PARP-1: Friend or Foe of DNA Damage and Repair in Tumorigenesis?

Oxidative stress induced by reactive oxygen species can result in DNA damage within cells and subsequently increase risk for carcinogenesis. This may be averted by repair of DNA damage through the base or nucleotide excision repair (BER/NER) pathways. PARP, a BER protein, is known for its role in DNA-repair. However, multiple lesions can occur within a small range of DNA, known as oxidative clustered DNA lesions (OCDLs), which are difficult to repair and may lead to the more severe DNA double-strand break (DSB). Inefficient DSB repair can then result in increased mutagenesis and neoplastic transformation. OCDLs occur more frequently within a variety of tumor tissues. Interestingly, PARP is highly expressed in several human cancers. Additionally, chronic inflammation may contribute to tumorigenesis through ROS-induced DNA damage. Furthermore, PARP can modulate inflammation through interaction with NFκB and regulating the expression of inflammatory signaling molecules. Thus, the upregulation of PARP may present a double-edged sword. PARP is needed to repair ROS-induced DNA lesions, but PARP expression may lead to increased inflammation via upregulation of NFκB signaling. Here, we discuss the role of PARP in the repair of oxidative damage versus the formation of OCDLs and speculate on the feasibility of PARP inhibition for the treatment and prevention of cancers by exploiting its role in inflammation.

ST6Gal-I sialyltransferase confers cisplatin resistance in ovarian tumor cells.

BACKGROUND: Platinum drugs, including cisplatin, are a frontline therapeutic in ovarian cancer treatment and acquired resistance to these agents is a major contributor to ovarian cancer morbidity and mortality. In this study a novel glycosylation-dependent mechanism for cisplatin resistance is described. Specifically, cisplatin-induced cell death is blocked by the activity of the ST6Gal-I sialyltransferase. ST6Gal-I modifies specific receptors by adding a negatively charged sialic acid sugar which influences diverse receptor functions. Overexpression of ST6Gal-I is a hallmark of ovarian and other cancers and its expression has been correlated to metastasis and poor prognosis. METHODS: Tumor cell viability and apoptotic induction were determined in cell lines with ST6Gal-I overexpression and knockdown. In addition, cell populations with acquired resistance to cisplatin were assayed for endogenous ST6Gal-I expression. RESULTS: We show that forced expression of ST6Gal-I in OV4 ovarian cancer cells that lack endogenous ST6Gal-I causes reduced activation of caspase 3 and increased cell viability following cisplatin treatment. Conversely, forced ST6Gal-I knockdown in Pa-1 cells with high endogenous ST6Gal-I increases cisplatin-induced caspase activation and cell death. A2780 ovarian cancer cells selected for stable cisplatin resistance display upregulated endogenous ST6Gal-I when compared with parental, cisplatin-sensitive, A2780 cells. Similarly, extended low dose cisplatin treatment of a Pa-1 polyclonal ST6Gal-I shRNA knockdown population led to selection for subclones with elevated ST6Gal-I expression. CONCLUSIONS: Receptor sialylation by ST6Gal-I confers a survival advantage for tumor cells in the presence of cisplatin. These collective findings support a role for ST6Gal-I in chemoresistance and highlight ST6Gal-I as a potential therapeutic target for platinum resistant tumors.

ST6Gal-I protein expression is upregulated in human epithelial tumors and correlates with stem cell markers in normal tissues and colon cancer cell lines.

The ST6Gal-I sialyltransferase adds an α2-6-linked sialic acid to the N-glycans of certain receptors. ST6Gal-I mRNA has been reported to be upregulated in human cancer, but a prior lack of antibodies has limited immunochemical analysis of the ST6Gal-I protein. Here, we show upregulated ST6Gal-I protein in several epithelial cancers, including many colon carcinomas. In normal colon, ST6Gal-I localized selectively to the base of crypts, where stem/progenitor cells are found, and the tissue staining patterns were similar to the established stem cell marker ALDH1. Similarly, ST6Gal-I expression was restricted to basal epidermal layers in skin, another stem/progenitor cell compartment. ST6Gal-I was highly expressed in induced pluripotent stem (iPS) cells, with no detectable expression in the fibroblasts from which iPS cells were derived. On the basis of these observations, we investigated further an association of ST6Gal-I with cancer stem cells (CSC). Selection of irinotecan resistance in colon carcinoma cells led to a greater proportion of CSCs compared with parental cells, as measured by the CSC markers CD133 and ALDH1 activity (Aldefluor). These chemoresistant cells exhibited a corresponding upregulation of ST6Gal-I expression. Conversely, short hairpin RNA (shRNA)-mediated attenuation of ST6Gal-I in colon carcinoma cells with elevated endogenous expression decreased the number of CD133/ALDH1-positive cells present in the cell population. Collectively, our results suggest that ST6Gal-I promotes tumorigenesis and may serve as a regulator of the stem cell phenotype in both normal and cancer cell populations.

Regulation of the metastatic cell phenotype by sialylated glycans.

