Current Cancer Drug Targets - Volume 12, Issue 1, 2012

The current paradigm for cancer therapy is undergoing a change from non-specific cytotoxic agents to more specific approaches based on unique molecular features of cancer cells. The identification and validation of disease relevant targets are crucial for the development of molecularly targeted anticancer therapies. Advances in our understanding of the molecular basis of cancer together with novel approaches to interfere with signal transduction pathways have opened new horizons for anticancer target discovery. In particular, the image-based large scale analysis of cellular phenotypes that arise from genetic or chemical perturbations paved the way for the identification and validation of disease relevant molecular targets independent of preconceived notions of mechanistic relationships. In addition, novel and sophisticated techniques of genome manipulation allow for the use of mouse models that faithfully recapitulate critical elements of human cancer for target validation in vivo. We believe that these advances will translate into more and better validated drug targets.

Unlike prostate and breast cancers, urothelial carcinoma of the urinary bladder is not yet considered as an endocrine-related neoplasm, and hormonal therapy for bladder cancer remains experimental. Nonetheless, there is increasing evidence indicating that nuclear hormone receptor signals are implicated in the development and progression of bladder cancer. Androgen-mediated androgen receptor (AR) signals have been convincingly shown to induce bladder tumorigenesis. Androgens also promote the growth of AR-positive bladder cancer cells, although it is controversial whether AR plays a dominant role in bladder cancer progression. Both stimulatory and inhibitory functions of estrogen receptor signals in bladder cancer have been reported. Various studies have also demonstrated the involvement of other nuclear receptors, including progesterone receptor, glucocorticoid receptor, vitamin D receptor, and retinoid receptors, as well as some orphan receptors, in bladder cancer. This review summarizes and discusses available data suggesting the modulation of bladder carcinogenesis and cancer progression via nuclear hormone receptor signaling pathways. These pathways have the potential to be an extremely important area of bladder cancer research, leading to the development of effective chemopreventive/therapeutic approaches, using hormonal manipulation. Considerable uncertainty remains regarding the selection of patients who are likely to benefit from hormonal therapy and optimal options for the treatment.

Angiogenesis is the formation of new blood vessels from the pre-existing vasculature. Besides its role in normal physiology, angiogenesis is significantly involved in many pathological conditions, including inflammation, cardiovascular diseases and cancer. Numerous studies have been undertaken in the area of tumor angiogenesis. It is known that pathological angiogenesis is necessary for tumors to proceed from avascular, dormant stage to vascular, sprouting stage and also contributes to their later invasion and metastasis. Playing a central role in tumor angiogenesis, vascular endothelial growth factor is considered as a key target in therapeutic approaches. This article aims to review the critical role of VEGF in tumor angiogenesis and the importance of VEGF-targeted strategies in cancer treatment.

Background: BRCA-1/2 germline mutations are responsible for early onset breast cancer and familial association. The underlying causes of the characteristic phenotypic behavior are not completely understood, but mammary stem cells appear to have a key role in this process. Materials and Methods: We have investigated the presence of mammary stem / progenitor cells in normal tissues and in tumor tissues obtained from women with and without BRCA1/2 germline mutations by utilizing ALDH-1 immunohistochemistry. Results: Isolated ALDH-1 positive cells were found in 15/28 (54%) of breast cancer samples from women with BRCA 1 or 2 mutations and in 33 /51 (65%) of matched sporadic breast cancer cases (p=0.5949, Chi Square test). While mammary stem cells were also detected in non-malignant breast lesions, only 41% of the tissues contained ALDH-1 positive cells (p=0.0371, Chi Square test). In patients with BRCA germline mutations ALDH-1 positive cells were more common in p53 positive (p=0.0028, Chi Square test) tumors, in high grade (p=0.0796), and in larger tumors (p=0.0604), while no such association was seen in sporadic cancer cases. In our patients, the expression of ALDH-1 positive cells in breast cancer was neither associated with disease-free and overall survival, nor time to metastasis. Conclusion: Breast cancers from BRCA mutation carriers do not harbor more ALHD-1 positive cells than sporadic tumors, and their more aggressive phenotype can thus not be explained by an increased stem cell pool. The presence of ALDH-1 in normal breast tissue suggests that additional factors determine the biological behavior of mammary stem cells.

