Side population is enriched in tumorigenic, stem-like cancer cells, whereas ABCG2+ and ABCG2- cancer cells are similarly tumorigenic

Recently, several human cancers including leukemia and breast and brain tumors were found to contain stem-like cancer cells called cancer stem cells (CSC). Most of these CSCs were identified using markers that identify putative normal stem cells. In some cases, stem-like cancer cells were identified using the flow cytometry-based side population technique. In this study, we first show that approximately 30% of cultured human cancer cells and xenograft tumors examined ( approximately 30 in total) possess a detectable side population. Purified side population cells from two cell lines (U373 glioma and MCF7 breast cancer) and a xenograft prostate tumor (LAPC-9) are more tumorigenic than the corresponding non-side population cells. These side population cells also possess some intrinsic stem cell properties as they generate non-side population cells in vivo, can be further transplanted, and preferentially express some "stemness" genes, including Notch-1 and beta-catenin. Because the side population phenotype is mainly mediated by ABCG2, an ATP-binding cassette half-transporter associated with multidrug resistance, we subsequently studied ABCG2+ and ABCG2- cancer cells with respect to their tumorigenicity in vivo. Although side population cells show increased ABCG2 mRNA expression relative to the non-side population cells and all cancer cells and xenograft tumors examined express ABCG2 in a small fraction (0.5-3%) of the cells, highly purified ABCG2+ cancer cells, surprisingly, have very similar tumorigenicity to the ABCG2- cancer cells. Mechanistic studies indicate that ABCG2 expression is associated with proliferation and ABCG2+ cancer cells can generate ABCG2- cells. However, ABCG2- cancer cells can also generate ABCG2+ cells. Furthermore, the ABCG2- cancer cells form more and larger clones in the long-term clonal analyses and the ABCG2- population preferentially expresses several "stemness" genes. Taken together, our results suggest that (a) the side population is enriched with tumorigenic stem-like cancer cells, (b) ABCG2 expression identifies mainly fast-cycling tumor progenitors, and (c) the ABCG2- population contains primitive stem-like cancer cells.

Highly purified CD44+ prostate cancer cells from xenograft human tumors are enriched in tumorigenic and metastatic progenitor cells

CD44 is a multifunctional protein involved in cell adhesion and signaling. The role of CD44 in prostate cancer (PCa) development and progression is controversial with studies showing both tumor-promoting and tumor-inhibiting effects. Most of these studies have used bulk-cultured PCa cells or PCa tissues to carry out correlative or overexpression experiments. The key experiment using prospectively purified cells has not been carried out. Here we use FACS to obtain homogeneous CD44(+) and CD44(-) tumor cell populations from multiple PCa cell cultures as well as four xenograft tumors to compare their in vitro and in vivo tumor-associated properties. Our results reveal that the CD44(+) PCa cells are more proliferative, clonogenic, tumorigenic, and metastatic than the isogenic CD44(-) PCa cells. Subsequent molecular studies demonstrate that the CD44(+) PCa cells possess certain intrinsic properties of progenitor cells. First, BrdU pulse-chase experiments reveal that CD44(+) cells colocalize with a population of intermediate label-retaining cells. Second, CD44(+) PCa cells express higher mRNA levels of several 'stemness' genes including Oct-3/4, Bmi, beta-catenin, and SMO. Third, CD44(+) PCa cells can generate CD44(-) cells in vitro and in vivo. Fourth, CD44(+) PCa cells, which are AR(-), can differentiate into AR(+) tumor cells. Finally, a very small percentage of CD44(+) PCa cells appear to undergo asymmetric cell division in clonal analyses. Altogether, our results suggest that the CD44(+) PCa cell population is enriched in tumorigenic and metastatic progenitor cells.

Hospital volume can serve as a surrogate for surgeon volume for achieving excellent outcomes in colorectal resection

OBJECTIVE

To examine the association of surgeon and hospital case volumes with the short-term outcomes of in-hospital death, total hospital charges, and length of stay for resection of colorectal carcinoma.

METHODS

The study design was a cross-sectional analysis of all adult patients who underwent resection for colorectal cancer using Maryland state discharge data from 1992 to 1996. Cases were divided into three groups based on annual surgeon case volume--low (< or =5), medium (5 to 10), and high (>10)--and hospital volume--low (<40), medium (40 to 70), and high (> or =70). Poisson and multiple linear regression analyses were used to identify differences in outcomes among volume groups while adjusting for variations in type of resections performed, cancer stage, patient comorbidities, urgency of admission, and patient demographic variables.

