Cloning and functional analysis of BAG-1: a novel Bcl-2-binding protein with anti-cell death activity

BAG-1: a novel biomarker predicting long-term survival in early-stage breast cancer

PURPOSE

Among women with early-stage breast cancer treated with lumpectomy and radiation therapy, 30% to 40% will develop metastatic disease, which is often fatal. A need exists therefore for biomarkers that distinguish patients at high risk of relapse. We performed a retrospective correlative analysis of BAG-1 protein expression in breast tumors derived from a cohort of early-stage breast cancer patients.

PATIENTS AND METHODS

Archival paraffin blocks from 122 women with stages I to II breast cancer treated with lumpectomy and radiation therapy (median follow-up, 12.1 years) were analyzed by immunohistochemical methods using monoclonal antibodies recognizing BAG-1 and other biomarkers, including Bcl-2, estrogen receptor, progesterone receptor, p53, and HER2/Neu. Immunostaining data were correlated with distant metastasis-free survival (DMFS) and overall survival (OS).

RESULTS

Cytosolic immunostaining for BAG-1 was upregulated in 79 (65%) of 122 invasive breast cancers (P <.001) compared with normal breast. Elevated BAG-1 was significantly associated with longer DMFS and OS, overall (stages 1 and II) and in node-negative (stage I only) patients, on the basis of univariate and multivariate analyses (DMFS, P =.005; OS, P =.01, in multivariate analysis of all patients; DMFS, P =.005; OS, P =.001, in multivariate analysis of node-negative patients). All other biomarkers failed to reach statistical significance in multivariate analysis. Clinical stage was an independent predictor of OS (P =.04) and DMFS (P =.02).

CONCLUSION

These findings provide preliminary evidence that BAG-1 represents a potential marker of improved survival in early-stage breast cancer patients, independent of the status of axillary lymph nodes.

Serial analysis of gene expression in differentiated cultures of human epidermal keratinocytes

Keratinocyte gene expression was surveyed more comprehensively than before, by means of serial analysis of gene expression. A total of 25,694 tags derived from expressed mRNA, were analyzed in a model for normal differentiation and in a model where cultured keratinocytes were stimulated for a prolonged period of time with tumor necrosis factor-alpha, thus mimicking aberrant differentiation in the context of cutaneous inflammation. Serial analysis of gene expression revealed many transcripts derived from unknown genes and a large number of genes that are not known to be expressed in keratinocytes; furthermore, these data provide quantitative information about the relative abundance of transcripts, allowing the identification of differentially expressed genes. A major part of the identified transcripts accounted for genes involved in energy metabolism and protein synthesis. A large proportion of all transcripts (6%) corresponded to genes associated with terminal differentiation and barrier formation. Another highly expressed functional group of genes (2% of all transcripts) corresponded to proteins involved in host protection such as antimicrobial proteins and proteinase inhibitors. Three of these genes were not known to be expressed in keratinocytes, and some were upregulated after prolonged tumor necrosis factor-alpha exposure. Our data on expressed genes in keratinocytes are consistent with the known function of human epidermis, and provide a first step to generate a transcriptome of human keratinocytes.

Inducible resistance to Fas-mediated apoptosis in B cells

Apoptosis produced in B cells through Fas (APO-1, CD95) triggering is regulated by signals derived from other surface receptors: CD40 engagement produces upregulation of Fas expression and marked susceptibility to Fas-induced cell death, whereas antigen receptor engagement, or IL-4R engagement, inhibits Fas killing and in so doing induces a state of Fas-resistance, even in otherwise sensitive, CD40-stimulated targets. Surface immunoglobulin and IL-4R utilize at least partially distinct pathways to produce Fas-resistance that differentially depend on PKC and STAT6, respectively. Further, surface immunoglobulin signaling for inducible Fas-resistance bypasses Btk, requires NF-kappaB, and entails new macromolecular synthesis. Terminal effectors of B cell Fas-resistance include the known anti-apoptotic gene products, Bcl-xL and FLIP, and a novel anti-apoptotic gene that encodes FAIM (Fas Apoptosis Inhibitory Molecule). faim was identified by differential display and exists in two alternatively spliced forms; faim-S is broadly expressed, but faim-L expression is tissue-specific. The FAIM sequence is highly evolu- tionarily conserved, suggesting an important role for this molecule throughout phylogeny. Inducible resistance to Fas killing is hypothesized to protect foreign antigen-specific B cells during potentially hazardous interactions with FasL-bearing T cells, whereas autoreactive B cells fail to become Fas-resistant and are deleted via Fas-dependent cytotoxicity. Inadvertent or aberrant acquisition of Fas-resistance may permit autoreactive B cells to escape Fas deletion, and malignant lymphocytes to impede anti-tumor immunity.

Regulation of Bcl-2-family proteins in myeloma cells by three myeloma survival factors: interleukin-6, interferon-alpha and insulin-like growth factor 1

As survival regulation is a key process in multiple myeloma biology, we have studied the Bcl-2 family proteins that can be regulated by three myeloma cell survival factors: interleukin-6 (IL-6), interferon-alpha (IFN-alpha) and insulin-like growth factor (IGF-1). Eleven myeloma cell lines, whose survival and proliferation are dependent on addition of IL-6, variably expressed 10 anti-apoptotic or pro-apoptotic proteins of the Bcl-2-family. When myeloma cells from four cell lines were IL-6 starved and activated with IL-6 or IFN-alpha, we observed that only Mcl-1 expression was up-regulated with myeloma cell survival induction. Nor was obvious regulation of these 10 pro-apoptotic or anti-apoptotic proteins found with IGF-1, another potent myeloma cell survival factor. Our results indicate that the myeloma cell survival activity of IL-6 linked to Bcl-xL regulation cannot be generalized and emphasize that Mcl-1 is the main target of IL-6 and IFN-alpha stimulation. However, other changes in the activity of the Bcl-2 protein family or other apoptosis regulators must be identified to elucidate the IGF-1 action mechanism. Cell Death and Differentiation (2000) 7, 1244 - 1252.

Hsp70-RAP46 interaction in downregulation of DNA binding by glucocorticoid receptor

Receptor-associating protein 46 (RAP46) is a cochaperone that regulates the transactivation function of several steroid receptors. It is transported into the nucleus by a liganded glucocorticoid receptor where it downregulates DNA binding and transactivation by this receptor. The N- and C-termini of RAP46 are both implicated in its negative regulatory function. In metabolic labelling experiments, we have shown that the N-terminus of RAP46 is modified by phosphorylation, but this does not contribute to the downregulation of glucocorticoid receptor activity. However, deletion of a sequence that binds 70 kDa heat shock protein (Hsp70) and the constitutive isoform of Hsp70 (Hsc70) at the C-terminus of RAP46 abrogated its negative regulatory action. Surface plasmon resonance studies showed that RAP46 binds the glucocorticoid receptor only when it has interacted with Hsp70/Hsc70, and confocal immunofluorescence analyses revealed a nuclear transport of Hsp70/Hsc70 by the liganded receptor. Together these findings demonstrate an important contribution of Hsp70/Hsc70 in the binding of RAP46 to the glucocorticoid receptor and suggest a role for this molecular chaperone in the RAP46-mediated downregulation of glucocorticoid receptor activity.

