The role of protein kinase C in G1 and G2/M phases of the cell cycle (review)

PKCη promotes senescence induced by oxidative stress and chemotherapy

Senescence is characterized by permanent cell-cycle arrest despite continued viability and metabolic activity, in conjunction with the secretion of a complex mixture of extracellular proteins and soluble factors known as the senescence-associated secretory phenotype (SASP). Cellular senescence has been shown to prevent the proliferation of potentially tumorigenic cells, and is thus generally considered a tumor suppressive process. However, some SASP components may act as pro-tumorigenic mediators on premalignant cells in the microenvironment. A limited number of studies indicated that protein kinase C (PKC) has a role in senescence, with different isoforms having opposing effects. It is therefore important to elucidate the functional role of specific PKCs in senescence. Here we show that PKCη, an epithelial specific and anti-apoptotic kinase, promotes senescence induced by oxidative stress and DNA damage. We further demonstrate that PKCη promotes senescence through its ability to upregulate the expression of the cell cycle inhibitors p21^Cip1 and p27^Kip1 and enhance transcription and secretion of interleukin-6 (IL-6). Moreover, we demonstrate that PKCη creates a positive loop for reinforcing senescence by increasing the transcription of both IL-6 and IL-6 receptor, whereas the expression of IL-8 is specifically suppressed by PKCη. Thus, the presence/absence of PKCη modulates major components of SASP. Furthermore, we show that the human polymorphic variant of PKCη, 374I, that exhibits higher kinase activity in comparison to WT-374V, is also more effective in IL-6 secretion, p21^Cip1 expression and the promotion of senescence, further supporting a role for PKCη in senescence. As there is now considerable interest in senescence activation/elimination to control tumor progression, it is first crucial to reveal the molecular regulators of senescence. This will improve our ability to develop new strategies to harness senescence as a potential cancer therapy in the future.

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

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

Phase II study of paclitaxel plus the protein kinase C inhibitor bryostatin-1 in advanced pancreatic carcinoma

PURPOSE

To determine the efficacy and toxicity of the protein kinase C inhibitor bryostatin-1 plus paclitaxel in patients with advanced pancreatic carcinoma.

METHODS

Each treatment cycle consisted of paclitaxel 90 mg/m by intravenous infusion over 1 hour on days 1, 8, and 16, plus bryostatin 25 mcg/m as a 1-hour intravenous infusion on days 2, 9, and 15, given every 28 days. Patients were evaluated for response after every 2 treatment cycles, and continued therapy until disease progression or prohibitive toxicity. The primary objective was to determine whether the combination produced a response rate of at least 30%.

RESULTS

Nineteen patients with locally advanced or metastatic pancreatic adenocarcinoma received a total of 52 cycles of therapy (range: 1-10). Patients received the combination as first-line therapy for advanced disease (N = 5) or after prior chemotherapy used alone or in combination with local therapy. No patients had a confirmed objective response. The median time to treatment failure was 1.9 months (95% confidence intervals: 1.2, 2.6 months). Reasons for discontinuing therapy included progressive disease or death in 14 patients (74%) or because of adverse events or patient choice in 5 patients (26%). The most common grade 3 to 4 toxicities included leukopenia in 26%, anemia in 11%, myalgias in 11%, gastrointestinal bleeding in 11%, infection in 10%, and thrombosis in 10%.

CONCLUSION

The combination of weekly paclitaxel and bryostatin-1 is not an effective therapy for patients with advanced pancreatic carcinoma.

