Deregulation of the Rho GTPase, Rac1, suppresses cyclin-dependent kinase inhibitor p21(CIP1) levels in androgen-independent human prostate cancer cells

Statins and prostate cancer-hype or hope? The biological perspective

Growing evidence suggests that men prescribed a statin for cholesterol control have a lower risk of advanced prostate cancer (PCa) and improved treatment outcomes; however, the mechanism by which statins elicit their anti-neoplastic effects is not well understood and is likely multifaceted. Statins are potent and specific inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR), the rate-limiting enzyme of the mevalonate (MVA) metabolic pathway. This two-part series is a review of the observational and experimental data on statins as anti-cancer agents in PCa. In this article, we describe the functional role that deregulated MVA metabolism plays in PCa progression and summarize the biological evidence and rationale for targeting the MVA pathway, with statins and other agents, for the treatment of PCa.

Hormone-Dependent Prostate Cancers are Dependent on Rac Signaling for Growth and Survival

Prostate cancer remains a common cause of cancer mortality in men. Initially, cancers are dependent of androgens for growth and survival. First line therapies reduce levels of circulating androgens or target the androgen receptor (AR) directly. Although most patients show durable responses, many patients eventually progress to castration-resistant prostate cancer (CRPC) creating a need for alternative treatment options. The Rac1 signaling pathway has previously been implicated as a driver of cancer initiation and disease progression. We investigated the role of HACE1, the E3 ubiquitin ligase for Rac1, in prostate cancer and found that HACE1 is commonly lost resulting in hyperactive Rac signaling leading to enhanced cellular proliferation, motility and viability. Importantly, we show that a Rac inhibitor can attenuate the growth and survival of prostate cancer cells. Rac signaling was also found to be critical in prostate cancers that express the AR. Rac inhibition in androgen dependent cells resulted in reduction of AR target gene expression suggesting that targeting Rac1 may be an alternative method for blocking the AR signaling axis. Finally, when used in combination with AR antagonists, Rac inhibition enhanced the suppression of AR target gene expression. Therefore, targeting Rac in prostate cancer has the potential to enhance the efficacy of approved AR therapies.

MicroRNAs in treatment-induced neuroendocrine differentiation in prostate cancer

Prostate cancer is a condition commonly associated with men worldwide. Androgen deprivation therapy remains one of the targeted therapies. However, after some years, there is biochemical recurrence and metastatic progression into castration-resistant prostate cancer (CRPC). CRPC cases are treated with second-line androgen deprivation therapy, after which, these CRPCs transdifferentiate to form neuroendocrine prostate cancer (NEPC), a highly aggressive variant of CRPC. NEPC arises via a reversible transdifferentiation process, known as neuroendocrine differentiation (NED), which is associated with altered expression of lineage markers such as decreased expression of androgen receptor and increased expression of neuroendocrine lineage markers including enolase 2, chromogranin A and synaptophysin. The etiological factors and molecular basis for NED are poorly understood, contributing to a lack of adequate molecular biomarkers for its diagnosis and therapy. Therefore, there is a need to fully understand the underlying molecular basis for this cancer. Recent studies have shown that microRNAs (miRNAs) play a key epigenetic role in driving therapy-induced NED in prostate cancer. In this review, we briefly describe the role of miRNAs in prostate cancer and CRPCs, discuss some key players in NEPCs and elaborate on miRNA dysregulation as a key epigenetic process that accompanies therapy-induced NED in metastatic CRPC. This understanding will contribute to better clinical management of the disease.

P-REX1-Independent, Calcium-Dependent RAC1 Hyperactivation in Prostate Cancer

The GTPase Rac1 is a well-established master regulator of cell motility and invasiveness contributing to cancer metastasis. Dysregulation of the Rac1 signaling pathway, resulting in elevated motile and invasive potential, has been reported in multiple cancers. However, there are limited studies on the regulation of Rac1 in prostate cancer. Here, we demonstrate that aggressive androgen-independent prostate cancer cells display marked hyperactivation of Rac1. This hyperactivation is independent of P-Rex1 activity or its direct activators, the PI3K product PIP₃ and Gβγ subunits. Furthermore, we demonstrate that the motility and invasiveness of PC3 prostate cancer cells is independent of P-Rex1, supporting the analysis of publicly available datasets indicating no correlation between high P-Rex1 expression and cancer progression in patients. Rac1 hyperactivation was not related to the presence of activating Rac1 mutations and was insensitive to overexpression of a Rac-GAP or the silencing of specific Rac-GEFs expressed in prostate cancer cells. Interestingly, active Rac1 levels in these cells were markedly reduced by elevations in intracellular calcium or by serum stimulation, suggesting the presence of an alternative means of Rac1 regulation in prostate cancer that does not involve previously established paradigms.

Inhibiting geranylgeranyl diphosphate synthesis reduces nuclear androgen receptor signaling and neuroendocrine differentiation in prostate cancer cell models

BACKGROUND

Following androgen deprivation for the treatment of advanced adenocarcinoma of the prostate, tumors can progress to neuroendocrine prostate cancer (NEPC). This transdifferentiation process is poorly understood, but trafficking of transcriptional factors and/or cytoskeletal rearrangements may be involved. We observed the role of geranylgeranylation in this process by treatment with digeranyl bisphosphonate (DGBP), a selective inhibitor of geranylgeranyl pyrophosphate synthase which blocks the prenylation of small GTPases such as Rho and Rab family proteins, including Cdc42 and Rac1.

METHODS

We examined the therapeutic potential of DGBP in LNCaP, C4-2B4, and 22Rv1 cell culture models. Cell morphology and protein expression were quantified to observe the development of the neuroendocrine phenotype in androgen-deprivation and abiraterone-treated LNCaP models of NEPC development. Luciferase reporter assays were utilized to examine AR activity, and immunofluorescence visualized the localization of AR within the cell.

RESULTS

Essential genes in the isoprenoid pathway, such as HMGCR, MVK, GGPS1, and GGT1, were highly expressed in a subset of castration resistant prostate cancers reported by Beltran et al. Under treatment with DGBP, nuclear localization of AR decreased in LNCaP, 22Rv1, and C4-2B4 cell lines, luciferase reporter activity was reduced in LNCaP and 22Rv1, and AR target gene transcription also decreased in LNCaP. Conversely, nuclear localization of AR was enhanced by the addition of GGOH. Finally, induction of the NEPC structural and molecular phenotype via androgen deprivation in LNCaP cells was inhibited by DGBP in a GGOH-dependent manner.