The super anti-apoptotic factor Bcl-xFNK constructed by disturbing intramolecular polar interactions in rat Bcl-xL

A powerful artificial anti-apoptotic factor will be useful for medical applications of the future therapies for many diseases by prolonging survival of sick cells. For constructing it, we designed the super anti-apoptotic factor by disturbing three intramolecular polar interactions among alpha-helix structures of Bcl-x(L). The resultant mutant Bcl-x(L), named Bcl-xFNK, was expected to make the pore-forming domain more mobile and flexible than the wild-type. When overexpressed in Jurkat cells, Bcl-xFNK was markedly more potent in prolonging survival following apoptosis-inducing treatment with a kind of cell death cytokines (anti-Fas), a protein kinase inhibitor (staurosporine), cell cycle inhibitors (TN-16, camptothecin, hydroxyurea, and trichostatin A), or oxidative stress (hydrogen peroxide and paraquat) than wild-type Bcl-x(L). Furthermore, the transfectants of bcl-xFNK became more resistant against a calcium ionophore and even a heat treatment than wild-type Bcl-x(L). In addition, Bcl-xFNK showed marked anti-apoptotic activity in Chinese hamster ovary and Jurkat cells deprived of serum. Thus, Bcl-xFNK may be the first mutant generated by site-directed mutagenesis of Bcl-x(L) with a gain-of-function phenotype. Interestingly, Bcl-xFNK was found to allow interleukin-3-dependent FDC-P1 to grow without interleukin-3, but not BaF/3. In Bcl-xFNK transfectants of FDC-P1 and Jurkat, the p42/p44 mitogen-activated protein kinase was activated by 2 to 5 times, but not in those of BaF/3 and Chinese hamster ovary. Bcl-xFNK might gain a new function to activate the mitogen-activated protein kinase in a cell-type specific manner. The findings of this study suggest that the central alpha5-alpha6 pore-forming region of anti-apoptotic factor Bcl-x(L) has a pivotal role in suppressing apoptosis.

A novel protein, RTN-XS, interacts with both Bcl-XL and Bcl-2 on endoplasmic reticulum and reduces their anti-apoptotic activity

Bcl-2 and Bcl-XL serve as critical inhibitors of apoptosis triggered by a broad range of stimuli, mainly acting on the mitochondria. We identified two members of the reticulon (RTN) family as Bcl-XL binding proteins, i.e., NSP-C (RTN1-C) and a new family member, RTN-XS, both of which did not belong to the Bcl-2 family and were predominantly localized on the endoplasmic reticulum (ER). RTN-XS interacted with both Bcl-XL and Bcl-2, increased the localization of Bcl-XL and Bcl-2 on the ER, and reduced the anti-apoptotic activity of Bcl-XL and Bcl-2. On the other hand, NSP-C interacted only with Bcl-XL, affected the localization of Bcl-XL, and reduced Bcl-XL activity, but had no effect on Bcl-2. These results suggest that RTN family proteins can modulate the anti-apoptotic activity of Bcl-XL and Bcl-2 by binding with them and can change their localization to the ER.

Regulation of death receptor-mediated apoptosis pathways

Apoptosis or programmed cell death can be induced by a variety of stimuli including activation of death receptors. This subgroup of the TNF/NGF-receptor-superfamily activates caspases, a family of aspartyl-specific cysteine-proteases, which are the main executioners of apoptosis. Depending on the cell type, signalling pathways downstream of the death receptors can be modulated by different proteins such as Bcl-2, FLIPs, chaperones and kinases. Deregulation of apoptosis has been associated with diseases as cancer, autoimmunity and AIDS. Therefore, the identification of modulators of apoptosis has several therapeutic implications.

Ribavirin-induced resistance to heat shock, inhibition of the Ras-Raf-1 pathway and arrest in G(1)

Ribavirin [1-(beta-D-ribofuranosyl)1,2,4-triazole-3-carboxamide (virazole)], a specific inhibitor of inositide 5'-monophosphate dehydrogenase (IMPDH), induces a strong depletion of GTP pools in IGR39 cells. After a 3-day treatment, the cell cycle was reversibly arrested in G(0)/G(1), suggesting the involvement of GTP in the cell cycle process. The reduction of the GTP cell content modified the appearance of the microtubule network, as examined using immunofluorescence. However, the dynamics of repolymerisation were not altered. When arrested in G(0)/G(1), cells displayed a surprising resistance to a 3-h period of heat shock at 45 degrees C. Considering the lack of coimmunoprecipitation of p21ras with Raf-1, the reduction of the level of GTP-associated p21ras and the decrease of the activation of the extracellular signal-regulated protein kinases (ERK), also known as mitogen-activated protein (MAP) kinase, in ribavirin-treated cells, we suggest a possible relationship between the expression of heat-shock proteins and the change, in GTP-depleted cells, of the regulation of Raf kinase by ras protein.

CAIR-1/BAG-3 forms an EGF-regulated ternary complex with phospholipase C-gamma and Hsp70/Hsc70

CAIR-1/BAG-3 forms an EGF-regulated ternary complex with Hsp70/Hsc70 and latent phospholipase C-gamma (PLC-gamma). The expression of CAIR-1, CAI stressed-1, was induced in A2058 human melanoma cells by continuous exposure to CAI, an inhibitor of nonvoltage-gated calcium influx. CAIR-1 sequence is identical, save 2 amino acids, to BAG-3 also cloned recently as Bis, a member of the bcl-2-associated athanogene family. We show that CAIR-1/BAG-3 binds to Hsp70/Hsc70 in intact cells and this binding is increased by short term exposure to CAI (P<0.007). CAIR-1/BAG-3 is phosphorylated in vivo in the absence of stimulation. Basal phosphorylation is inhibited by treatment with d-erythrosphingosine (d-ES), a broad inhibitor of the protein kinase C family. CAIR-1/BAG-3 contains several PXXP SH3 binding domains leading to the hypothesis that it is a partner protein of phospholipase C-gamma. PLC-gamma is bound to CAIR-1/BAG-3 in unstimulated cells. It is increased by CAI or d-ES (P=0.05) treatment, and abrogated by EGF (r2=0.99); d-ES treatment blocks the EGF-mediated dissociation. We show that CAIR-1/BAG-3 binds to PLC-gamma and Hsp70/Hsc70 through separate and distinct domains. Hsp70/Hsc70 binds to the BAG domain of BAGs-1 and -3. CAIR-1/BAG-3 from control and EGF-treated cell lysates bound selectively to the SH3 domain of PLC-gamma, but not its N-SH2 or C-SH2 domains. Confirming the SH3 interaction, PLC-gamma was pulled down by CAIR-1/BAG-3 PXXP-GST fusions, but GST-PXXP constructs confronted with lysates from EGF-treated cells did not bind PLC-gamma as was seen in intact cells. Hsp70/Hsc70 was brought down by the PLC-gamma SH3 construct equally from native and EGF-treated cells, but did not bind the PXXP construct under either condition. We propose that CAIR-1/BAG-3 may act as a multifunctional signaling protein linking the Hsp70/Hsc70 pathway with those necessary for activation of the EGF receptor tyrosine kinase signaling pathways.

