Cytokines as suppressors of apoptosis

Many cytokines have been isolated by their ability to induce growth and have been called growth factors. But these cytokines are also essential to induce cell viability, and cell viability and growth can be separately regulated. Using as examples myeloid hematopoietic cells, lymphocytes and neuronal cells, in vitro and in vivo studies have shown the role of cytokines in inducing viability of different cell types during development to mature cells. Some cytokines can act on more than one cell type. Cytokines induce viability of normal and cancer cells by suppressing the apoptotic machinery activated by wild-type p53, or by cytotoxic agents including irradiation and compounds used in cancer chemotherapy. Cytokines can be used to decrease apoptosis in normal cells and inhibition of cytokine activity may improve cancer therapy by enhancing apoptosis in cancer cells. The apoptosis suppressing function of cytokines is mediated by changing the balance in the activity of apoptosis inducing and suppressing genes. Apoptosis suppression is upstream of caspase activation in the apoptotic process. Cytokines can suppress multiple pathways leading to apoptosis, only some of which were suppressed by other agents such as some antioxidants, Ca²⁺-mobilizing compounds and protease inhibitors.     

microRNAs as oncogenes and tumor suppressors

microRNAs (miRNAs) are a new class of non-protein-coding, endogenous, small RNAs. They are important regulatory molecules in animals and plants. miRNA regulates gene expression by translational repression, mRNA cleavage, and mRNA decay initiated by miRNA-guided rapid deadenylation. Recent studies show that some miRNAs regulate cell proliferation and apoptosis processes that are important in cancer formation. By using multiple molecular techniques, which include Northern blot analysis, real-time PCR, miRNA microarray, up- or down-expression of specific miRNAs, it was found that several miRNAs were directly involved in human cancers, including lung, breast, brain, liver, colon cancer, and leukemia. In addition, some miRNAs may function as oncogenes or tumor suppressors. More than 50% of miRNA genes are located in cancer-associated genomic regions or in fragile sites, suggesting that miRNAs may play a more important role in the pathogenesis of a limited range of human cancers than previously thought. Overexpressed miRNAs in cancers, such as mir-17-92, may function as oncogenes and promote cancer development by negatively regulating tumor suppressor genes and/or genes that control cell differentiation or apoptosis. Underexpressed miRNAs in cancers, such as let-7, function as tumor suppressor genes and may inhibit cancers by regulating oncogenes and/or genes that control cell differentiation or apoptosis. miRNA expression profiles may become useful biomarkers for cancer diagnostics. In addition, miRNA therapy could be a powerful tool for cancer prevention and therapeutics.     

Nuclear tumor suppressors in space and time

Numerous studies have identified key binding partners and functional activities of nuclear tumor-suppressor proteins such as the retinoblastoma protein, p53 and BRCA1. Historically, less attention has been given to the subnuclear locations of these proteins. Here, we describe several recent studies that promote the view that regulated association with subcompartments of the nucleus is inherent to tumor-suppressor function.     

Interaction of two tumor suppressors: Phosphatase CTDSPL and Rb protein

Earlier we established that CTDSPL gene encoding small carboxy-terminal domain serine phosphatase can be considered a classical tumor suppressor gene. Besides, transfection of tumor cell line MCF-7 with CTDSPL led to the content decrease of inactive phosphorylated form of another tumor suppressor, retinoblastoma protein (Rb), and subsequently to cell cycle arrest at the G1/S boundary. This result implied that small phosphatase CTDSPL is able to specifically dephosphorylate and activate Rb protein. In order to add some fuel to this hypothesis, in the present work we studied the interaction of two tumor suppressors CTDSPL and Rb in vitro. GST pool-down assay revealed that CTDSPL is able to precipitate Rb protein from MCF-7 cell extracts, while surface plasmon resonance technique showed that interaction of the two proteins is direct. Results of this study reassert that phosphatase CTDSPL and Rb could be involved in the common mechanism of cell cycle regulation.     

Sweet escape: Sialic acids in tumor immune evasion

Sialic acids represent a family of sugar molecules derived from neuraminic acid that frequently terminate glycan chains and contribute to many biological processes. Already five decades ago, aberrantly high expression of sialic acids has been proposed to protect cancer cells from recognition and eradication by the immune system. Today, increased understanding at the molecular level demonstrates the broad immunomodulatory capacity of tumor-derived sialic acids that is, at least in part, mediated through interactions with immunoinhibitory Siglec receptors. Here we will review current studies from a sialic acid sugar perspective showing that tumor-derived sialic acids disable major killing mechanisms of effector immune cells, trigger production of immune suppressive cytokines and dampen activation of antigen-presenting cells and subsequent induction of anti-tumor immune responses. Furthermore, strategies to modulate sialic acid expression in cancer cells to improve cancer immunotherapy will be discussed.     

