Characterization of the mUBC9-binding sites required for E2A protein degradation

SRXN1 is a novel protective factor against methylmercury-induced apoptosis in C17.2 mouse neural stem cells

Methylmercury is an environmental pollutant that exhibits severe cerebral neurotoxicity and remains a worldwide problem. Motor and mental development disorders have been observed in children born to women who consumed relatively large amounts of methylmercury via contaminated fish during pregnancy. We previously found by RNA-sequencing analysis that treatment of C17.2 mouse neural stem cells with methylmercury induced the expression of SRXN1 (sulfiredoxin-1), a redox regulator. In this study, we examined the effect of methylmercury on SRXN1 expression and the role of SRXN1 in methylmercury-induced cell death. After C17.2 cells were treated with methylmercury, both mRNA and protein expression of SRXN1 increased in a time- and concentration-dependent manner. Because the induction of SRXN1 expression by methylmercury was suppressed by pretreatment with a transcription inhibitor, we searched the upstream region of the SRXN1 gene and found a binding sequence for transcription factor 3 (TCF3). Interestingly, the induction of SRXN1 expression by methylmercury was attenuated in cells in which TCF3 expression was suppressed by siRNA compared with control cells. This suggests that TCF3 is involved in the induction of SRXN1 expression by methylmercury. We previously reported that TCF3 overexpression suppressed methylmercury-induced apoptosis; in the present study, we found that SRXN1 overexpression also suppressed methylmercury-induced apoptosis, as assessed by cleaved caspase-3 expression. In summary, our results indicate that SRXN1 induced by TCF3-mediated expression is a novel protective factor against methylmercury-induced apoptosis.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43188-024-00277-6.

Hepatitis C virus core protein induces epithelial-mesenchymal transition in human hepatocytes by upregulating E12/E47 levels

Downregulation of E-cadherin is a hallmark of epithelial-mesenchymal transition (EMT), an essential component of cancer progression to more aggressive phenotypes characterized by tumor dedifferentiation, infiltration, and metastasis. However, the underlying mechanism for E-cadherin downregulation in hepatitis C virus (HCV)-associated hepatocellular carcinoma (HCC) is still unclear. In this study, we found that ectopic expression of HCV core protein or infection with HCV in human hepatocytes upregulated the levels of the transcriptional repressors, E12 and E47, resulting in inactivation of the E-cadherin promoter, containing E-box motifs, and subsequent repression of its expression. E12/E47 knock-down almost completely abolished the potential of HCV core protein to repress E-cadherin expression. HCV core protein inhibited ubiquitin-dependent proteasomal degradation of E12/E47 without affecting their expression at the transcriptional level. E12/E47 upregulation ultimately led to EMT in human hepatocytes, as demonstrated by morphological changes, altered expression levels of EMT markers, including E-cadherin, plakoglobin, and fibronectin, and increased capacity for cell detachment and migration. In conclusion, HCV core protein represses E-cadherin expression by upregulating E12/E47 levels to induce EMT in HCV-associated HCC.

Age effects on mouse and human B cells

Our laboratory has contributed to the areas of B cell receptor (BCR) and pre-BCR gene identification and transcription and has focused on the problem of the aged immune system in mice and humans for the last 15 years. We have found biomarkers for the decrease in B cell function in aged mice and humans. These include decreases in immunoglobulin (Ig) class switch (e.g., IgM to IgG), decreases in the enzyme AID (activation-induced cytidine deaminase) and decreases in the transcription factor E47. The E47 mRNA stability is decreased in old B cells due to decreased phospho-MAPKinase and phospho-TTP (tristetraprolin). Inflammation, e.g., TNF-α, which increases with age, impacts B cells directly by increasing their TNF-α and NF-κB and leads to the above decreased pathway. Both class switch and affinity maturation are decreased in elderly responses to the influenza vaccine and biomarkers we have found (numbers and percentages of switched memory B cells and AID in stimulated B cells in culture) can predict a beneficial or decreased immune response to the vaccine. Current and future avenues to improve the humoral immune response in the elderly are discussed.

