No abstract

pproximately 5% of breast cancers show a familial pattern of occurrence.

Activation of the stress-activated protein kinase (SAPK/JNK) by genotoxic agents is necessary for induction of apoptosis. We report here that ionizing radiation ionizing radiation exposure induces translocation of SAPK to mitochondria and association of SAPK with the anti-apoptotic Bcl-x L protein. SAPK phosphorylates Bcl-x L on threonine 47 (Thr-47) and threonine 115 (Thr-115) in vitro and in vivo. In contrast to wild-type Bcl-x L , a mutant Bcl-x L with the two threonines substituted by alanines (Ala-47, Ala-115) is a more potent inhibitor of ionizing radiation-induced apoptosis. These findings indicate that translocation of SAPK to mitochondria is functionally important for interactions with Bcl-x L in the apoptotic response to genotoxic stress.

Genotoxic stress exerts biological activity by activating downstream effectors, including the p53 tumor suppressor. p53 regulates cell-cycle checkpoint and induction of apoptosis in response to DNA damage; however, molecular mechanisms responsible for committing to these distinct functions remain to be elucidated. Recent studies demonstrated that phosphorylation of p53 at Ser46 is associated with induction of p53AIP1 expression, resulting in commitment to apoptotic cell death. In this regard, the role for Ser46 kinases in p53-dependent apoptosis has been established; however, the kinases responsible for Ser46 phosphorylation have yet to be identified. Here, we demonstrate that the dual-specificity tyrosine-phosphorylation-regulated kinase 2 (DYRK2) directly phosphorylates p53 at Ser46. Upon exposure to genotoxic stress, DYRK2 translocates into the nucleus for Ser46 phosphorylation. Consistent with these results, DYRK2 induces p53AIP1 expression and apoptosis in a Ser46 phosphorylation-dependent manner. These findings indicate that DYRK2 regulates p53 to induce apoptosis in response to DNA damage.

Cytochrome C is a mitochondrial protein that induces apoptosis when released into the cytosol or when added to cell-free extracts. Here we show that cells that overexpress the Bcl-2-related protein Bcl-xL fail to accumulate cytosolic cytochrome C or undergo apoptosis in response to genotoxic stress. Coimmunoprecipitation studies demonstrate that Bcl-xL associates with cytochrome C. Cytochrome C binds directly and specifically to Bcl-xL and not to the proapoptotic Bcl-xs protein. The results also demonstrate that Bcl-xs blocks binding of cytochrome C to Bcl-xL. Our findings support a role for Bcl-xL in protecting cells from apoptosis by inhibiting the availability of cytochrome C in the cytosol.