The tumor suppressor Par-4 activates an extrinsic pathway for apoptosis.

Prostate apoptosis response-4 (Par-4) is a proapoptotic protein with intracellular functions in the cytoplasm and nucleus. Unexpectedly, we noted Par-4 protein is spontaneously secreted by normal and cancer cells in culture, and by Par-4 transgenic mice that are resistant to spontaneous tumors. Short exposure to endoplasmic reticulum (ER) stress-inducing agents further increased cellular secretion of Par-4 by a brefeldin A-sensitive pathway. Secretion occurred independently of caspase activation and apoptosis. Interestingly, extracellular Par-4 induced apoptosis by binding to the stress response protein, glucose-regulated protein-78 (GRP78), expressed at the surface of cancer cells. The interaction of extracellular Par-4 and cell surface GRP78 led to apoptosis via ER stress and activation of the FADD/caspase-8/caspase-3 pathway. Moreover, apoptosis inducible by TRAIL, which also exerts cancer cell-specific effects, is dependent on extracellular Par-4 signaling via cell surface GRP78. Thus, Par-4 activates an extrinsic pathway involving cell surface GRP78 receptor for induction of apoptosis.

Arylquins target vimentin to trigger Par-4 secretion for tumor cell apoptosis.

The tumor suppressor protein prostate apoptosis response-4 (Par-4), which is secreted by normal cells, selectively induces apoptosis in cancer cells. We identified a 3-arylquinoline derivative, designated Arylquin 1, as a potent Par-4 secretagogue in cell cultures and mice. Mechanistically, Arylquin 1 binds vimentin, displaces Par-4 from vimentin for secretion and triggers the efficient paracrine apoptosis of diverse cancer cells. Thus, targeting vimentin with Par-4 secretagogues efficiently induces paracrine apoptosis of tumor cells.

Par-4 secretion: stoichiometry of 3-arylquinoline binding to vimentin.

Advanced prostate tumors usually metastasize to the lung, bone, and other vital tissues and are resistant to conventional therapy. Prostate apoptosis response-4 protein (Par-4) is a tumor suppressor that causes apoptosis in therapy-resistant prostate cancer cells by binding specifically to a receptor, Glucose-regulated protein-78 (GRP78), found only on the surface of cancer cells. 3-Arylquinolines or "arylquins" induce normal cells to release Par-4 from the intermediate filament protein, vimentin and promote Par-4 secretion that targets cancer cells in a paracrine manner. A structure-activity study identified arylquins that promote Par-4 secretion, and an evaluation of arylquin binding to the hERG potassium ion channel using a [(3)H]-dofetilide binding assay permitted the identification of structural features that separated this undesired activity from the desired Par-4 secretory activity. A binding study that relied on the natural fluorescence of arylquins and that used the purified rod domain of vimentin (residues 99-411) suggested that the mechanism behind Par-4 release involved arylquin binding to multiple sites in the rod domain.

Development of 6H-Chromeno[3,4-c]pyrido[3',2':4,5]thieno[2,3-e]pyridazin-6-ones as Par-4 Secretagogues.

Nitrosation and cyclization of 4-(3-aminothieno[2,3-b]pyridine-2-yl)-2H-chromen-2-ones 1 afforded substituted 6H-chromeno[3,4-c]pyrido[3',2':4,5]thieno[2,3-e]pyridazin-6-ones 2 that inhibited the intermediary filament protein, vimentin, at low micromolar concentrations. This inhibition promoted the secretion of Prostate Apoptosis Response-4 protein (Par-4), which selectively triggered apoptosis in prostate cancer cells such as CWR22Rv1, LNCaP-derivative C4-2B, PC-3 and its aggressive analog, PC-3 MM2.

Crizotinib induces Par-4 secretion from normal cells and GRP78 expression on the cancer cell surface for selective tumor growth inhibition.

