Advances in Cancer Drug Targets

Heat shock proteins (HSPs) are molecular chaperones that stabilize folding and conformation of normal as well as oncogenic proteins. These chaperones thereby prevent the formation of protein aggregates. HSPs are often overexpressed in human malignancies, including AML. HSP90 is the main chaperon required for the stabilization of multiple oncogenic kinases involved in the development of acute myelogenous leukemia (AML). HSP90 client proteins are involved in the regulation of apoptosis, proliferation, autophagy and cell cycle progression; several of these proteins are in addition considered as possible therapeutic targets for the treatment of AML. HSP90 inhibition thereby offers the possibility to modulate several of the intracellular regulatory pathways through targeting of a single molecule. Several inhibitors of HSP90 have been developed, and they are classified into four groups: geldanamycin derivatives, radicicol derivates, synthetic inhibitors and a final group of others. The HSP90 activity is in addition regulated by posttranscriptional modulation; HSP90 inhibition can thereby be indirectly achieved through increased acetylation caused by histone deacetylase inhibitors. Many of these agents have entered clinical trials, and the results from these initial studies have documented that HSP90 inhibition can mediate antileukemic effects in vivo. However, one would expect immunosuppressive side effects because HSP90 inhibitors have both direct and indirect inhibitory effects on T cell activation. Thus, future clinical studies are needed to clarify the efficiency and toxicity of HSP90 inhibitors in the treatment of human AML, including studies where HSP90 inhibitors are combined with conventional chemotherapy.

The spindle assembly checkpoint (SAC) is an essential control system of the eukaryotic cell cycle that ensures genome stability. The essence of this evolutionarily conserved mechanism is to delay mitosis progression until proper chromosome biorientation and attachment is achieved. Mutations in the genes encoding for diverse checkpoint proteins lead to the impairment of the mitotic checkpoint mechanism, thus resulting in the premature separation of sister chromatids and aneuploidy, a condition that is associated with various classes of cancer. The understanding of the organisation, structure and function of SAC components is essential for the molecular understanding of the process and to identify and evaluate new targets for cancer drug therapy.

The very early intuition by Paget about the molecular features of metastasis has been observed in the field of therapeutic opportunities only in the last few years with the development of targeted therapy. However, to date, the diagnosis of metastases is associated in the majority of cases with the loss of any therapeutic hope. According to present knowledge, metastatic spread is considered a part of a long process in which tumor cells gain new properties regarding their cellular function, including invasion and adaptive survival. This gain of function is based on the expression of new molecular markers that may be potential therapeutic targets in blocking tumor diffusion. The epidermal growth receptor family (ErbB) comprises four members that are frequently upregulated in advanced tumor stages and have been associated with the metastatic potential of several tumors. Several inhibitors specific for one ErbB receptor have been demonstrated to be effective antitumor agents in primary cancer, but their utilization is restricted to ErbB-overexpressing tumors and limited by toxicity problems, drug resistance and molecular desensitization. However, new studies indicate that ErbB inhibitors may provide a much-needed therapeutic option, mainly for patients with metastases. In order to illustrate the potential of ErbB family members as therapeutic targets in blocking metastases, we summarize the latest molecular evidence and the results of clinical trials.

Bisphosphonates are extensively used to treat cancer-induced bone disease in a range of solid and bone derived tumours, where they reduce the incidence of skeletal related events and improve patients’ quality of life. Recent reports indicate that bisphosphonates may also prevent recurrence of breast cancer at peripheral sites, suggesting that these drugs may have anti-cancer activity outside the skeleton and effects on a range of tumour cell types have been reported in vitro. These positive results have subsequently been supported by investigations of effects of bisphosphonates on tumour growth in vivo, both in bone and at peripheral sites. A reduction of tumour burden and in cancer-induced bone disease following bisphosphonate treatment is demonstrated in several model systems, including breast and prostate cancer, osteosarcoma and multiple myeloma. In addition, bisphosphonates have been shown to significantly reduce growth of human tumour cells implanted subcutaneously in immunocompromised mice. However, the majority of in vivo studies reporting anti-tumour effects have used high doses and frequent administration of bisphosphonates, and the clinical relevance of these data have therefore been the subject of considerable debate. Bisphosphonates may hold greater promise as anti-tumour agents when used in combination with cytotoxic drugs, and several in vivo studies have reported substantial increased inhibition of tumour growth and improved survival when bisphosphonates have been added to standard chemotherapy regimens. This review will summarise the published data on anti-tumour effects of bisphosphonates reported from in vivo models, alone and in combination with other anti-cancer agents, and highlight the main lessons learned and future challenges in this field.