Translational control of protein kinase Ceta by two upstream open reading frames

Protein kinase C (PKC) represents a family of serine/threonine kinases that play a central role in the regulation of cell growth, differentiation, and transformation. Posttranslational control of the PKC isoforms and their activation have been extensively studied; however, not much is known about their translational regulation. Here we report that the expression of one of the PKC isoforms, PKCeta, is regulated at the translational level both under normal growth conditions and during stress imposed by amino acid starvation, the latter causing a marked increase in its protein levels. The 5' untranslated region (5' UTR) of PKCeta is unusually long and GC rich, characteristic of many oncogenes and growth regulatory genes. We have identified two conserved upstream open reading frames (uORFs) in its 5' UTR and show their effect in suppressing the expression of PKCeta in MCF-7 growing cells. While the two uORFs function as repressive elements that maintain low basal levels of PKCeta in growing cells, they are required for its enhanced expression upon amino acid starvation. We show that the translational regulation during stress involves leaky scanning and is dependent on eIF-2alpha phosphorylation by GCN2. Our work further suggests that translational regulation could provide an additional level for controlling the expression of PKC family members, being more common than currently recognized.

Molecular pathways involved in the synergistic interaction of the PKC beta inhibitor enzastaurin with the antifolate pemetrexed in non-small cell lung cancer cells

Conventional regimens have limited impact against non-small cell lung cancer (NSCLC). Current research is focusing on multiple pathways as potential targets, and this study investigated molecular mechanisms underlying the combination of the PKCβ inhibitor enzastaurin with the multitargeted antifolate pemetrexed in the NSCLC cells SW1573 and A549. Pharmacologic interaction was studied using the combination-index method, while cell cycle, apoptosis induction, VEGF secretion and ERK1/2 and Akt phosphorylation were studied by flow cytometry and ELISAs. Reverse transcription–PCR, western blot and activity assays were performed to assess whether enzastaurin influenced thymidylate synthase (TS) and the expression of multiple targets involved in cancer signaling and cell cycle distribution. Enzastaurin-pemetrexed combination was highly synergistic and significantly increased apoptosis. Enzastaurin reduced both phosphoCdc25C, resulting in G2/M checkpoint abrogation and apoptosis induction in pemetrexed-damaged cells, and GSK3β and Akt phosphorylation, which was additionally reduced by drug combination (−58% in A549). Enzastaurin also significantly reduced pemetrexed-induced upregulation of TS expression, possibly through E2F-1 reduction, whereas the combination decreased TS in situ activity (>50% in both cell lines) and VEGF secretion. The effects of enzastaurin on signaling pathways involved in cell cycle control, apoptosis and angiogenesis, as well as on the expression of genes involved in pemetrexed activity provide a strong experimental basis to their evaluation as pharmacodynamic markers in clinical trials of enzastaurin-pemetrexed combination in NSCLC patients.

Analysis of protein kinase C delta (PKC delta) expression in endometrial tumors

Endometrial cancer is the most common gynecologic malignancy in the United States. However, its underlying molecular mechanisms are poorly understood; and few prognostic indicators have been identified. The protein kinase C (PKC) family has been shown to regulate pathways critical to malignant transformation; and in endometrial tumors, changes in PKC expression and activity have been linked to a more aggressive phenotype and poor prognosis. We have recently shown that PKC delta is a critical regulator of apoptosis and cell survival in endometrial cancer cells; however, PKC delta levels in endometrial tumors had not been determined. We used immunohistochemistry to examine PKC delta protein levels in normal endometrium and endometrioid carcinomas of increasing grade. Normal endometrium exhibited abundant nuclear and cytoplasmic staining of PKC delta confined to glandular epithelium. In endometrial tumors, decreased PKC delta expression, both in intensity and fraction of epithelial cells stained, was observed with increasing tumor grade, with PKC delta being preferentially lost from the nucleus. Consistent with these observations, endometrial cancer cell lines derived from poorly differentiated tumors exhibited reduced PKC delta levels relative to well-differentiated lines. Treatment of endometrial cancer cells with etoposide resulted in a translocation of PKC delta from cytoplasm to nucleus concomitant with induction of apoptosis. Decreased PKC delta expression, particularly in the nucleus, may compromise the ability of cells to undergo apoptosis, perhaps conferring resistance to chemotherapy. Our results indicate that loss of PKC delta is an indicator of endometrial malignancy and increasing grade of cancer. Thus, PKC delta may function as a tumor suppressor in endometrial cancer.