Hsp72-mediated suppression of c-Jun N-terminal kinase is implicated in development of tolerance to caspase-independent cell death

Pretreatment with mild heat shock is known to protect cells from severe stress (acquired thermotolerance). Here we addressed the mechanism of this phenomenon by using primary human fibroblasts. Severe heat shock (45 degrees C, 75 min) of the fibroblasts caused cell death displaying morphological characteristics of apoptosis; however, it was caspase independent. This cell death process was accompanied by strong activation of Akt, extracellular signal-regulated kinase 1 (ERK1) and ERK2, p38, and c-Jun N-terminal (JNK) kinases. Suppression of Akt or ERK1 and -2 kinases increased cell thermosensitivity. In contrast, suppression of stress kinase JNK rendered cells thermoresistant. Development of thermotolerance was not associated with Akt or ERK1 and -2 regulation, and inhibition of these kinases did not reduce acquired thermotolerance. On the other hand, acquired tolerance to severe heat shock was associated with downregulation of JNK. Using an antisense-RNA approach, we found that accumulation of the heat shock protein Hsp72 is necessary for JNK downregulation and is critical for thermotolerance. The capability of naive cells to withstand moderate heat treatment also appears to be dependent on the accumulation of Hsp72 induced by this stress. Indeed, exposure to 45 degrees C for 45 min caused only transient JNK activation and was nonlethal, while prevention of Hsp72 accumulation prolonged JNK activation and led to massive cell death. We also found that JNK activation by UV irradiation, interleukin-1, or tumor necrosis factor was suppressed in thermotolerant cells and that Hsp72 accumulation was responsible for this effect. Hsp72-mediated suppression of JNK is therefore critical for acquired thermotolerance and may play a role in tolerance to other stresses.

BAG-1 expression correlates highly with the malignant potential in early lesions (T1 and T2) of oral squamous cell carcinoma

BAG-1 is a Bcl-2-binding protein that functions as an anti-apoptotic molecule. In this report we show a possible correlation between BAG-1 expression levels and the probability of oral squamous cell carcinoma (SCC) progression. We investigated BAG-1 expression levels in 22 patients diagnosed with early lesions (T1 and T2) of oral SCCs using immunohistochemistry and western blotting. High steady-state levels of BAG-1 were detected in 13 out of 22 cases (59%). High BAG-1 expression was observed more frequently in cases with nodal metastasis (89%) than in those without nodal metastasis (38%) (P<0. 03), suggesting that BAG-1 expression levels may correlate with the pathological stage of oral SCCs. Furthermore, BAG-1 expression levels correlated with the WHO grade, i.e. 45% in grade-I cases as opposed to 72% in grade-II cases (P<0.02). These data suggest that an analysis of BAG-1 expression may be useful in establishing a prognosis for patients with oral SCCs, and especially in predicting the metastatic potential of SSCs.

The apoptotic pathway: a target for therapy in chronic lymphocytic leukemia

Cell division and apoptosis (programmed cell death) are the two major physiological processes which control the size of cell populations. Chronic lymphocytic leukaemia arises as a result of the clonal expansion of, usually B-, lymphocytes in which a dysregulation of apoptosis leads to prolonged cell survival. The same process becomes exaggerated with increasing drug resistance, the usual cause of treatment failure in this condition. The identification of points in the apoptotic pathway at which dysregulation occurs is beginning to open up new therapeutic opportunities where the conventional cytotoxic chemotherapy approach is found to fail. Although these strategies are still in their infancy they may increase understanding of the pathogenesis of the disorder and overcome the problem of drug resistance.

Expression of TRAIL (TNF-related apoptosis-inducing ligand) receptors in cervical cancer

Apoptosis is an intrinsic and fundamental biologic process that plays a critical role in the normal development of multicellular organisms and in the maintainance of tissue homeostasis. Some of the well known regulators of apoptosis are cytokines of the tumor necrosis factor (TNF) ligand family, such as Fas ligand (Fas L) and TNF, which induce apoptosis by activation of their corresponding receptors, Fas and TNFR-1. Recently, a new member of the TNF family known as TRAIL (TNF-related apoptosis-inducing ligand) was identified and shown to induce p53-independent apoptosis in a variety of tumor cell lines but not in normal cells. Four human receptors for TRAIL were also recently identified and designated TRAIL-R1, -R2, -R3, and -R4. The aim of this study is to examine whether TRAIL and TRAIL receptors (-R1, -R2, -R3) are expressed in uterine cervical cancer and whether it is correlated with apoptosis, TRAIL, and TRAIL receptors. The subjects were 20 patients who were diagnosed with cervical cancer. Western blotting was performed in nine cases and immunohistochemical staining for TRAIL and TRAIL receptors (-R1, -R2, -R3) and TUNEL method for detection of apoptosis was performed in 11 cases. There were proteins for TRAIL, TRAIL-R1, -R2, and -R3 in tissues from cervical cancer. All TRAIL receptors were expressed in both normal cervical epithelium and tumor cells, and TRAIL-R1 and -R2 were more strongly expressed in tumor cells than normal epithelium (P < 0.05). Apoptosis correlated with expression of TRAIL-R1 and -R2 (P < 0.05). This study suggests that TRAIL induces apoptosis in cervical cancer through its receptors.

Histone deacetylase inhibitors suppress IL-2–mediated gene expression prior to induction of apoptosis

Histone deacetylase (HDAC) inhibitors can induce transcriptional activation of a number of genes and induce cellular differentiation as histone acetylation levels increase. Although these inhibitors induce apoptosis in several cell lines, the precise mechanism by which they do so remains obscure. This study shows that HDAC inhibitors, sodium butyrate and trichostatin A (TSA), abrogate interleukin (IL)-2–mediated gene expression in IL-2–dependent cells. The HDAC inhibitors readily induced apoptosis in IL-2–dependent ILT-Mat cells and BAF-B03 transfectants expressing the IL-2 receptor βc chain, whereas they induced far less apoptosis in cytokine-independent K562 cells. However, these inhibitors similarly increased acetylation levels of histones in both cells. Although histone hyperacetylation is believed to lead to transcriptional activation, the results showed an abrogation of IL-2–mediated induction of c-myc,bag-1, and LC-PTP gene expression. This observed abrogation of gene expression occurred prior to phosphatidylserine externalization, a process that occurs in early apoptotic cells. Considering the biologic role played by IL-2–mediated gene expression in cell survival, these data suggest that its abrogation may contribute to the apoptotic process induced by HDAC inhibitors.

Histone deacetylase inhibitors suppress IL-2–mediated gene expression prior to induction of apoptosis

Histone deacetylase (HDAC) inhibitors can induce transcriptional activation of a number of genes and induce cellular differentiation as histone acetylation levels increase. Although these inhibitors induce apoptosis in several cell lines, the precise mechanism by which they do so remains obscure. This study shows that HDAC inhibitors, sodium butyrate and trichostatin A (TSA), abrogate interleukin (IL)-2–mediated gene expression in IL-2–dependent cells. The HDAC inhibitors readily induced apoptosis in IL-2–dependent ILT-Mat cells and BAF-B03 transfectants expressing the IL-2 receptor βc chain, whereas they induced far less apoptosis in cytokine-independent K562 cells. However, these inhibitors similarly increased acetylation levels of histones in both cells. Although histone hyperacetylation is believed to lead to transcriptional activation, the results showed an abrogation of IL-2–mediated induction of c-myc,bag-1, and LC-PTP gene expression. This observed abrogation of gene expression occurred prior to phosphatidylserine externalization, a process that occurs in early apoptotic cells. Considering the biologic role played by IL-2–mediated gene expression in cell survival, these data suggest that its abrogation may contribute to the apoptotic process induced by HDAC inhibitors.

The hepatocyte growth factor/Met pathway in development, tumorigenesis, and B-cell differentiation

This article summarizes the structure, signal transduction and physiologic functions of the HGF/Met pathway, as well as its role in tumor growth, invasion, and metastasis. Moreover, it highlights recent studies indicating a role for the HGF/Met pathway in antigen-specific B-cell development and B-cell neoplasia.

