Emerging targets for the prevention of pregnancy-associated breast cancer

Comment on: Lyons TR, et al. Nat Med 2011; 17:1109–15     

Protein tyrosine phosphatases as novel targets in breast cancer therapy

Breast cancer is linked to hyperactivation of protein tyrosine kinases (PTKs), and recent studies have unveiled that selective tyrosine dephosphorylation by protein tyrosine phosphatases (PTPs) of specific substrates, including PTKs, may activate or inactivate oncogenic pathways in human breast cancer cell growth-related processes. Here, we review the current knowledge on the involvement of PTPs in breast cancer, as major regulators of breast cancer therapy-targeted PTKs, such as HER1/EGFR, HER2/Neu, and Src. The functional interplay between PTKs and PTK-activating or -inactivating PTPs, and its implications in novel breast cancer therapies based on targeting of specific PTPs, are discussed.     

Claudins: emerging targets for cancer therapy

The claudin (CLDN) family of transmembrane proteins plays a critical role in the maintenance of epithelial and endothelial tight junctions. In addition to their function in preserving the structure of tight junctions, CLDNs might also play a role in the maintenance of the cytoskeleton and in cell signalling. Interestingly, several studies have recently reported specific CLDN family members to be overexpressed in a wide variety of cancer types. Although their functional role in cancer progression remains unclear, the differential expression of these proteins between tumour and normal cells, in addition to their membrane localisation, makes them prime candidates for cancer therapy. Preclinical studies have shown that tumour cells overexpressing CLDNs can be successfully targeted via several approaches, including the use of anti-CLDN antibodies as well as the cytolytic enterotoxin from Clostridium perfringens. Further studies are needed to determine the potential systemic toxicity of this approach considering the ubiquitous expression of CLDNs in the body, but CLDN-targeted therapeutics appear to have promise in the treatment of cancer.     

Molecular addresses in blood vessels as targets for therapy

We have isolated several organ- and tumor-homing peptides by using in vivo phage display. This technology involves the screening of peptide libraries in a living animal. The peptides that result from such a selection home to specific organs or tissues because they recognize molecular 'addresses', receptors that are differentially expressed in vascular beds. Targeted delivery of chemotherapeutics, pro-apoptotic peptides and cytokines to tumors using these peptides improved therapeutic efficacy in animal models. Translation of this technology into clinical applications will form the basis for targeting therapeutic and imaging agents in the context of cancer and other diseases.     

Proteasome deubiquitinases as novel targets for cancer therapy

The ubiquitin-proteasome system (UPS) is a conserved pathway regulating numerous biological processes including protein turnover, DNA repair, and intracellular trafficking. Tumor cells are dependent on a functioning UPS, making it an ideal target for the development of novel anti-cancer therapies. The development of bortezomib (Velcade^®) as a treatment for multiple myeloma and mantle cell lymphoma has verified this and suggests that targeting other components of the UPS may be a viable strategy for the treatment for cancer. We recently described a novel class of proteasome inhibitors that function by an alternative mechanism of action (D’Arcy et al., 2011). The small molecule b-AP15 blocks the deubiquitinase (DUB) activity of the 19S regulatory particle (19S RP) without inhibiting the proteolytic activities of the 20S core particle (20S CP). b-AP15 inhibits two proteasome-associated DUBs, USP14 and UCHL5, resulting in a rapid accumulation of high molecular weight ubiquitin conjugates and a functional proteasome shutdown. Interestingly, b-AP15 displays several differences to bortezomib including insensitivity to over-expression of the anti-apoptotic mediator Bcl-2 and anti-tumor activity in solid tumor models. In this review we will discuss the potential of proteasome deubiquitinase inhibitors as additions to the therapeutic arsenal against cancer.     