CONCLUSIONS

DGBP is a novel compound with the potential to reduce AR transcriptional activity and inhibit PCa progression to NEPC phenotype. These results suggest that DGBP may be used to block cell growth and metastasis in both hormone therapy sensitive and resistant paradigms.

Targeting protein geranylgeranylation slows tumor development in a murine model of prostate cancer metastasis

The isoprenoid biosynthetic pathway (IBP) plays a critical role in providing substrates and enzymes necessary for the post-translational modification and thus activation of a number of proteins involved in prostate cancer metastasis. Previous work by our lab found novel compound disodium [(6Z,11E,15E)-9-[bis(sodiooxy)phosphoryl]-17-hydroxy-2,6,12,16-tetramethyheptadeca-2,6,11,15-tetraen-9-yl]phosphonate (GGOHBP), which inhibits the IBP enzyme geranylgeranyl diphosphate synthase (GGDPS), reduced protein geranylgeranylation without altering protein farnesylation. This activity significantly reduced adrenal gland tumor burden in a murine model of human prostate cancer metastasis which relied on treatment of established disease. The present study determined the ability of GGDPS inhibition to slow the development of prostate cancer metastasis in a preventative murine model. Using tail vein injection of human derived PC-3 prostate cancer cells 4 d after initiating daily GGOHBP or vehicle treatments, we found GGOHBP significantly reduced whole body tumor burden, significantly slowed the development of tumors, and prolonged overall survival as compared to vehicle treated animals. The observed reduction in soft tissue tumor burden corresponded to a biochemical reduction in Rap1A geranylgeranylation, which for prostate cancer is important in its own merit and which serves as a surrogate marker for Rho family, i.e. Rac, protein modification. This effect was present in all treated mice pointing to strong target engagement, which was not observed in non-tumor burdened tissues or control mice. Our findings reiterate a role for protein geranylgeranylation in the development of prostate cancer metastasis in vivo.

Lipin-1 regulates cancer cell phenotype and is a potential target to potentiate rapamycin treatment

Lipogenesis inhibition was reported to induce apoptosis and repress proliferation of cancer cells while barely affecting normal cells. Lipins exhibit dual function as enzymes catalyzing the dephosphorylation of phosphatidic acid to diacylglycerol and as co-transcriptional regulators. Thus, they are able to regulate lipid homeostasis at several nodal points. Here, we show that lipin-1 is up-regulated in several cancer cell lines and overexpressed in 50 % of high grade prostate cancers. The proliferation of prostate and breast cancer cells, but not of non-tumorigenic cells, was repressed upon lipin-1 knock-down. Lipin-1 depletion also decreased cancer cell migration through RhoA activation. Lipin-1 silencing did not significantly affect global lipid synthesis but enhanced the cellular concentration of phosphatidic acid. In parallel, autophagy was induced while AKT and ribosomal protein S6 phosphorylation were repressed. We also observed a compensatory regulation between lipin-1 and lipin-2 and demonstrated that their co-silencing aggravates the phenotype induced by lipin-1 silencing alone. Most interestingly, lipin-1 depletion or lipins inhibition with propranolol sensitized cancer cells to rapamycin. These data indicate that lipin-1 controls main cellular processes involved in cancer progression and that its targeting, alone or in combination with other treatments, could open new avenues in anticancer therapy.

Infectivity and insertional mutagenesis of endogenous retrovirus in autoimmune NZB and B/W mice

Murine leukaemia virus has been suggested to contribute to both autoimmune disease and leukaemia in the NZB mouse and in the (NZB × NZW) F1 (abbreviated B/W) mouse. However, with apparently only xenotropic but no ecotropic virus constitutively expressed in these mice, few mechanisms could explain the aetiology of either disease in either mouse strain. Because pseudotyped and/or inducible ecotropic virus may play a role, we surveyed the ability of murine leukaemia virus in NZB, NZW and B/W mice to infect and form a provirus. From the spleen of NZB mice, we isolated circular cDNA of xenotropic and polytropic virus, which indicates ongoing infection by these viruses. From a B/W lymphoma, we isolated and determined the complete sequence of a putative ecotropic NZW virus. From B/W mice, we recovered de novo endogenous retroviral integration sites (tags) from the hyperproliferating cells of the spleen and the peritoneum. The tagged genes seemed to be selected to aid cellular proliferation, as several of them are known cancer genes. The insertions are consistent with the idea that endogenous retrovirus contributes to B-cell hyperproliferation and progression to lymphoma in B/W mice.

Targeting geranylgeranylation reduces adrenal gland tumor burden in a murine model of prostate cancer metastasis

The isoprenoid biosynthetic pathway (IBP) is critical for providing substrates for the post-translational modification of proteins key in regulating malignant cell properties, including proliferation, invasion, and migration. Inhibitors of the IBP, including statins and nitrogenous bisphosphonates, are used clinically for the treatment of hypercholesterolemia and bone disease respectively. The statins work predominantly in the liver, while the nitrogenous bisphosphonates are highly sequestered to bone. Inhibition of the entire IBP is limited by organ specificity and side effects resulting from depletion of all isoprenoids. We have developed a novel compound, disodium [(6Z,11E,15E)-9-[bis(sodiooxy)phosphoryl]-17-hydroxy-2,6,12,16-tetramethyheptadeca-2,6,11,15-tetraen-9-yl]phosphonate (GGOHBP), which selectively targets geranylgeranyl diphosphate synthase, reducing post-translational protein geranylgeranylation. Intracardiac injection of luciferase-expressing human-derived 22Rv1 PCa cells into SCID mice resulted in tumor development in bone (100 %), adrenal glands (72 %), mesentery (22 %), liver (17 %), and the thoracic cavity (6 %). Three weeks after tumor inoculation, daily subcutaneous (SQ) injections of 1.5 mg/kg GGOHBP or the vehicle were given for one month. Dissected tumors revealed a reduction in adrenal gland tumors corresponding to a 54 % (P < 0.005) reduction in total adrenal gland tumor weight of the treated mice as compared to vehicle-treated controls. Western blot analysis of the harvested tissues showed a reduction in Rap1A geranylgeranylation in adrenal glands and mesenteric tumors of the treated mice while non-tumorous tissues and control mice showed no Rap1A alteration. Our findings detail a novel bisphosphonate compound capable of preferentially altering the IBP in tumor-burdened adrenal glands of a murine model of PCa metastasis.