Protein inhibitor of activated STAT, PIASy regulates α-smooth muscle actin expression by interacting with E12 in mesangial cells

Phenotypic transformation of mesangial cells (MCs) is implicated in the development of glomerular disease; however, the mechanisms underlying their altered genetic program is still unclear. α-smooth muscle actin (α-SMA) is known to be a crucial marker for phenotypic transformation of MCs. Recently, E-boxes and the class I basic helix-loop-helix proteins, such as E12 have been shown to regulateα-SMA expression. Therefore, we tried to identify a novel E12 binding protein in MCs and to examine its role in glomerulonephritis. We found that PIASy, one of the protein inhibitors of activated STAT family protein, interacted with E12 by yeast two-hybrid screens and coimmunopreciptation assays. Overexpression of E12 significantly enhanced theα-SMA promoter activity, and the increase was blocked by co-transfection of PIASy, but not by a PIASy RING mutant. In vivo sumoylation assays revealed that PIASy was a SUMO E3 ligase for E12. Furthermore, transforming growth factor-β (TGF-β) treatment induced expression of both PIASy and E12, consistent with α-SMA expression. Moreover, reduced expression of PIASy protein by siRNA specific for PIASy resulted in increased TGF-β-mediated α-SMA expression. In vivo, PIASy and E12 were dramatically upregulated along with α-SMA and TGF-β in the proliferative phase of Thy1 glomerulonephritis. Furthermore, an association between PIASy and E12 proteins was observed at day 6 by IP-western blotting, but not at day 0. These results suggest that TGF-β up-regulates PIASy expression in MCs to down-regulateα-SMA gene transcription by the interaction with E12.

Increased expression of bHLH transcription factor E2A (TCF3) in prostate cancer promotes proliferation and confers resistance to doxorubicin induced apoptosis

E2A (TCF3) is a multifunctional basic helix loop helix (bHLH), transcription factor. E2A regulates transcription of target genes by homo- or heterodimerization with cell specific bHLH proteins. In general, E2A promotes cell differentiation, acts as a negative regulator of cell proliferation in normal cells and cancer cell lines and is required for normal B-cell development. Given the diverse biological pathways regulated/influenced by E2A little is known about its expression in cancer. In this study we investigated the expression of E2A in prostate cancer. Unexpectedly, E2A immuno-histochemistry demonstrated increased E2A expression in prostate cancer as compared to normal prostate. Silencing of E2A in prostate cancer cells DU145 and PC3 led to a significant reduction in proliferation due to G1 arrest that was in part mediated by increased CDKN1A(p21) and decreased Id1, Id3 and c-myc. E2A silencing in prostate cancer cell lines also resulted in increased apoptosis due to increased mitochondrial permeability and caspase 3/7 activation. Moreover, silencing of E2A increased sensitivity to doxorubicin induced apoptosis. Based on our results, we propose that E2A could be an upstream regulator of Id1 and c-Myc which are highly expressed in prostate cancer. These results for the first time demonstrate that E2A could in fact acts as a tumor promoter at least in prostate cancer.

Intron-1 rs3761548 is related to the defective transcription of Foxp3 in psoriasis through abrogating E47/c-Myb binding

Foxp3 is a master transcription factor (TF) for development and function of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Treg cells) and is critical for the transcription of target genes. But the transcriptional regulation of Foxp3 itself has not been fully understood until now. Here, we aimed to demonstrate the hypothesis that upstream single nucleotide polymorphism(s) (SNPs) of Foxp3 was/were responsible for the defective transcription of Foxp3 in psoriasis and to explore the mechanism behind this hypothesis. In this study, SNP of large sample was investigated for risk analysis. Mature algorithms, electrophoretic mobility shift and chromatin immunoprecipitation assays were used to identify TF binding site variations. Loss-of-function and overexpression assays and cell cycle blocker assay were performed to identify when and what kind of possible roles the candidate factors play. Our results showed that intron-1 rs3761548 was correlated with a significant susceptibility to psoriasis. The rs3761548 contributed to the decreased resting Foxp3 transcription and impaired acceleration of Foxp3 transcription levels after stimulation in psoriatic patients with genotype AA. We analysed and demonstrated potent new E47/c-Myb -dependent regulation elements in rs3761548, oppositely controlling Foxp3 gene transcription at G1 and G2/M phases of Treg cells in psoriatic patients. For patients with rs3761548 AA, the polymorphism causes loss of bindings to the E47 and c-Myb factors, leading to defective transcription of Foxp3 gene. Further identification of the networks and molecular mechanisms underlying Foxp3 transcription may provide new insights into Foxp3 transcriptional regulation and alternative therapeutic strategies to improve characteristics of autoimmune disorders.