Molecular markers of metastasis in breast cancer: current understanding and prospects for novel diagnosis and prevention

The early and clinically occult spread of viable tumour cells throughout the body is increasingly considered as a hallmark of cancer progression, because recent data suggest that these cells are precursors of subsequent distant relapse. Using monoclonal antibodies to epithelial cytokeratins or tumour-associated cell-membrane glycoproteins, individual carcinoma cells can be detected in cytological bone marrow preparations at frequencies of 10(-5) to 10(-6). Prospective clinical studies have shown that the presence of these immunostained micrometastatic cells in bone marrow, as a frequent site of overt metastases, is prognostically relevant with regard to relapse-free period and overall survival. This screening approach might therefore be used to improve tumour staging and to guide stratification of patients for adjuvant therapy in clinical trials. Another promising clinical application is the use of these micrometastatic cells to monitor response to adjuvant therapies, which at present can be assessed only retrospectively after an extended period of clinical follow-up. This review summarises current data on the clinical significance of occult metastatic breast cancer cells in bone marrow.     

The role of adjuvant clodronate therapy for prevention of bone metastasis in breast cancer

Bisphosphonates are effective against increased bone resorption because they inhibit osteoclast activity. The use of these drugs is well established for the treatment of metastatic breast and other cancers; they reduce skeletal complications, hypercalcemia, bone pain, and metastatic progression and they can improve the overall survival and quality of life. Preclinical observations and early clinical data indicate that early bisphosphonate treatment reduces the incidence and the extent of newly developed metastases in breast cancer. There is considerable interest in determining whether bisphosphonate treatment is to prevent the incidence of bone metastases and associated complications. To date three randomized, controlled clinical trials have examined the effect of long-term use of clodronate (1600 mg/d po.) on the incidence of bone metastases, other metastases, the survival of patients, and the side effects of the study drug as well. All the trials have observed significant reduction of the occurrence of bone metastases, although this reduction was significant only during the medication period. One of the trials mentioned have shown an unexpected reduction in non-osseous metastases, and two of them have revealed significant improvements in the death rates. These promising results need further evaluation by large clinical trials with longer treatment periods to establish the clinical role of adjuvant bisphosphonate treatment of primary breast cancer.     

Nutritional interactions: credentialing of molecular targets for cancer prevention

Dietary behavior has been identified as one of the most important modifiable determinants of cancer risk. Which personalized modifications are needed remains an area of considerable controversy. Part of this uncertainty may arise from interactions among dietary bioactive compounds and/or food combinations. These interactions may either enhance or negate the response to specific foods. Evidence suggests that the cancer-protective effects of an individual's diet may reflect the combined effects of various vitamins, minerals, and other bioactive components such as flavonoids, isothiocyanates, and/or allium compounds rather than from the effect of a single ingredient. A better understanding of physiologically important interactions is needed to determine the merit of combining foods for maximum efficacy for cancer prevention. Furthermore, the response is complicated, since multiple cellular processes associated with carcinogenesis can be modified simultaneously, including sites such as drug metabolism, DNA repair, cell proliferation, apoptosis, inflammation, differentiation, and angiogenesis. Current evidence suggests that bioactive food components can typically influence more than one process. It is essential to have a better understanding of how the response relates to exposures and credentialing which process is most involved in bringing about a change in tumor incidence and/or tumor behavior. Credentialing is being defined as a determination of which cellular process(es) and which bioactive food components are most important for bringing about a phenotypic change. Additional attention is needed to determine the critical intake of dietary components, their duration, and when they should be provided to optimize the desired physiological response. Further research is also needed on the molecular targets for bioactive components and whether genetic and epigenetic events dictate the direction and magnitude of the response.     

