Increased apoptosis in infiltrating mononuclear cells of colorectal cancer: a mechanism for tumor escape

Disturbance in apoptosis has been proposed as one of the mechanisms involved in the immune response targeting tumor outgrowth. How colorectal cancer cells escape from attack by the immune system is not yet fully understood. To investigate apoptotic molecules associated with colorectal cancer counterattack. Tissue samples of colon from 12 patients with colorectal cancer were collected and analyzed by immunostaining. In addition to tumorous tissues, corresponding nontumorous specimens of colon were obtained as controls. We examined the expression of Bcl-2, Bcl-xl, Bax, caspase-3, and inducible nitric oxide synthase in infiltrating mononuclear cells of colorectal cancer tissues and also in colorectal cancer tissues. The TUNEL assay was used to detect in situ apoptosis. Apoptosis was barely detectable in specimens of colorectal cancer, which was consistent with an increase in Bcl-2 level and a decrease in caspase-3 level. In contrast, infiltrating mononuclear cells of tumorous tissues showed a marked increase in apoptosis compared with those of nontumorous tissues. The increased apoptosis might have resulted from an imbalance of antiapoptotic and proapoptotic molecules, as reflected by reduction of Bcl-2 level and elevation of Bax level. The elevated caspase-3 levels found in this study could be a downstream effector of the Bcl-2 and Bax apoptotic pathways. A significant increase in inducible nitric oxide synthase observed in the infiltrating mononuclear cells might contribute to immunosuppression seen in colorectal cancer. It is tempting to speculate that aberrant expression of apoptotic molecules and inducible nitric oxide synthase in infiltrating mononuclear cells provides the underlying mechanism through which colorectal cancer cells escape attack by the immune system and subsequently grow without control.