Protein kinase Calpha expression confers retinoic acid sensitivity on MDA-MB-231 human breast cancer cells

The complexities of PKCα signaling in cancer

Protein kinase C α (PKCα) is a ubiquitously expressed member of the PKC family of serine/threonine kinases with diverse functions in normal and neoplastic cells. Early studies identified anti-proliferative and differentiation-inducing functions for PKCα in some normal tissues (e.g., regenerating epithelia) and pro-proliferative effects in others (e.g., cells of the hematopoietic system, smooth muscle cells). Additional well documented roles of PKCα signaling in normal cells include regulation of the cytoskeleton, cell adhesion, and cell migration, and PKCα can function as a survival factor in many contexts. While a majority of tumors lose expression of PKCα, others display aberrant overexpression of the enzyme. Cancer-related mutations in PKCα are uncommon, but rare examples of driver mutations have been detected in certain cancer types (e. g., choroid gliomas). Here we review the role of PKCα in various cancers, describe mechanisms by which PKCα affects cancer-related cell functions, and discuss how the diverse functions of PKCα contribute to tumor suppressive and tumor promoting activities of the enzyme. We end the discussion by addressing mutations and expression of PKCα in tumors and the clinical relevance of these findings.

All-trans retinoic acid and protein kinase C α/β1 inhibitor combined treatment targets cancer stem cells and impairs breast tumor progression

Breast cancer is the leading cause of cancer death among women worldwide. Blocking a single signaling pathway is often an ineffective therapy, especially in the case of aggressive or drug-resistant tumors. Since we have previously described the mechanism involved in the crosstalk between Retinoic Acid system and protein kinase C (PKC) pathway, the rationale of our study was to evaluate the effect of combining all-trans-retinoic acid (ATRA) with a classical PCK inhibitor (Gö6976) in preclinical settings. Employing hormone-independent mammary cancer models, Gö6976 and ATRA combined treatment induced a synergistic reduction in proliferative potential that correlated with an increased apoptosis and RARs modulation towards an anti-oncogenic profile. Combined treatment also impairs growth, self-renewal and clonogenicity potential of cancer stem cells and reduced tumor growth, metastatic spread and cancer stem cells frequency in vivo. An in-silico analysis of “Kaplan–Meier plotter” database indicated that low PKCα together with high RARα mRNA expression is a favorable prognosis factor for hormone-independent breast cancer patients. Here we demonstrate that a classical PKC inhibitor potentiates ATRA antitumor effects also targeting cancer stem cells growth, self-renewal and frequency.

Retinoic Acids in the Treatment of Most Lethal Solid Cancers

Although the use of oral administration of pharmacological all-trans retinoic acid (ATRA) concentration in acute promyelocytic leukaemia (APL) patients was approved for over 20 years and used as standard therapy still to date, the same use in solid cancers is still controversial. In the present review the literature about the top five lethal solid cancers (lung, stomach, liver, breast, and colon cancer), as defined by The Global Cancer Observatory of World Health Organization, and retinoic acids (ATRA, 9-cis retinoic acid, and 13-cis retinoic acid, RA) was compared. The action of retinoic acids in inhibiting the cell proliferation was found in several cell pathways and compartments: from membrane and cytoplasmic signaling, to metabolic enzymes, to gene expression. However, in parallel in the most aggressive phenotypes several escape routes have evolved conferring retinoic acids-resistance. The comparison between different solid cancer types pointed out that for some cancer types several information are still lacking. Moreover, even though some pathways and escape routes are the same between the cancer types, sometimes they can differently respond to retinoic acid therapy, so that generalization cannot be made. Further studies on molecular pathways are needed to perform combinatorial trials that allow overcoming retinoic acids resistance.

Down-regulation of PKCζ in renal cell carcinoma and its clinicopathological implications

Background

Metastatic renal cell carcinoma (RCC) is highly resistant to systemic chemotherapy. Unfortunately, nearly all patients die of the metastatic and chemoresistant RCC. Recent studies have shown the atypical PKCζ is an important regulator of tumorigenesis. However, the correlation between PKCζ expression and the clinical outcome in RCC patients is unclear. We examined the level of PKCζ expression in human RCC.

Methods

PKCζ mRNA and protein expressions were examined by real-time polymerase chain reaction (PCR) and immunohistochemistry (IHC) respectively in RCC tissues of 144 patients. Cellular cytotoxicity and proliferation were assessed by MTT.

