RAS signaling is frequently deregulated in human
neoplasms. However, RAS mutations have been found in only a small proportion of human
gastric cancers, implicating other mechanisms in the activation of RAS signaling in gastric
tumorigenesis. We have previously reported that decreased expression of
RAS protein activator like-1 (RASAL1), a member of the
RAS-GTPase-activating proteins that switch off RAS activity, contributes to colon
tumor progression. In our study, we explored the involvement of decreased RASAL1 expression in gastric
tumorigenesis. RASAL1 expression was reduced in 6 of 10
gastric cancer cell lines examined by immunoblotting. Knockdown of RASAL1 increased
mitogen-activated protein kinase signaling in response to
growth factor stimulation, and the forced expression of RASAL1 reduced proliferation of
gastric cancer cells. Immunohistochemical analyses in primary gastric
tumors showed that RASAL1 expression was reduced in 23 of 48 (48%) of the
gastric cancers but in none of the
adenomas (0/10). Methylation of the RASAL1 promoter region and loss of heterozygosity (LOH) at the RASAL1 locus were examined to investigate the causes of RASAL1 silencing. All cell lines with reduced RASAL1 had RASAL1 methylation, and two had LOH. In primary
gastric cancers, methylation or LOH was detected in 50% (6/12) of those with reduced RASAL1. Furthermore, RASAL1 expression was restored in some cell lines by
histone deacetylase inhibitor treatment. Our findings demonstrate that reduced RASAL1 expression, partly due to genetic and epigenetic changes, contributes to gastric
carcinogenesis, and also re-emphasize the importance of RAS signaling in
gastric cancer development.