CA215 and GnRH receptor as targets for cancer therapy

Two monoclonal antibodies (Mabs), RP215 and GHR106, were selected for the preclinical evaluations of anti-cancer drugs targeting various human cancers including those of the ovary, cervix, lung, and liver. Both Mabs were shown to react with pan cancer markers, which are over-expressed on the surface of almost all human cancers. RP215 Mab was shown to react with the carbohydrate-associated epitope(s) of cancer cell-expressed glycoproteins, mainly consisting of immunoglobulin superfamily (IgSF) proteins and mucins, generally known as CA215. GHR106 Mab was generated against the extracellular domain of human GnRH receptor, which is also highly expressed on the cancer cell surface. Preclinical studies were performed to evaluate the efficacy of these two Mabs as anti-cancer drugs for treating human cancers. High tumor specificity of RP215 Mab was demonstrated with immunohistochemical staining studies of various cancer cell lines, as well as normal and cancerous tissue sections. These two Mabs were shown to induce apoptosis as well as complement-dependent cytotoxicity upon treatment to many cultured cancer cells. Significant dose-dependent growth inhibition of tumor cells from several different tissue origins were demonstrated by nude mouse experiments. It was further demonstrated that GHR106 Mab can function as long-acting GnRH analogs in its biological actions. Efforts were made to generate human/mouse chimeric forms of the GHR106 Mab. Based on the results of these preclinical studies, we believe that these two Mabs, in chimeric or humanized forms, can be developed into suitable therapeutic agents for treatment of human cancers as anti-cancer drugs.     

Inhibiting tissue invasion and metastasis as targets for cancer therapy

Many of the steps involved in cancer spread are potential targets for anti-metastatic treatment. Until recently, research aimed at inhibiting metastasis has concentrated on the proteases, especially on urokinase-type plasminogen activator and collagenase IV However, recent data suggests that both adhesion proteins and motility factors could also serve as targets for new treatments to prevent cancer invasion and metastasis. Almost all the work to date using anti-metastatic agents has been carried out using either in vitro artificial membranes or with animal models. It is, however, likely that some of the inhibitors of experimental metastasis which are described will be evaluated in clinical trials in the near future.     

Nucleases in homologous recombination as targets for cancer therapy

Genomic DNA is constantly challenged from endogenous as well as exogenous sources. The DNA damage response (DDR) mechanism has evolved to combat these challenges and ensure genomic integrity. In this review, we will focus on repair of DNA double-strand breaks (DSB) by homologous recombination and the role of several nucleases and other recombination factors as suitable targets for cancer therapy. Their inactivation as well as overexpression have been shown to sensitize cancer cells by increasing toxicity to DNA-damaging agents and radiation or to be responsible for resistance of cancer cells. These factors can also be used in targeted cancer therapy by taking advantage of specific genetic abnormalities of cancer cells that are not present in normal cells and that result in cancer cell lethality.     

Hyaluronan-CD44 interactions as potential targets for cancer therapy

It is becoming increasingly clear that signals generated in tumor microenvironments are crucial to tumor cell behavior, such as survival, progression and metastasis. The establishment of these malignant behaviors requires that tumor cells acquire novel adhesion and migration properties to detach from their original sites and to localize to distant organs. CD44, an adhesion/homing molecule, is a major receptor for the glycosaminoglycan hyaluronan, which is one of the major components of the tumor extracellular matrix. CD44, a multistructural and multifunctional molecule, detects changes in extracellular matrix components, and thus is well positioned to provide appropriate responses to changes in the microenvironment, i.e. engagement in cell-cell and cell-extracellular matrix interactions, cell trafficking, lymph node homing and the presentation of growth factors/cytokines/chemokines to co-ordinate signaling events that enable the cell responses that change in the tissue environment. The potential involvement of CD44 variants (CD44v), especially CD44v4-v7 and CD44v6-v9, in tumor progression has been confirmed for many tumor types in numerous clinical studies. The downregulation of the standard CD44 isoform (CD44s) in colon cancer is postulated to result in increased tumorigenicity. CD44v-specific functions could be caused by their higher binding affinity than CD44s for hyaluronan. Alternatively, CD44v-specific functions could be caused by differences in associating molecules, which may bind selectively to the CD44v exon. This minireview summarizes how the interaction between hyaluronan and CD44v can serve as a potential target for cancer therapy, in particular how silencing CD44v can target multiple metastatic tumors.     

