Up-regulation of pro-apoptotic protein Bim and down-regulation of anti-apoptotic protein Mcl-1 cooperatively mediate enhanced tumor cell death induced by the combination of ERK kinase (MEK) inhibitor and microtubule inhibitor

Blockade of the ERK signaling pathway by ERK kinase (MEK) inhibitors selectively enhances the induction of apoptosis by microtubule inhibitors in tumor cells in which this pathway is constitutively activated. We examined the mechanism by which such drug combinations induce enhanced cell death by applying time-lapse microscopy to track the fate of individual cells. MEK inhibitors did not affect the first mitosis after drug exposure, but most cells remained arrested in interphase without entering a second mitosis. Low concentrations of microtubule inhibitors induced prolonged mitotic arrest followed by exit of cells from mitosis without division, with most cells remaining viable. However, the combination of a MEK inhibitor and a microtubule inhibitor induced massive cell death during prolonged mitosis. Impairment of spindle assembly checkpoint function by RNAi-mediated depletion of Mad2 or BubR1 markedly suppressed such prolonged mitotic arrest and cell death. The cell death was accompanied by up-regulation of the pro-apoptotic protein Bim (to which MEK inhibitors contributed) and by down-regulation of the anti-apoptotic protein Mcl-1 (to which microtubule and MEK inhibitors contributed synergistically). Whereas RNAi-mediated knockdown of Bim suppressed cell death, stabilization of Mcl-1 by RNAi-mediated depletion of Mule slowed its onset. Depletion of Mcl-1 sensitized tumor cells to MEK inhibitor-induced cell death, an effect that was antagonized by knockdown of Bim. The combination of MEK and microtubule inhibitors thus targets Bim and Mcl-1 in a cooperative manner to induce massive cell death in tumor cells with aberrant ERK pathway activation.     

Natural diterpenoid compound elevates expression of Bim protein, which interacts with antiapoptotic protein Bcl-2, converting it to proapoptotic Bax-like molecule

Overwhelming evidence indicates that Bax and Bak are indispensable for mediating cytochrome c release from mitochondria during apoptosis. Here we report a Bax/Bak-independent mechanism of cytochrome c release and apoptosis. We identified a natural diterpenoid compound that induced apoptosis in bax/bak double knock-out murine embryonic fibroblasts and substantially reduced the tumor growth from these cells implanted in mice. Treatment with the compound significantly increased expression of Bim, which migrated to mitochondria, altering the conformation of and forming oligomers with resident Bcl-2 to induce cytochrome c release and caspase activation. Importantly, purified Bim and Bcl-2 proteins cooperated to permeabilize a model mitochondrial outer membrane; this was accompanied by oligomerization of these proteins and deep embedding of Bcl-2 in the membrane. Therefore, the diterpenoid compound induces a structural and functional conversion of Bcl-2 through Bim to permeabilize the mitochondrial outer membrane, thereby inducing apoptosis independently of Bax and Bak. Because Bcl-2 family proteins play important roles in cancer development and relapse, this novel cell death mechanism can be explored for developing more effective anticancer therapeutics.     

The Bcl-2 Homology Domain 3 (BH3)-only Proteins Bim and Bid Are Functionally Active and Restrained by Anti-apoptotic Bcl-2 Family Proteins in Healthy Liver

