Current Cancer Drug Targets - Volume 10, Issue 4, 2010

Sphingosine kinase (SK) 1 and 2 are lipid kinases that phosphorylate sphingosine to form sphingosine-1 phosphate, a potent signalling molecule with pleiotrophic effects. SK1 is commonly up-regulated in tumours and its inhibition or genetic ablation has been shown to slow tumour growth as well as sensitise cancer cells to other chemotherapeutics. Therefore, SK1 is of particular interest as a target therapeutic intervention in cancer. Initial SK inhibitors were sphingosine derivatives and displayed efficacy in a number of disease models, establishing a premise for SK inhibition for anti-proliferative and anti-inflammatory therapies, even though these compounds had questionable specificity. More recently, a number of new SK inhibitors have been developed that display higher affinities and greater specificity for the SKs. Here we summarise the current small molecule inhibitors and related approaches for targeting the SKs, and their in vitro and in vivo efficacy. Furthermore, we highlight findings demonstrating the success of SK inhibition in cancer and a range of other disease models that promotes the continued interest in targeting the SKs for therapeutic benefit.

We recently demonstrated that poly(ADP-ribose) polymerase (PARP)-1 is involved in angiogenesis and tumour aggressiveness. In this study we have compared the influence of abrogation of PARP-1 expression by stable gene silencing to that of the pharmacological inhibition of cellular PARP activity using PARP-1/-2 inhibitors on the chemosensitivity of tumour cells to the wide spectrum methylating agent temozolomide (TMZ) and to the N3-adenine selective methylating agent {1-methyl-4-[1-methyl-4-(3-methoxysulfonylpropanamido)pyrrole-2-carboxamido]-pyrrole-2-carboxamido}propane (Me-Lex). Silencing of PARP-1 in melanoma or cervical carcinoma lines enhanced in vitro sensitivity to TMZ and Me- Lex, and induced a higher level of cell accumulation at the G2/M phase of cell cycle with respect to controls. GPI 15427, which inhibits both PARP-1 and PARP-2, increased sensitivity to TMZ and Me-Lex both in PARP-1-proficient and - deficient cells. However, it induced different cell cycle modulations depending on PARP-1 expression, provoking a G2/M arrest only in PARP-1 silenced cells. Treatment of PARP-1 silenced cells with TMZ or Me-Lex resulted in a more extensive phosphorylation of Chk-1 and p53 as compared to PARP-1 proficient cells. The combination of the methylating agents with GPI 15427 increased Chk-1 and p53 phosphorylation both in PARP-1 proficient or deficient cells. When mice challenged with PARP-1 silenced melanoma cells were treated with the TMZ and PARP inhibitor combination there was an additional reduction in tumour growth with respect to treatment with TMZ alone. These results suggest the involvement of PARP-2 or other PARPs, in the repair of DNA damage provoked by methylating agents, highlighting the importance of targeting both PARP-1 and PARP-2 for cancer therapy.

The cyclic-AMP response element-binding protein (CREB) is a nuclear transcription factor activated by phosphorylation at Ser133 by multiple serine/threonine (Ser/Thr) kinases. Upon phosphorylation, CREB binds the transcriptional co-activator, CBP (CREB-binding protein), to initiate CREB-dependent gene transcription. CREB is a critical regulator of cell differentiation, proliferation and survival in the nervous system. Recent studies have shown that CREB is involved tumor initiation, progression and metastasis, supporting its role as a proto-oncogene. Overexpression and overactivation of CREB were observed in cancer tissues from patients with prostate cancer, breast cancer, non-small-cell lung cancer and acute leukemia while down-regulation of CREB in several distinct cancer cell lines resulted in inhibition of cell proliferation and induction of apoptosis, suggesting that CREB may be a promising target for cancer therapy. Although CREB, as a transcription factor, is a challenging target for small molecules, various small molecules have been discovered to inhibit CREB phosphorylation, CREB-DNA, or CREB-CBP interaction. These results suggest that CREB is a suitable transcription factor for drug targeting and therefore targeting CREB could represent a novel strategy for cancer therapy.