Protein kinase inhibitors flavopiridol and 7-hydroxy-staurosporine down-regulate antiapoptosis proteins in B-cell chronic lymphocytic leukemia

Compounds that inhibit protein kinases are currently undergoing clinical evaluation for the treatment of a variety of malignancies. The kinase inhibitors flavopiridol and 7 hydroxy-staurosporine (UCN-01) were examined for their effects on B-cell chronic lymphocytic leukemia (B-CLL) cells in vitro (n = 49). Flavopiridol and UCN-01 induced concentration-dependent apoptosis of most B-CLL samples tested, with greater than 50% cell killing occurring at concentrations of less than 1 μmol/L, and with flavopiridol displaying more potent activity than UCN-01. Flavopiridol (0.1 μmol/L) and UCN-01 (1 μmol/L) also induced striking decreases in the levels of the antiapoptosis proteins Mcl-1, X-linked inhibitor of apoptosis (XIAP), and BAG-1 in nearly all cases of B-CLL and of Bcl-2 in approximately half of B-CLL specimens evaluated. In contrast, expression of the proapoptotic proteins Bax and Bak was not significantly influenced by these kinase inhibitors. Flavopiridol-induced decreases in the levels of antiapoptosis proteins Mcl-1 and XIAP preceded apoptosis and were not substantially affected by the addition of caspase inhibitors to cultures. In contrast, UCN-01–stimulated decreases in antiapoptosis proteins were slower, occurred concurrently with apoptosis, and were partially prevented by caspase inhibitors. The findings suggest that flavopiridol and UCN-01 induce apoptosis of B-CLL cells through different mechanisms. The potent apoptotic activities of flavopiridol and UCN-01 against cultured B-CLL cells suggest that they may be effective as single agents in the treatment of B-CLL or for sensitizing B-CLL cells to conventional cytotoxic drugs.

Protein kinase inhibitors flavopiridol and 7-hydroxy-staurosporine down-regulate antiapoptosis proteins in B-cell chronic lymphocytic leukemia

Compounds that inhibit protein kinases are currently undergoing clinical evaluation for the treatment of a variety of malignancies. The kinase inhibitors flavopiridol and 7 hydroxy-staurosporine (UCN-01) were examined for their effects on B-cell chronic lymphocytic leukemia (B-CLL) cells in vitro (n = 49). Flavopiridol and UCN-01 induced concentration-dependent apoptosis of most B-CLL samples tested, with greater than 50% cell killing occurring at concentrations of less than 1 μmol/L, and with flavopiridol displaying more potent activity than UCN-01. Flavopiridol (0.1 μmol/L) and UCN-01 (1 μmol/L) also induced striking decreases in the levels of the antiapoptosis proteins Mcl-1, X-linked inhibitor of apoptosis (XIAP), and BAG-1 in nearly all cases of B-CLL and of Bcl-2 in approximately half of B-CLL specimens evaluated. In contrast, expression of the proapoptotic proteins Bax and Bak was not significantly influenced by these kinase inhibitors. Flavopiridol-induced decreases in the levels of antiapoptosis proteins Mcl-1 and XIAP preceded apoptosis and were not substantially affected by the addition of caspase inhibitors to cultures. In contrast, UCN-01–stimulated decreases in antiapoptosis proteins were slower, occurred concurrently with apoptosis, and were partially prevented by caspase inhibitors. The findings suggest that flavopiridol and UCN-01 induce apoptosis of B-CLL cells through different mechanisms. The potent apoptotic activities of flavopiridol and UCN-01 against cultured B-CLL cells suggest that they may be effective as single agents in the treatment of B-CLL or for sensitizing B-CLL cells to conventional cytotoxic drugs.

Protooncogenes as mediators of apoptosis

Apoptosis has been well established as a vital biological phenomenon that is important in the maintenance of cellular homeostasis. Three major protooncogene families and their encoded proteins function as mediators of apoptosis in various cell types and are the subject of this chapter. Protooncogenic proteins such as c-Myc/Max, c-Fos/c-Jun, and Bcl-2/Bax utilize a synergetic effect to enhance their roles in the pro- or antiapoptotic action. These family members activate and repress the expression of their target genes, control cell cycle progression, and execute programmed cell death. Repression or overproduction of these protooncogenic proteins induces apoptosis, which may vary as a result of either cell type specificity or the nature of the apoptotic stimuli. The proapoptotic and antiapoptotic proteins exert their effects in the membrane of cellular organelles. Here they generate cell-type-specific signals that activate the caspase family of proteases and their regulators for the execution of apoptosis.

Regulatory cells potentiate the efficacy of IL-4 gene transfer by up-regulating Th2-dependent expression of protective molecules in the infectious tolerance pathway in transplant recipients

We have previously shown that the tolerant state in allograft recipients can be maintained and perpetuated by an "infectious" T cell-dependent regulatory mechanism. Hence, 1) treatment of LEW rats with RIB-5/2, a CD4 nondepleting mAb, produces indefinite survival of LBNF1 cardiac allografts; 2) donor-specific tolerance can be then transferred by spleen cells into new cohorts of test allograft recipients; and 3) putative regulatory CD4+ Th2-like cells are instrumental in this tolerance model. We now report on studies aimed at exposing mechanisms underlying the infectious tolerance pathway, with emphasis on the interactions between intragraft adenovirus-IL-4 gene transfer and systemic infusion of regulatory cells from tolerant hosts. Unlike individual treatment regimens, adjunctive therapy with adenovirus-IL-4 and suboptimal doses of regulatory spleen cells was strongly synergistic and extended donor-type test cardiac allograft survival to about 2 mo. RT-PCR-based expression of intragraft mRNA coding for IL-2 and IFN-gamma remained depressed, whereas that of IL-4 and IL-10 reciprocally increased selectively in the combined treatment group, data supported by ELISA studies. In parallel, only adjunctive treatment triggered intragraft induction of molecules with anti-oxidant (HO-1) and anti-apoptotic (Bcl-xL/Bag-1) but not with pro-apoptotic (CPP-32) functions, both in the early and late posttransplant phases. Hence, systemic infusion of regulatory cells potentiates the effects of local adenovirus-IL-4 gene transfer in transplant recipients. Th2-driven up-regulation of protective molecule programs at the graft site, such as of anti-oxidant HO-1 and/or anti-apoptotic Bcl-xL and Bag-1, may contribute, at least in part, to the maintenance of the infectious tolerance pathway in transplant recipients.

Distinct isoforms of the cofactor BAG-1 differentially affect Hsc70 chaperone function

In the mammalian cytosol and nucleus the activity of the molecular chaperone Hsc70 is regulated by chaperone cofactors that modulate ATP binding and hydrolysis by Hsc70. Among such cofactors is the anti-apoptotic protein BAG-1. Remarkably, BAG-1 is expressed as multiple isoforms, which are distinguished by their amino termini. We investigated whether distinct isoforms differ with respect to their Hsc70-regulating activity. By comparing the mainly cytosolic isoforms BAG-1M and BAG-1S, opposite effects of the two isoforms were observed in chaperone-assisted folding reactions. Whereas BAG-1M was found to inhibit the Hsc70-mediated refolding of nonnative polypeptide substrates, the BAG-1S isoform stimulated Hsc70 chaperone activity. The opposite effects are not due to differences in the regulation of the ATPase activity of Hsc70 by the two isoforms. Both isoforms stimulated ATP hydrolysis by Hsc70 in an Hsp40-dependent manner through an acceleration of ADP-ATP exchange. Our results reveal that the different amino termini of the distinct BAG-1 isoforms determine the outcome of an Hsc70-mediated folding event, most likely by transiently interacting with the polypeptide substrate. Employing isoforms of a cofactor with different substrate binding properties appears to provide the means to influence the chaperone function of Hsc70 in addition to modulating its ATPase cycle.

