M3 muscarinic receptor (M3R) activation promotes
colon cancer cell proliferation, migration, and invasion in vitro. Although over-expression of CHRM3, the gene encoding M3R, is reported in primary
colon cancers, expression of M3R itself has not been studied in colon
neoplasia. We compared M3R expression in normal colon to colon
adenomas, and primary and metastatic
colon cancers. Compared to adjacent normal colon, CHRM3 expression was increased up to 128-fold in 10 of 18 consecutive surgical
cancer specimens (56%) and associated with metastatic spread (P < 0.05). To analyze M3R
protein expression we interrogated 29 consecutive
paraffin-embedded
colon adenocarcinomas and adjacent normal colon using a specific anti-M3R antibody and immunoperoxidase staining. This revealed weak M3R expression in normal colonocytes, primarily on basolateral surfaces. In contrast, in 25 of 29
cancer tissues (86%) we observed both cytoplasmic and plasma membrane over-expression of M3R; compared to normal epithelium, mean M3R staining intensity was increased more than two-fold in
colon cancer (P < 0.001). M3R staining was also increased in 22 colon
adenomas compared to adjacent normal colon (P < 0.001). In contrast, M3R staining intensity was not increased in lymph node or liver
metastases. These findings suggest M3R expression plays an important role in early progression and invasion of colon
neoplasia but is less important once
tumors have spread.