Lung cancer is the leading cause of cancer-related deaths. Lung cancer cells develop resistance to apoptosis by suppressing the secretion of the tumor suppressor Par-4 protein (also known as PAWR) and/or down-modulating the Par-4 receptor GRP78 on the cell surface (csGRP78). We sought to identify FDA-approved drugs that elevate csGRP78 on the surface of lung cancer cells and induce Par-4 secretion from the cancer cells and/or normal cells in order to inhibit cancer growth in an autocrine or paracrine manner. In an unbiased screen, we identified crizotinib (CZT), an inhibitor of activated ALK/MET/ROS1 receptor tyrosine kinase, as an inducer of csGRP78 expression in ALK-negative, KRAS or EGFR mutant lung cancer cells. Elevation of csGRP78 in the lung cancer cells was dependent on activation of the non-receptor tyrosine kinase SRC by CZT. Inhibition of SRC activation in the cancer cells prevented csGRP78 translocation but promoted Par-4 secretion by CZT, implying that activated SRC prevented Par-4 secretion. In normal cells, CZT did not activate SRC and csGRP78 elevation but induced Par-4 secretion. Consequently, CZT induced Par-4 secretion from normal cells and elevated csGRP78 in the ALK-negative tumor cells to cause paracrine apoptosis in cancer cell cultures and growth inhibition of tumor xenografts in mice. Thus, CZT induces differential activation of SRC in normal and cancer cells to trigger the pro-apoptotic Par-4-GRP78 axis. As csGRP78 is a targetable receptor, CZT can be repurposed to elevate csGRP78 for inhibition of ALK-negative lung tumors.

Tumor Suppressor Par-4 Regulates Complement Factor C3 and Obesity.

Prostate apoptosis response-4 (Par-4) is a tumor suppressor that induces apoptosis in cancer cells. However, the physiological function of Par-4 remains unknown. Here we show that conventional Par-4 knockout (Par-4-/-) mice and adipocyte-specific Par-4 knockout (AKO) mice, but not hepatocyte-specific Par-4 knockout mice, are obese with standard chow diet. Par-4-/- and AKO mice exhibit increased absorption and storage of fat in adipocytes. Mechanistically, Par-4 loss is associated with mdm2 downregulation and activation of p53. We identified complement factor c3 as a p53-regulated gene linked to fat storage in adipocytes. Par-4 re-expression in adipocytes or c3 deletion reversed the obese mouse phenotype. Moreover, obese human subjects showed lower expression of Par-4 relative to lean subjects, and in longitudinal studies, low baseline Par-4 levels denoted an increased risk of developing obesity later in life. These findings indicate that Par-4 suppresses p53 and its target c3 to regulate obesity.

Ru(II)/amino acid complexes inhibit the progression of breast cancer cells through multiple mechanism-induced apoptosis.

For some cancer subtypes, such as triple-negative breast cancer, there are no specific therapies, which leads to a poor prognosis associated with invasion and metastases. Ruthenium complexes have been developed to act in all steps of tumor growth and its progression. In this study, we investigated the effects of Ruthenium (II) complexes coupled to the amino acids methionine (RuMet) and tryptophan (RuTrp) on the induction of cell death, clonogenic survival ability, inhibition of angiogenesis, and migration of MDA-MB-231 cells (human triple-negative breast cancer). The study also demonstrated that the RuMet and RuTrp complexes induce cell cycle blockage and apoptosis of MDA-MB-231 cells, as evidenced by an increase in the number of Annexin V-positive cells, p53 phosphorylation, caspase 3 activation, and poly(ADP-ribose) polymerase cleavage. Moreover, morphological changes and loss of mitochondrial membrane potential were detected. The RuMet and RuTrp complexes induced DNA damage probably due to reactive oxygen species production related to mitochondrial membrane depolarization. Therefore, the RuMet and RuTrp complexes acted directly on breast tumor cells, leading to cell death and inhibiting their metastatic potential; this reveals the potential therapeutic action of these drugs.

Par-4-dependent apoptosis by the dietary compound withaferin A in prostate cancer cells.

Deletion or mutation of the androgen receptor (AR) renders prostate tumors refractory to apoptosis by androgen ablation, the mainstay of prostate cancer therapy. To identify novel therapeutics that can induce apoptosis regardless of the AR status of prostate cancer cells, we screened dietary herbal compounds using a reporter assay for the prostate apoptosis response-4 (Par-4) gene, which induces p53- and PTEN-independent and cancer-selective apoptosis. One of the compounds, withaferin A (WA), a major constituent of the dietary compound Withania somnifera, induced Par-4-dependent apoptosis in androgen-refractory prostate cancer cells and regression of PC-3 xenografts in nude mice. Interestingly, restoration of wild-type AR in PC-3 (AR negative) cells abrogated both Par-4 induction and apoptosis by WA. Individually, WA and anti-androgens induced neither Par-4 nor apoptosis in androgen-responsive prostate cancer cells, yet in combination, WA and anti-androgen synergistically induced Par-4 and apoptosis in androgen-responsive prostate cancer cells. Thus, when judiciously combined with anti-androgens, WA inhibits survival of both androgen-responsive and androgen-refractory prostate cancer cells by a Par-4-dependent mechanism. As Par-4 up-regulation induces apoptosis in most tumor cells, our findings can be extended to high-throughput screens to identify synergistic combinations for both therapy-sensitive and therapy-resistant cancers.