Molecular mechanisms of inflammatory bone damage: emerging targets for therapy

Chronic inflammatory bone diseases, such as rheumatoid arthritis (RA), ankylosing spondylitis and periodontal disease, demonstrate the major impact of chronic inflammation on both bone metabolism and bone architecture. During the past decade, scientists have gained increasing insight into the link between inflammation and bone. As a result of new discoveries about the molecular mechanisms of inflammatory bone loss, several molecules have been identified that are attractive and novel targets for the treatment of inflammatory bone loss. These novel therapeutic approaches include anti-tumor necrosis factor (TNF)-alpha blocking agents, neutralizing antibodies against certain pro-inflammatory cytokines, such as interleukin (IL)-6 and IL-17, and a set of other promising targets that still require extensive research, such as the Wnt signaling network.     

Validation of HB-EGF and amphiregulin as targets for human cancer therapy

Aberrant expression levels of epidermal growth factor receptor (EGFR) and its cognate ligands have been recognized as one of the causes of cancer progression. To investigate the validity of EGFR ligands as targets for cancer therapy, we examined the expression of EGFR ligands and in vitro anti-tumor effects of small interference RNA (siRNA) for EGFR ligands in various cancer cells. HB-EGF expression was dominantly elevated in ovarian, gastric, and breast cancer, melanoma and glioblastoma cells, whereas amphiregulin was primarily expressed in pancreatic, colon, and prostate cancer, renal cell carcinoma and cholangiocarcinoma cells. Transfection of siRNAs for HB-EGF or amphiregulin into these cells significantly increased the numbers of apoptotic cells with attenuation of EGFR and ERK activation. In lung cancer cells, any EGFR ligand was not recognized as a validated target for cancer therapy. These results suggest that HB-EGF and amphiregulin are promising targets for cancer therapy.     

Myeloid-derived suppressor cells and proinflammatory cytokines as targets for cancer therapy

Myeloid-derived suppressor cells represent a heterogeneous population of immature myeloid cells. Under normal conditions, these cells differentiate into macrophages, dendritic cells, and granulocytes. However, in pathological states such as inflammation, infection, or tumor growth, there is an arrest of their differentiation that results in the accumulation of immature myeloid cells in the organism. In addition, these cells acquire a suppressor phenotype, expressing anti-inflammatory cytokines and reactive oxygen and nitrogen species, and suppress T-cell immune response. Myeloid-derived suppressor cells (MDSC) contribute to cancerogenesis by forming a favorable microenvironment for tumor growth. Proinflammatory cytokines, secreted by tumor cells and the tumor microenvironment, induce angiogenesis and metastasis and promote tumor growth. They also provide signals necessary for survival, accumulation, and function of MDSC. Understanding the mechanisms of myeloid suppressor cell development and the use of proinflammatory cytokine inhibitors may prove beneficial for tumor therapy.     

Use of biomarkers for breast cancer risk assessment and prevention

Epidemiologic models used for cancer risk prediction, such as the Gail model, are validated for populations undergoing regular screening but often have suboptimal individual predictive accuracy. Risk biomarkers may be employed to improve predictive accuracy based on the Gail or other epidemiologic models and, to the extent that they are reversible, may be used to assess response in phase I–II prevention trials. Risk biomarkers used as intermediate response endpoints include high mammographic breast density, intra-epithelial neoplasia, and cytomorphology with associated molecular markers such as Ki-67. At the present time these biomarkers may not be used to predict or monitor individual response to standard prevention interventions but are used in early phase clinical trials as preliminary indicators of efficacy.     

Proteomics of breast cancer: the quest for markers and therapeutic targets

Proteomics of breast cancer has already delivered significant data in terms of proteome profiling in addition to the identification of a few proteins of potential interest for diagnosis and treatment. With more pathological and experimental situations being studied, it now enters into a new phase dominated by the concepts of deep proteome analysis and the definition of protein-protein interaction networks leading to mammary cell deregulation and cancer progression. Together, what could be called "Systems Proteomics", integrating with information from the genomics and the physiopathology, is clearly emerging to become the frame for future investigations. However, difficulties ahead should not be underestimated. First, the proteome is complex, and current tools are still far from providing a definitive solution for its exploration. Second, breast cancer is a multifactorial disease which is so diverse that a great deal of time and efforts will be necessary to define its associated proteome modifications and translate it into practical applications for the clinic.