FHC和Bim相互作用抵抗细胞凋亡

FHC和Bim参与细胞铁代谢和由ROS引起的细胞凋亡过程.但是其具体的分子机制还未阐明.用pLexA-Bim L作为诱饵,筛选了一个基于pBD42AD的胎脑cDNA文库,发现FHC是一个新的Bim相互作用蛋白.酵母杂交实验发现Bim的相互作用片段为BH3功能域.上述相互作用进一步用免疫共沉淀和荧光共定位得以证实.在HEK293细胞过表达FHC可以减轻由Bim过表达或ROS所引起的细胞凋亡,而用FHC特异性siRNA调低FHC表达,则增加Bim过表达或ROS引起的细胞凋亡.研究首次报道了Bim和FHC的相互作用以及对细胞凋亡和氧化应激的影响,为进一步阐明FHC和Bim参与凋亡和ROS反应提供了新的线索.     

Caspase-1zeta, a new splice variant of the caspase-1 gene

Five alternatively spliced mRNA isoforms of human caspase-1 have been identified previously and we report here the cloning of a new isoform, named CASP1 zeta (zeta), from human ovarian surface epithelial cell cDNA. The new isoform zeta is identical to the alpha isoform but missing 79 nucleotides in the coding region of the prodomain of procaspase-1. Analysis of the cDNA sequence of the zeta isoform revealed an ORF of a shorter protein missing the 39 amino acids at the amino terminal of procaspase-1alpha, which comprises the important caspase activating recruitment domain (CARD), which is required for interactions between caspases and other proteins. Secondary structure analysis of procaspase-1 CARD predicted the truncation of the alpha1, the alpha2, and part of the alpha3 helix in the zeta isoform in comparison to the full-length alpha isoform. The new zeta isoform was expressed in many, but not all, adult human tissues by RT-PCR. In HEK293 cells, transient overexpression of wild-type caspase-1zeta induced apoptosis to levels similar to those of caspase-1alpha. However, mutational change at the caspase-1 active center of the Cys 246 of caspase-1zeta, as well as Cys 285 of caspase-1alpha, completely abolished their apoptotic activity. Our findings suggest that caspase-1zeta is a widespread, new proapoptotic isoform of caspase-1. They also demonstrate that the first 39 amino acids of the N-terminal of the CARD in procaspase-1 are not required for its apoptotic activity.     

Grb10 interacts with Bim L and inhibits apoptosis

Bim is a proapoptotic member of the Bcl-2 family and is primarily involved in the regulation of the intrinsic apoptotic pathway. However, the detail of regulation of Bim’s proapoptotic activity has not been clarified yet. Using Bim L as bait, we screened a human fetal cDNA library for interacting proteins and identified Grb10 as an interactor. This interaction was verified by co-immunoprecipitation and intracellular co-localization studies. The potential segment of Bim L that binds Grb10 was identified via a yeast mating test. Grb10 interacted with the DBD (dynein binding domain) of Bim and inhibited apoptosis triggered by overexpression of DBD containing Bim isoforms. The putative phosphorylation sites on DBD of Bim play a role for the anti-proapoptotic activity of Grb10. Our results suggest that Grb10 interacts with Bim L and inhibits its proapoptotic activity in a phosphorylation-dependant manner.     

Alternative splicing of Bim and Erk-mediated Bim(EL) phosphorylation are dispensable for hematopoietic homeostasis in vivo

