Reciprocal Regulation of SOCS 1 and SOCS3 Enhances Resistance to Ionizing Radiation in Glioblastoma Multiforme

Abstract

Purpose: The expression of suppressors of cytokine signaling 1 (SOCS1) and SOCS3 genes is dysregulated in several solid tumors, causing aberrant activation of cell growth and survival signaling pathways. In this study, we analyzed SOCS1 and SOCS3 gene expression in glioblastoma multiforme (GBM) and studied the role of each protein in GBM cell signaling and radiation resistance. Experimental Design: SOCS1and SOCS3 gene expression was analyzed in 10 GBM cell lines by reverse transcription-PCR and Western blotting. SOCS3 expression was also studied in 12 primary GBM tissues by immunohistochemistry.The methylation status of the SOCS1and SOCS3 loci was determined by methylation-specific PCR. Extracellular signal-regulated kinase (ERK)-mitogen-activated protein kinase (MAPK) activation in GBM cell lines overexpressing SOCS1 or lacking SOCS3 was determined by phosphorylated-specific Western blotting. Radiation responses in SOCS1-positive and SOCS3-deficient GBM cell lines and fibroblasts from wild-type and SOCS1or SOCS3 knockout mice were studied in a clonogenic survival assay.Results: All GBM cell lines tested lacked SOCS1expression, whereas GBM cell lines and primary GBM tumor samples constitutively expressed SOCS3. SOCS1 gene repression was linked to hypermethylation of the SOCS1genetic locus in GBM cells. Reintroduction of SOCS1or blocking SOCS3 expression sensitized cells to radiation and decreased the levels of activated ERK MAPKs in GBM cells. Conclusions: SOCS1and SOCS3 are aberrantly expressed in GBM cell lines and primary tissues. Altered SOCS gene expression leads to increased cell signaling through the ERK-MAPK pathway and may play a role in disease pathogenesis by enhancing GBM radioresistance.

Signal Transducer and Activator of Transcription-3: A Molecular Hub for Signaling Pathways in Gliomas

“…Our studies showed that human glioblastoma tissues expressed higher levels of SOCS-3 than did control brain tissues, and that SOCS-3 promoter activity and mRNA expression is inhibited by ectopic PIAS3 expression. 1 Moreover, constitutive expression of SOCS-3 in human glioblastoma tissues correlated with enhanced cell survival and radioresistance in vitro (105). This resistance was dependent on SOCS-3 expression, as evidenced by marked sensitivity to ionizing radiation in SOCS-3 À/À mouse embryonic fibroblasts compared with wild-type mouse embryonic fibroblasts (105).…”

“…In contrast, elevated SOCS-3 expression was reported in human breast cancer and melanoma tissues, as well as in a subset of classic Hodgkin's lymphoma cell lines and primary lymphoma cells (98)(99)(100)(101)(102)(103)(104). In several studies, SOCS-3 promoted cell growth and proliferation in a number of cancers (100,103,105), and elevated levels of SOCS-3 were reported to confer a growth advantage to a melanoma cell line (103). Lymphoma cells with low levels of endogenous SOCS-3 expression succumbed to proliferation arrest whereas cells that endogenously expressed higher levels of SOCS-3 did not undergo arrest (100).…”

“…1 Moreover, constitutive expression of SOCS-3 in human glioblastoma tissues correlated with enhanced cell survival and radioresistance in vitro (105). This resistance was dependent on SOCS-3 expression, as evidenced by marked sensitivity to ionizing radiation in SOCS-3 À/À mouse embryonic fibroblasts compared with wild-type mouse embryonic fibroblasts (105). Interestingly, U87-MG glioma cells that constitutively express SOCS-3 displayed significant radioresistance that was attenuated by expression of a dominant-negative STAT-3 construct (105).…”

“…This resistance was dependent on SOCS-3 expression, as evidenced by marked sensitivity to ionizing radiation in SOCS-3 À/À mouse embryonic fibroblasts compared with wild-type mouse embryonic fibroblasts (105). Interestingly, U87-MG glioma cells that constitutively express SOCS-3 displayed significant radioresistance that was attenuated by expression of a dominant-negative STAT-3 construct (105). Current results collectively show that constitutive expression of SOCS-3 and the associated radioresistance were both mediated by STAT-3, adding another aspect of tumorigenesis mediated by activated STAT-3 (i.e., radioresistance).…”

“…In addition, heightened DNA repair responses were detected in CD133 + glioblastoma cells (107). Because the expression of SOCS-3 in glioblastoma cells was also shown to correlate with resistance to ionizing radiation (105), it is tempting to speculate that CD133 + cells might also express elevated levels of SOCS-3, as well as activated STAT-3. Experiments to examine this parameter of STAT-3 signaling are currently being explored.…”