Cancer resistance in transgenic mice expressing the SAC module of Par-4.

Prostate apoptosis response-4 (Par-4) is a tumor-suppressor protein that induces apoptosis in cancer cells, but not in normal/immortalized cells. The cancer-specific proapoptotic action of Par-4 is encoded in its centrally located SAC domain. We report here the characterization of a novel mouse model with ubiquitous expression of the SAC domain. Although SAC transgenic mice displayed normal development and life span, they were resistant to the growth of spontaneous, as well as oncogene-induced, autochthonous tumors. Resistance to tumorigenesis was linked to inhibition of nuclear factor-kappaB activity and induction of apoptosis by the SAC domain. Collectively, our findings provide genetic evidence that the SAC domain of Par-4 confers cancer resistance in transgenic mice without compromising normal viability or aging, and may have therapeutic significance.

Critical role of prostate apoptosis response-4 in determining the sensitivity of pancreatic cancer cells to small-molecule inhibitor-induced apoptosis.

Role of prostate apoptosis response-4 (PAR-4) has been well described in prostate cancer. However, its significance in other cancers has not been fully elucidated. For the current study, we selected four pancreatic cancer cell lines (BxPC-3, Colo-357, L3.6pl, and HPAC) that showed differential endogenous expression of PAR-4. We found that nonpeptidic small-molecule inhibitors (SMI) of Bcl-2 family proteins (apogossypolone and TW-37; 250 nmol/L and 1 micromol/L, respectively) could induce PAR-4-dependent inhibition of cell growth and induction of apoptosis. Sensitivity to apoptosis was directly related to the expression levels of PAR-4 (R = 0.92 and R2 = 0.95). Conversely, small interfering RNA against PAR-4 blocked apoptosis, confirming that PAR-4 is a key player in the apoptotic process. PAR-4 nuclear localization is considered a prerequisite for cells to undergo apoptosis, and we found that the treatment of Colo-357 and L3.6pl cells with 250 nmol/L SMI leads to nuclear localization of PAR-4 as confirmed by 4',6-diamidino-2-phenylindole staining. In combination studies with gemcitabine, pretreatment with SMI leads to sensitization of Colo-357 cells to the growth-inhibitory and apoptotic action of a therapeutic drug, gemcitabine. In an in vivo setting, the maximum tolerated dose of TW-37 in xenograft of severe combined immunodeficient mice (40 mg/kg for three i.v. injections) led to significant tumor inhibition. Our results suggest that the observed antitumor activity of SMIs is mediated through a novel pathway involving induction of PAR-4. To our knowledge, this is the first study reporting SMI-mediated apoptosis involving PAR-4 in pancreatic cancer.

Neoadjuvant administration of hydroxychloroquine in a phase 1 clinical trial induced plasma Par-4 levels and apoptosis in diverse tumors.

Chloroquine and hydroxychloroquine (HCQ) are robust inducers of the tumor suppressor Par-4 secretion from normal cells. Secreted Par-4 causes paracrine apoptosis of tumor cells and inhibits metastasis in mice. We report the clinical results with pharmacodynamic analyses of our Phase I trial using neoadjuvant administration of HCQ in patients with surgically removable early stage solid tumors. This was a single-institution trial of oral HCQ (200 or 400 mg twice daily) given for 14 days prior to planned surgery. Dose escalation was based on isotonic regression to model safety and biological effect based on plasma Par-4 analysis. Eight of the nine patients treated with HCQ showed elevation in plasma Par-4 levels over basal levels. No toxicities were observed with these dose regimens. The resected tumors from the eight HCQ-treated patients with elevated plasma Par-4 levels, but not the resected tumor from the patient who failed to induce plasma Par-4 levels, exhibited TUNEL-positivity indicative of apoptosis. Resected tumors from all nine HCQ-treated patients showed p62/sequestosome-1 induction indicative of autophagy-inhibition by HCQ. Our findings indicate that both dose levels of HCQ were well-tolerated and that Par-4 secretion but not induction of the autophagy-inhibition marker p62 correlated with apoptosis induction in patients' tumors.