Roles of heterogeneous nuclear ribonucleoproteins A and B in cell proliferation

Overexpression of heterogeneous nuclear ribonucleoproteins (hnRNPs) A2 and B1 has been observed in a variety of tumour types, however, it is unknown whether this dysregulation is a consequence of, or a driving force for, unregulated cell proliferation. We have shown that the levels of hnRNPs A1, A2 and B1, but not A3, are modulated during the cell cycle of Colo16 squamous carcinoma cells and HaCaT immortalized keratinocytes, suggesting that A1, A2 and B1 are needed at particular cell cycle stages. However, the levels of hnRNP A1, A2 and B1 mRNAs were constant, indicating that regulation of protein levels was controlled at the level of translation. RNAi suppression of hnRNP A1 or A3 alone did not affect the proliferation of Colo16 cells but the proliferation rate was significantly reduced when both were suppressed simultaneously, or when either was suppressed together with hnRNP A2. Reducing hnRNP A2 expression in Colo16 and HaCaT cells by RNAi led to a non-apoptotic-related decrease in cell proliferation, reinforcing the view that this protein is required for cell proliferation. Suppression of hnRNP A2 in Colo16 cells was associated with increased p21 levels but p53 levels remained unchanged. In addition, expression of BRCA1 was downregulated, at both mRNA and protein levels. The observed effects of hnRNP A2 and its isoforms on cell proliferation and their correlation with BRCA1 and p21 expression suggest that these hnRNP proteins play a role in cell proliferation.

Discoidin domain receptor 2 mediates tumor cell cycle arrest induced by fibrillar collagen

During malignant invasion tumor cells establish contact with extracellular matrix proteins, including fibrillar collagen. In addition to providing a physical barrier against invasion, fibrillar collagen also restricts cell proliferation. It has been assumed that the growth regulatory activity of fibrillar collagen is the result of an indirect restrictive effect on cell spreading and cytoskeletal organization. Here we provide evidence for a direct inhibitory effect of fibrillar collagen on proliferation of human melanoma and fibrosarcoma cells that involves activation of the tyrosine kinase discoidin domain receptor 2 and is independent of effects on cell spreading. Cells plated in the presence of fibrillar collagen were growth arrested in the G0/G1 phase of the cell cycle. However treatment with the tyrosine kinase inhibitor genistein, down-regulation of discoidin domain receptor 2, or collagen deglycosylation that prevents discoidin domain receptor 2 activation allowed cells to enter the cell cycle in the presence of fibrillar collagen without a requirement for spreading and actin organization. Our data provide evidence for a novel direct mechanism by which cell contact with fibrillar collagen restricts proliferation.

[Bacterial cyclostatin, or how do bacteria manipulate the eukaryotic cell cycle]

Several bacterial proteins have been recently described that share the ability to inhibit the proliferation of cells in culture without causing early signs of cytotoxicity. Such observations suggest the existence of bacterial mechanisms of control of the eukaryotic cell cycle contributing to pathogenicity or adaptation to the host. This emerging concept of cellular microbiology is critically analyzed considering as a model the cytolethal distending toxins (CDT), a family of toxins whose mode of action on the cell cycle has been thoroughly studied over the last few years. CDTs activate a physiological G2 checkpoint in exposed cells, probably from an initial DNA alteration whose precise molecular nature has not yet been determined. Experimental data are lacking to extrapolate in vivo the antiproliferative effect of these bacterial proteins that we tentatively propose to call cyclostatins.