Bleomycin, a widely used anti-tumor agent, is well-known to cause singleand double-strand breaks in cellular DNA in vivo and in vitro leading finally to genomic instability of damaged cells. Bleomycin causes an increase of reactive oxygen species, resulting in oxidative stress and pulmonary fibrosis. Further, bleomycin induces apoptosis and senescence in epithelial as well as non-epithelial cells of the lung.

Caveolin-1 is a scaffold protein of caveolae, which are particularly abundant in alveolar epithelial type I cells, in endothelial and smooth muscle cells, and in fibroblasts of lung tissue. Caveolin-1 directly interacts with signaling molecules and effects diverse signaling pathways regulating cell proliferation, apoptosis, differentiation, migration and growth.

In this review we discuss aspects of bleomycin resistance. We summarize recent data about the effects of bleomycin in terms of lung cell biology and emphasize that bleomycin-induced injury of lung cells is accompanied by altered expression levels of caveolin-1. Caveolin-1 is involved in bleomycin-induced apoptosis and senescence of normal and lung cancer cells. Investigating the role of caveolin-1 may provide new tools for therapeutic interventions in lung disease and for the understanding of tumor biology.

Breast carcinogenesis is a multistep and multipath disease process which occurs in the epithelium lining of the ductal system in the vast majority of cases. Several studies have shown that the relative risk of breast cancer increases in every step of this progression and many tumour associated antigens or biomarkers appear during each phase of carcinogenesis. However, their ability to predict for a substantial likelihood of developing breast cancer remains unclear. The acquisition of breast tissue samples, representative of an individual’s cellular stability and subcellular biochemical and molecular state could lead to definition of surrogates for risk, early detection, pharmacodynamic determination and finally chemopreventive intervention. The intraductal approach includes nipple aspiration fluid (NAF), ductal lavage (DL) and mammary ductoscopy (MD). These techniques together with random periareolar fine needle aspiration (RPFNA) represent the available techniques for the sampling of breast fluid and exfoliated epithelial cells. At the moment, these procedures are not considered a screening procedure for early breast cancer detection but might provide a powerful research tool for studying breast carcinogenesis in vivo.

We summarize the current knowledge regarding the vast array of molecules involved at all stages of carcinogenesis and the possibility to utilize them as candidate biomarkers to refine risk assessment, and their possible use in prevention strategies.

Myostatin, a negative regulator of skeletal muscle mass, is increased in several conditions characterized by muscle wasting, among which cancer cachexia. Physiological inhibitors, such as follistatin, negatively regulate myostatin bioactivity.

Histone deacetylase inhibitors have been shown to improve muscle wasting and function in dystrophic mdx mice, mainly by modulating the myostatin/follistatin axis. The present study was aimed at investigating the efficacy of two histone deacetylase inhibitors, namely valproic acid and trichostatin A, in preventing muscle atrophy in C26 tumor-bearing mice and in C2C12 myotubes exposed to TNFα

The progressive muscle depletion that occurs in the C26 hosts was associated with increased expression of myostatin and muscle-specific ubiquitin ligases. Administration of valproic acid, but not trichostatin A, resulted in decreased muscle myostatin expression and increased follistatin levels. Neither agent, however, was able to effectively counteract muscle atrophy or ubiquitin ligase hyperexpression. By contrast, morphological analysis suggested that both valproic acid and trichostatin A are protective against TNFα-induced myotube atrophy.

