Frequent Wnt signaling activation and loss of SFRP genes in gastric cancer

Abstract

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The Wnt signaling pathway is essential to regulate cellular differentiation and proliferation, and its activation is a hallmark of colon cancer. So far, cumulative evidences have suggested frequent Wnt signaling activation in gastric cancer (GC), but we previously reported that mutations in APC or beta-catenin are quite infrequent in GC (Sasaki Y. Tumor Biol. 2001). Recently, we have found frequent hypermethylation and silencing of the secreted frizzled-related protein genes in colorectal cancer (CRC). We also reported evidences that inactivation of SFRPs may complement APC or beta-catenin mutations to allow constitutive Wnt signaling in CRC cells. In this study, we analyzed SFRPs expression and methylation in 15 GC cell lines. SFRP1 and SFRP2 are silenced in virtually all of the GC cells. By using methylation-specific PCR (MSP) and bisulfite sequencing, these cells showed complete methylation of the CpG islands associated with SFRP1 and 2. SFRP5 is also silenced and methylated in 12 of the 15 GC cells. Treatment with 5-aza-2’-deoxycytidine re-activated the silenced genes in these cells. FACScan and colony formation assay revealed that over-expression of SFRP1, 2 and 5 induced apoptosis and growth suppression of GC cells. Reporter assay revealed aberrant catenin-TCF transcription activity in 4 of 15 GC cells, which can be suppressed by overexpression of SFRP1, SFRP2 or SFRP5. Interestingly, we found that the frequency of nuclear beta-catenin accumulation is far higher than that of aberrant catenin-TCF transcription, and APC or beta-catenin mutations in GC cells. Taken together, we conclude that epigenetic inactivation of SFRPs is a frequent event in GC, and it may contribute to gastric tumorigenesis through facilitating Wnt signal activation.