RESULTS

During the 5-year period, 9739 resections were performed by 812 surgeons at 50 hospitals. The majority of surgeons (81%) and hospitals (58%) were in the low-volume group. The low-volume surgeons operated on 3461 of the 9739 total patients (36%) at an average rate of 1.8 cases per year. Higher surgeon volume was associated with significant improvement in all three outcomes (in-hospital death, length of stay, and cost). Medium-volume surgeons achieved results equivalent to high-volume surgeons when they operated in high- or medium-volume hospitals.

CONCLUSIONS

A skewed distribution of case volumes by surgeon was found in this study of patients who underwent resection for large bowel cancer in Maryland. The majority of these surgeons performed very few operations for colorectal cancer per year, whereas a minority performed >10 cases per year. Medium-volume surgeons achieved excellent outcomes similar to high-volume surgeons when operating in medium-volume or high-volume hospitals, but not in low-volume hospitals. The results of low-volume surgeons improved with increasing hospital volume but never equaled those of the high-volume surgeons.

NANOG promotes cancer stem cell characteristics and prostate cancer resistance to androgen deprivation

Cancer cell molecular mimicry of stem cells (SC) imbues neoplastic cells with enhanced proliferative and renewal capacities. In support, numerous mediators of SC self-renewal have been evinced to exhibit oncogenic potential. We have recently reported that shRNA-mediated knockdown of the embryonic stem cell (ESC) self-renewal gene NANOG significantly reduced the clonogenic and tumorigenic capabilities of various cancer cells. In this study, we sought to test the potential pro-tumorigenic functions of NANOG, particularly, in prostate cancer (PCa). Using quantitative RT-PCR, we first confirmed that PCa cells expressed NANOG mRNA primarily from the NANOGP8 locus on chromosome 15q14. We then constructed a lentiviral promoter reporter in which the -3.8 kb NANOGP8 genomic fragment was used to drive the expression of green fluorescence protein (GFP). We observed that NANOGP8-GFP⁺ PCa cells exhibited cancer stem cell (CSC) characteristics such as enhanced clonal growth and tumor regenerative capacity. To further investigate the functions and mechanisms of NANOG in tumorigenesis, we established tetracycline-inducible NANOG overexpressing cancer cell lines, including both prostate (Du145 and LNCaP) and breast (MCF-7) cancer cells. NANOG induction promoted drug-resistance in MCF-7 cells, tumor regeneration in Du145 cells, and, most importantly, castration-resistant tumor development in LNCaP cells. These pro-tumorigenic effects of NANOG were associated with key molecular changes, including an upregulation of molecules such as CXCR4, IGFBP5, CD133 and ALDH1. The present gain-of-function studies, coupled with our recent loss-of-function work, establish the integral role for NANOG in neoplastic processes and shed light on its mechanisms of action.

Functional evidence that the self-renewal gene NANOG regulates human tumor development

Tumor development has long been known to resemble abnormal embryogenesis. The embryonic stem cell (ESC) self-renewal gene NANOG is purportedly expressed by some epithelial cancer cells but a causal role in tumor development has remained unclear. Here, we provide compelling evidence that cultured cancer cells, as well as xenograft- and human primary prostate cancer cells express a functional variant of NANOG. NANOG mRNA in cancer cells is derived predominantly from a retrogene locus termed NANOGP8. NANOG protein is detectable in the nucleus of cancer cells and is expressed higher in patient prostate tumors than matched benign tissues. NANOGP8 mRNA and/or NANOG protein levels are enriched in putative cancer stem/progenitor cell populations. Importantly, extensive loss-of-function analysis reveals that RNA interference-mediated NANOG knockdown inhibits tumor development, establishing a functional significance for NANOG expression in cancer cells. Nanog short hairpin RNA transduced cancer cells exhibit decreased long-term clonal and clonogenic growth, reduced proliferation and, in some cases, altered differentiation. Thus, our results demonstrate that NANOG, a cell-fate regulatory molecule known to be important for ESC self-renewal, also plays a novel role in tumor development.