Tumor cells exhibit striking changes in cell surface glycosylation as a consequence of dysregulated glycosyltransferases and glycosidases. In particular, an increase in the expression of certain sialylated glycans is a prominent feature of many transformed cells. Altered sialylation has long been associated with metastatic cell behaviors including invasion and enhanced cell survival; however, there is limited information regarding the molecular details of how distinct sialylated structures or sialylated carrier proteins regulate cell signaling to control responses such as adhesion/migration or resistance to specific apoptotic pathways. The goal of this review is to highlight selected examples of sialylated glycans for which there is some knowledge of molecular mechanisms linking aberrant sialylation to critical processes involved in metastasis.

ST6Gal-I regulates macrophage apoptosis via α2-6 sialylation of the TNFR1 death receptor.

Macrophages play a central role in innate immunity, however mechanisms regulating macrophage survival are not fully understood. Herein we describe a novel apoptotic pathway involving α2-6 sialylation of the TNFR1 death receptor by the ST6Gal-I sialyltransferase. Variant glycosylation of TNFR1 has not previously been implicated in TNFR1 function, and little is known regarding the TNFR1 glycan composition. To study sialylation in macrophages, we treated U937 monocytic cells with PMA, which stimulates both macrophage differentiation and apoptosis. Interestingly, macrophage differentiation induces ST6Gal-I down-regulation, leading to reduced α2-6 sialylation of selected receptors. To prevent loss of α2-6 sialylation, we forced constitutive expression of ST6Gal-I, and found that this strongly inhibited PMA-induced apoptosis. Given that PMA-mediated apoptosis is thought to result from up-regulation of TNFα, which then activates TNFR1, we next evaluated the α2-6 sialylation of TNFR1. U937 cells with forced ST6Gal-I displayed TNFR1 with elevated α2-6 sialylation, and this was associated with diminished TNFα-stimulated apoptosis. Correspondingly, removal of α2-6 sialylation from TNFR1 through either neuraminidase treatment or expression of ST6Gal-I shRNA markedly enhanced TNFα-mediated apoptosis. To confirm the physiologic importance of TNFR1 sialylation, we generated overexpressing ST6Gal-I transgenic mice. Peritoneal macrophages from transgenic lines displayed TNFR1 with elevated α2-6 sialylation, and these cells were significantly protected against TNFα-stimulated apoptosis. Moreover, greater numbers of thioglycollate-induced peritoneal cells were observed in transgenic mice. These collective results highlight a new mechanism of TNFR1 regulation, and further intimate that loss of α2-6 sialylation during macrophage differentiation may limit macrophage lifespan by sensitizing cells to TNFα-stimulated apoptosis.

Sialylation of the Fas death receptor by ST6Gal-I provides protection against Fas-mediated apoptosis in colon carcinoma cells.

The glycosyltransferase, ST6Gal-I, adds sialic acid in an α2-6 linkage to the N-glycans of membrane and secreted glycoproteins. Up-regulation of ST6Gal-I occurs in many cancers, including colon carcinoma, and correlates with metastasis and poor prognosis. However, mechanisms by which ST6Gal-I facilitates tumor progression remain poorly understood due to limited knowledge of enzyme substrates. Herein we identify the death receptor, Fas (CD95), as an ST6Gal-I substrate, and show that α2-6 sialylation of Fas confers protection against Fas-mediated apoptosis. Intriguingly, differences in ST6Gal-I activity do not affect the function of DR4 or DR5 death receptors upon treatment with TRAIL, implicating a selective effect of ST6Gal-I on the Fas receptor. Using ST6Gal-I knockdown and forced overexpression colon carcinoma cell models, we find that α2-6 sialylation of Fas prevents apoptosis stimulated by FasL as well as the Fas-activating antibody, CH11, as evidenced by decreased activation of caspases 8 and 3. We also show that α2-6 sialylation of Fas does not alter the binding of CH11, but rather inhibits the capacity of Fas to induce apoptosis by blocking the association of FADD with Fas cytoplasmic tails, an event that initiates death-inducing signaling complex formation. Furthermore, α2-6 sialylation of Fas inhibits Fas internalization, which is required for apoptotic signaling. Although dysregulated Fas activity is a well known mechanism through which tumors evade apoptosis, the current study is the first to link Fas insensitivity to the actions of a specific sialyltransferase. This finding establishes a new paradigm by which death receptor function is impaired for the self-protection of tumors against apoptosis.

Chloride accumulation drives volume dynamics underlying cell proliferation and migration.

During brain development, progenitor cells migrate over long distances through narrow and tortuous extracellular spaces posing significant demands on the cell's ability to alter cell volume. This phenotype is recapitulated in primary brain tumors. We demonstrate here that volume changes occurring spontaneously in these cells are mediated by the flux of Cl- along with obligated water across the cell membrane. To do so, glioma cells accumulate Cl- to approximately 100 mM, a concentration threefold greater than predicted by the Nernst equation. Shunting this gradient through the sustained opening of exogenously expressed GABA-gated Cl- channels caused a 33% decrease in cell volume and impaired the ability of cells to migrate in a spatially constrained environment. Further, dividing cells condense their cytoplasm prior to mitosis, a phenomenon which is associated with the release of intracellular Cl- as indicated by a 40-mM decrease in [Cl-]i. These findings provide a new framework for considering the role of intracellular Cl- in glioma cells. Here, Cl- serves as an important osmotically active regulator of cell volume being the energetic driving force for volume changes required by immature cells in cell migration and proliferation. This mechanism that was studied in CNS malignancies may be shared with other immature cells in the brain as well.