Brian T. Kawasaki

CD44+ CD24(-) prostate cells are early cancer progenitor/stem cells that provide a model for patients with poor prognosis.

Recent evidence supports the hypothesis that cancer stem cells are responsible for tumour initiation and formation. Using flow cytometry, we isolated a population of CD44+CD24− prostate cells that display stem cell characteristics as well as gene expression patterns that predict overall survival in prostate cancer patients. CD44+CD24− cells form colonies in soft agar and form tumours in NOD/SCID mice when as few as 100 cells are injected. Furthermore, CD44+CD24− cells express genes known to be important in stem cell maintenance, such as BMI-1 and Oct-3/4. Moreover, we can maintain CD44+CD24− prostate stem-like cells as nonadherent spheres in serum-replacement media without substantially shifting gene expression. Addition of serum results in adherence to plastic and shifts gene expression patterns to resemble the differentiated parental cells. Thus, we propose that CD44+CD24− prostate cells are stem-like cells responsible for tumour initiation and we provide a genomic definition of these cells and the differentiated cells they give rise to. Furthermore, gene expression patterns of CD44+CD24− cells have a genomic signature that is predictive of poor patient prognosis. Therefore, CD44+CD24− LNCaP prostate cells offer an attractive model system to both explore the biology important to the maintenance and differentiation of prostate cancer stem cells as well as to develop the therapeutics, as the gene expression pattern in these cells is consistent with poor survival in prostate cancer patients.

Targeting cancer stem cells with phytochemicals.

Cancer, second only to heart disease, is the leading cause of death in the US. Although progress has been made in the early detection of cancer and in improvements of cancer therapies, the ability to provide long-term survival has been limited. Increasing evidence suggests that a minute, biologically unique population of cancer stem cells (SCs) exists in most neoplasms and may be responsible for tumor initiation, progression, metastasis, and relapse. Characterization of cancer SCs has led to the identification of key cellular activities that may make cancer SCs vulnerable to therapeutic interventions that target drug-effluxing capabilities, stem cell pathways, anti-apoptotic mechanisms, and induction of differentiation. Phytochemicals, compounds made from fruits, vegetables, and grains, possess anti-cancer properties and represent a promising therapeutic approach for the prevention and treatment of many cancers. This review summarizes the evidence for the cancer SC hypothesis and discusses the potential mechanisms by which phytochemicals might target cancer SCs.

Effects of the sesquiterpene lactone parthenolide on prostate tumor-initiating cells: An integrated molecular profiling approach.

Recent evidence suggests tumor‐initating cells (TICs), also called cancer stem cells, are responsible for tumor initiation and progression; therefore, they represent an important cell population for development of future anti‐cancer therapies. In this study, we show that the sesquiterpene lactone parthenolide (PTL) is cytotoxic to prostate TICs isolated from prostate cancer cell lines: DU145, PC3, VCAP, and LAPC4, as well as primary prostate TICs. Furthermore, PTL inhibited TIC‐driven tumor formation in mouse xenografts. Using an integrated molecular profiling approach encompassing proteomics, profiles of activated transcription factors and genomics we ascertained the effects of PTL on prostate cancer cells. In addition to the previously described effects of PTL, we determined that the non‐receptor tyrosine kinase src, and many src signaling components, including: Csk, FAK, β1‐arrestin, FGFR2, PKC, MEK/MAPK, CaMK, ELK‐1, and ELK‐1‐dependent genes are novel targets of PTL action. Furthermore, PTL altered the binding of transcription factors important in prostate cancer including: C/EBP‐α, fos related antigen‐1 (FRA‐1), HOXA‐4, c‐MYB, SNAIL, SP1, serum response factor (SRF), STAT3, X‐box binding protein‐1 (XBP1), and p53. In summary, we show PTL is cytotoxic to prostate TICs and describe the molecular events of PTL‐mediated cytotoxicity. Therefore, PTL represents a promising therapeutic for prostate cancer treatment. Prostate 69: 827–837, 2009.

Prostate Cancer Stem Cells

Despite the discovery over 60 years ago by Huggins and Hodges 1 that prostate cancers respond to androgen deprivation therapy, hormone‐refractory prostate cancer remains a major clinical challenge. There is now mounting evidence that solid tumours originate from undifferentiated stem cell‐like cells coexisting within a heterogeneous tumour mass that drive tumour formation, maintain tumour homeostasis and initiate metastases. This review focuses upon current evidence for prostate cancer stem cells, addressing the identification and properties of both normal and transformed prostate stem cells. Copyright

Activation of Signal Transducer and Activator of Transcription 3 through a Phosphomimetic Serine 727 Promotes Prostate Tumorigenesis Independent of Tyrosine 705 Phosphorylation

