Bcl2L12 inhibits post-mitochondrial apoptosis signaling in glioblastoma

Abstract

Glioblastoma (GBM) is an astrocytic brain tumor characterized by an aggressive clinical course and intense resistance to all therapeutic modalities. Here, we report the identification and functional characterization of Bcl2L12 (Bcl2-like-12) that is robustly expressed in nearly all human primary GBMs examined. Enforced Bcl2L12 expression confers marked apoptosis resistance in primary cortical astrocytes, and, conversely, its RNA interference (RNAi)-mediated knockdown sensitizes human glioma cell lines toward apoptosis in vitro and impairs tumor growth with increased intratumoral apoptosis in vivo. Mechanistically, Bcl2L12 expression does not affect cytochrome c release or apoptosome-driven caspase-9 activation, but instead inhibits post-mitochondrial apoptosis signaling at the level of effector caspase activation. One of Bcl2L12's mechanisms of action stems from its ability to interact with and neutralize caspase-7. Notably, while enforced Bcl2L12 expression inhibits apoptosis, it also engenders a pronecrotic state, which mirrors the cellular phenotype elicited by genetic or pharmacologic inhibition of post-mitochondrial apoptosis molecules. Thus, Bcl2L12 contributes to the classical tumor biological features of GBM such as intense apoptosis resistance and florid necrosis, and may provide a target for enhanced therapeutic responsiveness of this lethal cancer. Supplemental material is available at http://www.genesdev.org.

What drives intense apoptosis resistance and propensity for necrosis in glioblastoma? A role for Bcl2L12 as a multifunctional cell death regulator

“…1, left), while other heat shock proteins, caspases, and their (post-mitochondrial) inhibitors, such as members of the IAP family, are not differentially expressed. 6,17 In addition, primary human GBM tumor specimens revealed striking co-expression of Bcl2L12 and αB-crystallin, and Bcl2L12-driven (U87MG) xenografts showed significant upregulation of αB-crystallin, confirming a Bcl2L12-aB-crystallin signaling axis axis not only in glial cells in vitro, but also in GBM tumors in vivo. 17 The induction of αB-crystallin expression by Bcl2L12 appears to be highly relevant.…”

“…4,5 Detailed mRNA expression analyses and cell-culture based oncogene cooperation assays of genes within this chromosomal region pointed to Bcl2L12 as a putative oncoprotein based on its frequent mRNA upregulation in GBM and significant in vitro transforming activity. 6 Immunohistochemical tissue microarray (IHC-TMA) analysis confirmed robust Bcl2L12 expression in nearly all primary GBM with low or absent expression in lower-grade astrocytoma and normal brain tissue. 6 This robust expression profile in GBM prompted a multi-level functional characterization of Bcl2L12 to assess its impact on glioma-relevant signaling pathways.…”

“…6 Immunohistochemical tissue microarray (IHC-TMA) analysis confirmed robust Bcl2L12 expression in nearly all primary GBM with low or absent expression in lower-grade astrocytoma and normal brain tissue. 6 This robust expression profile in GBM prompted a multi-level functional characterization of Bcl2L12 to assess its impact on glioma-relevant signaling pathways.…”

“…6 Interestingly, Bcl2L12 lacks BH1, BH3 or BH4 and transmembrane motifs and additionally is characterized by a proline-rich stretch homologous to the Proline-rich domain in RRas and several PxxP tetrapeptide motifs, structural domains that may serve as binding sites for SH2/SH3 domain-containing signaling molecules (Fig. 1, lower right).…”

“…One model suggests that pro-death BH3-only proteins (e.g., tBid, Bad and Bim) bind Bcl-2 or Bcl-xL to displace multi-BH domain proteins, such as Bax and Bak, which then oligomerize and insert into the OOM to ultimately provoke cytochrome c release and subsequent caspase activation. 8 The nonclassical structural features of Bcl2L12 and the distant relationship to the Bcl-2 protein family, together with its prominent cytosolic and nuclear localization, 6 prompted us to consider that Bcl2L12 may possess distinct mechanisms of action which likely were to be independent of heterodimerization with pro-apoptotic Bcl-2-like proteins and protection of mitochondrial membrane integrity.…”