Results

PKCζ expression was significantly higher in normal than in cancerous tissues (P < 0.0001) by real-time PCR and IHC. Similarly, PKCζ expression was down-regulated in four renal cancer cell lines compared to immortalized benign renal tubular cells. Interestingly, an increase of PKCζ expression was associated with the elevated tumor grade (P = 0.04), but no such association was found in TNM stage (P = 0.13). Tumors with higher PKCζ expression were associated with tumor size (P = 0.048). Expression of higher PKCζ found a poor survival in patients with high tumor grade. Down-regulation of PKCζ showed the significant chemoresistance in RCC cell lines. Inactivation of PKCζ expression enhanced cellular resistance to cisplatin and paclitaxel, and proliferation in HK-2 cells by specific PKCζ siRNA and inhibitor.

Conclusions

PKCζ expression was associated with tumorigenesis and chemoresistance in RCC.

PKCα inhibited apoptosis by decreasing the activity of JNK in MCF-7/ADR cells

The development of multidrug resistance (MDR) in breast cancer patients is a serious therapeutic problem. The role of signal transduction in the development of MDR has drawn intensive attention recently. In this study, the role of c-Jun N-terminal kinase (JNK) pathway in MDR, specifically regulated by PKCα, was investigated in MCF-7/ADR cells. MTT, DNA ladder and flow cytometry were used to detect cell growth inhibition or apoptosis while Western blot was used to detect the activation of proteins. Compared with MCF-7 cells, the cell growth inhibition and apoptosis induced by tamoxifen (TAM) could not be detected in MCF-7/ADR cells, but the expression of PKCα in MCF-7/ADR cells was higher. And, Western blot results showed that JNK was activated by TAM in MCF-7 cells while not in MCF-7/ADR cells, even at very high doses. In addition, sp600125, the inhibitor of JNK, decreased the percentage of apoptosis induced by TAM in MCF-7 cells. These data showed that PKCα and JNK were key regulators in the apoptosis of MCF-7/ADR cells. Furthermore, PKCα being the upstream of JNK in inhibiting apoptosis was suggested by using Go6976, the specific PKCα inhibitor, in the presence or absence of sp600125. This study highlighted an important signaling pathway involved in MDR regulated by PKCα in MCF-7/ADR breast cancer cells and implied that JNK might be an important downstream target of PKCα in this cellular context.

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

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

Comparative effect of seeds of Rhynchosia volubilis and soybean on MG-63 human osteoblastic cell proliferation and estrogenicity

The seeds of Rhynchosia volubilis (SRV) (Leguminosae) and soybean have been used in oriental folk medicine to prevent postmenopausal osteoporosis. Their beneficial effects are caused by a high content of isoflavone, which function as partial agonists or antagonists of estrogen. To compare the estrogenic effects of SRV and soybean on the MG-63 osteoblastic cell proliferation, 70% methanol extracts of SRV or soybean were treated on MG-63 cells. Although biphasic over a concentration range of 0.001 mg/ml-0.1 mg/ml, both SRV and soybean extracts increased MG-63 cell proliferation. However SRV was more effective at increasing the cell proliferation that paralleled with the greater estrogenic effects as determined by estrogen receptor alpha (ERalpha) expression, an estrogenic response element (ERE)-luciferase activity and the selective expression of insulin-like growth factor-I (IGF-I). SRV-induced IGF-I expression resulted from increases in the mRNA levels. Despite the increased expression of ERbeta, ERE activity and IGF-I expression by soybean were lower than those by SRV. Furthermore, the comparable estrogenic effects between SRV and the combined treatment of genistein and daidzein standards at 0.5 x 10(-8) M, which is a concentration of these two isoflavones similar to that of SRV at 0.001 mg/ml, demonstrate that the greater estrogenicity of SRV for MG-63 cell proliferation is mediated by the synergism of low levels of isoflavones for the selective expression of IGF-I.