Hierarchical organization of prostate cancer cells in xenograft tumors: the CD44+alpha2beta1+ cell population is enriched in tumor-initiating cells

Prostate cancer cells are heterogeneous in their tumorigenicity. For example, the side population cells isolated from LAPC9 xenografts are 100 to 1,000 times more tumorigenic than the corresponding non-side population cells. Highly purified CD44(+) prostate cancer cells from several xenografts are also enriched in prostate cancer stem/progenitor cells. Because the CD44(+) prostate cancer cell population is still heterogeneous, we wonder whether we could further enrich for tumorigenic prostate cancer cells in this population using other markers. Integrin alpha2beta1 has been proposed to mark a population of normal human prostate stem cells. Therefore, we first asked whether the alpha2beta1(+/hi) cells in prostate tumors might also represent prostate cancer stem cells. Highly purified (> or =98%) alpha2beta1(+/hi) cells from three human xenograft tumors, Du145, LAPC4, and LAPC9, show higher clonal and clonogenic potential than the alpha2beta1(-/lo) cells in vitro. However, when injected into the nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse prostate or s.c., the alpha2beta1(+/hi) prostate cancer cells are no more tumorigenic than the alpha2beta1(-/lo) cells. Immunofluorescence studies reveal that CD44 and alpha2beta1 identify an overlapping and inclusive population of prostate cancer cells in that approximately 70% of alpha2beta1(+/hi) cells are CD44(+) and 20% to 30% of CD44(+) cells are distributed in the alpha2beta1(-/lo) cell population. Subsequently, we sorted out CD44(+)alpha2beta1(+/hi), CD44(+)alpha2beta1(-/lo), CD44(-)alpha2beta1(+/hi), and CD44(-)alpha2beta1(-/lo) cells from LAPC9 tumors and carried out tumorigenicity experiments. The results revealed a hierarchy in tumorigenic potential in the order of CD44(+)alpha2beta1(+/hi) approximately CD44(+)alpha2beta1(-/lo) > CD44(-)alpha2beta1(+/hi) >> CD44(-)alpha2beta1(-/lo). These observations together suggest that prostate cancer cells are organized as a hierarchy.

Arachidonate lipoxygenases as essential regulators of cell survival and apoptosis

Arachidonic acid (AA) metabolites derived from both cyclooxygenase (COX) and lipoxygenase (LOX) pathways transduce a variety of signals related to cell growth. Here, we report that the AA LOX pathway also functions as a critical regulator of cell survival and apoptosis. Rat Walker 256 (W256) carcinosarcoma cells express 12-LOX and synthesize 12(S)- and 15(S)-hydroxyeicosatetraenoic acids as their major LOX metabolites. W256 cells transfected with 12-LOX-specific antisense oligonucleotide or antisense oligonucleotides directed to conserved regions of LOXs underwent time- and dose-dependent apoptosis. Likewise, treatment of W256 cells with various LOX but not COX inhibitors induced apoptotic cell death, which could be partially inhibited by exogenous 12(S)- or 15(S)-hydroxyeicosatetraenoic acids. The W256 cell apoptosis induced by antisense oligos and LOX inhibitors was followed by a rapid downregulation of bcl-2 protein, a dramatic decrease in the bcl-2/bax ratio, and could be suppressed by bcl-2 overexpression. In contrast, p53, which is wild type in W256 cells, did not undergo alterations during apoptosis induction. The results suggest that the LOX pathway plays an important physiological role in regulating apoptosis.

The PSA(-/lo) prostate cancer cell population harbors self-renewing long-term tumor-propagating cells that resist castration

Prostate cancer (PCa) is heterogeneous and contains both differentiated and undifferentiated tumor cells, but the relative functional contribution of these two cell populations remains unclear. Here we report distinct molecular, cellular, and tumor-propagating properties of PCa cells that express high (PSA(+)) and low (PSA(-/lo)) levels of the differentiation marker PSA. PSA(-/lo) PCa cells are quiescent and refractory to stresses including androgen deprivation, exhibit high clonogenic potential, and possess long-term tumor-propagating capacity. They preferentially express stem cell genes and can undergo asymmetric cell division to generate PSA(+) cells. Importantly, PSA(-/lo) PCa cells can initiate robust tumor development and resist androgen ablation in castrated hosts, and they harbor highly tumorigenic castration-resistant PCa cells that can be prospectively enriched using ALDH(+)CD44(+)α2β1(+) phenotype. In contrast, PSA(+) PCa cells possess more limited tumor-propagating capacity, undergo symmetric division, and are sensitive to castration. Altogether, our study suggests that PSA(-/lo) cells may represent a critical source of castration-resistant PCa cells.

