The
BAD protein is a pro-apoptotic member of the Bcl-2 family whose ability to heterodimerize with survival
proteins such as Bcl-X(L) and to promote cell death is inhibited by phosphorylation.
Monoclonal antibodies were generated against the
human BAD protein and used to evaluate its expression by immunoblotting and immunohistochemistry in normal human tissues and by immunoblot analysis of the National Cancer Institute anti-
cancer drug screening panel of 60 human tumor cell lines.
BAD protein was detectable by immunoblotting in many normal tissues, with testis, breast, colon, and spleen being among those with the highest steady-state levels. Immunostaining of tissues revealed many examples of cell-type-specific expression of BAD, suggesting dynamic regulation of
BAD protein levels in vivo. In many types of normal cells, BAD immunoreactivity was associated with cytosolic organelles resembling mitochondria, suggesting that BAD is often heterodimerized with other Bcl-2 family
proteins in vivo. The relative levels of
BAD protein varied widely among established human tumor cell lines, with colon, lung, and
melanomas generally having the highest expression. As a group, hematopoietic and lymphoid lines contained the least
BAD protein. The
BAD protein derived from 11 of 41
tumor lines that expressed this
pro-apoptotic protein migrated in
gels as a clear doublet, consistent with the presence of hyperphosphorylated
BAD protein. Taken together, these findings define for the first time the normal cell-type-specific patterns of expression and intracellular locations of the
BAD protein in vivo and provide insights into the regulation of this pro-apoptotic Bcl-2 family
protein in human
tumors.