Lipopolysaccharide-induced liver apoptosis is increased in interleukin-10 knockout mice.

Although IL-10 down-regulates pro-inflammatory cytokine secretion by hepatic Kupffer cells, the mechanisms underlying its hepatoprotective effects are not fully clear. This study tested the hypothesis that IL-10 protects the liver against pro-inflammatory cytokines by counteracting their pro-apoptotic effects. Wild type and IL-10 knockout mice were treated with bacterial lipopolysaccharide and sacrificed 1, 4, 8, and 12 h later. Plasma ALT activity was measured as a marker of liver injury. Liver pathology and TUNEL response were assessed by histology. Plasma levels and whole liver mRNA levels were measured for TNF-alpha, IL-1 beta, TGF-beta1, IL-10, and their respective receptors. Hepatic mRNA levels were measured for several pro-apoptotic adaptors/regulators, including FasL, Fas receptor, FADD, TRADD, Bad, Bak, Bax, and Bcl-X(S), and anti-apoptotic regulators, including Bcl-w, Bcl-X(L), Bcl-2, and Bfl-1. Caspase-3 activity in the liver was determined as well as immunohistochemistry for IL-1RII, TGF-betaRII and Fas receptor. At all time points the livers from IL-10 knockout mice displayed a significantly increased number of apoptotic nuclei compared to wild type mice. Changes in plasma cytokine levels and their liver mRNA levels were consistent with suppression by IL-10 of pro-inflammatory cytokine secretion. In addition, pro-inflammatory cytokine receptor mRNA levels (TNF-alpha, TGF-beta, and IL-1 beta) were markedly up-regulated by LPS at all time points in IL-10 knockout mice as compared to wild type mice. Expression of the pro-inflammatory cytokine receptor IL-1RII was similarly increased as shown by immunostaining. The mRNA levels of a typical pro-apoptotic cytokine, TRAIL, were increased and LPS also up-regulated the mRNA expression of other apoptotic factors to a larger extent in IL-10 knockout mice than in their wild type counterparts, suggestive of an IL-10 anti-apoptotic effect. In the livers of knockout mice, markedly increased caspase-3 activity was already evident at the 1-h time point following LPS administration, while in the wild type animals this increase was delayed. Immunostaining also indicated that LPS increased hepatic expression of the pro-apoptotic receptors Fas and TGF-betaRII in IL-10 knockout mice. The data presented in this study show that: (i) IL-10 modulates not only the secretion of pro-inflammatory cytokines, but also the receptors of these cytokines, and ii) IL-10 protects the liver against LPS-induced injury at least in part by counteracting pro-inflammatory cytokine-induced liver apoptosis.

Identification of a unique core domain of par-4 sufficient for selective apoptosis induction in cancer cells.

Recent studies indicated that the leucine zipper domain protein Par-4 induces apoptosis in certain cancer cells by activation of the Fas prodeath pathway and coparallel inhibition of NF-kappaB transcriptional activity. However, the intracellular localization or functional domains of Par-4 involved in apoptosis remained unknown. In the present study, structure-function analysis indicated that inhibition of NF-kappaB activity and apoptosis is dependent on Par-4 translocation to the nucleus via a bipartite nuclear localization sequence, NLS2. Cancer cells that were resistant to Par-4-induced apoptosis retained Par-4 in the cytoplasm. Interestingly, a 59-amino-acid core that included NLS2 but not the C-terminal leucine zipper domain was necessary and sufficient to induce Fas pathway activation, inhibition of NF-kappaB activity, and apoptosis. Most important, this core domain had an expanded target range for induction of apoptosis, extending to previously resistant cancer cells but not to normal cells. These findings have identified a unique death-inducing domain selective for apoptosis induction in cancer cells (SAC domain) which holds promise for identifying key differences between cancer and normal cells and for molecular therapy of cancer.

A Naturally Generated Decoy of the Prostate Apoptosis Response-4 Protein Overcomes Therapy Resistance in Tumors.