Conditional reprogramming and long-term expansion of normal and tumor cells from human biospecimens

Historically, it has been difficult to propagate cells in vitro that are derived directly from human tumors or healthy tissue. However, in vitro preclinical models are essential tools for both the study of basic cancer biology and the promotion of translational research, including drug discovery and drug target identification. This protocol describes conditional reprogramming (CR), which involves coculture of irradiated mouse fibroblast feeder cells with normal and tumor human epithelial cells in the presence of a Rho kinase inhibitor (Y-27632). CR cells can be used for various applications, including regenerative medicine, drug sensitivity testing, gene expression profiling and xenograft studies. The method requires a pathologist to differentiate healthy tissue from tumor tissue, and basic tissue culture skills. The protocol can be used with cells derived from both fresh and cryopreserved tissue samples. As approximately 1 million cells can be generated in 7 d, the technique is directly applicable to diagnostic and predictive medicine. Moreover, the epithelial cells can be propagated indefinitely in vitro, yet retain the capacity to become fully differentiated when placed into conditions that mimic their natural environment.

Platelets and cancer metastasis: a causal relationship?

Cancer metastasis is a highly coordinated and dynamic multistep process in which cancer cells undergo extensive interactions with various host cells before they establish a secondary metastatic colony. Ample morphological studies have documented the close association of circulating tumor cells with host platelets. Several lines of evidence provide strong support for the concept that tumor cell-platelet interactions (i.e., TCIPA) significantly contribute to hematogenous metastasis. Clinically, cancer patients with advanced diseases are characterized by a variety of thromboembolic disorders including thrombocytosis. Pharmacologically, various anti-platelet agents/anticoagulants have demonstrated potent inhibitory effects on tumor cell-platelet interactions as well as spontaneous or experimental metastasis. Experimentally, interference with many of the intermediate steps of tumor cell-platelet interactions has resulted in diminished platelet aggregation induced by tumor cells and blocked cancer metastasis. Platelet interaction with tumor cells is a sequential process which involves two general types of mediators, i.e., membrane-bound molecules (adhesion molecules) and soluble release products. alpha IIb beta 3 integrin receptors present on both platelets as well as on tumor cells and 12(S)-HETE, a 12-lipoxygenase metabolite of arachidonic acid, are prototypical examples of each category. Mechanistically, platelets may contribute to metastasis by: (1) stabilizing tumor cell arrest in the vasculature, (2) stimulating tumor cell proliferation, (3) promoting tumor cells extravasation by potentiating tumor cell-induced endothelial cell retraction, and (4) enhancing tumor cell interaction with the extracellular matrix.

Cancer stem cells and radioresistance

PURPOSE

Radiation therapy has made significant contributions to cancer treatment. However, despite continuous improvements, tumor recurrence and therapy resistance still occur in a high proportion of patients. One underlying reason for this radioresistance might be attributable to the presence of cancer stem cells (CSC). The purpose of this review is to discuss CSC-specific mechanisms that confer radiation resistance.

CONCLUSIONS

We focus our discussions on breast cancer and glioblastoma multiforme (GBM) and conclude that both CSC-intrinsic and CSC-extrinsic factors as well as adaptive responses in CSC caused by irradiation and microenvironmental changes all make contributions to CSC-mediated radioresistance. Our discussions emphasize CSC as novel therapeutic targets in order to potentiate radiotherapy efficacy.

MicroRNA regulation of cancer stem cells

Cancer stem cells (CSC), or cancer cells with stem cell properties, have been reported in many human tumors and are thought to be responsible for tumor initiation, therapy resistance, progression, relapse, and metastasis. Despite their potential clinical importance, how CSCs are regulated at the molecular level is not well understood. MicroRNAs (miRNA), small noncoding RNAs that play critical roles in normal stem cell functions during development, have emerged as important regulators of CSCs as well. In this review, we summarize the current major findings of miRNA regulation of various CSCs and discuss our recent findings that miR-34a suppresses prostate CSCs and metastasis by directly repressing CD44. This recent progress has important implications for understanding how CSCs are intricately regulated by networks of miRNAs and for developing novel mechanism-based miRNA therapeutics that specifically target CSCs.