The pro-apoptotic BH3-only protein Bim has a major role in hematopoietic homeostasis, particularly in the lymphocyte compartment, where it strongly affects immune function. The three major Bim isoforms (Bim(EL), Bim(L) and Bim(S)) are generated by alternative splicing. Bim(EL), the most abundant isoform, contains a unique sequence that has been reported to be the target of phosphorylation by several MAP kinases. In particular, Erk1/2 has been shown to interact with Bim(EL) through the DEF2 domain of Bim(EL) and specifically phosphorylate this isoform, thereby targeting it for ubiquitination and proteasomal degradation. To examine the physiological importance of this mechanism of regulation and of the alternative splicing of Bim, we have generated several Bim knock-in mouse strains and analyzed their hematopoietic system. Although mutation in the DEF2 domain reduces Bim(EL) degradation in some circumstances, this mutation did not significantly increase Bim's pro-apoptotic activity in vivo nor impact on the homeostasis of the hematopoietic system. We also show that Bim(EL) and Bim(L) are interchangeable, and that Bim(S) is dispensable for the function of Bim. Hence, we conclude that physiological regulation of Bim relies on mechanisms independent of its alternative splicing or the Erk-dependent phosphorylation of Bim(EL).     

Degenerative disorders caused by Bcl-2 deficiency prevented by loss of its BH3-only antagonist Bim.

Apoptosis is triggered when proapoptotic members of the Bcl-2 protein family bearing only the BH3 association domain bind to Bcl-2 or its homologs and block their antiapoptotic activity. To test whether loss of the BH3-only protein Bim could prevent the cellular attrition caused by Bcl-2 deficiency, we generated mice lacking both genes. Mice without Bcl-2 have a fragile lymphoid system, become runted, turn gray, and succumb to polycystic kidney disease. Concomitant absence of Bim prevented all these disorders. Indeed, loss of even one bim allele restored normal kidney development, growth, and health. These results demonstrate that Bim levels set the threshold for initiation of apoptosis in several tissues and suggest that degenerative diseases might be alleviated by blocking BH3-only proteins.     

Bad plays a more significant role than Bid and Bim in mediating cell death signals in batch cultures of HEK 293 cells

The expression of three BH3-only proteins, Bad, Bid and Bim, were knocked down in HEK 293 cells using vectors that express corresponding siRNAs. When cultured in the presence of 10% (v/v) serum and a diminished glucose/nutrients environment, cells lacking any one of these BH3-only proteins showed delayed cell death compared to wild type cells. Remarkably, the culture life of Bad (−) cells was extended for an additional 5 days compared to WT HEK 293 cells. In the absence of serum, the suppression of either Bad, Bim or Bid expression delayed cell death under several stress conditions. Results presented in this paper offer an insight into the functions of BH3-only proteins in mediating the death signals under different stress conditions. Anup Padmanabhan and Sen Liu contributed equally to this work.     

BH3-only protein Bim inhibits activity of antiapoptotic members of Bcl-2 family when expressed in yeast

Proteins of the Bcl-2 family regulate programmed cell death in mammals by promoting the release of cytochrome c from mitochondria in response to various proapoptotic stimuli. The mechanism by which BH3-only members of the family activate multidomain proapoptotic proteins Bax and Bak to form a pore in mitochondrial membranes remains under dispute. We report that cell death promoting activity of BH3-only protein Bim can be reconstituted in yeast when both Bax and antiapoptotic protein Bcl-X(L) are present, suggesting that Bim likely activates Bax indirectly by inhibiting antiapoptotic proteins.     

Macrophage migration inhibitory factor (MIF) interacts with Bim and inhibits Bim-mediated apoptosis

The pro-apoptotic Bcl-2 family member Bim acts as a sensor for apoptotic stimuli and initiates apoptosis through the mitochondrial pathway. To identify novel regulators of Bim, we employed the yeast two-hybrid system and isolated the human gene encoding macrophage migration inhibitory factor (MIF), a ubiquitously expressed proinflammatory mediator that has also been implicated in cell proliferation, the cell cycle and carcinogenesis. The interaction between MIF and Bim was confirmed by both in vitro and in vivo protein interaction assays. Intriguingly, protein complexes between MIF and the three major Bim isoforms (BimEL/BimL/BimS) could be detected in HEK293 and K562 cells, especially in cells undergoing apoptosis. Moreover, exogenous expression of MIF partially inhibited Bim-induced apoptosis in HEK293 cells. SiRNA-mediated knockdown of MIF increased apoptosis in K562 cells exposed to the chemical oxidant diamide. Endogenous MIF may regulate the pro-apoptotic activity of Bim and inhibit the release of cytochrome c from mitochondria.     