BAG-1 promotes apoptosis induced by N-(4-hydroxyphenyl)retinamide in human cervical carcinoma cells

N-(4-hydroxyphenyl)retinamide (4-HPR) is a synthetic apoptosis-inducing retinoid with cancer chemopreventive properties and lower toxicity than all-trans retinoic acid. BAG-1 is an antiapoptotic gene that is overexpressed in cervical and other cancers. In this study, we examined whether BAG-1 can inhibit 4-HPR-induced apoptosis in the C33A cervical carcinoma cell line. Surprisingly, although it inhibited apoptosis induced by five different apoptotic stimuli, overexpression of BAG-1 enhanced apoptosis induced by 4-HPR, producing a 2.5-fold lower IC(50) of 4-HPR. The effects of BAG-1 on 4-HPR-induced apoptosis were mediated by enhancing the caspase-3 activation pathway. Deletion mutation experiments showed that the central ubiquitin homology domain of BAG-1 protein was necessary for its promotion of 4-HPR-induced apoptosis, whereas its C-terminal Hsp70/Hsc70-interacting domain was required for its inhibition of staurosporine-induced apoptosis. These in vitro results suggest that the effectiveness of 4-HPR against the development of malignancy may be due to the overexpression of BAG-1 in cancer cells.

Fas ligand gene transfer prolongs rat renal allograft survival and down-regulates anti-apoptotic Bag-1 in parallel with enhanced Th2-type cytokine expression

BACKGROUND

Fas ligand (FasL) induces apoptosis of cells bearing Fas receptor, and may play a role in the acquisition of immune privilege. We have previously shown that adenovirus (Ad)-mediated FasL gene transfer significantly prolongs survival in a strongly major histocompatibility complex-incompatible rat kidney allograft model. This study analyzes putative mechanisms of FasL-mediated effects, with particular emphasis on Th1 and Th2 immune activation and Bag-1 expression, a Bcl-2-binding anti-apoptotic protein.

METHODS

Kidney transplants were performed in Wistar-Furth to Lewis rat combination. Donor kidneys were perfused in situ with Ad-FasL or Ad-beta-Gal, and then transplanted. Kidney allografts were harvested at days 2, 7, and 56 and were evaluated by hematoxylin and eosin and immunohistochemical staining. The expression of FasL, Bag-1, and Th1/Th2 cytokine genes was assessed by Northern blots, Western blots, and competitive template reverse-transcriptase polymerase chain reaction, respectively.

RESULTS

Intragraft expression of FasL was enhanced, whereas that of anti-apoptotic Bag-1 gene was diminished throughout, in Ad-FasL-transduced well-functioning renal allografts, compared with Ad-beta-Gal-treated rejecting controls. In parallel, the expression of mRNA coding for IL-2 and IFN-gamma remained depressed, whereas that of IL-4 and IL-10 reciprocally and progressively increased in the Ad-FasL animal group.

CONCLUSIONS

Prolonged survival in Ad-FasL-transduced rat renal allograft model correlates with down-regulation of Bag-1, a novel anti-apoptotic gene, and preferential Th2-type cytokine elaboration profile at the graft site. Because Th1-like cells are sensitive to FasL-mediated cytotoxic effects, T-cell apoptosis may preferentially spare Th2-like cells, with resultant prolonged graft survival.

The Drosophila tumor necrosis factor receptor-associated factor-1 (DTRAF1) interacts with Pelle and regulates NFkappaB activity

A member of the tumor necrosis factor (TNF) receptor-associated factor (TRAF) family was identified in Drosophila. DTRAF1 contains 7 zinc finger domains followed by a TRAF domain, similar to mammalian TRAFs and other members of the family identified in data bases from Caenorhabditis elegans, Arabidopsis, and Dictyostelium. Analysis of DTRAF1 binding to different members of the human TNF receptor family showed that this protein can interact through its TRAF domain with the p75 neurotrophin receptor and weakly with the lymphotoxin-beta receptor. DTRAF1 can also self-associate and binds to human TRAF1, TRAF2, and TRAF4. Interestingly, DTRAF1 interacts with human cIAP-1 and cIAP-2 but not with Drosophila DIAP-1 and -2. By itself, DTRAF1 did not induce significant NFkappaB activation when overexpressed in mammalian cells, although it specifically increased NFkappaB induction by TRAF6. In contrast, TRAF2-mediated NFkappaB induction was partially inhibited by DTRAF1. Mutants of DTRAF1 lacking the N-terminal region inhibited NFkappaB induction by either TRAF2 or TRAF6. DTRAF1 specifically associated with the regulatory N-terminal domain of Pelle, a Drosophila homolog of the human kinase interleukin-1 receptor-associated kinase (IRAK). Interestingly, though Pelle and DTRAF1 individually were unable to induce NFkappaB in a human cell line, co-expression of Pelle and DTRAF1 resulted in significant NFkappaB activity. Interactions of DTRAF1 with human TRAF-, TNF receptor-, and IAP-family proteins imply strong evolutionary conservation of TRAF protein structure and function throughout Metazoan evolution.

Resistance to apoptosis is correlated with the reduced caspase-3 activation and enhanced expression of antiapoptotic proteins in human cervical multidrug-resistant cells

Recent studies have indicated that induction of apoptosis is the primary cytotoxic mechanism of most cancer chemotherapeutic agents, and abnormalities in the control of apoptosis can affect the sensitivity of malignant cells to multiple drugs. Here, we treated cells with cisplatin and other apoptotic stimuli and found that multidrug-resistant (MDR) endocervical HEN-16-2/CDDP cells, compared with drug-sensitive parental cells, were significantly more resistant to apoptosis and exhibited decreased proteolytic activation of caspase-3. The latter was further demonstrated by decreased cleavage of its substrate poly(ADP-ribose) polymerase (PARP). Further, Western blot analysis showed that MDR HEN-16-2/CDDP cells had significantly higher levels of the apoptosis-inhibiting proteins BAG-1 p50 and p33 isoforms and Bcl-X(L). This study provided the first evidence that overexpression of antiapoptotic BAG-1 p50 and p33 and Bcl-X(L) may cause resistance to apoptosis through reduction of caspase-3 activity in human cervical cells having an MDR phenotype.

Regions essential for the interaction between Bcl-2 and SMN, the spinal muscular atrophy disease gene product

The SMN gene is implicated in spinal muscular atrophy (SMA), and its product has been shown to interact with Bcl-2 protein to enhance its anti-apoptotic activity. In this study, we determined the regions that were essential for the interaction of Bcl-2 and SMN by co-immunoprecipitation of deletion mutants. Bcl-2 lacking its amino-terminal 20 amino acid residues or its carboxyl-terminal membrane-anchoring domain showed no or greatly reduced binding with SMN, respectively. However, Bcl-2 lacking other regions could still bind to SMN. Because Bcl-2 lacking the membrane-anchoring domain could bind to SMN in a yeast two-hybrid system, the amino-terminal region of Bcl-2 seems to be the most important domain for binding with SMN. A fragment of SMN encoded by exon 6 could bind to Bcl-2, but SMN lacking this region could not. From these results, we concluded that Bcl-2 and SMN proteins bound with each other at the amino-terminal region near the BH4 domain of Bcl-2 and the region encoded by exon 6 of SMN, both regions known to be important for their function.

