Functional expression of μ-opioid receptors in the human colon cancer cell line, HT-29, and their localization in human colon

We have investigated the functional expression of mu-opioid receptors (MORs) in the human colon cancer cell line, HT-29. As revealed by immunocytochemistry, immunoreactivity was present in both the cytoplasm and nuclei of the cells. Challenge with morphine for 24 h (1 nM to 1 mu M) barely affected cell proliferation, while the secretion of urokinase type plasminogen activator (a protease involved in invasion/metastasis) was markedly augmented by a concentration of 0.1 mu M. Human colon cancer tissue from 14 consecutively operated patients was investigated by immunohistochemistry. MORs were found in the nuclei of colonocytes and immune cells of the lamina propria in tumor-free tissue. In tumor tissue, immunoreactivity was found in the membrane and often in the nuclei of tumor cells. The current findings suggest that morphine administration could affect tumor progression by interfering with, for example, invasive properties. Our demonstration of a nuclear expression of the MORs appears to be a novel finding.