The role of microRNA in castration-resistant prostate cancer

INTRODUCTION

Castration-resistant prostate cancer (CRPC) has a historically low median survival rate, but recent advances and discoveries in microRNAs (miRNAs) have opened the potential for new prognostication modalities to enhance therapeutic success. As new chemotherapies and immunotherapies are developed, there is an increasing need for precision and stratification of CRPC to allow for optimization and personalization of therapy.

METHODS

A systematic literature review was conducted via electronic database resulting in the selection of 42 articles based on title, abstract, study format, and content by a consensus of all participating authors. Most selected articles were published between 2002 and 2013. In this review, we discuss the robustness of miRNAs as a biomarker platform, miRNAs associated with prostate cancer, and recent discoveries of miRNA associations with CRPC.

RESULTS

The associations discovered have been of interest owing to the ability to differentiate between CRPC and localized prostate cancer. With the evaluation of multiple miRNAs, it is possible to provide a profile regarding tumor characteristics. Furthermore, actions of miRNAs on CRPC tumor cells have the ability to suppress metastatic phenotypes.

CONCLUSION

miRNAs may have a growing role in CRPC prognostication and may potentially transform into a therapeutic potential.

Rac1-dependent lamellipodial motility in prostate cancer PC-3 cells revealed by optogenetic control of Rac1 activity

The lamellipodium, an essential structure for cell migration, plays an important role in the invasion and metastasis of cancer cells. Although Rac1 recognized as a key player in the formation of lamellipodia, the molecular mechanisms underlying lamellipodial motility are not fully understood. Optogenetic technology enabled us to spatiotemporally control the activity of photoactivatable Rac1 (PA-Rac1) in living cells. Using this system, we revealed the role of phosphatidylinositol 3-kinase (PI3K) in Rac1-dependent lamellipodial motility in PC-3 prostate cancer cells. Through local blue laser irradiation of PA-Rac1-expressing cells, lamellipodial motility was reversibly induced. First, outward extension of a lamellipodium parallel to the substratum was observed. The extended lamellipodium then showed ruffling activity at the periphery. Notably, PI(3,4,5)P₃ and WAVE2 were localized in the extending lamellipodium in a PI3K-dependent manner. We confirmed that the inhibition of PI3K activity greatly suppressed lamellipodial extension, while the ruffling activity was less affected. These results suggest that Rac1-induced lamellipodial motility consists of two distinct activities, PI3K-dependent outward extension and PI3K-independent ruffling.

Increased diacylglycerol kinase ζ expression in human metastatic colon cancer cells augments Rho GTPase activity and contributes to enhanced invasion

BACKGROUND

Unraveling the signaling pathways responsible for the establishment of a metastatic phenotype in carcinoma cells is critically important for understanding the pathology of cancer. The acquisition of cell motility is a key property of metastatic tumor cells and is a prerequisite for invasion. Rho GTPases regulate actin cytoskeleton reorganization and the cellular responses required for cell motility and invasion. Diacylglycerol kinase ζ (DGKζ), an enzyme that phosphorylates diacylglycerol to yield phosphatidic acid, regulates the activity of the Rho GTPases Rac1 and RhoA. DGKζ mRNA is highly expressed in several different colon cancer cell lines, as well as in colon cancer tissue relative to normal colonic epithelium, and thus may contribute to the metastatic process.

METHODS

To investigate potential roles of DGKζ in cancer metastasis, a cellular, isogenic model of human colorectal cancer metastatic transition was used. DGKζ protein levels, Rac1 and RhoA activity, and PAK phosphorylation were measured in the non-metastatic SW480 adenocarcinoma cell line and its highly metastatic variant, the SW620 line. The effect of DGKζ silencing on Rho GTPase activity and invasion through Matrigel-coated Transwell inserts was studied in SW620 cells. Invasiveness was also measured in PC-3 prostate cancer and MDA-MB-231 breast cancer cells depleted of DGKζ.

RESULTS

DGKζ protein levels were elevated approximately 3-fold in SW620 cells compared to SW480 cells. There was a concomitant increase in active Rac1 in SW620 cells, as well as substantial increases in the expression and phosphorylation of the Rac1 effector PAK1. Similarly, RhoA activity and expression were increased in SW620 cells. Knockdown of DGKζ expression in SW620 cells by shRNA-mediated silencing significantly reduced Rac1 and RhoA activity and attenuated the invasiveness of SW620 cells in vitro. DGKζ silencing in highly metastatic MDA-MB-231 breast cancer cells and PC-3 prostate cancer cells also significantly attenuated their invasiveness.

CONCLUSION

Elevated DGKζ expression contributes to increased Rho GTPase activation and the enhanced motility of metastatic cancer cells. These findings warrant further investigation of the clinical relevance of DGKζ upregulation in colon and other cancers. Interfering with DGKζ function could provide a means of inhibiting invasion and metastasis.

The microRNA -23b/-27b cluster suppresses the metastatic phenotype of castration-resistant prostate cancer cells

MicroRNAs (miRs) are small, endogenous, non-coding RNAs that regulate the stability and/or translation of complementary mRNA targets. MiRs have emerged not only as critical modulators of normal physiologic processes, but their deregulation may significantly impact prostate and other cancers. The expression of miR-23b and miR-27b, which are encoded by the same miR cluster (miR-23b/-27b), are downregulated in metastatic, castration-resistant tumors compared to primary prostate cancer and benign tissue; however, their possible role in prostate cancer progression is unknown. We found that ectopic expression of miR-23b/-27b in two independent castration-resistant prostate cancer cell lines resulted in suppression of invasion and migration, as well as reduced survival in soft agar (a measure of anoikis). However, there was no effect of miR-23b/-27b on cell proliferation suggesting that these miRs function as metastasis (but not growth) suppressors in prostate cancer. Conversely, inhibition of miR-23b/-27b in the less aggressive androgen-dependent LNCaP prostate cancer cell line resulted in enhanced invasion and migration also without affecting proliferation. Mechanistically, we found that introduction of miR-23b/-27b in metastatic, castration-resistant prostate cancer cell lines resulted in a significant attenuation of Rac1 activity without affecting total Rac1 levels and caused increased levels of the tumor suppressor E-cadherin. Inhibition of these miRs had the opposite effect in androgen-dependent LNCaP cells. These results suggest that miR-23b/-27b are metastasis suppressors that might serve as novel biomarkers and therapeutic agents for castration-resistant disease.

