Regulator of the apoptotic pathway BCL-2 Regulator of the apoptotic pathway Heather Anderson

Discovery of first antiapoptotic gene Tsujimoto, Cossman, Jaffe, and Croce first cloned portions of the Bcl-2 gene in 1985 Discovery of antiapoptotic activity and oncogenic potential of Bcl-2 by Vaux and Cory Adams in 1988 In 1989 experiments with transgenic mice showed that overexpression of Bcl-2 causes accumulation of B lymphocytes Later Vaux, Weissman, and Kim showed that Bcl-2 controlled an evolutionary conserved pathway

What is apoptosis? Process of programmed cell death Necrosis Morphological changes Necrosis Acute cellular injury Essential for defense against virus, development, and tissue homeostasis (http://en.wikipedia.org/wiki/Apoptosis)

Structure of Bcl-2 Contains 4 BH domains Located on the endoplasmic reticulum membrane, nuclear envelope, and outer membrane of the mitochondria Hydrophobic C terminal domain places it on the cytoplasmic face of the mitochondrial outer membrane BH3 BH4 Figure 3: Nature Reviews/ Cancer Volume 2 (© 2002 Nature Publishing Group)

Normal Apoptosis: mitochondrial pathway Bcl-2 inhibits apoptosis Upon cellular damage Bax/Bak migrates to mitochondria, inhibits Bcl-2, and form pores in the outer membrane Cytochrome c leaks out of the mitochondria and binds to Apaf-1 (apoptotic protease activating factor-1) Use ATP to form apoptosomes ad activate caspase-9 Caspase-9 cleaves caspase-3 and caspase-7 (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Apoptosis.html)

Stress pathway to apoptosis Bcl-2 pathway C. Elegan pathway Initiation of apoptosis also requires BH3 proteins Detect developmental death signals or intracellular damage Bind in groove on Bcl-2 and lead to heterodimerization Activation of apoptosis requires only one BH3 protein (EGL-1) Ced-9 directly binds to and inhibits Ced-4 EGL-1 displaces Ced-4 which then activates the Ced-3 caspase leading to apoptosis

Functions of Bcl-2 in the cell Prevents apoptosis and necrosis Controls homeostasis Maintains membrane integrity on mitochondria Inhibits Bax conformational change May also gate mitochondrial pore or compete for Bax target Required for survival of kidney, melanocyte stem cells, and mature lymphocytes

B-cell lymphoma B cell mutates and becomes cancerous Symptoms: Swollen lymph nodes, night sweats, tired, weight loss, and high temperatures Most common is follicular lymphoma Cleaved or non- cleaved cells Prognosis: median of 10 years (http://www.pathconsultddx.com/pathCon/diagnosis?pii=S1559-8675(06)70179-6) (http://www2.umdnj.edu/pathpweb/)

Bcl-2 mutations Bcl-2 is an oncogene, overexpression of this gene prevents apoptosis which leads to cancer 80 to 90% of follicular lymphomas, 30% diffuse large cell lymphomas Also found in chronic lymphocytic leukemia, acute lymphoblastic leukemia, acute promyelocytic leukemia, Burkitt lymphoma, and diffuse large B-cell lymphoma Transgenic mouse models

Bcl-2 mutation in B-cell lymphoma Bcl-2 is located on chromosome 18 Reciprocal translocation of chromosome 18 and 14 leads to overexpression of Bcl-2 Chromosome 14 contains the heavy antibody chain Cell death is delayed leading to cell proliferation Prevent activation of caspases (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/BCL-2.html)

Summary Bcl-2 inhibits apoptosis in the mitochondrial pathway Bcl-2 pathway is important in maintaining mitochondrial integrity and cell homeostasis especially with B-cells Translocation of chromosome 14 and 18 places Bcl-2 gene next to the antibody heavy chain enhancer High levels of Bcl-2 protect B-cells from early cell death, leading to B-cell lymphoma