An intrinsic pathway of apoptosis is regulated by the B-cell lymphoma-2 (Bcl-2) family proteins. We previously reported that a fine rheostatic balance between the anti- and pro-apoptotic multidomain Bcl-2 family proteins controls hepatocyte apoptosis in the healthy liver. The Bcl-2 homology domain 3 (BH3)-only proteins set this rheostatic balance toward apoptosis upon activation in the diseased liver. However, their involvement in healthy Bcl-2 rheostasis remains unknown. In the present study, we focused on two BH3-only proteins, Bim and Bid, and we clarified the Bcl-2 network that governs hepatocyte life and death in the healthy liver. We generated hepatocyte-specific Bcl-xL- or Mcl-1-knock-out mice, with or without disrupting Bim and/or Bid, and we examined hepatocyte apoptosis under physiological conditions. We also examined the effect of both Bid and Bim disruption on the hepatocyte apoptosis caused by the inhibition of Bcl-xL and Mcl-1. Spontaneous hepatocyte apoptosis in Bcl-xL- or Mcl-1-knock-out mice was significantly ameliorated by Bim deletion. The disruption of both Bim and Bid completely prevented hepatocyte apoptosis in Bcl-xL-knock-out mice and weakened massive hepatocyte apoptosis via the additional in vivo knockdown of mcl-1 in these mice. Finally, the hepatocyte apoptosis caused by ABT-737, which is a Bcl-xL/Bcl-2/Bcl-w inhibitor, was completely prevented in Bim/Bid double knock-out mice. The BH3-only proteins Bim and Bid are functionally active but are restrained by the anti-apoptotic Bcl-2 family proteins under physiological conditions. Hepatocyte integrity is maintained by the dynamic and well orchestrated Bcl-2 network in the healthy liver.     

Proapoptotic Activities of Protein Disulfide Isomerase (PDI) and PDIA3 Protein,a Role of the Bcl-2 Protein Bak

Protein disulfide isomerase (PDI) family proteins are classified as enzymatic chaperones for reconstructing misfolded proteins. Previous studies have shown that several PDI members possess potential proapoptotic functions. However, the detailed molecular mechanisms of PDI-mediated apoptosis are not completely known. In this study, we investigated how two members of PDI family, PDI and PDIA3, modulate apoptotic signaling. Inhibiting PDI and PDIA3 activities pharmacologically alleviates apoptosis induced by various apoptotic stimuli. Although a decrease of PDIA3 expression alleviates apoptotic responses, overexpression of PDIA3 exacerbates apoptotic signaling. Importantly, Bak, but not Bax, is essential for PDIA3-induced proapoptotic signaling. Furthermore, both purified PDI and PDIA3 proteins induce Bak-dependent, but not Bax-dependent, mitochondrial outer membrane permeabilization in vitro, probably through triggering Bak oligomerization on mitochondria. Our results suggest that both of PDI and PDIA3 possess Bak-dependent proapoptotic function through inducing mitochondrial outer membrane permeabilization, which provides a new mechanism linking ER chaperone proteins and apoptotic signaling.     

E2F1 inhibits MDM2 expression in a p53-dependent manner

MDM2 expression is down-regulated upon E2F1 over-expression, but the mechanism is not well defined. In the current study, we found that E2F1 inhibits MDM2 expression by suppressing its promoter activity. Although E2F1 binds to the MDM2 promoter, the inhibitory effect of E2F1 on the MDM2 promoter does not require the direct binding. We demonstrate that E2F1 inhibits MDM2 promoter activity in a p53-dependent manner. Knockdown of p53 in U2OS cells impairs the inhibitory effect of E2F1 on the MDM2 promoter. Consistent with this observation, E2F1 does not inhibit MDM2 promoter activity in p53-deficient H1299 cells, and the inhibition is restored when p53 is expressed exogenously. Both E2F1 and p53 are up-regulated after DNA damage stimulation. We show that such stimulation induces E2F1 to inhibit MDM2 promoter activity and promote p53 accumulation. Furthermore, inhibition of MDM2 by E2F1 promotes E2F1 induced apoptosis. These data suggest that E2F1 regulates the MDM2-p53 pathway by inhibiting p53 induced up-regulation of MDM2.     