Primary tumors are often heterogeneous, composed of therapy-sensitive and emerging therapy-resistant cancer cells. Interestingly, treatment of therapy-sensitive tumors in heterogeneous tumor microenvironments results in apoptosis of therapy-resistant tumors. In this study, we identify a prostate apoptosis response-4 (Par-4) amino-terminal fragment (PAF) that is released by diverse therapy-sensitive cancer cells following therapy-induced caspase cleavage of the tumor suppressor Par-4 protein. PAF caused apoptosis in cancer cells resistant to therapy and inhibited tumor growth. A VASA segment of Par-4 mediated its binding and degradation by the ubiquitin ligase Fbxo45, resulting in loss of Par-4 proapoptotic function. Conversely, PAF, which contains this VASA segment, competitively bound to Fbxo45 and rescued Par-4-mediated induction of cancer cell-specific apoptosis. Collectively, our findings identify a molecular decoy naturally generated during apoptosis that inhibits a ubiquitin ligase to overcome therapy resistance in tumors. Cancer Res; 77(15); 4039-50. ©2017 AACR.

Chloroquine-Inducible Par-4 Secretion Is Essential for Tumor Cell Apoptosis and Inhibition of Metastasis.

The induction of tumor suppressor proteins capable of cancer cell apoptosis represents an attractive option for the re-purposing of existing drugs. We report that the anti-malarial drug, chloroquine (CQ), is a robust inducer of Par-4 secretion from normal cells in mice and cancer patients in a clinical trial. CQ-inducible Par-4 secretion triggers paracrine apoptosis of cancer cells and also inhibits metastatic tumor growth. CQ induces Par-4 secretion via the classical secretory pathway that requires the activation of p53. Mechanistically, p53 directly induces Rab8b, a GTPase essential for vesicle transport of Par-4 to the plasma membrane prior to secretion. Our findings indicate that CQ induces p53- and Rab8b-dependent Par-4 secretion from normal cells for Par-4-dependent inhibition of metastatic tumor growth.

Microarray gene analysis of the liver in a rat model of chronic, voluntary alcohol intake.

The mechanisms underlying alcoholic liver disease are not fully understood. It has been established that alcohol interferes with transcriptional and translational regulatory steps of cell function. To understand such an effect, assessment of alcohol-induced changes in the simultaneous expression of a large number of genes may prove very useful. The purpose of the current study was to test a large number of genes ( approximately 8700) for possible changes in expression induced by alcohol alone or in addition to treatment with lipopolysaccharide (LPS), a putative mediator of alcohol effects on the liver. Male rats were fed an alcohol-containing liquid diet (Lieber-DeCarli) for 14-15 weeks, injected with Escherichia coli LPS (0.8 mg x kg(-1)), and killed 24 h later. Blood samples were taken for determination of plasma liver enzyme activity, and liver samples were obtained for histologic evaluation and total RNA extraction. Total RNA was analyzed for gene expression (Rat Toxicology U34 Array; Affymetrix, Santa Clara, CA). Of 8740 genes on the microchip, 2259 were expressed in the liver. Seven hundred ninety-eight genes underwent significant changes induced by either alcohol or LPS, but listed in this article are only those that significantly increased or decreased expression twofold or more. The genes were assigned to functional groups and reviewed. Gene changes were discussed from two viewpoints: relevance to established hypotheses of alcohol and LPS mechanisms of action and revealing of novel mechanisms of alcohol-induced liver injury. Application of DNA microarray technology to the study of alcohol-induced liver injury generated novel theoretical and experimental approaches to alcohol-induced liver injury.

Paracrine apoptotic effect of p53 mediated by tumor suppressor Par-4.

The guardian of the genome, p53, is often mutated in cancer and may contribute to therapeutic resistance. Given that p53 is intact and functional in normal tissues, we harnessed its potential to inhibit the growth of p53-deficient cancer cells. Specific activation of p53 in normal fibroblasts selectively induced apoptosis in p53-deficient cancer cells. This paracrine effect was mediated by p53-dependent secretion of the tumor suppressor Par-4. Accordingly, the activation of p53 in normal mice, but not p53(-)/(-) or Par-4(-)/(-) mice, caused systemic elevation of Par-4, which induced apoptosis of p53-deficient tumor cells. Mechanistically, p53 induced Par-4 secretion by suppressing the expression of its binding partner, UACA, which sequesters Par-4. Thus, normal cells can be empowered by p53 activation to induce Par-4 secretion for the inhibition of therapy-resistant tumors.

