Ep. Holinger et al., Bak BH3 peptides antagonize Bcl-x(L) function and induce apoptosis throughcytochrome c-independent activation of caspases, J BIOL CHEM, 274(19), 1999, pp. 13298-13304
The Bcl-2 homology 3 (BH3) domain is crucial for the death-inducing and dim
erization properties of pro-apoptotic members of the Bcl-2 protein family,
including Bak, Bar, and Bad. Here we report that synthetic peptides corresp
onding to the BH3 domain of Bak bind to Bcl-x(L), antagonize its anti-apopt
otic function, and rapidly induce apoptosis when delivered into intact cell
s via fusion to the Antennapedia homeoprotein internalization domain. Treat
ment of HeLa cells with the Antennapedia-BH3 fusion peptide resulted in pep
tide internalization and induction of apoptosis within 2-3 h, as indicated
by caspase activation and subsequent poly(ADP-ribose) polymerase cleavage,
as well as morphological characteristics of apoptosis, A point mutation wit
hin the BH3 peptide that blocks its ability to bind to Bcl-x(L) abolished i
ts apoptotic activity, suggesting that interaction of the BH3 peptide with
Eel-a-related death suppressors, such as Bcl-x(L), may be critical for its
activity in cells. While overexpression of Bcl-x(L) can block BH3-induced a
poptosis, treatment with BH3 peptides resensitized Bcl-x(L)-expressing cell
s to Fas-mediated apoptosis, BH3-induced apoptosis was blocked by caspase i
nhibitors, demonstrating a dependence on caspase activation, but was not ac
companied by a dramatic early loss of mitochondrial membrane potential or d
etectable translocation of cytochrome c from mitochondria to cytosol, These
findings demonstrate that the BH3 domain itself is capable of inducing apo
ptosis in whole cells, possibly by antagonizing the function of Eel-a-relat
ed death suppressors.