转录因子E2F1蛋白纯化及甲基化鉴定

目的:构建人E2F1基因原核表达质粒p GEX-KG-E2F1,并在大肠杆菌中诱导表达。随后验证纯化得到的E2F1蛋白可作为底物被甲基化转移酶修饰。方法:构建原核表达质粒p GEX-KG-E2F1,在大肠杆菌BL-21中经异丙基硫代半乳糖苷(IPTG)诱导表达,利用GST亲和层析法纯化表达的E2F1蛋白。随后将纯化的E2F1蛋白作为底物,组蛋白甲基化转移酶SET7/9作为酶进行体外同位素标记放射自显影实验,检测纯化的E2F1蛋白能否被甲基化。结果:酶切鉴定和测序结果证明成功构建了原核表达载体p GEX-KG-E2F1,SDS-PAGE检测结果证明实现了人E2F1基因在大肠杆菌中的可溶性表达,放射自显影证明纯化得到的E2F1蛋白可作为底物被甲基化转移酶SET7/9甲基化。结论:成功构建了转录因子E2F1体外甲基化体系,为筛选新的能甲基化E2F1的酶奠定基础。     

Inhibition of Protein Phosphatase 2A (PP2A) Prevents Mcl-1 Protein Dephosphorylation at the Thr-163/Ser-159 Phosphodegron,Dramatically Reducing Expression in Mcl-1-amplified Lymphoma Cells

Abundant, sustained expression of prosurvival Mcl-1 is an important determinant of viability and drug resistance in cancer cells. The Mcl-1 protein contains PEST sequences (enriched in proline, glutamic acid, serine, and threonine) and is normally subject to rapid turnover via multiple different pathways. One of these pathways involves a phosphodegron in the PEST region, where Thr-163 phosphorylation primes for Ser-159 phosphorylation by glycogen synthase kinase-3. Turnover via this phosphodegron-targeted pathway is reduced in Mcl-1-overexpressing BL41-3 Burkitt lymphoma and other cancer cells; turnover is further slowed in the presence of phorbol ester-induced ERK activation, resulting in Mcl-1 stabilization and an exacerbation of chemoresistance. The present studies focused on Mcl-1 dephosphorylation, which was also found to profoundly influence turnover. Exposure of BL41-3 cells to an inhibitor of protein phosphatase 2A (PP2A), okadaic acid, resulted in a rapid increase in phosphorylation at Thr-163 and Ser-159, along with a precipitous decrease in Mcl-1 expression. The decline in Mcl-1 expression preceded the appearance of cell death markers and was not slowed in the presence of phorbol ester. Upon exposure to calyculin A, which also potently inhibits PP2A, versus tautomycin, which does not, only the former increased Thr-163/Ser-159 phosphorylation and decreased Mcl-1 expression. Mcl-1 co-immunoprecipitated with PP2A upon transfection into CHO cells, and PP2A/Aα knockdown recapitulated the increase in Mcl-1 phosphorylation and decrease in expression. In sum, inhibition of PP2A prevents Mcl-1 dephosphorylation and results in rapid loss of this prosurvival protein in chemoresistant cancer cells.     

Death Receptor 6 Induces Apoptosis Not through Type I or Type II Pathways, but via a Unique Mitochondria-dependent Pathway by Interacting with Bax Protein

Cells undergo apoptosis through two major pathways, the extrinsic pathway (death receptor pathway) and the intrinsic pathway (the mitochondrial pathway). These two pathways can be linked by caspase-8-activated truncated Bid formation. Very recently, death receptor 6 (DR6) was shown to be involved in the neurodegeneration observed in Alzheimer disease. DR6, also known as TNFRSF21, is a relatively new member of the death receptor family, and it was found that DR6 induces apoptosis when it is overexpressed. However, how the death signal mediated by DR6 is transduced intracellularly is not known. To this end, we have examined the roles of caspases, apoptogenic mitochondrial factor cytochrome c, and the Bcl-2 family proteins in DR6-induced apoptosis. Our data demonstrated that Bax translocation is absolutely required for DR6-induced apoptosis. On the other hand, inhibition of caspase-8 and knockdown of Bid have no effect on DR6-induced apoptosis. Our results strongly suggest that DR6-induced apoptosis occurs through a new pathway that is different from the type I and type II pathways through interacting with Bax.