Genetics of Apoptosis

4.2

BIK

Our group has recently discovered that the BH3-only protein, BIK, is
transcriptionally upregulated by p53 (Mathai et al., 2002). In healthy cells, BIK is
undectable, but after oncogenic or genotoxic stress, BIK accumulates almost
exclusively at the ER membrane. Adenoviral delivery of BIK potently triggers Cyt.c
release and apoptosis independently of p53, but, unlike other BH3-only molecules,
BIK carries out this function from the ER. Our recent studies suggest that BIK may
activate mitochondrial release of Cyt.c and other death pathways by activating factors
in both ER and cytosol (Germain et al., submitted). Thus, a post-mitochondrial
supernatant (primarily containing cytosol and ER microsomes), but not an S-100
fraction, isolated from cells expressing BIK in the presence of caspase inhibitors, can
release Cyt.c from mitochondria isolated from cells lacking BIK expression. In vivo,
BIK induces Cyt.c release independently of caspases. The ER and cytosolic factors
contributing to BIK-induced Cyt.c release are currently under investigation. Given
the current model on the mechanism of action of BH3-only molecules at the
mitochondria, it is surprising to find that BIK functions at the ER. As discussed above,
this could reflect functions of BAX/BAK at the ER, or provide a mechanism to
neutralize ER Bcl-2/Bcl-xL, or represent hitherto unknown actions of certain BH3-
only molecules at the ER required for communicating to the mitochondria. In this
context, it is noteworthy that p53 upregulates numerous BH3-only proteins after
genotoxic stress (Vogelstein et al., 2000); therefore, the actions of BIK on the ER,
and of PUMA or NOXA at the mitochondria may be required in concert to trigger
efficient Cyt.c release and caspase activation.

4.3

The reticulon family

The reticulon family members NSP-C and RTN-xs were identified as Bcl-xL-
interacting proteins in a yeast two-hybrid screen (Tagami et al., 2000). Both proteins
contain two transmembrane domains and localize to the ER. RTN-xs can interact
with both Bcl-xL and Bcl-2 in cotransfected cells, and interactions between the
endogenous proteins are weakly detected. NSP-C, however, interacts with Bcl-xL,
but not Bcl-2. Overexpression of RTN-xs increases the amount of Bcl-2 and Bcl-xL
at the ER and inhibits the ability of both proteins to protect against tunicamycinand
staurosporin-induced apoptosis (Tagami et al., 2000). It remains to be determined
whether reticulon family members inhibit the ability of Bcl-2/Bcl-xL to alter ER Ca^2

• homeostasis or some other function of these proteins at the ER.

116 GENETICS OF APOPTOSIS