Altogether, these results suggest that modulation of the myostatin/follistatin axis can prevent TNFα-associated myofiber atrophy, although it is not sufficient to correct muscle atrophy in cancer cachexia.

The inability of the host immune system to control tumor growth appears to result from dominant mechanisms of immune suppression that prevent the immune system from effectively responding in a way that consistently results in tumor rejection. Among the many possible mediators of tumoral immune escape, the immunoregulatory enzyme, indoleamine 2,3-dioxygenase (IDO), has recently gained considerable attention. IDO is a heme-containing, monomeric oxidoreductase that catalyzes the first and rate-limiting step in the degradation of the essential amino acid tryptophan to N-formyl-kynurenine. Tryptophan depletion as well as the accumulation of its metabolites results in a strongly inhibitory effect on the development of immune responses by blocking T cell activation, inducing T cell apoptosis and promoting the differentiation of naïve T cells into cells with a regulatory phenotype (Tregs). Recent data obtained from preclinical tumor models demonstrate that IDO inhibition can significantly enhance the antitumor activity of various chemotherapeutic and immunotherapeutic agents. These results, coupled with data showing that increased IDO expression is an independent prognostic variable for reduced overall survival in cancer patients, suggest that IDO inhibition may represent an effective strategy to treat malignancies, either alone or in combination with chemotherapeutics or other immune based therapies. This review will focus on the role of IDO as a mediator of peripheral immune tolerance, evidence that IDO becomes dysregulated in human cancers, and the latest progress on the development of IDO inhibitors as a novel anti-cancer therapy.

Appropriate subcellular localization of proteins is crucial for regulating their functions. Both p53 and the BH3-only Bid play roles in the development and the treatment of hepatocellular carcinoma. Bid genomic loci contain p53-binding DNA response elements and Bid can mediate p53-dependent transactivation. However, how these molecules function in the liver cells is not completely known. In this study, liver cells were stimulated by etoposide to damage DNA, and the location and the interaction of Bid and p53 were examined by Western blot, immonocchemical staining, immunoprecipitation and siRNA methods. Our data showed that etoposide-induced DNA damage significantly induced p53 and Bid nuclear export. When cells were stimulated by etoposide, p53 could, through the interaction with Bid, cause translocation of Bid from the nucleus to the cytoplasm and on to its ultimate location in the mitochondria. Moreover, p53 physically associated with Bid, and both p53 and Bid cooperatively promoted cell death induced by etoposide. Knockdown of Bid expression notably attenuated cell death induced by etoposide and also released p53 from the mitochondria. In conclusion, these findings reveal a novel mechanism how p53 facilitates Bid nuclear export and how both of them interact in the nucleus and the mitochondria to induce apoptosis in response to etoposide-induced DNA damage.

Anthracyclines are an important reagent in many chemotherapy regimes for treating a wide range of tumors. One of the primary mechanisms by which anthracycline acts involves DNA damage caused by inhibition of topoisomerase II. Enzymatic detoxification of anthracycline is a critical factor in determining anthracycline resistance. The natural product, daunorubicin, a toxic analogue of anthracycline, is reduced to the less toxic daunorubicinol by the catalytic action of AKR1B10, which is overexpressed in most cases of smoking associated squamous cell carcinoma (SCC) and adenocarcinoma. In addition, AKR1B10 was discovered as an enzyme overexpressed in human liver, cervical, and endometrial cancer cases in samples from uterine cancer patients. The expression of AKR1B10 was also found to be associated with tumor recurrence after surgery and with keratinization of squamous cell carcinoma in cervical cancer. It is estimated to have the potential as a tumor intervention target for colorectal cancer cells (HCT-8) and as a diagnostic marker for non-small-cell lung cancer. This chapter presents the chemical mechanism of action of daunorubicin and a method to improve the effectiveness of daunorubicin by modulating the catalytic activity of AKR1B10.