Vav3-rac1 signaling regulates prostate cancer metastasis with elevated Vav3 expression correlating with prostate cancer progression and posttreatment recurrence

Prostate cancer remains the second leading cause of cancer death in men in the Western world. Yet current therapies do not significantly improve the long-term survival of patients with distant metastasis. In this study, we investigated the role of the guanine nucleotide exchange factor Vav3 in prostate cancer progression and metastasis and found that Vav3 expression correlated positively with prostate cancer cell migration and invasion. Stimulation of the receptor tyrosine kinase EphA2 by ephrinA1 resulted in recruitment and tyrosine phosphorylation of Vav3, leading to Rac1 activation as well as increased migration and invasion in vitro. Reduction of Vav3 resulted in fewer para-aortic lymph nodes and bone metastasis in vivo. Clinically, expression of Vav3 and EphA2 was elevated in late-stage and metastatic prostate cancers. Among patients with stage IIB or earlier prostate cancer, higher Vav3 expression correlated with lower cumulative biochemical failure-free survival, suggesting that Vav3 may represent a prognostic marker for posttreatment recurrence of prostate cancer. Together, our findings provide evidence that the Vav3-mediated signaling pathway may serve as a therapeutic target for prostate cancer metastasis.

WAVE regulatory complex activation by cooperating GTPases Arf and Rac1

The WAVE regulatory complex (WRC) is a critical element in the control of actin polymerization at the eukaryotic cell membrane, but how WRC is activated remains uncertain. While Rho GTPase Rac1 can bind and activate WRC in vitro, this interaction is of low affinity, suggesting other factors may be important. By reconstituting WAVE-dependent actin assembly on membrane-coated beads in mammalian cell extracts, we found that Rac1 was not sufficient to engender bead motility, and we uncovered a key requirement for Arf GTPases. In vitro, Rac1 and Arf1 were individually able to bind weakly to recombinant WRC and activate it, but when both GTPases were bound at the membrane, recruitment and concomitant activation of WRC were dramatically enhanced. This cooperativity between the two GTPases was sufficient to induce WAVE-dependent bead motility in cell extracts. Our findings suggest that Arf GTPases may be central components in WAVE signalling, acting directly, alongside Rac1.

Epigenetic regulation of phosphatidylinositol 3,4,5-triphosphate-dependent Rac exchanger 1 gene expression in prostate cancer cells

Aberrant up-regulation of P-Rex1 expression plays important roles in cancer progression and metastasis. The present study investigated the regulatory mechanism underlying P-Rex1 gene expression in prostate cancer cells. We showed that P-Rex1 expression was much higher in metastatic prostate cancer cells than in prostate epithelial cells and non-metastatic prostate cancer cells. Histone deacetylase (HDAC) inhibitors or silence of endogenous HDAC1 and HDAC2 markedly elevated P-Rex1 transcription in non-metastatic prostate cancer cells, whereas overexpression of recombinant HDAC1 in metastatic prostate cancer cells suppressed P-Rex1 expression. HDAC inhibitor trichostatin A (TSA) also significantly increased P-Rex1 promoter activity and caused acetylated histones to accumulate and associate with the P-Rex1 promoter. One Sp1 site, essential for basal promoter activity, was identified as critical for the TSA effect. TSA treatment did not alter the DNA-binding activity of Sp1 toward the P-Rex1 promoter; however, it facilitated the dissociation of the repressive HDAC1 and HDAC2 from the Sp1 binding region. Interestingly, HDAC1 association with Sp1 and with the P-Rex1 promoter were much weaker in metastatic prostate cancer PC-3 cells than in non-metastatic prostate cancer cells, and HDAC inhibitors only had very modest stimulatory effects on P-Rex1 promoter activity and P-Rex1 expression in PC-3 cells. Altogether, our studies demonstrate that HDACs could regulate P-Rex1 gene transcription by interaction with Sp1 and by region-specific changes in histone acetylation within the P-Rex1 promoter. Disassociation of HDACs from Sp1 on the P-Rex1 promoter may contribute to aberrant up-regulation of P-Rex1 in cancer.

Differential roles of Smad2 and Smad3 in the regulation of TGF-β1-mediated growth inhibition and cell migration in pancreatic ductal adenocarcinoma cells: control by Rac1

BACKGROUND

Progression of pancreatic ductal adenocarcinoma (PDAC) is largely the result of genetic and/or epigenetic alterations in the transforming growth factor-beta (TGF-β)/Smad signalling pathway, eventually resulting in loss of TGF-β-mediated growth arrest and an increase in cellular migration, invasion, and metastasis. These cellular responses to TGF-β are mediated solely or partially through the canonical Smad signalling pathway which commences with activation of receptor-regulated Smads (R-Smads) Smad2 and Smad3 by the TGF-β type I receptor. However, little is known on the relative contribution of each R-Smad, the possible existence of functional antagonism, or the crosstalk with other signalling pathways in the control of TGF-β1-induced growth inhibition and cell migration. Using genetic and pharmacologic approaches we have inhibited in PDAC cells endogenous Smad2 and Smad3, as well as a potential regulator, the small GTPase Rac1, and have analysed the consequences for TGF-β1-mediated growth inhibition and cell migration (chemokinesis).