Cell-of-Origin of Cancer versus Cancer Stem Cells: Assays and Interpretations

A tumor originates from a normal cell that has undergone tumorigenic transformation as a result of genetic mutations. This transformed cell is the cell-of-origin for the tumor. In contrast, an established clinical tumor is sustained by subpopulations of self-renewing cancer cells operationally called cancer stem cells (CSC) that can generate, intraclonally, both tumorigenic and nontumorigenic cells. Identifying and characterizing tumor cell-of-origin and CSCs should help elucidate tumor cell heterogeneity, which, in turn, should help understand tumor cell responses to clinical treatments, drug resistance, tumor relapse, and metastatic spread. Both tumor transplantation and lineage-tracing assays have been helpful in characterizing these cancer cell populations, although each system has its strengths and caveats. In this article, we briefly review and summarize advantages and limitations of both assays in support of a combinatorial approach to accurately define the roles of both cancer-initiating and cancer-propagating cells. As an aside, we also wish to clarify the definitions of cancer cell-of-origin and CSCs, which are often interchangeably used by mistake.

Lack of replicative senescence in cultured rat oligodendrocyte precursor cells

Most mammalian somatic cells are thought to have a limited proliferative capacity because they permanently stop dividing after a finite number of divisions in culture, a state termed replicative cell senescence. Here we show that most oligodendrocyte precursor cells purified from postnatal rat optic nerve can proliferate indefinitely in serum-free culture if prevented from differentiating; various cell cycle-inhibitory proteins increase, but the cells do not stop dividing. The cells maintain high telomerase activity and p53- and Rb-dependent cell cycle checkpoint responses, and serum or genotoxic drugs induce them to acquire a senescence-like phenotype. Our findings suggest that some normal rodent precursor cells have an unlimited proliferative capacity if cultured in conditions that avoid both differentiation and the activation of checkpoint responses that arrest the cell cycle.

Adhesion molecules and tumor cell interaction with endothelium and subendothelial matrix

Cancer metastasis poses the greatest challenge to the eradication of malignancy. The majority of clinical and experimental evidence indicates that metastasis is a non-random, organ-specific process. Tumor cell interaction with endothelium and subendothelial matrix constitutes the most crucial factor in determining the organ preference of metastasis. A plethora of cell surface adhesion molecules, which encompass four major families (i.e., integrins, cadherins, immunoglobulins and selectins) and many other unclassified molecules, mediate tumor-host interactions. Adhesion molecules and adhesion processes are involved in most, if not all, of the intermediate steps of the metastatic cascade. Decreased E-cadherin expression and increased CD44 expression are clearly correlated with the acquisition of the invasive capacity of primary tumor cells. Similarly, altered expression pattern of many other adhesion molecules such as upregulated expression of the laminin receptors and depressed expression of fibronectin receptors (alpha 5 beta 1) appears to be involved in tumor cell invasion into the subendothelial matrix. Tumor cell-endothelium interactions involve several well-defined sequential steps that can be analyzed by the 'Docking and Locking' hypothesis at the molecular level. Tumor cell-matrix interactions are determined by the repertoire of adhesion receptors of tumor cells and the unique composition of organ-specific matrices. Our experimental data, together with others', suggest that the integrin alpha IIb beta 3 is one of the major players in these tumor-host interactions. Tumor-host interaction is a dynamic process which is constantly modulated by a host of factors including various cytokines, growth factors and arachidonate metabolites such as 12(S)-HETE. Delineation of the molecular mechanisms of tumor-host interactions may provide additional means to intervene in the metastatic process.

Prostate cancer stem/progenitor cells: identification, characterization, and implications

Several solid tumors have now been shown to contain stem cell-like cells called cancer stem cells (CSC). These cells, although generally rare, appear to be highly tumorigenic and may be the cells that drive tumor formation, maintain tumor homeostasis, and mediate tumor metastasis. In this Perspective, we first provide our insight on how a CSC should be defined. We then summarize our current knowledge of stem/progenitor cells in the normal human prostate (NHP), an organ highly susceptible to hyperproliferative diseases such as benign prostate hyperplasia (BPH) and prostate cancer (PCa). We further review the evidence that cultured PCa cells, xenograft prostate tumors, and patient tumors may contain stem/progenitor cells. Along with our discussion, we present several methodologies that can be potentially used to identify putative tumor-reinitiating CSC. Finally, we present a hypothetical model for the hierarchical organization of human PCa cells and discuss the implications of this model in helping understand prostate carcinogenesis and design novel diagnostic, prognostic, and therapeutic approaches.

