Human right and left colon differ in epithelial cell apoptosis and in expression of Bak, a pro-apoptotic Bcl-2 homologue

Abstract

Background: Propensity to colonic neoplasia differs between the right and left colon.

Aims: To examine whether this difference may be related to regional differences in epithelial apoptosis, in expression of a proapoptotic regulatory protein, Bak, and in proliferation.

Patients: Individuals with no history of colorectal neoplasia.

Methods: Archival blocks of colorectal tissues were immunostained for proliferating cells (antibody to Ki-67 antigen), and Bak expression (polyclonal antiserum). Cells containing DNA strand breaks, a marker of apoptosis, were identified by terminal deoxyuridine nucleotidyl nick end labelling (TUNEL).

Results: There were fewer TUNEL positive epithelial cells in the right colon (mean 1.2 (SE 0.1)% of all epithelial cells) than the left colon (2.2 (0.1)%, p<0.0001) or rectum (2.2 (0.3)%, p<0.05). Bak expression was less common in the right colon (mean 46 (2.3)% of epithelial cells immunoreactive) than the left colon (66 (2.7)%, p<0.0001), or rectum (67 (2.3)%, p<0.001). Bak expression and TUNEL positivity were highly positively correlated (p<0.0001). In contrast to apoptosis, mean whole crypt proliferation labelling index was similar throughout the colorectum (right colon: 15.6 (3.2)%; left colon: 13. 5 (1.2)%; rectum: 13.3 (2.3)%).

Conclusion: The percentage of proliferating colonic epithelial cells is constant throughout the colon, but fewer epithelial cells undergo Bak mediated apoptosis in the right than in the left colon or rectum. This suggests that colonocytes may be lost by methods other than apoptosis in the right colon.

Figure 1

TUNEL staining showing dark, positively stained, nuclei of cells carrying DNA strand breaks at the surface of colonic crypts, more frequent in the left colon (A) than in the right colon (B). Some TUNEL positive cells are also evident in the lamina propria, especially in the left colon. Original magnification ×200.

Figure 2

Differences in the regional distribution of TUNEL positive epithelial cells within the colorectum. Individual data points are shown with means and standard errors represented by the bars.

Figure 3

Expression of Bak in colonic crypts from left colon (A) and right colon (B). Bak is expressed in the cytoplasm of superficial colonocytes throughout the colon but the immunointensity and percentage of cells staining with Bak antibody is greater in the left compared with the right colon. At higher magnification (C), Bak immunostaining is evident in the supranuclear and infranuclear regions of surface colonocytes. Original magnification ×200 (A,B); ×400 (C).

Figure 4

Percentage of Bak immunoreactive epithelial cells in different regions of the normal human colorectum. Individual data points are shown with mean and standard errors shown as bars.

Figure 5

Correlation between Bak immunoreactivity and TUNEL positive cells in normal human colonic epithelium.

Figure 6

Immunostaining with the Mib-1 antibody to Ki-67 shows the nuclei of proliferating cells in the base of normal colonic crypts from right colon. Original magnification ×200.