Regulation of tumor suppressors by nuclear-cytoplasmic shuttling

Tumor suppressor proteins control the proliferation and survival of normal cells; consequently, their inactivation by gene mutations can initiate or drive cancer progression. Most tumor suppressors have been identified by genetic screening, and in many cases their function and regulation are poorly understood. Ten such proteins were recently shown to contain nuclear transport signals that facilitate their shuttling between the nucleus and cytoplasm. This type of dynamic intracellular movement not only regulates protein localization, but also often impacts on function. Here, we review the pathways by which tumor suppressors such as APC, p53, VHL, and BRCA1 cross the nuclear envelope and the impact of regulated nuclear import/export on protein function.     

The Drosophila tumor suppressors Expanded and Merlin differentially regulate cell cycle exit, apoptosis, and Wingless signaling

Mutations that inactivate either merlin (mer) or expanded (ex) result in increased cell growth and proliferation in Drosophila. Both Mer and Ex are members of the Band 4.1 protein superfamily, and, based on analyses of mer ex double mutants, they are proposed to function together in at least a partially redundant manner upstream of the Hippo (Hpo) and Warts (Wts) proteins to regulate cell growth and division. By individually analyzing ex and mer mutant phenotypes, we have found important qualitative and quantitative differences in the ways Mer and Ex function to regulate cell proliferation and cell survival. Though both mer and ex restrict cell and tissue growth, ex clones exhibit delayed cell cycle exit in the developing eye, while mer clones do not. Conversely, loss of mer substantially compromises normal developmental apoptosis in the pupal retina, while loss of ex has only mild effects. Finally, ex has a role in regulating Wingless protein levels in the eye that is not obviously shared by either mer or hpo. Taken together, our data suggest that Mer and Ex differentially regulate multiple downstream pathways.     

Phosphorylation-dependent interaction between tumor suppressors Dlg and Lgl

The tumor suppressors Discs Large (Dlg), Lethal giant larvae (Lgl) and Scribble are essential for the establishment and maintenance of epithelial cell polarity in metazoan. Dlg, Lgl and Scribble are known to interact strongly with each other genetically and form the evolutionarily conserved Scribble complex. Despite more than a decade of extensive research, it has not been demonstrated whether Dlg, Lgl and Scribble physically interact with each other. Here, we show that Dlg directly interacts with Lgl in a phosphorylation-dependent manner. Phosphorylation of any one of the three conserved Ser residues situated in the central linker region of Lgl is sufficient for its binding to the Dlg guanylate kinase (GK) domain. The crystal structures of the Dlg4 GK domain in complex with two phosphor-Lgl2 peptides reveal the molecular mechanism underlying the specific and phosphorylation-dependent Dlg/Lgl complex formation. In addition to providing a mechanistic basis underlying the regulated formation of the Scribble complex, the structure of the Dlg/Lgl complex may also serve as a starting point for designing specific Dlg inhibitors for targeting the Scribble complex formation.     

Tyrosine phosphatases as a superfamily of tumor suppressors in colorectal cancer

Phosphorylation and dephosphorylation processes catalyzed by numerous kinases and phosphorylases are essential for cell homeostasis and may lead to disturbances in a variety of vital cellular pathways, such as cell proliferation and differentiation, and thus to complex diseases including cancer. As over 80 % of all oncogenes encode protein tyrosine kinases (PTKs), protein tyrosine phosphatases (PTPs), which can reverse the effects of tyrosine kinases, are very important tumor suppressors. Alterations in tyrosine kinase and phosphatase genes including point mutations, changes in epigenetic regulation, as well as chromosomal aberrations involving regions critical to these genes, are frequently observed in a variety of cancers. Colorectal cancer (CRC) is one of the most common cancers in humans. CRCs occur in a familial (about 15 % of all cases), hereditary (about 5%) and sporadic (almost 75-80 %) form. As genetic-environmental interrelations play an important role in the susceptibility to sporadic forms of CRCs, many studies are focused on genetic alterations in such tumors. Mutational analysis of the tyrosine phosphatome in CRCs has identified somatic mutations in PTPRG, PTPRT, PTPN3, PTPN13 and PTPN14. The majority of these mutations result in a loss of protein function. Also, alterations in the expression of these genes, such as decreased expression of PTPRR, PTPRO, PTPRG and PTPRD, mediated by epigenetic mechanisms have been observed in a variety of tumors. Since cancer is a social and global problem, there will be a growing number of studies on alterations in the candidate cancer genes, including protein kinases and phosphatases, to determine the origin, biology and potential pathways for targeted anticancer therapy.     