Heat shock protein 32 (Hsp32), also known as heme oxygenase 1 (HO-1), has recently been identified as a potential target in various hematologic malignancies. We provide evidence that Hsp32 is constitutively expressed in primary leukemic cells in patients with acute myeloid leukemia (AML) and in various AML cell lines (HL60, U937, KG1). Expression of Hsp32 mRNA was demonstrable by qPCR, and expression of the Hsp32 protein by immunocytochemistry and Western blotting. The stem cell-enriched CD34+/CD38+ and CD34+/CD38- fractions of AML cells were found to express Hsp32 mRNA in excess over normal CD34+ progenitor cells. Two Hsp32-targeting drugs, pegylated zinc-protoporphyrin (PEG-ZnPP) and styrene-maleic-acid-copolymer-micelle-encapsulated ZnPP (SMAZnPP), were found to inhibit cytokine-dependent and spontaneous proliferation in all 3 AML cell lines as well as in primary AML cells. Growth inhibitory effects of SMA-ZnPP and PEG-ZnPP were dose-dependent with IC50 values ranging between 1 and 20 μM, and were accompanied by apoptosis as evidenced by light- and electron microscopy, Tunel assay, and caspase-3 activation. Finally, we were able to demonstrate that SMA-ZnPP inhibits cytokine-dependent proliferation of CD34+/CD38+ and CD34+/CD38- AML progenitor cells in vitro in all patients as well as leukemiainitiation of AML stem cells in NOD-SCID IL-2Rγ-/- (NSG) mice in vivo. Together, our data suggest that Hsp32 plays an important role as a survival factor in leukemic stem cells and as a potential new target in AML.

Tumor-specific delivery of ligand-directed prodrugs can increase the therapeutic window of chemotherapeutics by maintaining efficacy whilst decreasing toxic side effects. We have previously described a series of synthetic Nalkylated isatin cytotoxins that destabilize microtubules and induce apoptosis with 10-fold greater potency than conventional anti-mitotics in vitro. Here, we report the characterization, in vitro cytotoxicity and in vivo efficacy of a lead compound, 5,7-dibromo-N-(p-hydroxymethylbenzyl)isatin (N-AI) conjugated via an esterase-labile linker (N-AIE) to two proven targeting ligands, transferrin (Tf) and plasminogen activator inhibitor type 2 (PAI-2/serpinB2). N-AI was released from N-AIE and the targeting ligands Tf/PAI-2 in an esterase-dependent manner at 37 °C and both Tf- and PAI-2-N-AIE conjugates were stable at physiological pH. Human cancer cell lines which vary in their expression levels of Tf receptor (TfR/CD71) and PAI-2 target, receptor bound urokinase (uPA) selectively internalized the conjugates. Tf-N-AIE was up to 24 times more active than the free drug and showed clear selectivity patterns based on TfR levels. PAI-2-N-AIE showed equivalent activity compared to the parent drug and strong selectivity patterns for uPA levels. In preliminary in vivo experiments, the PAI-2- and Tf-N-AIE conjugates were efficacious at 1/20th and 1/10th of the dose of the free N-AI, respectively, in a metastatic, orthotopic human breast tumor xenograft mouse model. Thus, this strategy specifically delivers and concentrates a novel class of isatin-based, tubulin destabilizing agents to tumors in vivo and warrants further detailed preclinical investigation.

Liver cancer is one of the most common malignant cancers worldwide. Systemic chemotherapy remains the major treatment option, but with severe adverse effects. Combinations of systemic with targeted treatments may provide effective therapeutics. The objectives of this study were to demonstrate if insulin-like growth factor-I receptor (IGF-IR) might serve as a functional target for liver cancer treatment and to investigate the chemo-sensitizing activity of IGF-IR downregulation. IGF-IR knockdown was achieved by stable transfection of liver cancer cells with IGF-IR small interfering RNA (siRNA). IGF-IR knockdown resulted in reduced growth, clonogenic survival, adhesion and migration of liver cancer cells, and increased sensitivities of liver cancer cells to apoptosis-inducing agents and chemotherapeutic drugs in vitro. In the animal studies, both IGF-IR knockdown and adriamycin (ADM) treatment significantly reduced the growth of liver tumors. IGF-IR knockdown enhanced the effect of ADM on tumor growth by further reducing tumor angiogenesis and inducing tumor cell apoptosis. The final tumor sizes in the IGFIR-siRNA, ADM-treated EGFP, and ADM-treated IGFIR-siRNA groups were significantly reduced by 52.5%, 33.8%, and 86.3%, respectively, compared with that in the EGFP control, suggesting that the ADM and the IGF-IR knockdown inhibit the growth of liver tumors in a synergistic manner. These results support that IGF-IR may serve as a functional molecular target for liver cancer treatment, and that the combination of systemic chemotherapy with targeted IGF-IR suppression may provide an effective treatment strategy for liver cancer.