The proapoptotic activity of the Bcl-2 family member Bim is regulated by interaction with the dynein motor complex

Bcl-2 family members that have only a single Bcl-2 homology domain, BH3, are potent inducers of apoptosis, and some appear to play a critical role in developmentally programmed cell death. We examined the regulation of the proapoptotic activity of the BH3-only protein Bim. In healthy cells, most Bim molecules were bound to LC8 cytoplasmic dynein light chain and thereby sequestered to the microtubule-associated dynein motor complex. Certain apoptotic stimuli disrupted the interaction between LC8 and the dynein motor complex. This freed Bim to translocate together with LC8 to Bcl-2 and to neutralize its antiapoptotic activity. This process did not require caspase activity and therefore constitutes an initiating event in apoptosis signaling.     

Proapoptotic Bcl-2 family member Bim is involved in the control of mast cell survival and is induced together with Bcl-XL upon IgE-receptor activation

Mast cells play critical roles in the regulation of acute and chronic inflammations. Apoptosis is one of the mechanisms that limit and resolve inflammatory responses. Mast cell survival can be controlled by growth factors and activation of the IgE-receptor FcvarepsilonRI. Members of the Bcl-2 protein family are critical regulators of apoptosis and our study provides evidence that the proapoptotic BH3-only family member Bim is essential for growth factor deprivation-induced mast cell apoptosis and that Bim levels increase upon FcvarepsilonRI activation. Bim deficiency or Bcl-2 overexpression delayed or even prevented cytokine withdrawal-induced mast cell apoptosis in culture. The prosurvival protein Bcl-XL and the proapoptotic Bim were both induced upon FcvarepsilonRI activation. These results suggest that Bim and possibly also other BH3-only proteins control growth factor withdrawal-induced mast cell apoptosis and that the fate of mast cells upon FcvarepsilonRI activation depends on the relative levels of pro- and antiapoptotic Bcl-2 family members.     

B cell receptor-mediated apoptosis of human lymphocytes is associated with a new regulatory pathway of Bim isoform expression

Studies in Bim-deficient mice have shown that the proapoptotic molecule Bim plays a key role in the control of B cell homeostasis and activation. However, the role of Bim in human B lymphocyte apoptosis is unknown. We show in this study that, depending on the degree of cross-linking, B cell receptors can mediate both Bim-dependent and apparent Bim-independent apoptotic pathways. Cross-linked anti-mu Ab-mediated activation induces an original pathway governing the expression of the various Bim isoforms. This new pathway involves the following three sequential steps: 1) extracellular signal-regulated kinase-dependent phosphorylation of the BimEL isoform, which is produced in large amounts in healthy B cells; 2) proteasome-mediated degradation of phosphorylated BimEL; and 3) increased expression of the shorter apoptotic isoforms BimL and BimS.     

Identification of novel isoforms of the BH3 domain protein Bim which directly activate Bax to trigger apoptosis

Bim (Bcl-2-interacting mediator of cell death) is a member of the BH3 domain-only subgroup of Bcl-2 family members, for which three splice variants have been described. Bim is expressed in many healthy cell types, where it is maintained in an inactive conformation through binding to the microtubule-associated dynein motor complex. Upon certain apoptotic stimuli, Bim is released from microtubules and mediates caspase-dependent apoptosis through a mechanism that is still unclear. Here, we have identified and characterized novel splice variants of human Bim mRNA. In particular, we show that a newly discovered, small protein isoform, BimAD, is also able to induce apoptosis strongly in several human cell lines. BimAD and the previously characterized isoform BimS are shown to be capable of heterodimerizing in vivo with both death antagonists (Bcl-2 and Bcl-X(L)) and death agonists (Bax). Mutants of BimAD that bind to Bax but not to Bcl-2 still promote apoptosis, indicating that Bim can regulate apoptosis through direct activation of the Bax-mediated cell death pathway without interaction with antiapoptotic Bcl-2 family members. Furthermore, we have shown that the interaction of the BimS and BimAD isoforms with Bax leads to a conformational change in this protein analogous to that triggered by the BH3-only protein Bid.     