Identification of RANBP16 and RANBP17 as novel interaction partners for the bHLH transcription factor E12

The ubiquitously expressed basic helix-loop-helix (bHLH) transcription factors E12 and E47, products of alternative splicing of the E2A/TCF3 gene, regulate diverse biological processes including cell growth, differentiation and development. To search for novel protein interactions for E12, we utilized the bHLH domain of E12 as a bait in yeast two-hybrid screening. Yeast two-hybrid, mammalian two-hybrid, and co-immunoprecipitation analyses demonstrate specific interaction of E12 with RANBP17, a novel member of the importin-beta superfamily; this interaction maps to the CRM1 homology region of RANBP17. Ectopic expression of RANBP17 leads to a approximately 3-fold increase in E2A/MyoD mediated transactivation of an E-box regulated luciferase reporter gene. Interaction and transactivation studies also revealed similar functions for RANBP16/XPO7. Furthermore, ectopic expression of either RANBP16 or RANBP17 resulted in increased level of endogenous transcript for the cyclin-dependent kinase inhibitor, p21(Waf1/Cip1), a well-characterized E2A target gene. Together, these biochemical and functional data reveal RANBP16 and RANBP17 as novel regulators of E2A protein action.

Mechanisms for decreased function of B cells in aged mice and humans

The immune system has been known for some time to be compromised in aged individuals, e.g., both mice and humans, and in both humoral and cellular responses. Our studies have begun to elucidate intrinsic B lymphocyte defects in Ig class switch recombination, activation-induced cytidine deaminase, and E47 transcription factor expression. These defects occur in both mice and humans. Our studies have also shown that tristetraprolin is one of the key players in regulating the decreased E47 mRNA stability in aged B lymphocytes. These and current studies should lead to improvements in B lymphocyte function in aged populations.

The type 2 dengue virus envelope protein interacts with small ubiquitin-like modifier-1 (SUMO-1) conjugating enzyme 9 (Ubc9)

Dengue viruses are mosquito-borne flaviviruses and may cause the life-threatening dengue hemorrhagic fever and dengue shock syndrome. Its envelope protein is responsible mainly for the virus attachment and entry to host cells. To identify the human cellular proteins interacting with the envelope protein of dengue virus serotype 2 inside host cells, we have performed a screening with the yeast-two-hybrid-based "Functional Yeast Array". Interestingly, the small ubiquitin-like modifier-1 conjugating enzyme 9 protein, modulating cellular processes such as those regulating signal transduction and cell growth, was one of the candidates interacting with the dengue virus envelope protein. With co-precipitation assay, we have demonstrated that it indeed could interact directly with the Ubc9 protein. Site-directed mutagenesis has demonstrated that Ubc9 might interact with the E protein via amino acid residues K51 and K241. Furthermore, immunofluorescence microscopy has shown that the DV2E-EGFP proteins tended to progress toward the nuclear membrane and co-localized with Flag-Ubc9 proteins around the nuclear membrane in the cytoplasmic side, and DV2E-EGFP also shifted the distribution of Flag-Ubc9 from evenly in the nucleus toward concentrating around the nuclear membrane in the nucleic side. In addition, over-expression of Ubc9 could reduce the plaque formation of the dengue virus in mammalian cells. This is the first report that DV envelope proteins can interact with the protein of sumoylation system and Ubc9 may involve in the host defense system to prevent virus propagation.

E2A protein degradation by the ubiquitin-proteasome system is stage-dependent during muscle differentiation

The E2A proteins are basic helix-loop-helix transcription factors that regulate proliferation and differentiation in many cell types. In muscle cells, the E2A proteins form heterodimers with muscle regulatory factors such as MyoD, which then bind to DNA and regulate the transcription of target genes essential for muscle differentiation. We now demonstrate that E2A proteins are primarily localized in the nucleus in both C2C12 myoblasts and myotubes, and are degraded by the ubiquitin proteasome system evidenced by stabilization following treatment with the proteasome inhibitor, MG132. During the differentiation from myoblast to myotube, the cellular abundance of E2A proteins is relatively unaltered, despite significant changes (each approximately 5-fold) in the relative rates of protein synthesis and protein degradation via the ubiquitin-proteasome system. The rate of ubiquitin-proteasome-mediated E2A protein degradation depends on the myogenic differentiation state (t 1/2 approximately 2 h in proliferating myoblasts versus t 1/2 > 10 h in differentiated myotubes), and is also associated with cell cycle in non-muscle cells. Our findings reveal an important role for both translational and post-translational regulatory mechanisms in mediating the complex program of muscle differentiation determined by the E2A proteins.

