SOCS3 promoter methylation is mutually exclusive to EGFR amplification in gliomas and promotes glioma cell invasion through STAT3 and FAK activation.

The suppressor of cytokine signaling 3 (SOCS3) gene is one of eight structurally related genes of the SOCS family and has been suggested to function as a tumor suppressor by inhibition of the JAK/STAT signaling pathway. We investigated 60 human gliomas of different histological types for SOCS3 alterations and found frequent SOCS3 promoter hypermethylation and transcriptional downregulation. However, SOCS3 promoter hypermethylation was virtually absent in primary glioblastomas, which are characterized by frequent epidermal growth factor receptor (EGFR) amplification and overexpression. Assessment of the relationship between SOCS3 and EGFR aberrations revealed that SOCS3 promoter hypermethylation was inversely related to both the EGFR gene dosage as well as the EGFR protein expression, thus suggesting SOCS3 inactivation as a mechanism substituting for EGFR activation in a subset of gliomas. In support of this hypothesis, stable shRNA-mediated SOCS3 knock-down in U251 glioblastoma cells resulted in an activation of EGFR-related signaling pathways, i.e. an increase in the activation levels of STAT3, FAK and to a lesser extent MAPK, while the AKT phosphorylation levels remained unaffected. Functionally, SOCS3-depletion caused strongly increased tumor cell invasion with no obvious effect on tumor cell proliferation. In summary, our findings suggest that SOCS3 inactivation by promoter hypermethylation is mutually exclusive to EGFR activation in gliomas and preferentially promotes glioma cell invasion through STAT3 and FAK activation.