Loss of Expression and Function of SOCS3 Is an Early Event in HNSCC: Altered Subcellular Localization as a Possible Mechanism Involved in Proliferation, Migration and Invasion
The suppressors of cytokine signaling (SOCS) are inhibitors of cytokine signaling that function via the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway. Recently, methylation of SOCS-1 and SOCS-3 has been implicated in the tumorigenesis of liver and lung cancer. This study was performed to elucidate the role of SOCS-1 and SOCS-3 in squamous cell carcinoma of the head and neck (HNSCC) and its precursor lesions. HNSCC of 94 patients and corresponding normal mucosa, lymph node metastases as well as 16 high- and 21 low-grade squamous cell dysplasias were studied by using methylation-specific PCR (MSP) for the SOCS-1 and SOCS-3 promoter after microdissection. The presence of SOCS-3 mRNA transcripts was confirmed by semiquantitative real-time PCR, and the SOCS-3 protein was analysed immunohistochemically. SOCS-3 hypermethylation was found in 85/94 HNSCC (90%) and in 10/16 high-grade and 9/21 low-grade dysplasias (63 and 43%, respectively). SOCS-1 promoter hypermethylation was detected in 10/94 HNSCC samples (11%) and in 2/16 high-grade and 1/21 low-grade dysplasias (13 and 5%, respectively). Lymph node metastases exhibited an identical methylation status as the primary tumors. Methylation of the SOCS-3 promoter correlated with downregulation of SOCS-3 transcripts and protein expression in these tumors and various cell lines. In the cell lines tested, SOCS-3 and SOCS-1 transcripts increased upon treatment with the demethylation compound 5-aza-2-deoxycytidine (5-AZA-DC). Overexpression of wild-type SOCS-3 in carcinoma cells with methylated SOCS-3 resulted in the induction of apoptosis and growth suppression as well as downregulation of STAT3, bcl-2 as well as bcl-xL. Our data suggest that promoter methylation and subsequent transcript downregulation of SOCS-3 transcripts and, to a much lesser extent, SOCS-1 are involved in the multistep carcinogenesis of HNSCC. During its involvement in tumor growth, restoration of SOCS-3 may hold treatment potential for HNSCC.