RNA synthesis, as determined by [3H]uridine incorporation, was also found in the presence of the kinase inhibitor staurosporine (ED50 approximately 2 x 10(-8) M) and three structurally related derivatives with increased selectivity for PKC (RO 317549, ED50 approximately 1 x 10(-6) M; RO 318220, ED50 approximately 1 x 10(-6) M and CGP 41251, ED50 approximately 4 x 10(-6) M). The markedly lower potency of the latter three compounds as compared to staurosporine suggests that this suppression of EPO gene induction was not mediated by inhibition of PKC. In summary the data indicate that PKC alpha is a negative modulator of EPO gene expression in hepatocytes. A kinase other than PKC, however, appears to be an essential element of hypoxic signalling.

Inhibition of estrogen stimulated mitogenesis by 3-phenylacetylamino-2,6-piperidinedione and its para-hydroxy analog

3-Phenylactetylamino-2,6-piperidinedione (A10) inhibited estradiol stimulated cell growth in the MCF-7 (E3) human breast tumor cell line in vivo and in vitro. While high concentrations of A10 were needed to inhibit cell proliferation (IC50 = 3 x 10(-3) M in vitro), the compound demonstrated little toxicity. The effect appeared specific since a hydrolysis product of A10, phenylacetylglutamine, demonstrated no growth inhibitory activity at similar concentrations in MCF-7 (E3) cells in vitro. A computer designed analog, p-hydroxy A10, was more potent than A10 in inhibiting activity in MCF-7 (E3) cells in vitro. The IC50 for p-hydroxy A10 was 7 x 10(-6) M which was comparable to that of the antiestrogen, tamoxifen (IC50 1 x 10(-7) M). All three compounds caused a decline in estrogen receptor levels in a dose-dependent fashion. A10 also inhibited estradiol induction of progesterone receptors. Examination of protein kinase activity following an acute exposure to a 10(-11) M growth stimulatory dose of estradiol revealed a 168% increase in protein kinase activity over that of untreated control cells. A10 in a dose-responsive fashion inhibited the estradiol stimulated increase in protein kinase activity. The protein kinase activity was also inhibited by p-hydroxy A10. These activities of A10 and p-hydroxy A10 coupled with the low toxicity and novelty of the basic A10 structure provide an exciting possibility of developing a new class of clinically useful antineoplastic drugs with minimal side effects.

Regulation of prolactin receptor expression by the tumour promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate in human breast cancer cells

In both the normal and malignant human breast, cellular sensitivity to the proliferative and differentiative activities of the lactogenic hormones is conferred by expression of the prolactin receptor (PRLR). The PRLR is regulated by steroid hormones; however, recent findings have suggested that PRLR may also be regulated by protein kinase C. To examine this possibility we have studied the effect of various modulators of PKC activity on PRLR binding activity and gene expression in five PRLR positive human breast cancer cell lines. Treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA), a tumour promoter and modulator of PKC activity, decreased PRLR binding activity in all cell lines examined. In MCF-7 cells, 10 nM TPA caused a 70% loss of PRLR mRNA after 12 h, paralleled 3 h later by a comparable loss of cell surface PRLR. Mezerein, a non-phorbol ester modulator of PKC activity and 1,2-dioctanoyl-sn-glycerol, a permeant analogue of the endogenous activator of PKC, also reduced PRLR binding activity and gene expression in a time- and concentration-dependent manner. Cycloheximide failed to abrogate the TPA-induced decline in PRLR mRNA levels, indicating that this process was not dependent upon continuing protein synthesis. No change in the stability of PRLR mRNA was observed during 24 h of TPA treatment and TPA reduced the rate of PRLR gene transcription within 3 h of treatment. These results demonstrate that modulators of PKC activity reduce PRLR binding activity and gene expression, implicating this signal transduction pathway in PRLR regulation.

Effect of tumour-promoting phorbol ester, thrombin and vasopressin on translocation of three distinct protein kinase C isoforms in human platelets and regulation by calcium

Protein kinase C (PKC) acts in synergy with Ca2+ mobilization for the activation of platelets. Three different PKC subtypes that specifically react with antibodies to alpha- beta- and zeta-PKC have been detected in human platelets. We have compared the subcellular redistribution of these isoforms in platelets after exposure to the tumour-promoting phorbol ester phorbol 12-myristate 13-acetate (PMA) and to two physiological agonists, thrombin and vasopressin. In the presence of PMA, beta-PKC is most rapidly translocated to membranes, followed by zeta-PKC and alpha-PKC [membrane contents of 39 +/- 6, 31 +/- 4 and 24 +/- 4% (means +/- S.E.M.) respectively after 2 min incubation]. In contrast, both thrombin and vasopressin induced a biphasic translocation of PKC isoforms. For both agonists, the first phase of translocation occurred within 1 min and was identical for the three isoforms. However, during the second phase, the translocation of zeta-PKC by thrombin and vasopressin differed [membrane contents (mean +/- S.E.M.) of 24 +/- 3 and 46 +/- 4% respectively after 10 min]. These results suggest a differential activation of zeta-PKC by vasopressin and thrombin. PMA-induced translocation of alpha-PKC was decreased from 278 +/- 27 to 198 +/- 24 (mean +/- S.E.M., P = 0.02; percentage increase over control value) in the presence of 1 mM-EDTA, whereas chelation of intracellular Ca2+ by Quin2-AM does not influence this response. These results suggest that the PMA-induced translocation of alpha-PKC depends on the presence of 1 mM concentration of extracellular Ca2+. In addition, the chelation of either extracellular or intracellular Ca2+ inhibited both vasopressin- and thrombin-induced translocation of all three isoforms, suggesting that Ca2+ is an important requirement for the translocation of alpha-, beta- and zeta-PKC by physiological agonists. In conclusion, the translocation of PKC varies between different isoforms and between different agonists.

Phorbol ester inhibits erythropoietin production in human hepatoma cells (Hep G2)

Using the human hepatoma cell line Hep G2, we have studied a possible role of protein kinase C (PKC) activity for regulation of erythropoietin (EPO) production. During a 72-h incubation, EPO production by the cells was stimulated sevenfold by exposure to low oxygen tension (1%) and threefold by exposure to cobaltous chloride (100 microM). The phorbol ester phorbol 12-myristate-13 acetate (PMA) led to a concentration-dependent inhibition of basal and stimulated EPO formation (ED50 10 nM). This decrease of EPO production, which was apparent already after 1 h of incubation with PMA, reached its maximal effect after 24 h and held on for 72 h. It was paralleled by an inhibition of the increase of EPO mRNA levels in response to stimulation. A 24-h preincubation of the cells with PMA (100 nM) virtually blunted the effect of hypoxia on EPO formation. Recovery of EPO synthesis after removal of PMA took 48-72 h. The effect of PMA on EPO production was mimicked by phorbol 12,13-dibutyrate (ED50 1 microM) but not by 4 alpha-phorbol 12,13-didecanoate. The synthetic diacylglycerol analogues oleolyl-acetylglycerol and dioctanoylglycerol (2-200 microM) also had no effect on either basal or stimulated EPO production. Treatment with PMA caused a translocation of the alpha-isoenzyme of PKC from the cytosol to the membrane after 1 h and a disappearance of the membrane-bound form after 24 h of incubation. Staurosporine and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, two structurally different inhibitors of PKC activity, inhibited basal and stimulated EPO production with ED50 values of 9 nM and 50 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)