Ubiquitin-dependent proteolysis of trihydrophobin 1 (TH1) by the human papilloma virus E6-associated protein (E6-AP)

Neuronal UBE3A substrates hold therapeutic potential for Angelman syndrome

Emerging therapies for Angelman syndrome, a severe neurodevelopmental disorder, are focused on restoring UBE3A gene expression in the brain. Further therapeutic opportunities may arise from a better understanding of how UBE3A gene products-both long and short isoforms of the ubiquitin ligase E3A (UBE3A)-function in neurons. Great strides have been made recently toward identifying ubiquitin substrates of UBE3A in vitro and in heterologous expression systems. From this work, a particularly close relationship between UBE3A and subunits of the 19S regulatory particle of the proteasome has become evident. We propose that further research cognizant of isoform-specific UBE3A functional roles will be instrumental in elucidating key UBE3A/substrate relationships within distinct neuronal compartments, lending to the discovery of novel therapeutic targets and valuable clinical biomarkers for the treatment of Angelman syndrome.

Inhibition of hepatitis B virus replication by cIAP2 involves accelerating the ubiquitin-proteasome-mediated destruction of polymerase

Cellular inhibitor of apoptosis protein 2 (cIAP2) is a potent suppressor of apoptotic cell death. We have shown previously that cIAP2 is involved in the tumor necrosis factor alpha (TNF-α)-induced anti-hepatitis B virus (HBV) response; however, the mechanism for this antiviral effect remains unclear. In the present study, we demonstrate that cIAP2 can significantly reduce the levels of HBV DNA replication intermediates but not the total viral RNA or core protein levels. Domain-mapping analysis revealed that the carboxy-terminal domains of cIAP2 were indispensable for this anti-HBV ability and that an E3 ligase-deficient mutant of cIAP2 (termed cIAP2*) completely lost its antiviral activity. We further identified HBV polymerase as the target of cIAP2. Overexpression of cIAP2 but not cIAP2* reduced polymerase protein levels, while cIAP2 knockdown increased polymerase expression. In addition, we observed that cIAP2 promoted the degradation of the viral polymerase through a proteasome-dependent pathway. Further experiments demonstrated that cIAP2 can bind to polymerase and promote its polyubiquitylation. Finally, we found that cIAP2 downregulated the encapsidation of HBV pregenomic RNA. Taken together, these data reveal a novel mechanism for the inhibition of HBV replication by cIAP2 via acceleration of the ubiquitin-proteasome-mediated decay of polymerase and reduction of the encapsidation of HBV pregenomic RNA, making this mechanism a novel strategy for HBV therapy.

Angelman syndrome protein UBE3A interacts with primary microcephaly protein ASPM, localizes to centrosomes and regulates chromosome segregation

Many proteins associated with the phenotype microcephaly have been localized to the centrosome or linked to it functionally. All the seven autosomal recessive primary microcephaly (MCPH) proteins localize at the centrosome. Microcephalic osteodysplastic primordial dwarfism type II protein PCNT and Seckel syndrome (also characterized by severe microcephaly) protein ATR are also centrosomal proteins. All of the above findings show the importance of centrosomal proteins as the key players in neurogenesis and brain development. However, the exact mechanism as to how the loss-of-function of these proteins leads to microcephaly remains to be elucidated. To gain insight into the function of the most commonly mutated MCPH gene ASPM, we used the yeast two-hybrid technique to screen a human fetal brain cDNA library with an ASPM bait. The analysis identified Angelman syndrome gene product UBE3A as an ASPM interactor. Like ASPM, UBE3A also localizes to the centrosome. The identification of UBE3A as an ASPM interactor is not surprising as more than 80% of Angelman syndrome patients have microcephaly. However, unlike in MCPH, microcephaly is postnatal in Angelman syndrome patients. Our results show that UBE3A is a cell cycle regulated protein and its level peaks in mitosis. The shRNA knockdown of UBE3A in HEK293 cells led to many mitotic abnormalities including chromosome missegregation, abnormal cytokinesis and apoptosis. Thus our study links Angelman syndrome protein UBE3A to ASPM, centrosome and mitosis for the first time. We suggest that a defective chromosome segregation mechanism is responsible for the development of microcephaly in Angelman syndrome.

E6AP ubiquitin ligase mediates ubiquitin-dependent degradation of peroxiredoxin 1

E6-associated protein (E6AP) is a cellular ubiquitin protein ligase that mediates ubiquitylation and degradation of tumor suppressor p53 in conjunction with the high-risk human papillomavirus E6 protein. We previously reported that E6AP targets annexin A1 protein for ubiquitin-dependent proteasomal degradation. To gain a better understanding of the physiological function of E6AP, we have been seeking to identify novel substrates of E6AP. Here, we identified peroxiredoxin 1 (Prx1) as a novel E6AP-binding protein using a tandem affinity purification procedure coupled with mass spectrometry. Prx1 is a 25-kDa member of the Prx family, a ubiquitous family of antioxidant peroxidases that regulate many cellular processes through intracellular oxidative signal transduction pathways. Immunoprecipitation analysis showed that E6AP binds Prx1 in vivo. Pull-down experiments showed that E6AP binds Prx1 in vitro. Ectopic expression of E6AP enhanced the degradation of Prx1 in vivo. In vivo and in vitro ubiquitylation assays revealed that E6AP promoted polyubiquitylation of Prx1. RNAi-mediated downregulation of endogenous E6AP increased the level of endogenous Prx1 protein. Taken together, our data suggest that E6AP mediates the ubiquitin-dependent proteasomal degradation of Prx1. Our findings raise a possibility that E6AP may play a role in regulating Prx1-dependent intracellular oxidative signal transduction pathways.

