Current Signal Transduction Therapy - Volume 8, Issue 3, 2013

Cancer stem cells (CSCs) are rare tumor cells that exhibit stem cell properties such as self-renewal capacity and pluripotency. In recent years, cancer stem cells have been recognized as key tumor-initiating cells that are therapyresistant and highly tumorogenic and therefore may play a pivotal role in cancer recurrence following chemotherapy. While chemotherapy is often capable of inducing cell death in tumors and reducing the tumor bulk, remaining cells can re-grow and many cancer patients experience recurrence and ultimately death. Herein, we discuss the mechanisms of chemoresistance identified in CSCs and methods of treating chemoresistant cancers driven by CSCs. These mechanisms include: aberrant ABC transporter expression/activity, aldehyde dehydrogenase (ALDH) activity, B-cell lymphoma-2 (BCL2) related chemoresistance, enhanced DNA damage response, activation of pro-survival signaling pathways and epigenetic deregulations. Developmental pathways, such as the Wnt/β-catenin pathway, direct the differentiation of normal stem cells promoting: proliferation, genomic instability and DNA damage tolerance in CSCs. The Notch/- secretase/Jagged and BMP signaling pathways are important regulators of differentiation. These signaling pathways represent novel attractive targets for drug discovery. CSCs can be forced to differentiate, lose their properties and become more sensitive to chemotherapy. Growing evidence emphasizes the interplay between metabolic disturbances, epigenomic changes and cancer. Epigenetic-based mechanisms are reversible and the possibility of "resetting" the abnormal cancer epigenome by applying pharmacological compounds provides a new and attractive approach. A number of compounds targeting epigenetic enzymes were screened for their ability to induce differentiation or to make CSCs more sensitive to therapy. Despite controversies surrounding the CSC hypothesis, there is substantial evidence for their role in cancer and a number of drugs intended to specifically target CSCs have entered clinical trials. Understanding how tumor-initiating cells escape chemotherapy will help to develop more specific and personalized approaches to treating cancer that may improve clinical outcomes for cancer patients.

The emergence of low molecular weight kinase inhibitors as “targeted” drugs has led to remarkable advances in the treatment of cancer patients. The clinical benefits of these tumor therapies, however, vary widely in patient populations and with duration of treatment. Intrinsic and acquired resistance against such drugs limits their efficacy. In addition to the well studied mechanisms of resistance based upon drug transport and metabolism, genetic alterations in drug target structures and the activation of compensatory cell signaling have received recent attention. Adaptive responses can be triggered which counteract the initial dependence of tumor cells upon a particular signaling molecule and allow only a transient inhibition of tumor cell growth. These compensating signaling mechanisms are often based upon the relief of repression of regulatory feedback loops. They might involve cell autonomous, intracellular events or they can be mediated via the secretion of growth factor receptor ligands into the tumor microenvironment and signal induction in an auto- or paracrine fashion. The transcription factors Stat3 and Stat5 mediate the biological functions of cytokines, interleukins and growth factors and can be considered as endpoints of multiple signaling pathways. In normal cells this activation is transient and the Stat molecules return to their non-phosphorylated state within a short time period. In tumor cells the balance between activating and de-activating signals is disturbed resulting in the persistent activation of Stat3 or Stat5. The constant activation of Stat3 induces the expression of target genes, which cause the proliferation and survival of cancer cells, as well as their migration and invasive behavior. Activating components of the Jak-Stat pathway have been recognized as potentially valuable drug targets and important principles of compensatory signaling circuit induction during targeted drug treatment have been discovered in the context of kinase inhibition studies in HNSCC cells [1]. The treatment of HNSCC with a specific inhibitor of c-Src, initially resulted in reduced Stat3 and Stat5 activation and subsequently an arrest of cell proliferation and increased apoptosis. However, the inhibition of c-Src only caused a persistent inhibition of Stat5, whereas the inhibition of Stat3 was only transient. The activation of Stat3 was restored within a short time period in the presence of the c-Src inhibitor. This process is mediated through the suppression of PStat5 activity and the decrease in the expression of the Stat5 dependent target gene SOCS2, a negative regulator of Jak2. Jak2 activity is enhanced upon SOCS2 downregulation and causes the reactivation of Stat3. A similar observation has been made upon inhibition of Bmx, bone marrow kinase x-linked, activated in the murine glioma cell lines Tu-2449 and Tu-9648. Its inhibition resulted in a transient decrease of P-Stat3 and the induction of a compensatory Stat3 activation mechanism, possibly through the relief of negative feedback inhibition and Jak2 activation. These observations indicate that the inhibition of a single tyrosine kinase might not be sufficient to induce lasting therapeutic effects in cancer patients. Compensatory kinases and pathways might become activated and maintain the growth and survival of tumor cells. The definition of these escape pathways and their preemptive inhibition will suggest effective new combination therapies for cancer.