RESULTS

SiRNA-mediated silencing of Smad3 in the TGF-β responsive PDAC cell line PANC-1 reduced TGF-β1-induced growth inhibition but increased the migratory response, while silencing of Smad2 enhanced growth inhibition but decreased chemokinesis. Interestingly, siRNA-mediated silencing of the small GTPase Rac1, or ectopic expression of a dominant-negative Rac1 mutant largely mimicked the effect of Smad2 silencing on both TGF-β1-induced growth inhibition, via upregulation of the cdk inhibitor p21WAF1, and cell migration. Inhibition of Rac1 activation reduced both TGF-β1-induction of a Smad2-specific transcriptional reporter and Smad2 C-terminal phosphorylation in PDAC cells while Smad3-specific transcriptional activity and Smad3 C-terminal phosphorylation appeared increased. Disruption of autocrine TGF-β signalling in PANC-1 cells rendered cells less susceptible to the growth-suppressive effect of Rac1 inhibition, suggesting that the decrease in "basal" proliferation upon Rac1 inhibition was caused by potentiation of autocrine TGF-β growth inhibition.

CONCLUSIONS

In malignant cells with a functional TGF-β signalling pathway Rac1 antagonizes the TGF-β1 growth inhibitory response and enhances cell migration by antagonistically regulating Smad2 and Smad3 activation. This study reveals that Rac1 is prooncogenic in that it can alter TGF-β signalling at the R-Smad level from a tumour-suppressive towards a tumour-promoting outcome. Hence, Rac1 might represent a viable target for therapeutic intervention to inhibit PDAC progression.

Inhibition of the Rho GTPase, Rac1, decreases estrogen receptor levels and is a novel therapeutic strategy in breast cancer

Rac1, a Rho GTPase, modulates diverse cellular processes and is hyperactive in some cancers. Estrogen receptor-alpha (ERα) in concert with intracellular signaling pathways regulates genes associated with cell proliferation, tumor development, and breast cancer cell survival. Therefore, we examined the possibility of Rac1 and ERα crosstalk in breast cancer cells. We found that Rac1 enhanced ERα transcriptional activity in breast cancer cells. Vav3, a Rho guanine nucleotide exchange factor that activates Rac1, was an upstream mediator, and P21/Cdc42/Rac1 activating kinase-1 (Pak-1) was a downstream effector of Rac1 enhancement of ERα activity. These results suggest that Rac1 may prove to be a therapeutic target. To test this hypothesis, we used a small molecule Rac inhibitor, EHT 1864, and found that EHT 1864 inhibited ERα transcriptional activity. Furthermore, EHT 1864 inhibited estrogen-induced cell proliferation in breast cancer cells and decreased tamoxifen-resistant breast cancer cell growth. EHT 1864 decreased activity of the promoter of the ERα gene resulting in down-regulation of ERα mRNA and protein levels. Therefore, ERα down-regulation by EHT 1864 is the likely mechanism of EHT 1864-mediated inhibition of ERα activity and estrogen-stimulated breast cancer cell proliferation. Since ERα plays a critical role in the pathogenesis of breast cancer and the Rac inhibitor EHT 1864 down-regulates ERα expression and breast cancer cell proliferation, further investigation of the therapeutic potential of Rac1 targeting in the treatment of breast cancer is warranted.

CaM kinase kinase beta-mediated activation of the growth regulatory kinase AMPK is required for androgen-dependent migration of prostate cancer cells

While patients with advanced prostate cancer initially respond favorably to androgen ablation therapy, most experience a relapse of the disease within 1-2 years. Although hormone-refractory disease is unresponsive to androgen-deprivation, androgen receptor (AR)-regulated signaling pathways remain active and are necessary for cancer progression. Thus, both AR itself and the processes downstream of the receptor remain viable targets for therapeutic intervention. Microarray analysis of multiple clinical cohorts showed that the serine/threonine kinase Ca2+/calmodulin-dependent protein kinase kinase β (CaMKKβ) is both highly expressed in the prostate and further elevated in prostate cancers. Using cellular models of prostate cancer, we have determined that androgens (a) directly increase the expression of a CaMKKβ splice variant and (b) increase functional CaMKKβ protein levels as determined by the phosphorylation of both CaMKI and AMP-activated protein kinase (AMPK), two of CaMKKβ's primary substrates. Importantly, inhibition of the CaMKKβ-AMPK, but not CaMKI, signaling axis in prostate cancer cells by pharmacological inhibitors or siRNA-mediated knockdown blocks androgen-mediated migration and invasion. Conversely, overexpression of CaMKKβ alone leads to both increased AMPK phosphorylation and cell migration. Given the key roles of CaMKKβ and AMPK in the biology of prostate cancer cells, we propose that these enzymes are potential therapeutic targets in prostate cancer.

p66Shc--a longevity redox protein in human prostate cancer progression and metastasis : p66Shc in cancer progression and metastasis

p66Shc, a 66 kDa proto-oncogene Src homologous-collagen homologue (Shc) adaptor protein, is classically known in mediating receptor tyrosine kinase signaling and recently identified as a sensor to oxidative stress-induced apoptosis and as a longevity protein in mammals. The expression of p66Shc is decreased in mice and increased in human fibroblasts upon aging and in aging-related diseases, including prostate cancer. p66Shc protein level correlates with the proliferation of several carcinoma cells and can be regulated by steroid hormones. Recent advances point that p66Shc protein plays a role in mediating cross-talk between steroid hormones and redox signals by serving as a common convergence point in signaling pathways on cell proliferation and apoptosis. This article first reviews the unique function of p66Shc protein in regulating oxidative stress-induced apoptosis. Subsequently, we discuss its novel role in androgen-regulated prostate cancer cell proliferation and metastasis and the mechanism by which it mediates androgen action via the redox signaling pathway. The data together indicate that p66Shc might be a useful biomarker for the prognosis of prostate cancer and serve as an effective target for its cancer treatment.

Activation of Rac1 is closely related to androgen-independent cell proliferation of prostate cancer cells both in vitro and in vivo

We and others previously showed that signaling through cSrc or atypical protein kinase C (aPKC) pathway regulates the proliferation of prostate cancer cells and is associated with their progression to castrate-resistance in vivo. However, the interrelation of these two kinases has been largely unexplored. In the present study, we show that androgen-induced activation of cSrc regulates the activity of aPKC through the small molecular weight G protein Rac1 in androgen-dependent LNCaP cells. Knockdown of cSrc in those cells reduces the phosphorylation of aPKC and the abundance of activated form of Rac1. Additionally, the treatment of those cells with Rac1 inhibitor repressed cell cycle progression at G(1)/S transition. In fact, forced expression of a constitutively active Rac1 mutant in LNCaP cells promoted cell proliferation under androgen-depleted conditions both in vitro and in vivo. Moreover, LNCaP C4-2 and AILNCaP cells, the syngeneic androgen-independent sublines from LNCaP cells, harbored abundant Rac1-GTP. Importantly, the inhibition of Rac1 suppressed cell proliferation and induced apoptotic cell death in all prostate cancer cell lines tested irrespective of their androgen-dependence. In immunohistochemical evaluation of tumor specimens from prostate cancer patients, Rac1 pathway appeared to be activated in the majority of castrate-resistant diseases. Collectively, our present results both in vitro and in vivo highly implicate that Rac1 can be a potential therapeutic target for patients with advanced prostate cancer, especially those with castrate-resistant status.