MyoD synergizes with the E-protein HEB beta to induce myogenic differentiation

The MyoD family of basic helix-loop-helix transcription factors function as heterodimers with members of the E-protein family to induce myogenic gene activation. The E-protein HEB is alternatively spliced to generate alpha and beta isoforms. While the function of these molecules has been studied in other cell types, questions persist regarding the molecular functions of HEB proteins in skeletal muscle. Our data demonstrate that HEB alpha expression remains unchanged in both myoblasts and myotubes, whereas HEB beta is upregulated during the early phases of terminal differentiation. Upon induction of differentiation, a MyoD-HEB beta complex bound the E1 E-box of the myogenin promoter leading to transcriptional activation. Importantly, forced expression of HEB beta with MyoD synergistically lead to precocious myogenin expression in proliferating myoblasts. However, after differentiation, HEB alpha and HEB beta synergized with myogenin, but not MyoD, to activate the myogenin promoter. Specific knockdown of HEB beta by small interfering RNA in myoblasts blocked differentiation and inhibited induction of myogenin transcription. Therefore, HEB alpha and HEB beta play novel and central roles in orchestrating the regulation of myogenic factor activity through myogenic differentiation.

E12 and E47 modulate cellular localization and proteasome-mediated degradation of MyoD and Id1

Programs of tissue differentiation are likely controlled by factors regulating gene expression and protein degradation. In muscle, the degradation of the muscle transcription factor MyoD and its inhibitor Id1 occurs via the ubiquitin-proteasome system. E12 and E47, splice products of the E2A gene, interact with MyoD to activate transcription of the muscle program and are also degraded by the ubiquitin-proteasome system (t(1/2) = approximately 6 h). E12 and E47 each contain two regions of basic amino acids, which, when mutated, lead to cytoplasmic accumulation of the proteins. These NLS mutants (E12(NLS), E47(NLS)) are degraded with a half-life similar to the wild-type proteins. In nonmuscle cells, cotransfection of either E12 or E47 with MyoD extended MyoD's half-life from approximately 1 to approximately 4 h. In addition, cotransfection of either E12 or E47 with Id1 led to a marked reduction in Id1's degradation rate from t(1/2) of approximately 1 to approximately 8 h. Furthermore, the cotransfection of NLS deficient mutants of MyoD or Id1 with E12 or E47 resulted in altered intracellular localization of the proteins largely dependent upon the E12 or E47 moiety. Cotransfection of wild-type MyoD or Id1 with NLS deficient mutants of E12 or E47 also led to an altered intracellular localization of MyoD and Id1. These results demonstrate in vivo that E12 and E47 modulate both MyoD and Id1 degradation and may have implications for the physiological regulation of muscle development.

RNA Stability of the E2A-Encoded Transcription Factor E47 Is Lower in Splenic Activated B Cells from Aged Mice

We have demonstrated previously that DNA binding and protein expression of the E2A-encoded transcription factor E47 are lower in nuclear extracts of activated splenic B cells from old mice. In the present study, we address how E47 protein expression is regulated in aging. Results herein show that E2A mRNA levels were decreased in stimulated splenic B cells from old as compared with young mice. RNA stability assays showed that the rate of E2A mRNA decay was accelerated in stimulated splenic B cells from old mice, but E47 protein degradation rates were comparable in young vs aged B cells, indicating that the regulation of E47 expression in activated splenic B cells occurs primarily by mRNA stability. The rates of decay of other mRNAs showed that the increased mRNA degradation in aged splenic activated B cells is not a general phenomenon but restricted to a subset of mRNAs. We next investigated the signal transduction pathways controlling E2A mRNA expression and stability and found that p38 MAPK regulates E2A mRNA expression through increased mRNA stability and is down-regulated in aged activated B cells. Results show that inhibition of p38 MAPK significantly reduces E2A mRNA stability in both young and old B cells, further stressing the role of p38 MAPK in E2A RNA stabilization. These studies demonstrate that the transcription factor E2A, critical for many aspects of B cell function, is regulated by a novel mechanism in aging.