Although early stage disease detection and treatment options for clear cell renal cell cancer (ccRCC) improved in recent years, prognosis of patients with late stage ccRCC remains poor, mostly due to development of tyrosine kinase inhibitors and mammalian target of rapamycin inhibitors resistance followed by disease progression. Cancer stem cells (ccRCC-CSC) model has focused a significant attention in recent years as a potential explanation for the tumor heterogeneity, drug resistance, disease recurrence and metastasis of ccRCC and other cancers. Cancer stem cells have been proposed to be responsible for tumor initiation, repopulation and growth that cause patients to succumb to renal cancer. Precise identification of ccRCC-CSC populations and definition of hierarchy of cells within ccRCC tumor including tumor initiating cells and tumor progenitor cells will facilitate accurate characterization of drug targets and ultimately contribute to more personalized and effective care. This mini-review discusses the potential strategies to inhibit the signaling pathways underlying stemness in an effort to treat renal cancer. Mechanism that could be exploited as a therapeutic target against drug resistant ccRCC-CSCs is summarized.

Mammalian target of rapamycin (mTOR) is a kinase protein involved in PI3K/AKT signaling with a central role in the processes of cell growth, survival and angiogenesis. Frequent mutations of this pathway make upstream and downstream components novel targets for tailored therapy design. Two mTOR inhibitors – everolimus and temsirolimus - enable an increase in overall survival (OS) or progression-free survival (PFS) time in a treatment of renal cancer. Despite recent advances in renal cancer treatment, resistance to targeted therapy is common. Understanding of molecular mechanisms is the basis of drug resistance which can facilitate prediction of success or failure in combinational or sequential targeted therapy. The article provides current knowledge on the mTOR signaling network and gives insight into the mechanisms of resistance to mTOR inhibitors from the complex perspective of RCC biology. The mechanisms of resistance developed not only by cancer cells, but also by interactions with tumor microenvironment are analyzed to emphasize the role of angiogenesis in ccRCC pathogenesis. As recent studies have shown the role of PI3K/AKT-mTOR pathway in proliferation and differentiation of cancer stem cells, we discuss cancer stem cell hypothesis and its possible contribution to ccRCC resistance. In the context of drug resistance, we also elaborate on a new approach considering ccRCC as a metabolic disease. In conclusion we speculate on future developments in agents targeting the mTOR pathway taking into consideration the singular biology of ccRCC.

Clear - cell renal cell carcinoma (ccRCC) is a histological subtype of renal cell carcinoma - the most prevalent adult kidney cancer. Causes of ccRCC are not completely understood and therefore number of available therapies is limited. As a consequence of tumor chemo- and radioresistance as well as restrictions in offered targeted therapies, overall response rate is still unsatisfactory. Moreover, a significant group of patients (circa 1/4) does not respond to the targeted first-line treatment, while in other cases, after an initial period of stable improvement, disease progression occurs. Owing to this, more data on resistance mechanisms are needed, especially those concerning widely used, relatively lately approved and more successful than previous therapies - tyrosine kinase inhibitors (TKIs). Up to date, five TKIs have been licensed for ccRCC treatment: sunitinib (SUTENT®, Pfizer Inc.), sorafenib (Nexavar®, Bayer HealthCare/Onyx Pharmaceuticals), pazopanib (Votrient®, GlaxoSmithKline), axitinib (Inlyta®, Pfitzer Inc.) and tivozanib (AV-951®, AVEO Pharmaceuticals). Researchers have specified different subsets of tyrosine kinase inhibitors potential resistance mechanisms in clear-cell renal cell carcinoma. In most papers published until now, drug resistance is divided into intrinsic and acquired, and typically multi-drug resistance (MDR) protein is described. Herein, the authors focus on molecular analysis concerning acquired, non-genetic resistance to TKIs, with insight into specific biological processes.

The present article highlights diverse roles of stem cells in healthy state, oncology and regenerative medicine in comparison to the Clear Cell Renal Cell Carcinoma (ccRCC) “stem cells”. Furthermore, we support the notion of urgent need to change the terminology of “cancer stem cells” into tumor-propagating cells (TPCs), and to diversify terms of kidney stem and kidney progenitor cell. We present the role of normal stem and progenitor cells in kidney development and regeneration in comparison to their markers. Moreover, we would like to introduce a hypothesis about the TPCs origin, and the role epithelial to mesenchymal transition in metastasis. We described current knowledge about various approaches to identification of renal TPCs. Finally, we present future perspectives in respect to current studies.