Valproic acid, in combination with all-trans retinoic acid and 5-aza-2'-deoxycytidine, restores expression of silenced RARbeta2 in breast cancer cells

Epigenetic silencing of tumor suppressor genes has been established as an important process of carcinogenesis. The retinoic acid (RA) receptor beta2 (RARbeta2) gene is one such tumor suppressor gene often silenced during carcinogenesis. The combined use of histone deacetylase and DNA methyltransferase inhibitors has been shown to reverse the epigenetic silencing of numerous growth regulatory genes. Valproic acid (VPA), which has long been used in the treatment of epilepsy, was shown recently to be an effective histone deacetylase inhibitor that can induce differentiation of neoplastically transformed cells. In this study, we show for the first time that VPA, in combination with RA and the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine (Aza-dC), can overcome the epigenetic barriers to transcription of a prototypical silenced tumor suppressor gene, RARbeta2, in human breast cancer cells. Chromatin immunoprecipitation assays show that the combination of VPA, RA, and Aza-dC increases histone acetylation at the silenced RARbeta2 promoter of MCF-7 breast cancer cells. Furthermore, reverse transcription-PCR analyses reveal cell type-specific effects in the actions of VPA on RARbeta2 expression in cultured human breast cancer cells. Finally, we show that VPA, in combination with RA and Aza-dC, inhibits the proliferation of both estrogen receptor alpha-positive (MCF-7) and estrogen receptor alpha-negative (MDA-MB-231) breast cancer cell lines. These data suggest that VPA may ultimately be useful in combination therapies in the treatment of human breast cancers.

Differential effects of all-trans-retinoic acid (RA) on Erk1/2 phosphorylation and cAMP accumulation in normal and malignant human prostate epithelial cells: Erk1/2 inhibition restores RA-induced decrease of cell growth in malignant prostate cells

OBJECTIVE

All-trans-retinoic acid (RA) regulates cellular growth, differentiation and apoptosis in human prostate by binding to RA receptors. Non-genomic retinoid effects on signal transduction kinases in the cytoplasm are also described in several cells but they are still unknown in prostate cells.

METHODS

Using an epithelial cell line derived from normal human prostate (EPN), and normal (NPEC) and malignant (CPEC) epithelial primary cultures of human prostate, we have examined effects of RA on both extracellular signal-regulated kinase 1/2 (Erk1/2) and cAMP accumulation. Then we have verified the effect of the inhibition of Erk1/2 on RA-induced growth arrest and apoptosis in malignant cells.

RESULTS

In NPEC and in EPN treated with RA for up to 24 h, Western blot analyses of Erk1/2 phosphorylation show that RA causes a rapid activation of Erk1/2 within 5 min, which is maintained for 30 min, followed by a return to basal levels. In CPEC, the activated phosphorylation levels persist up to 24 h. While basal cAMP levels are not affected by 30 min treatment with RA in both EPN and NPEC, levels are increased in CPEC. Forskolin-induced cAMP levels are decreased by RA in all cell types. CPEC were incubated for up to 96 h with RA with and without the inhibitor of Erk1/2, UO126. CPEC incubated with RA and UO126 for 72 h showed a significant arrest of cell growth and after 96 h apoptosis in 11% of cells.

CONCLUSIONS

We show rapid effects of RA on cytoplasmic messenger pathways in human prostate, and that responses can differ between normal and malignant cells. The inhibition of these pathways could improve the efficiency of RA in prostate cancer growth control.

Synergistic effects of acyclic retinoid and OSI-461 on growth inhibition and gene expression in human hepatoma cells

Hepatoma is one of the most frequently occurring cancers worldwide. However, effective chemotherapeutic agents for this disease have not been developed. Acyclic retinoid, a novel synthetic retinoid, can reduce the incidence of postsurgical recurrence of hepatoma and improve the survival rate. OSI-461, a potent derivative of exisulind, can increase intracellular levels of cyclic GMP, which leads to activation of protein kinase G and induction of apoptosis in cancer cells. In the present study, we examined the combined effects of acyclic retinoid plus OSI-461 in the HepG2 human hepatoma cell line. We found that the combination of as little as 1.0 micromol/L acyclic retinoid and 0.01 micromol/L OSI-461 exerted synergistic inhibition of the growth of HepG2 cells. Combined treatment with low concentrations of these two agents also acted synergistically to induce apoptosis in HepG2 cells through induction of Bax and Apaf-1, reduction of Bcl-2 and Bcl-xL, and activation of caspase-3, -8, and -9. OSI-461 enhanced the G0-G1 arrest caused by acyclic retinoid, and the combination of these agents caused a synergistic decrease in the levels of expression of cyclin D1 protein and mRNA, inhibited cyclin D1 promoter activity, decreased the level of hyperphosphorylated forms of the Rb protein, induced increased cellular levels of the p21(CIP1) protein and mRNA, and stimulated p21(CIP1) promoter activity. Moreover, OSI-461 enhanced the ability of acyclic retinoid to induce increased cellular levels of retinoic acid receptor beta and to stimulate retinoic acid response element-chloramphenicol acetyltransferase activity. A hypothetical model involving concerted effects on p21(CIP1) and retinoic acid receptor beta expression is proposed to explain these synergistic effects. Our results suggest that the combination of acyclic retinoid plus OSI-461 might be an effective regimen for the chemoprevention and chemotherapy of human hepatoma and possibly other malignancies.