Novel mechanism of apoptosis resistance in cancer mediated by extracellular PAR-4.

Tumor suppressor PAR-4 acts in part by modulating sensitivity to apoptosis, but the basis for its activity is not fully understood. In this study, we describe a novel mechanism of antiapoptosis by NF-κB, revealing that it can block PAR-4-mediated apoptosis by downregulating trafficking of the PAR-4 receptor GRP78 from the endoplasmic reticulum to the cell surface. Mechanistic investigations revealed that NF-κB mediated this antiapoptotic mechanism by upregulating expression of UACA, a proinflammatory protein in certain disease settings. In clinical specimens of cancer, a strong correlation existed between NF-κB activity and UACA expression, relative to normal tissues. UACA bound to intracellular PAR-4 in diverse cancer cells, where it prevented translocation of GRP78 from the endoplasmic reticulum to the cell surface. This pathway of antiapoptosis could be inhibited by suppressing levels of NF-κB or UACA expression, which enhanced endoplasmic reticulum stress and restored GRP78 trafficking to the cell surface, thereby sensitizing cancer cells to apoptosis by extracellular PAR-4 or GRP78 agonistic antibody. In summary, our results identify a novel intracellular pathway of apoptosis mediated by NF-κB through UACA elevation, which by attenuating endoplasmic reticulum stress and GRP78 translocation to the cell surface can blunt the sensitivity of cancer cells to apoptosis.

Prostate apoptosis response-4 is involved in the apoptosis response to docetaxel in MCF-7 breast cancer cells.

Experimental evidence indicates that prostate apoptosis response-4 (Par-4, also known as PAWR) is a key regulator of cancer cell survival and may be a target for cancer-selective targeted therapeutics. Par-4 was first identified in prostate cancer cells undergoing apoptosis. Both intracellular and extracellular Par-4 have been implicated in apoptosis. Relatively little is known about the role of Par-4 in breast cancer cell apoptosis. In this study, we sought to investigate the effects of Par-4 expression on cell proliferation, apoptosis and drug sensitivity in breast cancer cells. MCF-7 cells were stably transfected with expression vectors for Par-4, or transiently transfected with siRNA for Par-4 knockdown. Cell proliferation assays were performed using MTT and apoptosis was evaluated using acridine orange staining, fluorescence microscopy and flow cytometry. Par-4 overexpression reduced MCF-7 proliferation rates. Conversely, Par-4 knockdown led to increased MCF-7 proliferation. Par-4 downregulation also led to increased BCL-2 and reduced BID expression. Par-4 overexpression did not affect the cell cycle profile. However, MCF-7 cells with increased Par-4 expression showed reduced ERK phosphorylation, suggesting that the inhibition of cell proliferation promoted by Par-4 may be mediated by the MAPK/ERK1/2 pathway. MCF-7 cells with increased Par-4 expression showed a marginal increase in early apoptotic cells. Importantly, we found that Par-4 expression modulates apoptosis in response to docetaxel in MCF7 breast cancer cells. Par-4 exerts growth inhibitory effects on breast cancer cells and chemosensitizes them to docetaxel.

Systemic Par-4 inhibits non-autochthonous tumor growth.

The tumor suppressor protein Par-4 (Prostate apoptosis response-4) is spontaneously secreted by normal and cancer cells. Extracellular Par-4 induces caspase-dependent apoptosis in cancer cell cultures by binding, via its effector SAC domain, to cell surface GRP78 receptor. However, the functional significance of extracellular Par-4/SAC has not been validated in animal models. We show that Par-4/SAC-transgenic mice express systemic Par-4/SAC protein and are resistant to the growth of non-autochthonous tumors. Consistently, secretory Par-4/SAC pro-apoptotic activity can be transferred from these cancer-resistant transgenic mice to cancer-susceptible mice by bone marrow transplantation. Moreover, intravenous injection of recombinant Par-4 or SAC protein inhibits metastasis of cancer cells. Collectively, our findings indicate that extracellular Par-4/SAC is systemically functional in inhibition of tumor growth and metastasis progression, and may merit investigation as a therapy.