Ubiquitin-proteasome-mediated degradation, intracellular localization, and protein synthesis of MyoD and Id1 during muscle differentiation

Mammalian skeletal myogenesis results in the differentiation of myoblasts to mature syncytial myotubes, a process regulated by an intricate genetic network of at least three protein families: muscle regulatory factors, E proteins, and Id proteins. MyoD, a key muscle regulatory factor, and its negative regulator Id1 have both been shown to be degraded by the ubiquitin-proteasome system. Using C2C12 cells and confocal fluorescence microscopy, we showed that MyoD and Id1 co-localize within the nucleus in proliferating myoblasts. In mature myotubes, in contrast, they reside in distinctive subcellular compartments, with MyoD within the nucleus and Id1 exclusively in the cytoplasm. Cellular abundance of Id1 was markedly diminished from the very onset of muscle differentiation, whereas MyoD abundance was reduced to a much lesser extent and only at the later stages of differentiation. These reductions in MyoD and Id1 protein levels seem to result from a change in the rate of protein synthesis rather than the rate of degradation. In vivo protein stability studies revealed that the rates of ubiquitin-proteasome-mediated MyoD and Id1 degradation are independent of myogenic differentiation state. Id1 and MyoD were both rapidly degraded, each with a t 1/2 approximately = 1 h in myoblasts and in myotubes. Furthermore, relative protein synthesis rates for MyoD and Id1 were significantly diminished during myoblast to myotube differentiation. These results provide insight as to the interaction between MyoD and Id1 in the process of muscle differentiation and have implications for the involvement of the ubiquitin-proteasome-mediated protein degradation and protein synthesis in muscle differentiation and metabolism under abnormal and pathological conditions.

Decreased E47 in senescent B cell precursors is stage specific and regulated posttranslationally by protein turnover

The E2A-encoded transcription factor E47 is crucial to B lymphopoiesis. Senescent BALB/c mice ( approximately 2 years old) had reduced pre-B cells ex vivo. Pro-B/early pre-B cells from these aged mice, both ex vivo and in vitro, were deficient in E47 protein. In vitro, IL-7 expanded pro-B/early pre-B cells from young BALB/c mice expressed E47 protein that was relatively stable over a 5-h period. Cultured senescent pro-B/early pre-B cells exhibited reduced E47 protein stability with approximately 50-90% loss of E47 over the same time period. Degradation of E47 was effectively blocked by the proteasome inhibitor lactacystin as well as calpain I and II inhibitors; E2A proteins were also shown to undergo ubiquitination. Although senescent B cell precursors expressed less E47 protein, E47 mRNA levels and turnover were normal. Therefore, E47 protein levels are reduced relatively early in B lineage differentiation in senescence and the decline in E47 protein occurs via increased protein degradation by proteasome and, possibly, calpain pathways. In contrast, normal E47 protein levels were observed within the highly reduced pre-B cell pool in aged mice. This suggests that pre-B cells in senescence undergo selection based on E47 expression. Increased degradation rates and lower steady-state levels were also observed for the transcription factors Pax-5/BSAP, Bob-1, and Ikaros, but this was not a general property of all proteins in aged B cell precursors. Therefore, altered turnover of multiple, select proteins crucial to B cell development may contribute to diminished B lymphopoiesis in old age.

Age-related differences in the E2A-encoded transcription factor E47 in bone marrow-derived B cell precursors and in splenic B cells

We have investigated the effects of aging on the E2A-encoded transcription factor E47, a key regulator of B cell functions, in B cell precursors and in splenic B cells. Here, we show that old mice can be classified as severely depleted, moderately depleted or not depleted mice, according to the percentage of pre-B cells in their bone marrow. IL-7-expanded populations of pro-B/early pre-B cells from bone marrow of both severely depleted and moderately depleted old mice exhibit a reduced E47 DNA-binding and expression compared to young mice, and this defect in severely depleted old mice is more dramatic than that in moderately depleted old mice. However, mRNA levels were comparable, suggesting that E47 in the bone marrow is not transcriptionally regulated. In the spleen, activated B cells from both severely depleted and moderately depleted old mice show a lower E47 DNA-binding and expression than young mice. However, in contrast to precursor B cells, E47 DNA-binding and expression are similarly and only moderately reduced in both severely depleted and in moderately depleted mice. The mRNA levels were found to be decreased in stimulated splenic B cells from old as compared to young mice, suggesting that E47 mRNA in the spleen may be both transcriptionally and/or post-transcriptionally regulated.