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

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

Protein kinase C signaling mediates a program of cell cycle withdrawal in the intestinal epithelium

Members of the protein kinase C (PKC) family of signal transduction molecules have been widely implicated in regulation of cell growth and differentiation, although the underlying molecular mechanisms involved remain poorly defined. Using combined in vitro and in vivo intestinal epithelial model systems, we demonstrate that PKC signaling can trigger a coordinated program of molecular events leading to cell cycle withdrawal into G(0). PKC activation in the IEC-18 intestinal crypt cell line resulted in rapid downregulation of D-type cyclins and differential induction of p21(waf1/cip1) and p27(kip1), thus targeting all of the major G(1)/S cyclin-dependent kinase complexes. These events were associated with coordinated alterations in expression and phosphorylation of the pocket proteins p107, pRb, and p130 that drive cells to exit the cell cycle into G(0) as indicated by concomitant downregulation of the DNA licensing factor cdc6. Manipulation of PKC isozyme levels in IEC-18 cells demonstrated that PKCalpha alone can trigger hallmark events of cell cycle withdrawal in intestinal epithelial cells. Notably, analysis of the developmental control of cell cycle regulatory molecules along the crypt-villus axis revealed that PKCalpha activation is appropriately positioned within intestinal crypts to trigger this program of cell cycle exit-specific events in situ. Together, these data point to PKCalpha as a key regulator of cell cycle withdrawal in the intestinal epithelium.

Involvement of p21(Waf1/Cip1) in protein kinase C alpha-induced cell cycle progression

Protein kinase C (PKC) plays an important role in the regulation of glioma growth; however, the identity of the specific isoform and mechanism by which PKC fulfills this function remain unknown. In this study, we demonstrate that PKC activation in glioma cells increased their progression through the cell cycle. Of the six PKC isoforms that were present in glioma cells, PKC alpha was both necessary and sufficient to promote cell cycle progression when stimulated with phorbol 12-myristate 13-acetate. Also, decreased PKC alpha expression resulted in a marked decrease in cell proliferation. The only cell cycle-regulatory molecule whose expression was rapidly altered and increased by PKC alpha activity was the cyclin-cyclin-dependent kinase (CDK) inhibitor p21(Waf1/Cip1). Coimmunoprecipitation studies revealed that p21(Waf1/Cip1) upregulation was accompanied by an incorporation of p21(Waf1/Cip1) into various cyclin-CDK complexes and that the kinase activity of these complexes was increased, thus resulting in cell cycle progression. Furthermore, depletion of p21(Waf1/Cip1) by antisense strategy attenuated the PKC-induced cell cycle progression. These results suggest that PKC alpha activity controls glioma cell cycle progression through the upregulation of p21(Waf1/Cip1), which facilitates active cyclin-CDK complex formation.

Cellular activation triggered by the autosomal dominant polycystic kidney disease gene product PKD2

Autosomal dominant polycystic kidney disease (ADPKD) is caused by germ line mutations in at least three ADPKD genes. Two recently isolated ADPKD genes, PKD1 and PKD2, encode integral membrane proteins of unknown function. We found that PKD2 upregulated AP-1-dependent transcription in human embryonic kidney 293T cells. The PKD2-mediated AP-1 activity was dependent upon activation of the mitogen-activated protein kinases p38 and JNK1 and protein kinase C (PKC) epsilon, a calcium-independent PKC isozyme. Staurosporine, but not the calcium chelator BAPTA [1,2-bis(o-aminophenoxy)ethane-N,N,N', N'-tetraacetate], inhibited PKD2-mediated signaling, consistent with the involvement of a calcium-independent PKC isozyme. Coexpression of PKD2 with the interacting C terminus of PKD1 dramatically augmented PKD2-mediated AP-1 activation. The synergistic signaling between PKD1 and PKD2 involved the activation of two distinct PKC isozymes, PKC alpha and PKC epsilon, respectively. Our findings are consistent with others that support a functional connection between PKD1 and PKD2 involving multiple signaling pathways that converge to induce AP-1 activity, a transcription factor that regulates different cellular programs such as proliferation, differentiation, and apoptosis. Activation of these signaling cascades may promote the full maturation of developing tubular epithelial cells, while inactivation of these signaling cascades may impair terminal differentiation and facilitate the development of renal tubular cysts.