BAG-1M: a potential specificity determinant of corticosteroid receptor action

BAG-1M is a eukaryotic cochaperone that associates with several proteins, including the glucocorticoid receptor (GR). It down-regulates GR-mediated transactivation by a mechanism that requires its prior recruitment by the liganded receptor from cytoplasm into the nucleus. In the nucleus, it uses a repeated sequence motif ([EEX4]8) at its NH2 terminus to inhibit DNA binding, as well as transactivation functions of the receptor. The mineralocorticoid receptor (MR), a structural and functional homologue of the GR, is unable to translocate BAG-1M into the nucleus, and its transactivation function is also not affected by this protein. This differential regulation of GR and MR activity could be relevant in classic mineralocorticoid tissues such as the kidney in which GR activity needs to be repressed to allow the MR to exert its action. In in situ hybridization studies, we show that BAG-1M is expressed in the kidney. Its expression pattern, especially in the developing kidney, correlated well with that of the GR. We therefore postulate that BAG-1M may be a specificity determinant in GR and MR action, and may feature prominently in the control of GR activity in kidney development.

Specific chaperone-like activity of inhibitor of caspase-activated DNase for caspase-activated DNase

Caspase-activated DNase (CAD) is the enzyme that causes DNA fragmentation during apoptosis. CAD forms aggregates when it is synthesized in the absence of an inhibitor of CAD (ICAD). Here, using renaturation systems of chemically denatured CAD, we report that ICAD-L, a long form of ICAD, has a chaperone-like activity specific for CAD. Murine CAD carries 14 cysteines, most of which were found to be in reduced form. Reducing agents enhanced the production of the functional CAD in an in vitro translation system. The denatured CAD could be efficiently renatured under highly reducing conditions only in the presence of ICAD-L. This process was ATP-independent. In contrast, reticulocyte lysates stimulated ICAD-L- and ATP-dependent renaturation of denatured CAD without requiring a high concentration of reducing agents. These results indicate that ICAD-L works not only as a specific inhibitor but also as a specific chaperone for CAD.

Inhibition of homodimerization of poly(ADP-ribose) polymerase by its C-terminal cleavage products produced during apoptosis

The biochemical role of the C-terminal fragment of poly(ADP-ribose) polymerase (PARP) was investigated in HeLa cells undergoing UV-mediated apoptosis. During the course of apoptosis, the C-terminal cleavage product of PARP interacted with intact PARP and down-regulated PARP activity by blocking the homodimerization of PARP. The basic leucine zipper motif in the auto-modification domain of the C-terminal fragment of PARP represented the site of association, and Leu(405) was critical to the ability of the basic leucine zipper motif to associate with intact PARP. The expression of the C-terminal fragment of PARP stimulated UV-mediated apoptosis. These results suggest that the C-terminal cleavage product of PARP produced during apoptosis blocks the homodimerization of PARP and inhibits the cellular PARP activity. The inhibition of the cellular PARP activity might prevent cellular NAD(+) depletion and stimulate apoptosis by maintaining the basal cellular energy level required for the completion of apoptosis.

The immunomodulatory role of CD4-positive cytotoxic T-lymphocytes in health and disease

Among the CD4-positive (CD4+) T-lymphocytes a population exists which expresses cytolytic activity. These 'killer' cells belong to the T helper type 1 (Th1) subset and if activated, express Fas-ligand (FasL) which induces apoptosis in Fas-positive target cells. The major targets of these CD4+ cytotoxic T-lymphocytes (CTL) are cells of the immune system, such as T, B cells and macrophages which express Fas upon activation. Thus, CD4+ CTL play a major immunoregulatory part through the elimination of activated myeloid and lymphoid cells during and upon completion of an immune response. In certain diseases, such as in HIV-infection and some autoimmune disorders, the functional activity of CD4+ CTL is disturbed preferentially at the level of FasL-Fas interaction, further emphasizing their important immunoregulatory role. Furthermore, Fas-ligand expressing tumors can evade the attack of Fas-positive CD4+ CTL and other effector cells, thereby giving them an opportunity to expand.

Isoform-specific expression of BAG-1 in mouse development

BAG-1 is a family of proteins with diverse activities that range in cultured cells from protection against programmed cell death through to regulation of steroid hormone action. At least three proteins (BAG-1L, BAG-1M and BAG-1) are encoded by the Bag-1 mRNA through the use of alternative translation-initiation sites. To assess the in vivo function of these factors, we have used in situ hybridization and immunohistochemical techniques to determine the distribution of Bag-1 transcript and proteins during mouse development. Bag-1 mRNA was identified in several organs with cartilaginous tissues showing the highest expression levels. The level of expression at some of these sites was downregulated during the course of development. In the immunohistochemical studies, antibodies directed against the BAG-1 proteins stained all the sites identified in the in situ hybridization studies although isoform-specific differences were observed. BAG-1L specific antibody showed ubiquitous staining as early as day 10.5 post-coitum but there was a progressive restriction during subsequent stages of embryogenesis. On the contrary, an antibody that preferentially recognized the other isoforms only stained the mouse myocardium in the early developmental stages before finally recognizing additional organs later on in development. These results demonstrate a stage- and site-specific expression of the BAG-1 isoforms during mouse development.

Adaptive responses of the endothelium to stress

It is now established that endothelial cells acquire several functional properties in response to a diverse array of extracellular stimuli. This expression of an altered phenotype is referred to as endothelial cell activation, and it includes several activities that promote inflammation and coagulation. While it is recognized that endothelial cell activation has a principal role in host defense, recent studies also demonstrate that endothelial cells are capable of complex molecular responses that protect the endothelium against various forms of stress including heat shock, hypoxia, oxidative stress, shock, ischemia-reperfusion injury, toxins, wounds, and mechanical stress. In this review, we examine endothelial cell genotypic and phenotypic responses to stress. Also, we highlight important cellular stress responses that, although not yet demonstrated directly in endothelial cells, likely exist as part of the repertoire of stress responses in endothelium. A detailed understanding of the molecular mechanisms mediating the adaptive responses of endothelial cells to stress should facilitate the development of novel therapeutics to aid in the management of diverse surgical diseases and their complications.

Pw1/Peg3 is a potential cell death mediator and cooperates with Siah1a in p53-mediated apoptosis

Induction of wild-type p53 in mouse fibroblasts causes cell cycle arrest at the G(1) phase, whereas coexpression of p53 and the protooncogene c-myc induces apoptosis. Although p53 transcriptional activity generally is required for both pathways, the molecular components mediating p53-dependent apoptosis are not well understood. To identify factors that could mediate p53-induced cell death, we used a comparative RNA differential display procedure. We have identified Pw1/Peg3 as a gene product induced during p53/c-myc-mediated apoptosis. Pw1/Peg3 is not induced during p53-mediated G(1) growth arrest nor by c-myc alone. Although it is not clear whether the induction of Pw1/Peg3 depends on p53 activity, we show that Pw1/Peg3 interacts with a p53-inducible gene product Siah1a. We demonstrate that coexpression of Pw1/Peg3 with Siah1a induces apoptosis independently of p53 whereas expression of Pw1/Peg3 or Siah1a separately has no effect on cell death. These data suggest that Siah1a and Pw1/Peg3 cooperate in the p53-mediated cell death pathway. Furthermore, we show that inhibiting Pw1/Peg3 activity blocks p53-induced apoptosis. The observation that Pw1/Peg3 is necessary for the p53 apoptotic response suggests a pivotal role for this gene in determining cell death versus survival.