Upregulation of PIP3-dependent Rac exchanger 1 (P-Rex1) promotes prostate cancer metastasis

Excessive activation of G-protein-coupled receptor (GPCR) and receptor tyrosine kinase (RTK) pathways has been linked to prostate cancer metastasis. Rac activation by guanine nucleotide exchange factors (GEFs) plays an important role in directional cell migration, a critical step of tumor metastasis cascades. We found that the upregulation of P-Rex1, a Rac-selective GEF synergistically activated by Gbetagamma freed during GPCR signaling, and PIP3, generated during either RTK or GPCR signaling, strongly correlates with metastatic phenotypes in both prostate cancer cell lines and human prostate cancer specimens. Silencing endogenous P-Rex1 in metastatic prostate cancer PC-3 cells selectively inhibited Rac activity and reduced cell migration and invasion in response to ligands of both epidermal growth factor receptor and G-protein-coupled CXC chemokine receptor 4. Conversely, expression of recombinant P-Rex1, but not its 'GEF-dead' mutant, in non-metastatic prostate cancer cells, such as CWR22Rv1, increased cell migration and invasion through Rac-dependent lamellipodia formation. More importantly, using a mouse xenograft model, we showed that the expression of P-Rex1, but not its mutant, induced lymph node metastasis of CWR22Rv1 cells without an effect on primary tumor growth. Thus, by functioning as a coincidence detector of chemotactic signals from both GPCRs and RTKs, P-Rex1-dependent activation of Rac promotes prostate cancer metastasis.

Chromosome Y variants from different inbred mouse strains are linked to differences in the morphologic and molecular responses of cardiac cells to postpubertal testosterone

BACKGROUND

We have reported previously that when chromosome Y (chrY) from the mouse strain C57BL/6J (ChrYC57) was substituted for that of A/J mice (ChrYA), cardiomyocytes from the resulting "chromosome substitution" C57BL/6J-chrYA strain were smaller than that of their C57BL/6J counterparts. In reverse, when chrYA from A/J mice was substituted for that of chrYC57, cardiomyocytes from the resulting A/J-chrYC57 strain were larger than in their A/J counterparts. We further used these strains to test whether: 1) the origin of chrY could also be linked to differences in the profile of gene expression in the hearts of adult male mice, and 2) post-pubertal testosterone could play a role in the differential morphologic and/or molecular effects of chrYC57 and chrYA.

RESULTS

The increased size of cardiomyocytes from adult male C57BL/6J mice compared to C57BL/6J-chrYA resulted from the absence of hypertrophic effects of post-pubertal testosterone on cells from the latter strain. However, gene profiling revealed that the latter effect could not be explained on the basis of an insensitivity of cells from C57BL/6J-chrYA to androgens, since even more cardiac genes were affected by post-pubertal testosterone in C57BL/6J-chrYA hearts than in C57BL/6J. By testing for interaction between the effects of surgery and strain, we identified 249 "interaction genes" whose expression was affected by post-pubertal testosterone differentially according to the genetic origin of chrY. These interaction genes were found to be enriched within a limited number of signaling pathways, including: 1) p53 signaling, which comprises the interacting genes Ccnd1, Pten and Cdkn1a that are also potential co-regulators of the androgen receptors, and 2) circadian rhythm, which comprises Arntl/Bmal1, which may in turn regulate cell growth via the control of Cdkn1a.

CONCLUSION

Although post-pubertal testosterone increased the size of cardiomyocytes from male C56BL/6J mice but not that from their C57BL/6J-chrYA counterparts, it affected gene expression in the hearts from both strains. However, several cardiac genes responded to post-pubertal testosterone in a strict strain-selective manner, which provides possible mechanisms explaining how chrY may, in part via interference with androgen regulatory events, be linked to morphologic differences of cardiac cells of adult male mice.

A novel role of Shc adaptor proteins in steroid hormone-regulated cancers

Tyrosine phosphorylation plays a critical role in growth regulation, and its aberrant regulation can be involved in carcinogenesis. The association of Shc (Src homolog and collagen homolog) adaptor protein family members in tyrosine phosphorylation signaling pathway is well recognized. Shc adaptor proteins transmit activated tyrosine phosphorylation signaling that suggest their plausible role in growth regulation including carcinogenesis and metastasis. In parallel, by sharing a similar mechanism of carcinogenesis, the steroids are involved in the early stage of carcinogenesis as well as the regulation of cancer progression and metastatic processes. Recent evidence indicates a cross-talk between tyrosine phosphorylation signaling and steroid hormone action in epithelial cells, including prostate and breast cancer cells. Therefore, the members of Shc proteins may function as mediators between tyrosine phosphorylation and steroid signaling in steroid-regulated cell proliferation and carcinogenesis. In this communication, we discuss the novel roles of Shc proteins, specifically p52(Shc) and p66(Shc), in steroid hormone-regulated cancers and a novel molecular mechanism by which redox signaling induced by p66(Shc) mediates steroid action via a non-genomic pathway. The p66(Shc) protein may serve as an effective biomarker for predicting cancer prognosis as well as a useful target for treatment.