Retinoids arrest breast cancer cell proliferation: retinoic acid selectively reduces the duration of receptor tyrosine kinase signaling

Retinoic acid (RA) induces cell cycle arrest of hormone-dependent human breast cancer (HBC) cells. Previously, we demonstrated that RA-induced growth arrest of T-47D HBC cells required the activity of the RA-induced protein kinase, protein kinase Calpha (PKCalpha) [J. Cell Physiol. 172 (1997) 306]. Here, we demonstrate that RA treatment of T-47D cells interfered with growth factor signaling to downstream, cytoplasmic and nuclear targets. RA treatment did not inhibit epidermal growth factor (EGF) receptor activation but resulted in rapid inactivation. The lack of sustained EGFR activation was associated with transient rather than sustained association of the EGFR with the Shc adaptor proteins and activation of Erk 1/2 and with compromised induction of expression of immediate early response genes. Inhibiting the activity of PKCalpha, a retinoic acid-induced target gene, prevented the effects of RA on cell proliferation and EGF signaling. Constitutive expression of PKCalpha, in the absence of RA, decreased cell proliferation and decreased EGF signaling. RA treatment increased steady-state levels of the protein tyrosine phosphatase PTP-1C and all measured effects of RA on EGF receptor function were reversed by the tyrosine phosphate inhibitor orthovanadate. These results indicate that RA-induced target genes, particularly PKCalpha, prevent sustained growth factor signaling, uncoupling activated receptor tyrosine kinases and nuclear targets that are required for cell cycle progression.

Enhanced retinoid-induced apoptosis of MDA-MB-231 breast cancer cells by PKC inhibitors involves activation of ERK

Retinoids are vitamin A derivatives, which cause growth inhibition, differentiation and/or apoptosis in various cell types, including some breast cancer cells. In general, estrogen receptor (ER)-positive cells are retinoic acid (RA) sensitive, whereas ER-negative cells are resistant. In this report, we show that ER-negative MDA-MB-231 cells are strongly growth inhibited by retinoids in combination with a PKC inhibitor. While neither RA nor GF109203X (GF) has a significant growth inhibitory effect in these cells, RA+GF potently suppress proliferation. We found that RA+GF induce apoptosis, as shown by an increase in fragmented DNA, Annexin-V-positive cells and caspase-3 activation. Apoptosis was also induced by GF in combination with two synthetic retinoids. Expression of phosphorylated as well as total PKC was decreased by GF and this was potentiated by RA. In addition, treatment with GF caused a strong and sustained activation of ERK1/2 and p38-MAPK, as well as a weaker activation of JNK. Importantly, inhibition of ERK but not p38 or JNK suppressed apoptosis induced by RA+GF, indicating that activation of ERK is specifically required. In support of this novel finding, the ability of other PKC inhibitors to cause apoptosis in combination with RA correlates with ability to cause sustained activation of ERK.

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.

The Protein Kinase C Signaling Pathway Regulates a Molecular Switch between Transactivation and Transrepression Activity of the Peroxisome Proliferator-Activated Receptor α

Peroxisome proliferator-activated receptor (PPAR) alpha is a nuclear receptor implicated in several physiological processes such as lipid and lipoprotein metabolism, glucose homeostasis, and the inflammatory response. PPARalpha is activated by natural fatty acids and synthetic compounds like fibrates. PPARalpha activity has been shown to be modulated by its phosphorylation status. PPARalpha is phosphorylated by kinases such as the MAPKs and cAMP-activated protein kinase A. In this report, we show that protein kinase C (PKC) inhibition impairs ligand-activated PPARalpha transcriptional activity. Furthermore, PKC inhibition decreases PPARalpha ligand-induction of its target genes including PPARalpha itself and carnitine palmitoyltransferase I. By contrast, PKC inhibition enhances PPARalpha transrepression properties as demonstrated using the fibrinogen-beta gene as model system. Finally, PKC inhibition decreases PPARalpha phosphorylation activity of hepatocyte cell extracts. In addition, PPARalpha purified protein is phosphorylated in vitro by recombinant PKCalpha and betaII. The replacement of serines 179 and 230 by alanine residues reduces the phosphorylation of the PPARalpha protein. The PPARalpha S179A-S230A protein displays an impaired ligand-induced transactivation activity and an enhanced trans-repression activity. Altogether, our data indicate that the PKC signaling pathway acts as a molecular switch dissociating the transactivation and transrepression functions of PPARalpha, which involved phosphorylation of serines 179 and 230.

