Reversal of Blimp-1-mediated apoptosis by A1, a member of the Bcl-2 family.

Blimp-1 (B lymphocyte-induced maturation protein 1) is strongly expressed during the late stages of B cell differentiation to immunoglobulin-secreting plasma cells. Overexpression of Blimp-1 in B lymphoma cells has been reported to induce either growth arrest and cell death or Ig secretion and terminal differentiation, depending on the developmental stage of the recipient lymphomas. By using a retroviral expression system we show that Blimp-1-transduced immature WEHI 231 murine B lymphoma cells produce J chain, increased levels of the secretory form of micro heavy chain mRNA and secrete IgM for a short period of time. Concomitantly, they exhibit altered ratios of c-myc/mad4 mRNA levels, a reduction in the expression of the anti-apoptotic bcl-2 family member A1 and a distinct growth disadvantage, followed by cell death. Reintroduction of A1 by retroviral transduction greatly extends the life span of Blimp-1-expressing WEHI 231 cells which continue to secrete IgM. These data suggest that levels of A1 may determine the checkpoint between death and survival of Blimp-1-expressing B cells at different stages of differentiation.     

The HSV-1 Us3 protein kinase is sufficient to block apoptosis induced by overexpression of a variety of Bcl-2 family members

The Us3 protein kinase encoded by herpes simplex virus type-1 (HSV-1) suppresses apoptosis in infected cells and is sufficient to block apoptosis induced by overexpression of Bad [Proc. Natl. Acad. Sci. 98 (2001) 10410]. While Us3 can induce phosphorylation of Bad, phosphorylation of Bad is dispensable for Us3 anti-apoptotic function [J. Virol. 77 (2003) 6567]. We extend the findings with Bad to demonstrate that Us3 blocks apoptosis induced by overexpression of Bid, a factor parallel to Bad in the apoptotic pathway, and Bax, a factor downstream of Bad in the apoptotic pathway. A previous report suggested that Us3 exerts its effects at a premitochondrial stage [J. Virol. 75 (2001) 5491], but our results suggest that Us3 exerts anti-apoptotic effects downstream of the mitochondria. We show that the kinase activity of Us3 is necessary for Us3 anti-apoptotic effects, because a catalytically inactive form of Us3 was unable to block apoptosis. A second function of Us3, primary envelopment during viral egress, is conserved in the Us3 homologue of Pseudorabies virus (PRV) [J. Gen. Virol. 82 (2001) 2363]. Experiments published here demonstrate that PRV Us3 can also block apoptosis induced by Bax, suggesting that the anti-apoptotic activity of Us3 is conserved across alpha-herpesviruses.     

Chk2/Cds1 protein kinase blocks apoptosis during early development of Xenopus laevis.

Early Xenopus laevis embryos possess cell cycles that do not arrest at checkpoints in response to damaged DNA. At the midblastula transition (MBT), embryos with damaged DNA undergo apoptosis. After the MBT, DNA damage triggers cell cycle arrest rather than apoptosis. The transition from checkpoint-unregulated to checkpoint-regulated cycles makes Xenopus embryos compelling for studying mechanisms regulating response to genomic damage. The DNA damage checkpoint is mediated by the Chk2/Cds1 kinase. Conflicting evidence implicates Chk2 as an inhibitor or promoter of apoptosis. To better understand the developmental function of Chk2, we expressed wild-type (wt) and dominant-negative (DN) Chk2 in Xenopus embryos. Wt-Chk2 created a pre-MBT checkpoint due to degradation of Cdc25A and phosphorylation of cyclin-dependent kinases. Embryos expressing DN-Chk2 developed normally until gastrulation and then underwent apoptosis. Conversely, low doses of wt-Chk2 blocked radiation-induced apoptosis. Therefore, Chk2 operates at a switch between cell cycle arrest or apoptosis in response to genomic assaults.     

Rheb GTPase is a direct target of TSC2 GAP activity and regulates mTOR signaling

Tuberous sclerosis complex (TSC) is a genetic disease caused by mutation in either TSC1 or TSC2. The TSC1 and TSC2 gene products form a functional complex and inhibit phosphorylation of S6K and 4EBP1. These functions of TSC1/TSC2 are likely mediated by mTOR. Here we report that TSC2 is a GTPase-activating protein (GAP) toward Rheb, a Ras family GTPase. Rheb stimulates phosphorylation of S6K and 4EBP1. This function of Rheb is blocked by rapamycin and dominant-negative mTOR. Rheb stimulates the phosphorylation of mTOR and plays an essential role in regulation of S6K and 4EBP1 in response to nutrients and cellular energy status. Our data demonstrate that Rheb acts downstream of TSC1/TSC2 and upstream of mTOR to regulate cell growth.