Aberrantly activated signal transducer and activator of transcription 3 (Stat3) is implicated in the development of various human cancers. Y705 phosphorylation is conventionally thought to be required for Stat3 signal-dependent activation and seems to play an essential role in some malignancies. Recently, it was shown that Stat3 is activated through novel and noncanonical mechanisms, including phosphorylation at S727. Here, we investigate S727 phosphorylation of Stat3 and its subsequent effects in prostate cancer development, independent of Y705 phosphorylation, using mutated Stat3 in the human prostate cancer cell line LNCaP. We show mutation of S727 to the phosphomimetic residue Glu, and inactivation of Y705 (Y705F/S727E) resulted in a remarkable growth advantage in low-serum, enhanced anchorage-independent growth in soft agar, and increased tumorigenicity in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice, possibly by direct activation of downstream proto-oncogenes c-myc, mcl-1, and survivin. Y705F/S727E mutant cells were more invasive than Y705F/S727A (inactivation of Y705 and S727) mutant cells, and more Y705F/S727E mutant Stat3 was localized in the nuclei relative to Y705F/S727A mutant Stat3 at the steady state. Furthermore, the Y705F/S727E but not the Y705F/S727A mutant induced anchorage-independent growth of noncancerous prostate epithelial cells (RWPE-1). We further show that Stat3 is phosphorylated at S727 in 65% of malignant prostate tissues (n = 20) relative to 25% of normal prostate tissues (n = 4). Moreover, there is a positive correlation between phosphoS727-Stat3 expression and Gleason score in these prostate cancer tissues (P = 0.05). Our data suggest for the first time that S727 phosphorylation is sufficient to activate Stat3, thereby driving prostate tumorigenesis independent of Y705 phosphorylation.

Cancer stem cells, CD200 and immunoevasion

The limited success seen in cancer immunotherapy signifies that an alternative approach is required. Advances in cancer biology have identified a biologically unique subpopulation of cells, termed cancer stem cells (CSC), that survive after conventional therapy. CSCs are the putative cancer-initiating cells responsible for tumor initiation, progression and metastasis. CSCs might be able to evade the immune system by generating a tolerogenic response facilitated by the immunosuppressive factor CD200. This article reviews the biological importance of CSCs and the potentially important role of CD200 in tumor immunology. Moreover, we discuss the prospective role CD200 plays in the ability of a CSC to escape the immune system. Future immunotherapy must consider targeting CSCs to achieve curative responses.

Gossypol Induces Apoptosis by Activating p53 in Prostate Cancer Cells and Prostate Tumor–Initiating Cells

Prostate cancer continues to represent a burgeoning medical problem in the United States. Recent studies suggest that gossypol, a bioactive phytochemical produced by cotton plants, is a promising agent against prostate cancer. The current studies were undertaken to examine the chemotherapeutic efficacy of gossypol on human prostate cancer cell lines and prostate tumor–initiating cells. Gossypol reduced the viability of three prostate cancer cell lines (LAPC4, PC3, and DU145) with an IC50 between 3 and 5 μmol/L. Additionally, gossypol was effective at inhibiting prostate tumor–initiating cell-driven tumor growth in a nonobese diabetic/severe combined immunodeficient xenograft model. Our integrated molecular profiling approach encompassing proteomics, activated transcription factors, and genomics suggests that the decrease in viability was associated with increased DNA damage and the induction of apoptosis. Exposure of DU145 cells to gossypol (1–10 μmol/L) resulted in the activation of 13 proteins and 7 transcription factors, and the expression of 17 genes involved in the mitochondrial pathway of apoptosis. These studies show for the first time that gossypol treatment induces DNA damage and activates p53. Collectively, these data support the use of gossypol as a novel agent for prostate cancer. Mol Cancer Ther; 9(2); 461–70

Stem cells in prostate cancer: resolving the castrate-resistant conundrum and implications for hormonal therapy.

Androgen deprivation therapy (ADT) is initial systemic therapy for advanced prostate cancer and is used as an adjuvant to local therapy for high risk disease, but responses in advanced disease are transient. Prostate cancer stem cells are a small fraction of tumor cells that give rise to malignant cells. Initial or acquired stem cell resistance to castration must therefore underlie castrate-resistant prostate cancer. We sought to review the evidence on cancer stem cells and androgen deprivation therapy to determine if prostate cancer stem cell resistance occurs from the outset, or if it is an acquired resistance. Prostate cancer stem cells do not express androgen receptor (AR) and hence should not be directly responsive to androgen deprivation therapy. However, castrate-resistant tumors that are derived from stem cells, have molecular changes such as amplification of the androgen receptor gene, or other genetic changes resulting in gain-of-function changes in AR, implying an acquired resistance to androgen deprivation. The origins of castrate-resistant tumors, with mechanisms such as androgen receptor gene amplification from androgen receptor negative prostate cancer stem cells, is an apparent conundrum. Insight into how this occurs may lead to new treatments that overcome or delay castrateresistance. Herein, we review the evidence on cancer stem cells, the benefits of ADT, the biological basis of response to ADT, and mechanisms of castrate-resistance. We also explore the apparent conundrum of why AR-negative prostate cancer stem cells can give rise to castrate-resistant prostate cancer. We propose possible explanations that may resolve this conundrum and discuss implications for hormonal therapy.