Metastasis is a complex process that propagates cells from the primary or initial site of the cancer occurrence to distant parts of the body. Cancer cells break from the cancer site and circulate through the bloodstream or lymph vessels, allowing them to reach nearly all parts of the body. These circulating tumour cells (CTCs) contain specialized metastasisinitiating cells (MICs) that reside in the biological heterogeneous primary tumour. Researchers have hypothesized that metastasis of renal cell carcinoma is initiated by circulation of MICs in patients’ blood and bone marrow. Based on the cancer stem/progenitor cell concept of carcinogenesis, understanding the molecular phenotypes of metastasis-initiating cells (MICs) in renal cancer could play a vital role in developing strategies for therapeutic interventions in renal cancer. Existence of MICs among CTCs in renal carcinoma has not been proven in large scale. However, some studies have reported that specialized markers are found on the surface of circulating cells from the primary tumour. In mice, MICs have been isolated from CTCs using such markers, which have then been transplanted into xenograft model to show whether they give rise to metastasis in different organs. Considering these findings, in this review we have attempted to summarize the studies connected with MICs and their gene expression profiles that are responsible for metastasis in renal cancer.

The successful treatment of renal cancer remains a therapeutic challenge. Clear Cell Renal Cell Carcinoma (ccRCC) is resistant to conventional radio and chemotherapy, but complete response has been observed after immunotherapy with high-dose interleukin-2 (IL-2) and interferon (IFN)-α. Nevertheless, immunotherapy strategies have shown response rates in the range of 5 to 10%. For the past 20 years, the mechanisms of treatment resistance have been studied, and immune escape of tumours in cancer development and spread has been a broadly investigated phenomenon. Multiple studies have revealed that genomic abnormalities of ccRCC promote the loss of major histocompatibility complex (MHC) molecules on the renal cancer cell surface, resulting in immune response resistance. Studies have shown that IFN-α-induced signalling pathways are deregulated in ccRCC cells and promote immune escape. Polymorphisms of multiple genes, including STAT3, have been shown to trigger immune-response deregulation. Investigation and understanding of the mechanisms of renal cell cancer immunotherapy resistance are extremely important for the design of rational combinatorial approaches and other novel therapies in the future. This mini-review focuses on immunotherapy resistance mechanisms in ccRCC.

Malignant gliomas are common primary tumors of the central nervous system, characterized by aggressive cell proliferation, diffuse infiltration and resistance to conventional therapy. Glioblastoma (former Glioblastoma multiforme, GBM), grade IV astrocytoma, is the most aggressive tumor, with a median survival of around 14 months. New therapies against this devastating and invariably fatal disease are needed. Stem-like cell populations have been identified in a number of malignancies including glioblastoma. These rare stem cells (called also glioma-initiating cells) are believed to be responsible not only for tumor initiation and progression but also resistance to therapeutic agents and tumor recurrence. Recently, the population of cells within glioblastoma with stem-like properties has gained increasing attention as a target to refine treatment strategies. This chapter aims to summarize the recent data regarding isolation, biology and mechanisms of resistance of glioblastoma stem-like cells to therapy.

Our current understanding of cancer-stem cells (CSCs) is that they are slow growing, generally mesenchymallike cells capable of generating tumors. Convincing evidence for the existence of such cells comes from recent lineage tracing experiments. CSCs have been reported as being resistant to conventional drug treatment and have been considered as being responsible for failure of chemotherapy. Recently, several databases aiming the genetic characterization of a large number of cancer cell lines have been made publicly available. In addition to gene expression data, these databases contain cytotoxicity information for all cell lines for a number of drugs as well. It is possible to classify known cell lines derived from a given tumor, based on how similar they are to CSCs, or in other words, to define their stem-ness, using gene-lists that define such cells. Using two such, independently generated, gene lists we found that breast cancer cell lines could be categorized into two distinct groups which we designate CSC-like and non-CSC-like. We then identified drugs to which the two groups were most sensitive to. We also generated sensitivity profiles for all drugs, within one such database, to identify chemotherapeutics with preferential action on breast cancer. We believe this is a straight-forward approach for swiftly identifying drugs that would selectively target a subpopulation of cells for any given tumor type.