Modulating protein kinase C (PKC) to increase the efficacy of chemotherapy: stepping into darkness

The identification of molecules that promote chemotherapeutic resistance would allow rationally designed approaches to abrogate this resistance, thereby possibly improving clinical outcomes for patients with cancer. In this regard, the PKC family is attractive for targeting, because it is comprised of a family of isoforms that play key roles in multiple cellular processes and can contribute to cellular transformation. Encouraging in vitro data originally showed that approaches to modulate PKC activity through small-molecule inhibitors or genetic manipulation could affect tumor cell survival. Recently, some of these approaches have begun clinical testing. Early-stage clinical trials revealed scattered clinical responses to these agents, but the most recent clinical trials have shown that combining modulators of PKC with standard chemotherapy does not improve outcome over chemotherapy alone. In this review, we will trace the development of these approaches, and discuss possible explanations for the recent negative results. Importantly, we will suggest guidelines for the clinical evaluation of PKC modulators.

Effects of Acyclic Retinoid on Growth, Cell Cycle Control, Epidermal Growth Factor Receptor Signaling, and Gene Expression in Human Squamous Cell Carcinoma Cells

We described recently the growth inhibitory effects of the novel compound acyclic retinoid (ACR) in human hepatoma cell lines (M. Suzui et al., Cancer Res., 62: 3997-4006, 2002). In this study we examined the cellular and molecular effects of ACR on human squamous cell carcinoma (SCC) cells. ACR inhibited growth of the esophageal SCC cell line HCE7, and the head and neck SCC cell lines YCU-N861 and YCU-H891, with IC(50) values of approximately 10, 25, and 40 microM, respectively. Detailed studies were then done with HCE7 cells. Treatment of these cells with 10 microM ACR caused an increase of cells in G(0)-G(1) and induced apoptosis. This was associated with two phases of molecular events. During phase 1, which occurred within 6-12 h, there was an increase in the retinoic acid receptor beta (RARbeta) and p21(CIP1) proteins, and their corresponding mRNAs, and a decrease in the hyperphosphorylated form of the retinoblastoma protein. During phase 2, which occurred at approximately 24 h, there was a decrease in the cellular level of transforming growth factor alpha, and the phosphorylated (i.e., activated) forms of the epidermal growth factor receptor, Stat3, and extracellular signal-regulated kinase proteins, and a decrease in both cyclin D1 protein and mRNA. Reporter assays indicated that ACR inhibited the transcriptional activity of the cyclin D1, c-fos, and activator protein promoters. On the other hand, ACR markedly stimulated the activity of a retinoic acid response element-CAT reporter when the cells were cotransfected with a RARbeta expression vector. A hypothetical model explaining these two phases is presented. The diverse effects that we obtained with ACR suggest that this agent might be useful in the chemoprevention and/or therapy of human SCCs.

Cell growth inhibition by all-trans retinoic acid in SKBR-3 breast cancer cells: Involvement of protein kinase C? and extracellular signal-regulated kinase mitogen-activated protein kinase

All-trans retinoic acid (ATRA), a synthetic derivative of vitamin A, inhibits the growth of breast cancer cells. To elucidate the mechanism by which ATRA causes cell growth inhibition, we examined changes in cell cycle and intracellular signaling pathways, focusing on protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). Using the estrogen receptor-negative, retinoid receptor-positive breast cancer cell line SKRB-3, we found that treatment with ATRA significantly decreased the expression of PKCalpha, as well as reducing ERK MAPK phosphorylation. ATRA treatment leads to dephosphorylation of Rb, and consequently to G(1) arrest. Marked changes in the expression of cyclins (particularly cyclins A and E) were observed in SKBR-3 cells treated with ATRA. Using a series of pharmacological and molecular approaches, we found evidence that ATRA-induced SKBR-3 cell growth inhibition involves the deregulation of the PKCalpha-MAPK pathway. These data suggest that retinoids interfered with signal transduction pathways that are crucial for cell cycle progression, and highlight the complexities of the biological effects of retinoid derivatives.