Bcl2L12-mediated inhibition of effector caspase-3 and caspase-7 via distinct mechanisms in glioblastoma.
Glioblastoma multiforme (GBM) is a highly aggressive brain cancer that is characterized by the paradoxical features of intense apoptosis resistance yet a marked propensity to undergo necrosis. Bcl2L12 (for Bcl2-Like12) is a nuclear and cytoplasmic oncoprotein that is universally overexpressed in primary GBM and functions to block postmitochondrial apoptosis signaling by neutralizing effector caspase-3 and caspase-7 maturation. This postmitochondrial block in apoptosis engenders the alternate cell fate of cellular necrosis, thus providing a molecular explanation for GBM's classical features. Whereas Bcl2L12-mediated neutralization of caspase-7 maturation involves physical interaction, the mechanism governing Bcl2L12-mediated inhibition of caspase-3 activity is not known. The nuclear localization of Bcl2L12 prompted expression profile studies of primary astrocytes engineered to overexpress Bcl2L12. The Bcl2L12 transcriptome revealed a striking induction of the small heat shock protein alpha-basic-crystallin (alphaB-crystallin/HspB5), a link reinforced by robust alphaB-crystallin expression in Bcl2L12-expressing orthotopic glioma and strong coexpression of alphaB-crystallin and Bcl2L12 proteins in human primary GBMs. On the functional level, enforced alphaB-crystallin or Bcl2L12 expression enhances orthotopic tumor growth. Conversely, RNAi-mediated knockdown of alphaB-crystallin in Bcl2L12-expressing astrocytes and glioma cell lines with high endogenous alphaB-crystallin showed enhanced apoptosis, yet decreased necrotic cell death with associated increased caspase-3 but not caspase-7 activation. Mirroring this specific effect on effector caspase-3 activation, alphaB-crystallin selectively binds pro-caspase-3 and its cleavage intermediates in vitro and in vivo. Thus, alphaB-crystallin is a Bcl2L12-induced oncoprotein that enables Bcl2L12 to block the activation of both effector caspases via distinct mechanisms, thereby contributing to GBM pathogenesis and its hallmark biological properties.
Stegh
Alexander H
AH
Department of Medical Oncology, Dana-Farber Cancer Institute, Boston, MA 02115, USA.
Kesari
Santosh
S
Mahoney
John E
JE
Jenq
Harry T
HT
Forloney
Kristin L
KL
Protopopov
Alexei
A
Louis
David N
DN
Chin
Lynda
L
DePinho
Ronald A
RA
D017209:Apoptosis / Q000502:physiology; D001253:Astrocytes / Q000378:metabolism; D015153:Blotting, Western; D053148:Caspase 3 / Q000378:metabolism*; D053179:Caspase 7 / Q000378:metabolism*; D053938:DNA Fragmentation; D020869:Gene Expression Profiling; D005909:Glioblastoma / Q000378:metabolism*; D006801:Humans; D009124:Muscle Proteins / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000235:genetics; D038203:alpha-Crystallin B Chain / Q000378:metabolism*
C430100:BCL2L12 protein, human; D009124:Muscle Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D034741:RNA, Small Interfering; D038203:alpha-Crystallin B Chain; D053148:Caspase 3; D053179:Caspase 7