Cytosolic Accumulation of HSP60 during Apoptosis with or without Apparent Mitochondrial Release

Most heat shock proteins (HSPs) have pro-survival functions. However, the role of HSP60, a mitochondrial matrix protein, is somewhat controversial with both pro-survival and pro-apoptotic functions reported. Here we show that in numerous apoptotic systems HSP60 protein accumulates in the cytosol. In BMD188-induced cell death, HSP60 accumulates in the cytosol with significant mitochondrial release. In contrast, in apoptosis induced by multiple other inducers, the cytosolic HSP60 accumulates without an apparent mitochondrial release. The short interfering RNA-mediated knockdown experiments revealed that in BMD188-induced apoptosis, HSP60 has a pro-death function and that the pro-death role of HSP60 seems to involve caspase-3 maturation and activation in the cytosol. In contrast, HSP60 appears to play a pro-survival role in other apoptotic systems where there is no apparent mitochondrial release as its knockdown promotes cell death. In these latter apoptotic systems HSP60 does not associate with active caspase-3. In both cases, HSP60 does not appreciably interact with Bax. Taken together, our results suggest the following: 1) cytosolic accumulation of HSP60 represents a common phenomenon during apoptosis induction; 2) cytosolic HSP60 accumulation during apoptosis occurs either with or without apparent mitochondrial release; and 3) the cytosolically accumulated HSP60 possesses either pro-survival or pro-death functions, which involves differential interactions with caspase-3.

MicroRNA-141 suppresses prostate cancer stem cells and metastasis by targeting a cohort of pro-metastasis genes

MicroRNAs play important roles in regulating tumour development, progression and metastasis. Here we show that one of the miR-200 family members, miR-141, is under-expressed in several prostate cancer (PCa) stem/progenitor cell populations in both xenograft and primary patient tumours. Enforced expression of miR-141 in CD44⁺ and bulk PCa cells inhibits cancer stem cell properties including holoclone and sphere formation, as well as invasion, and suppresses tumour regeneration and metastasis. Moreover, miR-141 expression enforces a strong epithelial phenotype with a partial loss of mesenchymal phenotype. Whole-genome RNA sequencing uncovers novel miR-141-regulated molecular targets in PCa cells including the Rho GTPase family members (for example, CDC42, CDC42EP3, RAC1 and ARPC5) and stem cell molecules CD44 and EZH2, all of which are validated as direct and functionally relevant targets of miR-141. Our results suggest that miR-141 employs multiple mechanisms to obstruct tumour growth and metastasis.

MicroRNAs have important roles in regulating tumor development, progression and metastasis. Here, the authors demonstrate the tumor-suppressive functions of miRNA141 in prostate cancer stem cells mediated by directly targeting CD44, Rho GTPase protein family members, and EZH2.

PC3 human prostate carcinoma cell holoclones contain self-renewing tumor-initiating cells

Primary keratinocytes exhibit three typical clonal morphologies represented by holoclones, meroclones, and paraclones, with holoclones containing self-renewing stem cells, and meroclones and paraclones containing more mature and differentiated cells. Interestingly, long-term-cultured human epithelial cancer cells in clonal cultures also form holoclones, meroclones, and paraclones, and tumor cell holoclones have been hypothesized to harbor stem-like cells or cancer stem cells. However, the key question of whether tumor cell holoclones genuinely contain tumor-initiating cells has not been directly addressed. Here, using PC3 human prostate carcinoma cells as a model, we provide direct experimental evidence that tumor cell holoclones contain stem-like cells that can initiate serially transplantable tumors. Importantly, holoclones derived from either cultured PC3 cells or holoclone-initiated tumors can be serially passaged and regenerate all three types of clones. In contrast, meroclones and paraclones cannot be continuously propagated and fail to initiate tumor development. Phenotypic characterizations reveal high levels of CD44, alpha(2)beta(1) integrin, and beta-catenin expression in holoclones, whereas meroclones and paraclones show markedly reduced expression of these stem cell markers. The present results have important implications in understanding morphologic heterogeneities and tumorigenic hierarchies in human epithelial cancer cells.