Genomic organization and characterization of the promoter for the E2A gene

Although E2A gene products are ubiquitously expressed, E2A-deficient mice display selective abnormalities in lymphocyte development, suggesting a certain requirement of the E2A gene products during lymphocyte development. To gain insights into the mechanisms of E2A transcriptional regulation, we isolated the genomic clones which are composed of four exons and one noncoding exon and span approximately 16 kb. The promoter region of E2A gene lacks a TATA box, and primer extension analysis showed several transcription initiation sites, a feature that characterizes TATA-less promoters. The transient transfection assay using the 5'-flanking region (positions -2994/+62) revealed that both positive (-357/-158) and negative (-831/-358) regulatory segments control E2A transcription in B-cell (WEHI-231) and T-cell (DO11.10) lines. However, contribution of a certain segment to promoter activity was different between lymphocytes and fibroblasts (NIH-3T3). Sequential deletion analysis of the constructs spanning the positive regulatory segments showed that the segment -257/-238 played a critical role in the basal promoter activity of the E2A gene, although other segments within -337 to -158 also appeared to be involved. Mutational analysis using the -257/-238 fragment failed to demonstrate a single cis-element responsible for the basal promoter activity, suggesting that E2A promoter requires the interaction of multiple regulatory elements. Electrophoretic mobility shift assay (EMSA) demonstrated a highly specific complex comprised of a positive regulatory segment (-267/-238) and putative transcription factor(s), which might be necessary for the basal promoter activity of the E2A gene.

Identification and characterization of a novel Pyk2/related adhesion focal tyrosine kinase-associated protein that inhibits alpha-synuclein phosphorylation

alpha-Synuclein is a presynaptic protein involved in the pathogenesis of several neurodegenerative diseases, such as Parkinson's disease. Pyk2/related adhesion focal tyrosine kinase (RAFTK) tyrosine kinase is an upstream regulator of Src family kinases in the central nervous system that is involved in alpha-synuclein phosphorylation. The present study reports the cloning and characterization of a novel adaptor protein, Pyk2/RAFTK-associated protein (PRAP), that specifically binds to Pyk2/RAFTK and inhibits alpha-synuclein tyrosine phosphorylation. PRAP contains a coiled-coil domain, a pleckstrin homology domain, and a SH3 domain; the SH3 domain binds to the proline-rich domain of Pyk2/RAFTK. PRAP was observed to be present throughout the brain, including substantia nigra dopaminergic neurons, in which it localized to the cytoplasm. PRAP was found to function as a substrate for Src family kinases, such as c-Src or Fyn, but not for Pyk2/RAFTK. Hyperosmotic stress induced phosphorylation of tyrosine 125 of alpha-synuclein via Pyk2/RAFTK, which acted through Src family kinases. Such phosphorylation was inhibited by PRAP expression, suggesting that PRAP negatively regulates alpha-synuclein phosphorylation following cell stress. In conclusion, PRAP functions as a downstream target for Pyk2/RAFTK and plays a role in alpha-synuclein phosphorylation.

Levels of E2A protein expression in B cell precursors are stage-dependent and inhibited by stem cell factor (c-kit ligand)

OBJECTIVE

The E2A-encoded proteins E47 and E12 are crucial to the development of pro-B and pre-B cells. The expression of E2A protein and mRNA during early B lymphopoiesis was determined and effects of stem cell factor (SCF; Steel factor; c-kit ligand) on E2A expression were evaluated.

MATERIALS AND METHODS

Ex vivo murine pro-B cells and pre-B cells were isolated and in vitro B cell precursors were derived after culture of bone marrow with rmIL-7. Levels of E2A proteins were determined by Western analysis and mRNA by RT-PCR. E2A expression in vitro was also assessed in cultures supplemented with IL-7 +/- recombinant murine SCF (rmSCF), insulin-like growth factor-1 (rhIGF-1), or Flt3-ligand (rhFlt3-L). Turnover of E2A proteins was assessed following cycloheximide treatment.

RESULTS

Ex vivo, pro-B cells had lower E47 protein levels than did pre-B cells but had comparable E2A mRNA levels. E2A protein, but not mRNA, levels were reduced in pro-B cells upon culture in vitro with IL-7 + rmSCF. This was associated with increased turnover of E2A proteins. In contrast, culture with IL-7 + IGF-1 or Flt3-L had minimal effects on E2A protein levels.

CONCLUSION

Pre-B cells expressed higher levels of E2A protein than did pro-B cells and this mainly resulted from posttranscriptional regulation. Exogenous SCF inhibited E2A protein, but not mRNA, expression in cultured B cell precursors, possibly by increasing E2A protein turnover. The capacity to respond to SCF may influence the levels of E2A during B-cell development.