SmgGDS is up-regulated in prostate carcinoma and promotes tumour phenotypes in prostate cancer cells

SmgGDS is a guanine nucleotide exchange factor with the unique ability to activate multiple small GTPases, implicating it in cancer development and progression. Here, we investigated the role of SmgGDS in prostate cancer by studying the expression of SmgGDS in benign and malignant prostatic tissues. We also probed SmgGDS function in three prostate carcinoma cell lines using small interfering RNA (siRNA). Immunohistochemical analysis revealed that SmgGDS levels were elevated in prostatic intraepithelial neoplasia (PIN), prostate carcinoma, and metastatic prostate carcinoma. In addition, expression of SmgGDS positively correlated with that of cyclooxygenase-2 (COX-2), a protein believed to promote the development of prostate carcinoma. Reduction of SmgGDS expression in prostate carcinoma cells inhibited proliferation and migration, irrespective of androgen receptor status. These effects were accompanied by a reduction in COX-2 expression and in activity of NF-kappaB, a known regulator of COX-2. Taken together, these findings suggest that SmgGDS promotes the development and progression of prostate cancer, possibly associated with NF-kappaB-dependent up-regulation of COX-2.

Ligand-independent activation of androgen receptors by Rho GTPase signaling in prostate cancer

Prostate cancer invariably recurs after androgen deprivation therapy. Growth of this recurrent/androgen-independent form of prostate cancer may be due to increased androgen receptor (AR) transcriptional activity in the absence of androgen. This ligand-independent AR activation is promoted by some growth factors but the mechanism is not well understood. Vav3, a Rho guanosine triphosphatase guanine nucleotide exchange factor, which is activated by growth factors, is up-regulated in human prostate cancer. We show here that Vav3 levels increase during in vivo progression of prostate cancer to androgen independence. Vav3 strikingly enhanced growth factor activation of AR in the absence of androgen. Because Vav3 may be chronically activated in prostate cancer by growth factor receptors, we examined the effects of a constitutively active (Ca) form of Vav3 on AR transcriptional activity. Ca Vav3 caused nuclear localization and ligand-independent activation of AR via the Rho guanosine triphosphatase, Rac1. Ca Rac1 activation of AR occurred, in part, through MAPK/ERK signaling. Expression of active Rac1 conferred androgen-independent growth of prostate cancer cells in culture, soft agar, and mice. These findings suggest that Vav3/Rac 1 signaling is an important modulator of ligand-independent AR transcriptional activity in prostate cancer progression.

alpha-CaMKII controls the growth of human osteosarcoma by regulating cell cycle progression

Osteosarcoma is the most frequent type of primary bone cancer in children and adolescents. These malignant osteoid forming tumors are characterized by their uncontrolled hyperproliferation. Here, we investigate the role of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in the growth of human osteosarcoma. We show that alpha-CaMKII is expressed in human osteosarcoma cell lines and in primary osteosarcoma tissue derived from patients. The pharmacologic inhibition of CaMKII in MG-63 and 143B human osteosarcoma cells by KN-93 resulted in an 80 and 70% decrease in proliferation, respectively, and induced cell cycle arrest in the G(0)/G(1) phase. The in vivo administration of KN-93 to mice xenografted with human osteosarcoma cells significantly decreased intratibial and subcutaneous tumor growth. Mechanistically, KN-93 and alpha-CaMKII siRNA increased p21((CIP/KIP)) gene expression, protein levels, and decreased the phosphorylation of retinoblastoma protein and E2F transactivation. Furthermore, the inhibition of CaMKII decreased membrane-bound Tiam1 and GTP-bound Rac1, which are known to be involved in p21 expression and tumor growth in a variety of solid malignant neoplasms. Our results suggest that CaMKII plays a critical role in the growth of osteosarcoma, and its inhibition could be an attractive therapeutic target to combat conventional high-grade osteosarcoma in children.

Establishment and characterization of androgen-independent human prostate cancer cell lines, LN-REC4 and LNCaP-SF, from LNCaP

AIM

To investigate the mechanisms of androgen-independent growth in prostate cancer (PCa), we established two PCa cell lines, LN-REC4 and LNCaP-SF, from the androgen-dependent PCa cell line, LNCaP.

MATERIALS AND METHODS

LN-Pre and LN-REC4 cells were generated from LNCaP tumors grown on intact and castrated severe combined immunodeficient (SCID) mouse, respectively. After we cultured LNCaP cells under a steroid-free conditions for 6 months in vitro, LNCaP-SF cells were established. To show the character of LN-REC4 and LNCaP-SF cells, androgen sensitivity was investigated through examination of growth rate, and prostate-specific antigen (PSA), androgen receptor (AR), p21, p27, and cyclin D1 expression were examined by reverse transcription-polymerase chain reaction (RT-PCR). Angiogenesis assay in vitro was carried out using conditioned medium. To examine the expression level of vascular endothelial growth factor (VEGF), RT-PCR and enzyme-linked immunosorbent assay were also done.

RESULTS AND CONCLUSIONS

LN-REC4 cells proliferated better than LNCaP cells in castrated mice and did well irrespective of castration, although responsiveness for androgen of LN-REC4 cells attenuated less than that of LNCaP cells in vitro. LNCaP-SF cells in castrated mice proliferated more rapidly than in normal mice. The PSA expression in LNCaP-SF cells was still induced by androgen. Expression of AR, p21, p27 and cyclin D1 were not changed in LN-REC4 and LNCaP-SF cells. Angiogenesis assay showed that both cells stimulated angiogenesis. LN-REC4 induced VEGF more than LNCaP and LN-Pre cells. However, expression of VEGF per cell in LNCaP-SF was lower than LNCaP cells, suggesting that other factors might be involved in angiogenesis. These cell lines might be a useful tool for researching androgen-independent growth and treatments of recurred PCa.

Interaction of Pyk2 and PTP-PEST with leupaxin in prostate cancer cells

We have identified the presence of leupaxin (LPXN), which belongs to the paxillin extended family of focal adhesion-associated adaptor proteins, in prostate cancer cells. Previous studies have demonstrated that LPXN is a component of the podosomal signaling complex found in osteoclasts, where LPXN was found to associate with the protein tyrosine kinases Pyk2 and c-Src and the cytosolic protein tyrosine phosphatase-proline-, glutamate-, serine-, and threonine-rich sequence (PTP-PEST). In the current study, LPXN was detectable as a 50-kDa protein in PC-3 cells, a bone-derived metastatic prostate cancer cell line. In PC-3 cells, LPXN was also found to associate with Pyk2, c-Src, and PTP-PEST. A siRNA-mediated inhibition of LPXN resulted in decreased in vitro PC-3 cell migration. A recombinant adenoviral-mediated overexpression of LPXN resulted in an increased association of Pyk2 with LPXN, whereas a similar adenoviral-mediated overexpression of PTP-PEST resulted in decreased association of Pyk2 and c-Src with LPXN. The overexpression of LPXN in PC-3 cells resulted in increased migration, as assessed by in vitro Transwell migration assays. On the contrary, the overexpression of PTP-PEST in PC-3 cells resulted in decreased migration. The overexpression of LPXN resulted in increased activity of Rho GTPase, which was decreased in PTP-PEST-overexpressing cells. The increase in Rho GTPase activity following overexpression of LPXN was inhibited in the presence of Y27632, a selective inhibitor of Rho GTPase. In conclusion, our data demonstrate that LPXN forms a signaling complex with Pyk2, c-Src, and PTP-PEST to regulate migration of prostate cancer cells.