Early mitochondrial activation and cytochrome c up-regulation during apoptosis

Apoptosis induced by many stimuli requires the mitochondrial respiratory chain (MRC) function. While studying the molecular mechanisms underlying this MRC-dependent apoptotic pathway, we find that apoptosis in multiple cell types induced by a variety of stimuli is preceded by an early induction of MRC proteins such as cytochrome c (which is encoded by a nuclear gene) and cytochrome c oxidase subunit II (COX II) (which is encoded by the mitochondrial genome). Several non-MRC proteins localized in the mitochondria, e.g. Smac, Bim, Bak, and Bcl-2, are also rapidly up-regulated. The up-regulation of many of these proteins (e.g. cytochrome c, COX II, and Bim) results from transcriptional activation of the respective genes. The up-regulated cytosolic cytochrome c rapidly translocates to the mitochondria, resulting in an accumulation of holocytochrome c in the mitochondria accompanied by increasing holocytochrome c release into the cytosol. The increased cytochrome c transport from cytosol to the mitochondria does not depend on the mitochondrial protein synthesis or MRC per se. In contrast, cytochrome c release from the mitochondria involves dynamic changes in Bcl-2 family proteins (e.g. up-regulation of Bak, Bcl-2, and Bcl-x(L)), opening of permeability transition pore, and loss of mitochondrial membrane potential. Overexpression of cytochrome c enhances caspase activation and promotes cell death in response to apoptotic stimulation, but simple up-regulation of cytochrome c using an ecdysone-inducible system is, by itself, insufficient to induce apoptosis. Taken together, these results suggest that apoptosis induced by many stimuli involves an early mitochondrial activation, which may be responsible for the subsequent disruption of MRC functions, loss of Deltapsi(m), cytochrome c release, and ultimately cell death.

Concise Review: NANOG in Cancer Stem Cells and Tumor Development: An Update and Outstanding Questions

The homeobox domain transcription factor NANOG, a key regulator of embryonic development and cellular reprogramming, has been reported to be broadly expressed in human cancers. Functional studies have provided strong evidence that NANOG possesses protumorigenic attributes. In addition to promoting self-renewal and long-term proliferative potential of stem-like cancer cells, NANOG-mediated oncogenic reprogramming may underlie clinical manifestations of malignant disease. In this review, we examine the molecular origin, expression, biological activities, and mechanisms of action of NANOG in various malignancies. We also consider clinical implications such as correlations between NANOG expression and cancer prognosis and/or response to therapy. We surmise that NANOG potentiates the molecular circuitry of tumorigenesis, and thus may represent a novel therapeutic target or biomarker for the diagnosis, prognosis, and treatment outcome of cancer. Finally, we present critical pending questions relating NANOG to cancer stem cells and tumor development.

12-lipoxygenases and 12(S)-HETE: role in cancer metastasis

Arachidonic acid metabolites have been implicated in multiple steps of carcinogenesis. Their role in tumor cell metastasis, the ultimate challenge for the treatment of cancer patients, are however not well-documented. Arachidonic acid is primarily metabolized through three pathways, i.e., cyclooxygenase, lipoxygenase, and P450-dependent monooxygenase. In this review we focus our attention on one specific lipoxygenase, i.e., 12-lipoxygenase, and its potential role in modulating the metastatic process. In mammalian cells there exist three types of 12-lipoxygenases which differ in tissue distribution, preferential substrates, and profile of their metabolites. Most of these 12-lipoxygenases have been cloned and sequenced, and the molecular and biochemical determinants responsible for catalysis of specific substrates characterized. Solid tumor cells express 12-lipoxygenase mRNA, possess 12-lipoxygenase protein, and biosynthesize 12(S)-HETE [12(S)-hydroxyeicosatetraenoic acid], as revealed by numerous experimental approaches. The ability of tumor cells to generate 12(S)-HETE is positively correlated to their metastatic potential. A large collection of experimental data suggest that 12(S)-HETE is a crucial intracellular signaling molecule that activates protein kinase C and mediates the biological functions of many growth factors and cytokines such as bFGF, PDGF, EGF, and AMF. 12(S)-HETE plays a pivotal role in multiple steps of the metastatic 'cascade' encompassing tumor cell-vasculature interactions, tumor cell motility, proteolysis, invasion, and angiogenesis. The fact that 12-lipoxygenase is expressed in a wide diversity of tumor cell lines and 12(S)-HETE is a key modulatory molecule in metastasis provides the rationale for targeting these molecules in anti-cancer and anti-metastasis therapeutic protocols.