Expression of RALT, a feedback inhibitor of ErbB receptors, is subjected to an integrated transcriptional and post-translational control

Over-expression studies have demonstrated that RALT (receptor associated late transducer) is a feedback inhibitor of ErbB-2 mitogenic and transforming signals. In growth-arrested cells, expression of endogenous RALT is induced by mitogenic stimuli, is high throughout mid to late G1 and returns to baseline as cells move into S phase. Here, we show that physiological levels of RALT effectively suppress ErbB-2 mitogenic signals. We also investigate the regulatory mechanisms that preside to the control of RALT expression. We demonstrate that pharmacological ablation of extracellular signal-regulated kinase (ERK) activation leads to blockade of RALT expression, unlike genetic and/or pharmacological interference with the activities of PKC, Src family kinases, p38 SAPK and PI-3K. Tamoxifen-dependent activation of an inducible Raf : ER chimera was sufficient to induce RALT expression. Thus, activation of the Ras-Raf-ERK pathway is necessary and sufficient to drive RALT expression. The RALT protein is labile and was found to accumulate robustly upon pharmacological inhibition of the proteasome. We were able to detect ubiquitin-conjugated RALT species in living cells, suggesting that ubiquitinylation targets RALT for proteasome-dependent degradation. Such an integrated transcriptional and post-translational control is likely to provide RALT with the ability to fluctuate timely in order to tune ErbB signals.

Yeast PalA/AIP1/Alix homolog Rim20p associates with a PEST-like region and is required for its proteolytic cleavage

The Saccharomyces cerevisiae zinc finger protein Rim101p is activated by cleavage of its C-terminal region, which resembles PEST regions that confer susceptibility to proteolysis. Here we report that Rim20p, a member of the broadly conserved PalA/AIP1/Alix family, is required for Rim101p cleavage. Two-hybrid and coimmunoprecipitation assays indicate that Rim20p binds to Rim101p, and a two-hybrid assay shows that the Rim101p PEST-like region is sufficient for Rim20p binding. Rim101p-Rim20p interaction is conserved in Candida albicans, supporting the idea that interaction is functionally significant. Analysis of Rim20p mutant proteins indicates that some of its broadly conserved regions are required for processing of Rim101p and for stability of Rim20p itself but are not required for interaction with Rim101p. A recent genome-wide two-hybrid study (T. Ito, T. Chiba, R. Ozawa, M. Yoshida, M. Hattori, and Y. Sakaki, Proc. Natl. Acad. Sci. USA 98:4569-4574, 2000) indicates that Rim20p interacts with Snf7p and that Snf7p interacts with Rim13p, a cysteine protease required for Rim101p proteolysis. We suggest that Rim20p may serve as part of a scaffold that places Rim101p and Rim13p in close proximity.

Friend of GATA 2 physically interacts with chicken ovalbumin upstream promoter-TF2 (COUP-TF2) and COUP-TF3 and represses COUP-TF2-dependent activation of the atrial natriuretic factor promoter

Friend of GATA (FOG)-2 is a multi-zinc finger transcriptional corepressor protein that binds specifically to GATA4. Gene targeting studies have demonstrated that FOG-2 is required for normal cardiac morphogenesis, including the development of the coronary vasculature, left ventricular compact zone, and heart valves. To better understand the molecular mechanisms by which FOG-2 regulates these cardiac developmental programs, we screened a mouse day 11 embryo library using a yeast two-hybrid interaction trap with the fifth and sixth zinc fingers of FOG-2 as bait. Using this approach, we isolated clones encoding the orphan nuclear receptors chicken ovalbumin upstream promoter-transcription factor (COUP-TF) 2 and COUP-TF3. COUP-TF2-null embryos die during embryonic development with defective angiogenesis and cardiac defects, a pattern that partly resembles the FOG-2-null phenotype. The interaction between COUP-TF2 and FOG-2 in mammalian cells was confirmed by co-immunoprecipitation of these proteins from transfected COS-7 cells. The sites of binding interaction between COUP-TF2 and FOG-2 were mapped to zinc fingers 5 and 6 and fingers 7 and 8 of FOG-2 and to the carboxyl terminus of the COUP-TF proteins. Binding to COUP-TF2 was specific because FOG-2 did not interact with the ligand-binding domains of retinoid X receptor alpha, glucocorticoid receptor, and peroxisome proliferating antigen receptor gamma, which are related to the COUP-TF proteins. Full-length FOG-2 markedly enhanced transcriptional repression by GAL4-COUP-TF2(117-414), but not by a COUP-TF2 repression domain mutant. Moreover, FOG-2 repressed COUP-TF2dependent synergistic activation of the atrial natriuretic factor promoter by both GATA4 and the FOG-2-independent mutant GATA4-E215K. Taken together, these findings suggest that FOG-2 functions as a corepressor for both GATA and COUP-TF proteins.