Androgen receptor auto-regulates its expression by a negative feedback loop through upregulation of IFI16 protein

Expression of androgen receptor (AR) in prostate epithelial cells is thought to regulate cell proliferation, differentiation, and survival. However, the molecular mechanisms remain unclear. We report that re-expression of AR in PC-3 human prostate cancer cell line resulted in upregulation of IFI16 protein, a negative regulator of cell growth. We found that the IFI16 protein bound to AR in a ligand-dependent manner and the DNA-binding domain (DBD) of the AR was sufficient to bind IFI16. Furthermore, re-expression of IFI16 protein in LNCaP prostate cancer cells, which do not express IFI16 protein, resulted in downregulation of AR expression and an inhibition of the expression of AR target genes. Our observations identify a role for IFI16 protein in AR-mediated functions.

S179D prolactin increases vitamin D receptor and p21 through up-regulation of short 1b prolactin receptor in human prostate cancer cells

In this study, we further investigated the mechanisms by which pseudophosphorylated prolactin (S179D PRL) inhibits the growth of human prostate cancer cells. When treated with S179D PRL for 3 days, LnCAP cells responded by increasing expression of the vitamin D receptor (VDR) and the cell cycle regulatory molecule, p21, whereas PC3 and DU145 cells did not. After 5 days of treatment, both PC3 and DU145 cells responded. Untreated LnCAP cells express the short 1b form (SF1b) of the human prolactin receptor, but DU145 and PC3 cells express only low amounts of this receptor until elevated by treatment with S179D PRL. DU145 and PC3 cells become sensitive to the negative effects of S179D PRL on cell number after induction of the SF1b. Transfection of either SF1b or SF1a into PC3 or DU145 cells made them sensitive to S179D PRL in the 3-day time frame, a finding that was not duplicated by transfection with the long form of the receptor. Treatment of LnCAP cells with S179D PRL increased long-term activation of extracellular signal-regulated kinase 1/2 (ERK1/2). This did not occur in PC3 and DU145 cells until transfection with SF1a/SF1b. Blockade of ERK signaling eliminated S179D PRL-stimulated expression of the VDR and p21 in LnCAP cells and transfected PC3 and DU145 cells. We conclude that initiation of alternative splicing to produce SF1b, and subsequent altered signaling, contribute to the growth inhibitory mechanisms of S179D PRL. This is the first indication of a role for short prolactin receptors in the regulation of cell proliferation.

Lost in transcription: p21 repression, mechanisms, and consequences

The cyclin-dependent kinase inhibitor p21WAF1/CIP1 is a major player in cell cycle control and it is mainly regulated at the transcriptional level. Whereas induction of p21 predominantly leads to cell cycle arrest, repression of p21 may have a variety of outcomes depending on the context. In this review, we concentrate on transcriptional repression of p21 by cellular and viral factors, and delve in detail into its possible biological implications and its role in cancer. It seems that the major mode of p21 transcriptional repression by negative regulators is the interference with positive transcription factors without direct binding to the p21 promoter. Specifically, the negative factors may either inhibit binding of positive regulators to the promoter or hinder their transcriptional activity. The ability of p21 to inhibit proliferation may contribute to its tumor suppressor function. Because of this, it is not surprising that a number of oncogenes repress p21 to promote cell growth and tumorigenesis. However, p21 is also an inhibitor of apoptosis and p21 repression may also have an anticancer effect. For example, c-Myc and chemical p21 inhibitors, which repress p21, sensitize tumor cells to apoptosis by anticancer drugs. Further identification of factors that repress p21 is likely to contribute to the better understanding of its role in cancer.

c-Myc suppresses p21WAF1/CIP1 expression during estrogen signaling and antiestrogen resistance in human breast cancer cells

Estrogen rapidly induces expression of the proto-oncogene c-myc. c-Myc is required for estrogen-stimulated proliferation of breast cancer cells, and deregulated c-Myc expression has been implicated in antiestrogen resistance. In this report, we investigate the mechanism(s) by which c-Myc mediates estrogen-stimulated proliferation and contributes to cell cycle progression in the presence of antiestrogen. The MCF-7 cell line is a model of estrogen-dependent, antiestrogen-sensitive human breast cancer. Using stable MCF-7 derivatives with inducible c-Myc expression, we demonstrated that in antiestrogen-treated cells, the elevated mRNA and protein levels of p21(WAF1/CIP1), a cell cycle inhibitor, decreased upon either c-Myc induction or estrogen treatment. Expression of p21 blocked c-Myc-mediated cell cycle progression in the presence of antiestrogen, suggesting that the decrease in p21 is necessary for this process. Using RNA interference to suppress c-Myc expression, we further established that c-Myc is required for estrogen-mediated decreases in p21(WAF1/CIP1). Finally, we observed that neither c-Myc nor p21(WAF1/CIP1) is regulated by estrogen or antiestrogen in an antiestrogen-resistant MCF-7 derivative. The p21 levels in the antiestrogen-resistant cells increased when c-Myc expression was suppressed, suggesting that loss of p21 regulation was a consequence of constitutive c-Myc expression. Together, these studies implicate p21(WAF1/CIP1) as an important target of c-Myc in breast cancer cells and provide a link between estrogen, c-Myc, and the cell cycle machinery. They further suggest that aberrant c-Myc expression, which is frequently observed in human breast cancers, can contribute to antiestrogen resistance by altering p21(WAF1/CIP1) regulation.