Distinct microRNA expression profiles in prostate cancer stem/progenitor cells and tumor-suppressive functions of let-7

MiRNAs regulate cancer cells, but their potential effects on cancer stem/progenitor cells are still being explored. In this study, we used quantitative real-time-PCR to define miRNA expression patterns in various stem/progenitor cell populations in prostate cancer, including CD44+, CD133+, integrin α2β1+, and side population cells. We identified distinct and common patterns in these different tumorigenic cell subsets. Multiple tumor-suppressive miRNAs were downregulated coordinately in several prostate cancer stem/progenitor cell populations, namely, miR-34a, let-7b, miR-106a, and miR-141, whereas miR-301 and miR-452 were commonly overexpressed. The let-7 overexpression inhibited prostate cancer cell proliferation and clonal expansion in vitro and tumor regeneration in vivo. In addition, let-7 and miR-34a exerted differential inhibitory effects in prostate cancer cells, with miR-34a inducing G1 phase cell-cycle arrest accompanied by cell senescence and let-7 inducing G2-M phase cell-cycle arrest without senescence. Taken together, our findings define distinct miRNA expression patterns that coordinately regulate the tumorigenicity of prostate cancer cells.

Target to apoptosis: a hopeful weapon for prostate cancer

BACKGROUND

Prostate cancer is the most commonly diagnosed neoplasm and the second leading cause of male death in this country. Multiple genetic and epigenetic factors have been implicated in the oncogenesis and progression of prostate cancer. However, the molecular mechanisms underlying the disease remain largely unknown. The major difficulty in the clinical management of prostate cancer stems from the reality that reliable and accurate diagnostic/prognostic biomarkers are not available and that effective treatment regimens for hormone-resistant prostate cancers are yet to be developed.

METHODS

The present review, through extensive literature research, summarizes the most recently accumulated experimental and clinical data on the relationship between apoptosis and prostate cancer. We analyze the possibility of inducing prostate cancer cell apoptosis by: 1) androgen ablation by castration or biochemical antagonists: 2) chemotherapeutic drugs or natural/synthetic chemicals; 3) manipulation of apoptosis-related oncoproteins; and 4) modulation of intracellular signal transducers.

RESULTS

1) Prostate cancer, like most other solid tumors, represents a very heterogeneous entity. Most prostate cancers, at the time of clinical diagnosis, present themselves as mixtures of androgen-dependent and androgen-independent cells. 2) Most prostate cancers respond initially to androgen ablation since the population of androgen-dependent cells undergoes rapid apoptosis upon androgen withdrawal. However, androgen ablation rarely cures patients, most of whom will experience recurrence due to takeover of the tumor mass by androgen-independent tumor cells as well as the emergence of apoptosis-resistant clones as a result of further genetic alterations such as bcl-2 amplification. 3) On the other hand, although androgen-independent prostate cancer cells do not undergo apoptosis upon androgen blocking, they do maintain the appropriate molecular machinery of apoptosis. Therefore, certain conventional chemotherapy drugs can eliminate androgen-independent cancer cells by inducing apoptosis. 4) However, most drugs used in chemotherapy induce apoptosis or mediate cytotoxicity only in proliferating cancer cells. Human prostate cancer cells demonstrate very slow growth kinetics. Thus, novel chemical/natural products need be identified to eradicate those nonproliferating cancer cells. In this regard, the angiogenesis inhibitor, linomide, and a plant extract, beta-lapachone, demonstrate very promising apoptosis-inducing effects on prostate cancer cells in a proliferation-independent manner. 5) An alternative way to modulate the apoptotic response is by interfering with the expression levels of essential regulatory molecule of apoptosis. Bcl-2 and p53 represent two prime targets for such manipulations. 6) Finally, modulation of signal transduction pathways (e.g., intracellular Ca2+ levels, PKC activity) involved in apoptosis may also induce and/or enhance the apoptotic response of prostate cancer cells.

CONCLUSIONS

Modulation of apoptotic response represents a novel mechanism-based approach which may help identify novel drugs and/or develop new therapeutic regimens for the treatment of prostate cancers.

Evidence that axon-derived neuregulin promotes oligodendrocyte survival in the developing rat optic nerve

It was previously shown that newly formed oligodendrocytes depend on axons for their survival, but the nature of the axon-derived survival signal(s) remained unknown. We show here that neuregulin (NRG) supports the survival of purified oligodendrocytes and aged oligodendrocyte precursor cells (OPCs) but not of young OPCs. We demonstrate that axons promote the survival of purified oligodendrocytes and that this effect is inhibited if NRG is neutralized. In the developing rat optic nerve, we provide evidence that delivery of NRG decreases both normal oligodendrocyte death and the extra oligodendrocyte death induced by nerve transection, whereas neutralization of endogenous NRG increases the normal death. These results suggest that NRG is an axon-associated survival signal for developing oligodendrocytes.