Tumor-associated B7-H1 promotes T-cell apoptosis: a potential mechanism of immune evasion

B7-H1, a recently described member of the B7 family of costimulatory molecules, is thought to be involved in the regulation of cellular and humoral immune responses through the PD-1 receptor on activated T and B cells. We report here that, except for cells of the macrophage lineage, normal human tissues do not express B7-H1. In contrast, B7-H1 is abundant in human carcinomas of lung, ovary and colon and in melanomas. The pro-inflammatory cytokine interferon-gamma upregulates B7-H1 on the surface of tumor cell lines. Cancer cell-associated B7-H1 increases apoptosis of antigen-specific human T-cell clones in vitro, and the apoptotic effect of B7-H1 is mediated largely by one or more receptors other than PD-1. In addition, expression of B7-H1 on mouse P815 tumor increases apoptosis of activated tumor-reactive T cells and promotes the growth of highly immunogenic B7-1(+) tumors in vivo. These findings have implications for the design of T cell-based cancer immunotherapy.     

Loss of the tuberous sclerosis complex tumor suppressors triggers the unfolded protein response to regulate insulin signaling and apoptosis

Mammalian target of rapamycin, mTOR, is a major sensor of nutrient and energy availability in the cell and regulates a variety of cellular processes, including growth, proliferation, and metabolism. Loss of the tuberous sclerosis complex genes (TSC1 or TSC2) leads to constitutive activation of mTOR and downstream signaling elements, resulting in the development of tumors, neurological disorders, and at the cellular level, severe insulin/IGF-1 resistance. Here, we show that loss of TSC1 or TSC2 in cell lines and mouse or human tumors causes endoplasmic reticulum (ER) stress and activates the unfolded protein response (UPR). The resulting ER stress plays a significant role in the mTOR-mediated negative-feedback inhibition of insulin action and increases the vulnerability to apoptosis. These results demonstrate ER stress as a critical component of the pathologies associated with dysregulated mTOR activity and offer the possibility to exploit this mechanism for new therapeutic opportunities.     

Roles of tumor suppressors in regulating tumor-associated inflammation

Loss or silencing of tumor suppressors (TSs) promotes neoplastic transformation and malignant progression. To date, most work on TS has focused on their cell autonomous effects. Recent evidence, however, demonstrates an important noncell autonomous role for TS in the control of tumor-associated inflammation. We review evidence from clinical data sets and mouse model studies demonstrating enhanced inflammation and altered tumor microenvironment (TME) upon TS inactivation. We discuss clinical correlations between tumor-associated inflammation and inactivation of TS, and their therapeutic implications. This review sets forth the concept that TS can also suppress tumor-associated inflammation, a concept that provides new insights into tumor–host interactions. We also propose that in some cases the loss of TS function in cancer can be overcome through inhibition of the resulting inflammatory response, regardless whether it is a direct or an indirect consequence of TS loss.     

VHL and p53: tumor suppressors team up to prevent cancer

In a recent issue of Molecular Cell, Roe et al. (2006) report that the von Hippel-Lindau (VHL) protein is a positive regulator of p53, thus providing insight into the mechanisms by which VHL loss of function leads to cancer.     

TRAIL death receptors as tumor suppressors and drug targets

Various ways of targeting TRAIL-death receptors for the treatment of a diverse set of malignancies are being explored in ongoing clinical trials. Recent data of ours and others suggest that loss of the only death signaling receptor in mice (TRAIL-R) is associated with susceptibility to various stages of lymphomagenesis and carcinogenesis, perhaps in a complex cell- and model-specific manner (1,2). Myc-overexpressing B cell lymphomas with an intact TRAIL-R locus displayed a number of gene expression changes indicating resistance to TRAIL-R signaling. Herein we show some data on the use of recombinant human TRAIL (rhTRAIL) and γ-radiation (10 Gy) in combination in an autochthonous mouse model for hepatocellular carcinoma. As cell death signaling through the death receptors is evolutionary conserved from zebra fish to man, novel genetically engineered mouse tumor models may prove useful in establishing in vivo models that excel our fundamental understanding of resistance to TRAIL-death receptor signaling, off-target effects from TRAIL-death receptor targeting compounds and help in identifying a clinically cogent rationale for efficient targeting of TRAIL death receptors in patients. Once established, mouse tumor models may prove to be a useful tool in understanding TRAIL-death receptor signaling.