The ubiquitin-related BAG-1 provides a link between the molecular chaperones Hsc70/Hsp70 and the proteasome

The BAG-1 protein modulates the chaperone activity of Hsc70 and Hsp70 in the mammalian cytosol and nucleus. Remarkably, BAG-1 possesses a ubiquitin-like domain at its amino terminus, suggesting a link to the ubiquitin/proteasome system. Here we show that BAG-1 is indeed associated with the 26 S proteasome in HeLa cells. Binding of the chaperone cofactor to the proteolytic complex is regulated by ATP hydrolysis and is not mediated by Hsc70 and Hsp70. The presented findings reveal a role of BAG-1 as a physical link between the Hsc70/Hsp70 chaperone system and the proteasome. In fact, targeting of BAG-1 to the proteasome promotes an association of the chaperones with the proteolytic complex in vitro and in vivo. A regulatory function of the chaperone cofactor at the interface between protein folding and protein degradation is thus indicated.

Growth suppression of Escherichia coli by induction of expression of mammalian genes with transmembrane or ATPase domains

Growth inhibition of Escherichia coli host cells is frequently observed when some mammalian genes are induced to express exogenously. To find common features of these mammalian genes, an assay was designed for the isolation of these genes which show growth-inhibitory effect on E. coli by induction of expression. Of 38,000 clones derived from a mouse brain cDNA library, 64 cDNA clones were systematically selected out by this method, of which 45 clones had putative open reading frames encoding proteins with putative membrane-associated regions or ATP-binding/ATPase activities. These results show that a fraction of membrane-associated proteins or ATP-binding/ATPase genes can be isolated from cDNA libraries by our simple method.

Apoptosis or plasma cell differentiation of CD38-positive B-chronic lymphocytic leukemia cells induced by cross-linking of surface IgM or IgD

Previously, we demonstrated that B-chronic lymphocytic leukemia (B-CLL) cells could be divided into 2 groups depending on the expression of CD38 by the malignant cells. The 2 groups differed in their signal-transducing capacities initiated by cross-linking of surface IgM; only in CD38-positive cells was an efficient signal delivered, invariably resulting in cell apoptosis. In this study, we investigated the effect of surface IgD cross-linking in 10 patients with CD38-positive B-CLL. Exposure of the malignant cells to goat antihuman δ-chain antibodies (Gaδ-ab) caused [Ca++]i mobilization and tyrosine kinase phosphorylation in a manner not different from that observed after goat antihuman μ-chain antibody (Gaμ-ab) treatment in vitro. However, Gaδ-ab-treated cells failed to undergo apoptosis and instead displayed prolonged survival in culture and differentiated into plasma cells when rIL2 was concomitantly present. Cross-linking of surface IgD failed to induce proliferation of the malignant cells in vitro. Moreover, treatment with Gaδ-ab did not prevent apoptosis of B-CLL cells induced by Gaμ-ab. Collectively, these experiments demonstrated that IgM and IgD expressed by the same cell may deliver opposite signals under particular circumstances and provide some clues for the understanding of the pathophysiology of B-CLL.

A family of ubiquitin-like proteins binds the ATPase domain of Hsp70-like Stch

We have isolated two human ubiquitin-like (UbL) proteins that bind to a short peptide within the ATPase domain of the Hsp70-like Stch protein. Chap1 is a duplicated homologue of the yeast Dsk2 gene that is required for transit through the G2/M phase of the cell cycle and expression of the human full-length cDNA restored viability and suppressed the G2/M arrest phenotype of dsk2Delta rad23Delta Saccharomyces cerevisiae mutants. Chap2 is a homologue for Xenopus scythe which is an essential component of reaper-induced apoptosis in egg extracts. While the N-terminal UbL domains were not essential for Stch binding, Chap1/Dsk2 contains a Sti1-like repeat sequence that is required for binding to Stch and is also conserved in the Hsp70 binding proteins, Hip and p60/Sti1/Hop. These findings extend the association between Hsp70 members and genes encoding UbL sequences and suggest a broader role for the Hsp70-like ATPase family in regulating cell cycle and cell death events.

Bag1 functions in vivo as a negative regulator of Hsp70 chaperone activity

Studies on the Hsp70 chaperone machine in eukaryotes have shown that Hsp70 and Hsp40/Hdj1 family proteins are sufficient to prevent protein misfolding and aggregation and to promote refolding of denatured polypeptides. Additional protein cofactors include Hip and Bag1, identified in protein interaction assays, which bind to and modulate Hsp70 chaperone activity in vitro. Bag1, originally identified as an antiapoptotic protein, forms a stoichiometric complex with Hsp70 and inhibits completely Hsp70-dependent in vitro protein refolding of an unfolded polypeptide. Given its proposed involvement in multiple cell signaling events as a regulator of Raf1, Bcl2, or androgen receptor, we wondered whether Bag1 functions in vivo as a negative regulator of Hsp70. In this study, we demonstrate that Bag1, expressed in mammalian tissue culture cells, has pronounced effects on one of the principal activities of Hsp70, as a molecular chaperone essential for stabilization and refolding of a thermally inactivated protein. The levels of Hsp70 and Bag1 were modulated either by transient transfection or conditional expression in stably transfected lines to achieve levels within the range detected in different mammalian tissue culture cell lines. For example, a twofold increase in the concentration of Bag1 reduced Hsp70-dependent refolding of denatured luciferase by a factor of 2. This effect was titratable, and higher levels of wild-type but not a mutant form of Bag1 further inhibited Hsp70 refolding by up to a factor of 5. The negative effects of Bag1 were also observed in a biochemical analysis of Bag1- or Hsp70-overexpressing cells. The ability of Hsp70 to maintain thermally denatured firefly luciferase in a soluble state was reversed by Bag1, thus providing an explanation for the in vivo chaperone-inhibitory effects of Bag1. Similar effects on Hsp70 were observed with other cytoplasmic isoforms of Bag1 which have in common the carboxyl-terminal Hsp70-binding domain and differ by variable-length amino-terminal extensions. These results provide the first formal evidence that Bag1 functions in vivo as a regulator of Hsp70 and suggest an intriguing complexity for Hsp70-regulatory events.

BAG-1, a novel Bcl-2-interacting protein, activates expression of human JC virus

Transcription of the human polyomavirus JC virus (JCV) genome is regulated by cellular proteins and the large tumour (T) antigen. Earlier studies led to the identification of nuclear factor-1 (NF-1)-binding sites in the JCV enhancer by DNase I protection assays of extracts from retinoic acid (RA)-differentiated P19 embryonal carcinoma (EC) cells. In this study, a cDNA clone that encodes a protein capable of binding to the JCV NF-1 sites was isolated from an RA-differentiated EC cell cDNA library. Sequence analysis revealed that the cDNA isolated was identical to the previously described Bcl-2-interacting protein BAG-1 (Bcl-2-associated athano gene-1). Results from RNA studies indicated that BAG-1 is expressed in several cell types. Co-transfection of a recombinant BAG-1 expression plasmid with JCV promoters indicated that BAG-1 stimulates transcription of the JCV(E) promoter and to a lesser extent the JCV(L) promoter. Mutations in the NF-1 sites in the JCV(E) promoter eliminated the activation by BAG-1. Thus, BAG-1 is a novel transcription factor that may play a role in JCV expression.