Negative role of trihydrophobin 1 in breast cancer growth and migration

Trihydrophobin 1 (TH1) is a member of the negative elongation factor complex, which is involved in transcriptional pausing. Although the negative elongation factor complex attenuates the estrogen receptor α-mediated transcription, little is known about the relationship between TH1 and tumor progression. Here, we report that the protein level of TH1 was negatively correlated with the aggressiveness of human breast cancer. Immunohistochemical analysis revealed that TH1 expression in clinical stage III-IV primary breast cancer tissues was statistically significantly lower than that in stage I-II breast tissues (P < 0.01), and especially inversely associated with lymph node metastasis (P < 0.001). Furthermore, we showed that overexpression of TH1 in MDA-MB-231 breast cancer cells inhibited, and knockdown of TH1 in MCF-7 cells enhanced, cell proliferation and migratory ability. Moreover, upregulation of TH1 in MDA-MB-231 cells resulted in the decrease of cyclin D1, β-catenin, and ERK activity, and the increase of p21. In contrast, knockdown of TH1 in MCF-7 cells enhanced the expression of cyclin D1 and β-catenin, increased the activity of ERK, and downregulated the expression of p21. Additionally, overexpression of TH1 in MDA-MB-231 cells prevented. However, knockdown of TH1 in MCF-7 cells induced a number of molecular and cellular alterations associated with epithelial-mesenchymal transition. Taken together, our results suggest that TH1 might play an important role in regulation of proliferation and invasion in human breast cancer, and could be a potential target for human breast cancer treatment.

Trihydrophobin 1 attenuates androgen signal transduction through promoting androgen receptor degradation

The androgen-signaling pathway plays critical roles in normal prostate development, benign prostatic hyperplasia, established prostate cancer, and in prostate carcinogenesis. In this study, we report that trihydrophobin 1 (TH1) is a potent negative regulator to attenuate the androgen signal-transduction cascade through promoting androgen receptor (AR) degradation. TH1 interacts with AR both in vitro and in vivo, decreases the stability of AR, and promotes AR ubiquitination in a ligand-independent manner. TH1 also associates with AR at the active androgen-responsive prostate-specific antigen (PSA) promoter in the nucleus of LNCaP cells. Decrease of endogenous AR protein by TH1 interferes with androgen-induced luciferase reporter expression and reduces endogenous PSA expression. Taken together, these results indicate that TH1 is a novel regulator to control the duration and magnitude of androgen signal transduction and might be directly involved in androgen-related developmental, physiological, and pathological processes.

Trihydrophobin 1 Interacts with PAK1 and Regulates ERK/MAPK Activation and Cell Migration

The Rac1/Cdc42 effector, p21-activated kinase (PAK), is activated by various signaling cascades, including receptor-tyrosine kinases and integrins, and regulates a number of processes such as cell proliferation and motility. PAK activity has been shown to be required for maximal activation of the canonical RAF-MEK-MAPK signaling cascade, possibly because of PAK co-activation of RAF and MEK. Here we have shown that trihydrophobin 1 (TH1), originally identified as a negative regulator of A-RAF kinase, also interacted with PAK1 in cultured cells. Confocal microscopy assay indicated that TH1 colocalized with PAK1 in both the cytoplasm and nucleus, which is consistent with our previous results. GST pulldown and coimmunoprecipitation experiments demonstrated that TH1 interacted directly with PAK1 and bound selectively to the carboxyl-terminal kinase domain of PAK1, and the ability of the binding was enhanced along with activation of PAK1. The binding pattern of PAK1 implies that this interaction was mediated in part by PAK1 kinase activity. As indicated by in vitro kinase activity assays and Western blot detections, TH1 inhibited PAK1 kinase activity and negatively regulated MAPK signal transduction. Interestingly, TH1 bound with MEK1/ERK in cells and in vitro without directly suppressing their kinase activity. Furthermore, we observed that TH1 localized to focal adhesions and filopodia in the leading edge of cells, where TH1 reduced cell migration through affecting actin and adhesion dynamics. Based on these observations, we propose a model in which TH1 interacts with PAK1 and specifically restricts the activation of MAPK modules through the upstream region of the MAPK pathway, thereby influencing cell migration.

Cdc34-mediated degradation of ATF5 is blocked by cisplatin

ATF5, a member of activating transcription factor (ATF)/cAMP-response element-binding protein (CREB) family of b-ZIP transcription factors, contributes to neural cell differentiation and is involved in cell apoptosis in response to cisplatin and a number of environment factors. However, the mechanisms governing the regulation of ATF5 protein during apoptosis are largely unknown. In this study we reported that ATF5 protein was a substrate of the ubiquitin-proteasome pathway. Interestingly, the ubiquitin-dependent degradation of exogenous ATF5 protein was independent of lysine residues. Instead, the addition of a large N-terminal enhanced green fluorescence protein tag increased the stability of ATF5 protein, and the free amino acid group of the N-terminal methionine of ATF5 protein was a site for ubiquitinylation, indicating that exogenous ATF5 was degraded via the ubiquitin-proteasome system through N-terminal ubiquitinylation. Furthermore, cisplatin increased ATF5 protein expression via preventing its ubiquitin-dependent degradation, which might be associated with its promoting the nucleus-to-cytoplasm translocation of E2 ubiquitin-conjugating enzyme Cdc34 and reducing the interaction between ATF5 and Cdc34. In summary, a down-regulation of proteasome-mediated degradation of ATF5 might contribute to cisplatin-induced apoptosis, providing a new mechanism of cisplatin-induced apoptosis.