Once prostate cancer becomes castration resistant, cancer cells may rapidly gain the ability to invade and to metastasize to lymph nodes and distant organs. The progression through hormone-dependent to hormone-independent/ castration-resistant and metastatic PCa is poorly understood. In this review paper, we provide an overview on the cellular and molecular mechanisms underlying the process of tumor cell invasion and metastasis in prostate cancer. We specifically presented the most recent findings on the role of multiple cellular signaling pathways including androgen receptor (AR), mitogen-activated protein kinases (MAPK), Akt, transforming growth factor β (TGFβ), interleukin-6 (IL- 6) and vascular endothelial growth factor (VEGF) in the development of hormone-independent/castration-resistant prostate cancer. In addition, we also discussed the recent findings on signatures of gene expression during prostate cancer progression. Our overviews on the novel findings will help to gain better understanding of the complexity of molecular mechanisms that may play an essential role for the development of castration-resistant and metastatic prostate cancer. It will also shed light on the identifying specific targets and design effective therapeutic drug candidates.

Androgen deprivation therapy has been the major treatment for advanced prostate cancer (PCa) and has shown to prolong life. However, remissions are temporary and patients almost inevitably progress to become castration-resistant prostate cancer (CRPC). CRPC is almost incurable even when treated with docetaxel that may have a slight life prolonging effect on CRPC patients. Interestingly, most of CRPC still express androgen receptor (AR) and depend on the AR for growth. Recently it has been suggested that AR may act as a tumor suppressor in normal prostatic epithelial cells, while in PCa cells AR becomes oncogenic, even under androgen deprivation states. The mechanisms for the latter are still under intensive investigations. A number of studies showed that, in fact, AR signaling is increased under an androgen-depleted environment. The mechanisms suggested in these studies including AR mutations, AR overexpression by gene amplification and other mechanisms that allow activation by low androgen levels or by other endogenous steroids, increased local de novo synthesis of androgens will be discussed. Moreover, developments and tests in clinical trials in CRPC of a number of novel agents interrupting AR signaling mediated PCa growth will also be discussed.

Human mesenchymal stem cells (hMSCs) consist of cells that can differentiate into mesenchymal tissues, including osteoblasts, adipocytes and chondrocytes. hMSCs constitute a particular stem cell niche in the stromal compartment of the bone marrow, and play a role in maintaining the normal function of haematopoietic stem cells. Furthermore, hMSCs localise to solid tumours, and can modulate cancer cell function through secretion of paracrine signals. While hMSCs, either in the bone marrow, or in the microenvironment of a tumour, will be targeted by DNA damaging agents used in cancer therapy, the response of the hMSC population to DNA damage is not well understood. As progenitor cells, genomic DNA damage to hMSCs during cancer therapy could generate a population of surviving cells that can go on to give rise to secondary tumours. A better understanding of the response of hMSCs to DNA damage could provide new insights into the effects of cancer treatments, as well as into the development of treatment-associated secondary cancers. This article reviews the relationship of hMSCs to cancer, with a focus on the response of hMSCs to DNA damaging agents.

Alterations in tyrosine kinase expression or functionality have been linked to tumor growth, and detailed analysis of tyrosine kinase pathways has led to the development of novel anticancer drugs based on their inhibition. The aim of the present work was to examine the cytotoxicity and cellular alterations correlated with multidrug resistance mechanisms induced by three tyrosine kinase inhibitors, lapatinib, sorafenib and gefitinib. The study was performed on three breast cancer cell lines (BRC-230, MCF-7 and SkBr3). Drug-induced growth inhibition was detected by Sulforhodamine B analysis. Apoptosis, cytosolic calcium alteration, extrusion pump activity and mitochondrial membrane depolarization were assessed by flow cytometry. Drug efflux-related gene expression was analyzed by RT-PCR and drug target protein expression was evaluated by Western Blot. Lapatinib and gefitinib induced a cytotoxic effect and mitochondrial membrane depolarization in BRC-230 and SkBr3 cells, while sorafenib induced apoptosis and a high and rapid dissipation of mitochondrial potential in all cell lines. Moreover, all three drugs produced a rapid cytosolic calcium mobilization from endoplasmic reticulum stores in the investigated cell lines and a strong decrease in multidrug transporter activity in BRC- 230 and MCF-7 cells. Mitochondrial membrane depolarization and inhibition of multidrug transporter activity induced by tyrosine kinase inhibitors were independent of cytosolic calcium mobilization. These data suggest that the investigated drugs possess mechanisms of action that are independent of drug target expression, opening up further possibilities for the development of new therapeutic strategies.