Loss of bag-1 immunoreactivity in rat brain after transient middle cerebral artery occlusion

Although bag-1 is a strong apoptosis repressor protein, its functions in normal or injured brains are not fully understood. In the present study, we investigated expression of bag-1 protein in rat brain after transient middle cerebral artery (MCA) occlusion, and compared the results with that of terminal deoxynucleotidyl transferase-mediated dUTP-biotin end labeling (TUNEL). Immunohistochemical analysis revealed that neuronal, choroid plexus, and ependymal cells were positively stained in the sham control brain. After 90 min of transient MCA occlusion, immunoreactivity for bag-1 progressively decreased from 3 to 48 h in the nuclei of neurons. Western blot analysis revealed that immunoreactive bag-1 was markedly decreased in the nuclear fraction. In contrast, cytosolic and mitochondrial fractions showed no or only slight change after the ischemia. TUNEL positive cells appeared at 48 h after the reperfusion, which was preceded by loss of bag-1 immunoreactivity. The present results suggest that bag-1 plays some roles in normal neuronal function, and its loss may be involved in neuronal cell death after ischemia.

To be, or not to be, that is the question. Apoptosis in human trophoblast

Apoptosis, the morphology of cell suicide, may result from programmed cell death or may be a response to exogenous stimuli. Apoptosis can be induced in cultured trophoblast and can be identified in the trophoblast of placental villi. The trophoblast regulates maternal-fetal gas, nutrient and waste product exchange; therefore, the presence of apoptosis in this key cellular interface highlights the importance of understanding what controls apoptosis in the placenta. In this review, we describe the signal transduction pathways that trigger apoptosis in other systems, identify key genetic controls for the process and outline the final common pathway which effects execution in cells committed to suicide. Multiplicity, redundancy and cross talk among pathways characterize the surface membrane signals and exogenous stimuli that trigger apoptosis in other cells. As each step in the apoptotic process is discussed, we describe what is known about the step in human placental villi. Recent studies suggest that a further understanding of receptor-mediated signalling pathways, the Bcl-2 regulators and the caspases and substrates involved in placental apoptosis will surely provide insights into both normal placental development and the placental dysfunction associated with some abnormal pregnancies.

Mitochondria and neuronal survival

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca(2+), generate reactive oxygen species, and undergo Ca(2+)-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

Differential effects of the hsp70-binding protein BAG-1 on glucocorticoid receptor folding by the hsp90-based chaperone machinery

The heat shock protein hsp70/hsc70 is a required component of a five-protein (hsp90, hsp70, Hop, hsp40, and p23) minimal chaperone system reconstituted from reticulocyte lysate that forms glucocorticoid receptor (GR).hsp90 heterocomplexes. BAG-1 is a cofactor that binds to the ATPase domain of hsp70/hsc70 and that modulates its chaperone activity. Inasmuch as BAG-1 has been found in association with several members of the steroid receptor family, we have examined the effect of BAG-1 on GR folding and GR.hsp90 heterocomplex assembly. BAG-1 was present in reticulocyte lysate at a BAG-1:hsp70/hsc70 molar ratio of approximately 0.03, and its elimination by immunoadsorption did not affect GR folding and GR. hsp90 heterocomplex assembly. At low BAG-1:hsp70/hsc70 ratios, BAG-1 promoted the release of Hop from the hsp90-based chaperone system without inhibiting GR.hsp90 heterocomplex assembly. However, at molar ratios approaching stoichiometry with hsp70, BAG-1 produced a concentration-dependent inhibition of GR folding to the steroid-binding form with corresponding inhibition of GR.hsp90 heterocomplex assembly by the minimal five-protein chaperone system. Also, there was decreased steroid-binding activity in cells that were transiently or stably transfected with BAG-1. These observations suggest that, at physiological concentrations, BAG-1 modulates assembly by promoting Hop release from the assembly complex; but, at concentrations closer to those in transfected cells and some transformed cell lines, hsp70 is continuously bound by BAG-1, and heterocomplex assembly is blocked.

Bis, a Bcl-2-binding protein that synergizes with Bcl-2 in preventing cell death

Bcl-2 is the best characterized inhibitor of apoptosis, although the molecular basis of this action is not fully understood. Using a protein interaction cloning procedure, we identified a human gene designated as bis (mapped to chromosome 10q25) that encoded a novel Bcl-2-interacting protein. Bis protein showed no significant homology with Bcl-2 family proteins and had no prominent functional motif. Co-immunoprecipitation analysis confirmed that Bis interacted with Bcl-2 in vivo. DNA transfection experiments indicated that Bis itself exerted only weak anti-apoptotic activity, but was synergistic with Bcl-2 in preventing Bax-induced and Fas-mediated apoptosis. These results suggest that Bis is a novel modulator of cellular anti-apoptotic activity that functions through its interaction with Bcl-2.

High level expression of differentially localized BAG-1 isoforms in some oestrogen receptor-positive human breast cancers

Sensitivity to oestrogens and apoptosis are critical determinants of the development and progression of breast cancer and reflect closely linked pathways in breast epithelial cells. For example, induction of BCL-2 oncoprotein expression by oestrogen contributes to suppression of apoptosis and BCL-2 and oestrogen receptor (ER) are frequently co-expressed in tumours. BAG-1/HAP is a multifunctional protein which complexes with BCL-2 and steroid hormone receptors (including the ER), and can suppress apoptosis and influence steroid hormone-dependent transcription. Therefore, analysis of expression of BAG-1 in human breast cancer is of considerable interest. BAG-1 was readily detected by immunostaining in normal breast epithelial cells and most ER-positive tumours, but was undetectable or weakly expressed in ER-negative tumours. BAG-1 positive cells showed a predominantly cytoplasmic or cytoplasmic plus nuclear distribution of staining. A correlation between ER and BAG-1 was also evident in breast cancer derived cell lines, as all lines examined with functional ER expression also expressed high levels of BAG-1. In addition to the prototypical 36 kDa BAG-1 isoform, breast cancer cells expressed higher molecular weight isoforms and, in contrast to BCL-2, BAG-1 expression was independent of oestrogens. BAG-1 isoforms were differentially localized to the nucleus or cytoplasm and this was also independent of oestrogens. These results demonstrate a close association between BAG-1 and functional ER expression and suggest BAG-1 may be useful as a therapeutic target or prognostic marker in breast cancer.

A nuclear action of the eukaryotic cochaperone RAP46 in downregulation of glucocorticoid receptor activity

RAP46 is a eukaryotic cochaperone that associates with several proteins, including the heat shock protein hsp70/hsc70 and the glucocorticoid receptor (GR). Here we show a downregulation of GR-mediated transactivation by RAP46 via a mechanism independent of a cytoplasmic action of this cochaperone. We demonstrate a specific cytoplasmic-nuclear recruitment of RAP46 by the liganded GR that results in inhibition of the transactivation function of the receptor. A repeated sequence motif [EEX(4)](8) at the NH(2) terminus of RAP46 or BAG-1L, a larger isoform of RAP46, is responsible for this downregulation of GR activity. BAG-1, a shorter isoform with only a duplication of the [EEX(4)] sequence, does not inhibit GR activity. The [EEX(4)](8) motif, when linked to an otherwise unrelated protein, abrogated the inhibitory action of endogenous RAP46 on GR-mediated transactivation. The nuclear effects of RAP46 and BAG-1L are specific since GR-mediated inhibition of AP-1 activity was not affected. These studies identify the [EEX(4)](8) sequence as a signature motif for inhibition of GR-mediated transactivation and demonstrate a specific nuclear action of a eukaryotic cochaperone in the regulation of GR activity.