Inhibiting proteasomes in human HepG2 and LNCaP cells increases endogenous androgen receptor levels

Treating HepG2 cells with MG132 for 4 h to inhibit proteasomal activity increased androgen receptor immunoreactivity in two major bands with molecular weights of 102 and 110 kDa by 77% each (P < 0. 05). MG132 treatment also increased the overall level of polyubiquitinated proteins between 66 and 220 kDa by 140% (P < 0.05). Antiubiquitin immunoreactivity comigrating with the androgen receptor bands was also increased by MG132 treatment. Two other proteasome inhibitors, lactacystin and epoxomycin, caused similar increases in the androgen receptor in HepG2 cells. Proteosome-inhibition studies conducted in LNCaP cells also showed that the two major androgen receptor bands with molecular weights of 102 and 110 kDa were increased by 85 and 115%, respectively (P < 0. 05 for both) by MG132 treatment. Overall levels of polyubiquitinated proteins with molecular weights between 66 and 220 kDa increased 365%. Ubiquitin immunoreactivity comigrating with the androgen receptor bands was also significantly increased. Thus inhibiting proteasomes in two human androgen-responsive cell lines increases endogenous androgen receptor levels as well as androgen receptor-associated ubiquitin-modified immunoreactivity. The regulation of steady-state levels of endogenous androgen receptor by proteasomal degradation could be involved in its rapid turnover in the absence of ligand and would provide a mechanism for limiting androgen responses. A PEST sequence similar to one in the vitamin D receptor is present in the hinge region of all known mammalian androgen receptors, suggesting that it may function in proteasome-mediated androgen receptor turnover.

Ubiquitin-like proteins: new wines in new bottles

Ubiquitin is a small polypeptide that covalently modifies other cellular proteins and targets them to the proteasome for degradation. In recent years, ubiquitin-dependent proteolysis has been demonstrated to play a critical role in the regulation of many cellular processes, such as cell cycle progression, cell signaling, and immune recognition. The recent discovery of three new ubiquitin-like proteins, NEDD8, Sentrin/SUMO, and Apg12, has further broadened the horizon of this type of post-translational protein modification. This review will focus on the biology and biochemistry of the Sentrin/SUMO and NEDD8 modification pathways, which are clearly distinct from the ubiquitination pathway and have unique biological functions.

Regulation of microphthalmia-associated transcription factor MITF protein levels by association with the ubiquitin-conjugating enzyme hUBC9

The basic helix-loop-helix/leucine zipper (bHLH/ZIP) microphthalmia-associated transcription factor (MITF) regulates transcription of genes encoding enzymes essential for melanin biosynthesis in melanocytes and retinal pigmented epithelial cells. To determine how MITF activity is regulated, we used the yeast two-hybrid system to identify proteins expressed by human melanoma cells that interact with MITF. The majority of clones that showed positive interaction with a 158-amino-acid region of MITF containing the bHLH/ZIP domain (aa 168-325) encoded the ubiquitin conjugating enzyme hUBC9. The association of MITF with hUBC9 was further confirmed by an in vitro GST pull-down assay. Although hUBC9 is known to interact preferentially with SENTRIN/SUMO1, in vitro transcription/translation analysis demonstrated greater association of MITF with ubiquitin than with SENTRIN. Importantly, cotransfection of MITF and hUBC9 expression vectors resulted in MITF protein degradation. MITF protein was stabilized by the proteasome inhibitor MG132, indicating the role of the ubiquitin-proteasome system in MITF degradation. Serine 73, which is located in a region rich in proline, glutamic acid, serine, and threonine (PEST), regulates MITF protein stability, since a serine to alanine mutation prevented hUBC9-mediated MITF (S73A) degradation. Furthermore, we identified lysine 201 as a potential ubiquitination site. A lysine to arginine mutation abolished MITF (K201R) degradation by hUBC9 in vivo. Our experiments indicate that by targeting MITF for proteasome degradation, hUBC9 is a critical regulator of melanocyte differentiation.