Colocalization of BAX and BCL-2 in small intestine and kidney biopsies with different degrees of DNA fragmentation

Morphological changes associated with apoptosis are closely correlated with the expression of specific proteins. However, the cause-effect relationshi ps between the expression of these proteins and DNA degradation are barely known. For studying expression of apoptosis-related proteins in relation to different degrees of DNA fragmentation, the small intestine with its spati ally organized continuum of proliferation, differentiation and death is a v ery useful preparation. Enterocytes towards the apex of the villi become in creasingly susceptible to apoptosis. Here, this "apoptotic gradient" is use d to demonstrate the presence of BAX and BCL-2 proteins in the cytoplasm of cells at the onset of apoptosis. In semithin serial sections of the small intestine, BAX, BCL-2 and DNA fragmentation were demonstrated. BAX and BCL- 2 are always colocalized and only in cells with fragmented DNA. The gradien t of BAX or BCL-2 staining is similar to the gradient of DNA fragmentation. Immunoreactivity for BCL-2 or BAX is most intense in cells that are prone to become apoptotic next in the course of cellular turnover but not in cell s in an advanced apoptotic state, showing strongly condensed chromatin. Whe n using the same technique on semithin sections of kidney biopsies, contain ing epithelia with low cellular turnover, we found DNA fragmentation mainly in the epithelial cells of the distal tubules. Similar to the situation in the enterocytes, BAX staining was confined to the cytoplasm of epithelial cells with a moderate degree of DNA fragmentation and reduced in epithelial cells with a high degree of DNA fragmentation. In contrast to the situatio n in the small intestine, very low levels of BCL-2 were found. The results suggest that expression of BCL-2 and BAX is related to cell damage as indic ated by DNA fragmentation but not to advanced stages of cellular death, as indicated by chromatin condensation and cellular shrinkage.