Cyclic-AMP-dependent protein kinase A regulates apoptosis by stabilizing the BH3-only protein Bim

The proapoptotic Bcl2 homology domain 3(BH3)-only protein Bim is controlled by stringent post-translational regulation, predominantly through alterations in phosphorylation status. To identify new kinases involved in its regulation, we carried out a yeast two-hybrid screen using a non-spliceable variant of the predominant isoform--Bim(EL)--as the bait and identified the regulatory subunit of cyclic-AMP-dependent protein kinase A--PRKAR1A--as an interacting partner. We also show that protein kinase A (PKA) is a Bim(EL) isoform-specific kinase that promotes its stabilization. Inhibition of PKA or mutation of the PKA phosphorylation site within Bim(EL) resulted in its accelerated proteasome-dependent degradation. These results might have implications for human diseases that are characterized by abnormally increased PKA activity, such as the Carney complex and dilated cardiomyopathy.     

Multisite phosphorylation regulates Bim stability and apoptotic activity

The proapoptotic BH3-only protein Bim is established to be an important mediator of signaling pathways that induce cell death. Multisite phosphorylation of Bim by several members of the MAP kinase group is implicated as a regulatory mechanism that controls the apoptotic activity of Bim. To test the role of Bim phosphorylation in vivo, we constructed mice with a series of mutant alleles that express phosphorylation-defective Bim proteins. We show that mutation of the phosphorylation site Thr-112 causes decreased binding of Bim to the antiapoptotic protein Bcl2 and can increase cell survival. In contrast, mutation of the phosphorylation sites Ser-55, Ser-65, and Ser-73 can cause increased apoptosis because of reduced proteasomal degradation of Bim. Together, these data indicate that phosphorylation can regulate Bim by multiple mechanisms and that the phosphorylation of Bim on different sites can contribute to the sensitivity of cellular apoptotic responses.     

Promoter-, cell-, and ligand-specific transactivation responses of the VDRB1 isoform

The vitamin D receptor (VDR) mediates the effects of 1,25(OH)(2)D(3), the active form of vitamin D. The human VDRB1 isoform differs from the originally described VDR by an N-terminal extension of 50 amino acids. Here we investigate cell-, promoter-, and ligand-specific transactivation by the VDRB1 isoform. Transactivation by these isoforms of the cytochrome P450 CYP24 promoter was compared in kidney (HEK293 and COS1), tumor-derived colon (Caco-2, LS174T, and HCT15), and mammary (HS578T and MCF7) cell lines. VDRB1 transactivation in response to 1,25(OH)(2)D(3) was greater in COS1 and HCT15 cells (145%), lower in HEK293 and Caco-2 cells (70-85%) and similar in other cell lines tested. By contrast, on the cytochrome P450 CYP3A4 promoter, 1,25(OH)(2)D(3)-induced VDRB1 transactivation was significantly lower than VDRA in Caco-2 (68%), but comparable to VDRA in HEK293 and COS1 cells. Ligand-dependence of VDRB1 differential transactivation was investigated using the secondary bile acid lithocholic acid (LCA). On the CYP24 promoter LCA-induced transactivation was similar for both isoforms in COS1, whereas in Caco-2 and HEK293 cells VDRB1 was less active. On the CYP3A4 promoter, LCA activation of VDRB1 was comparable to VDRA in all the cell lines tested. Mutational analysis indicated that both the 1,25(OH)(2)D(3) and LCA-regulated activities of both VDR isoforms required a functional ligand-dependent activation function (AF-2) domain. In gel shift assays VDR:DNA complex formation was stronger in the presence of 1,25(OH)(2)D(3) than with LCA. These results indicate that regulation of VDRB1 transactivation activity is dependent on cellular context, promoter, and the nature of the ligand.