APOBEC3B expression is promoted by lincNMR collaborating with TGF-ß-Smad pathway.

Long non-coding RNAs (lncRNAs) participate in carcinogenesis and cancer malignancies. Transforming growth factor-ß (TGF-ß) is involved in various cellular processes including cancer progression. We performed comprehensive RNA sequencing analyses to identify lncRNAs regulated by TGF-ß and found that lincNMR (long intergenic noncoding RNA-nucleotide metabolism regulator, also identified as MAP3K9-DT) was induced by TGF-ß in various cell lines. There are several variants of lincNMR (hereafter lincNMRs) in the lincNMR/MAP3K9-DT locus, and their expression was increased by TGF-ß. TGF-ß-mediated induction of lincNMRs was decreased by depletion of Smad2/3 in Huh7, suggesting that the TGF-ß-Smad pathway is involved in lincNMRs expression. We also found that APOBEC3B but not other APOBEC family members were a target gene of lincNMRs. APOBEC3B, a cytidine deaminase, promotes C to U mutation and highly expressed in various human cancers. Although it is associated with cancer progression, regulatory mechanisms of APOBEC3B expression have not been fully elucidated. We performed RNA immunoprecipitation assays and proved that lincNMRs bound to endogenous Smad2 in Huh7 cells. The increased activity of the promoter of APOBEC3B induced by overexpression of Smad2/3 was inhibited by depletion of lincNMRs. These data suggest that lincNMRs participate in APOBEC3B expression by collaborating with TGF-ß-Smad pathway. High expression of lincNMRs was positively correlated with high expression of APOBEC3B in various cancer cell lines. Overexpression of APOBEC3B as well as lincNMR was found in human cancers such as hepatic and lung cancers and was associated with their poor prognosis, suggesting that lincNMR may contribute to tumor malignancy via enhanced expression of APOBEC3B.

Homologous recombination is reduced in female embryonic stem cells by two active X chromosomes.

The reactivation of X-linked genes is observed in some primary breast tumors. Two active X chromosomes are also observed in female embryonic stem cells (ESCs), but whether double doses of X-linked genes affect DNA repair efficiency remains unclear. Here, we establish isogenic female/male ESCs and show that the female ESCs are more sensitive to camptothecin and have lower gene targeting efficiency than male ESCs, suggesting that homologous recombination (HR) efficiency is reduced in female ESCs. We also generate Xist-inducible female ESCs and show that the lower HR efficiency is restored when X chromosome inactivation is induced. Finally, we assess the X-linked genes with a role in DNA repair and find that Brcc3 is one of the genes involved in a network promoting proper HR. Our findings link the double doses of X-linked genes with lower DNA repair activity, and this may have relevance for common diseases in female patients, such as breast cancer.

Expression of Tumor Suppressor FHIT Is Regulated by the LINC00173-SNAIL Axis in Human Lung Adenocarcinoma.

Long non-coding RNAs (lncRNAs) play a critical role in a variety of human diseases such as cancer. Here, to elucidate a novel function of a lncRNA called LINC00173, we investigated its binding partner, target gene, and its regulatory mechanism in lung adenocarcinoma, including the A549 cell line and patients. In the A549 cell line, RNA immunoprecipitation (RIP) assays revealed that LINC00173 efficiently binds to SNAIL. RNA-seq and RT-qPCR analyses revealed that the expression of FHIT was decreased upon LINC00173 depletion, indicating that FHIT is a target gene of LINC00173. Overexpression of SNAIL suppressed and depletion of SNAIL increased the expression of FHIT, indicating that SNAIL negatively regulates FHIT. The downregulation of FHIT expression upon LINC00173 depletion was restored by additional SNAIL depletion, revealing a LINC00173-SNAIL-FHIT axis for FHIT regulation. Data from 501 patients with lung adenocarcinoma also support the existence of a LINC00173-SNAIL-FHIT axis, as FHIT expression correlated positively with LINC00173 (p = 1.75 × 10-6) and negatively with SNAIL (p = 7.00 × 10-5). Taken together, we propose that LINC00173 positively regulates FHIT gene expression by binding to SNAIL and inhibiting its function in human lung adenocarcinoma. Thus, this study sheds light on the LINC00173-SNAIL-FHIT axis, which may be a key mechanism for carcinogenesis and progression in human lung adenocarcinoma.

Inhibiting Skp2 E3 Ligase Suppresses Bleomycin-Induced Pulmonary Fibrosis.

Idiopathic pulmonary fibrosis (IPF) is a progressive disease with poor prognosis and no curative therapies. SCF-Skp2 E3 ligase is a target for cancer therapy, but there have been no reports about Skp2 as a target for IPF. Here we demonstrate that Skp2 is a promising therapeutic target for IPF. We examined whether disrupting Skp2 suppressed pulmonary fibrosis in a bleomycin (BLM)-induced mouse model and found that pulmonary fibrosis was significantly suppressed in Skp2-deficient mice compared with controls. The pulmonary accumulation of fibrotic markers such as collagen type 1 and fibronectin in BLM-infused mice was decreased in Skp2-deficient mice. Moreover, the number of bronchoalveolar lavage fluid cells accompanied with pulmonary fibrosis was significantly diminished. Levels of the Skp2 target p27 were significantly decreased by BLM-administration in wild-type mice, but recovered in Skp2-/- mice. In vimentin-positive mesenchymal fibroblasts, the decrease of p27-positive cells and increase of Ki67-positive cells by BLM-administration was suppressed by Skp2-deficency. As these results suggested that inhibiting Skp2 might be effective for BLM-induced pulmonary fibrosis, we next performed a treatment experiment using the Skp2 inhibitor SZL-P1-41. As expected, BLM-induced pulmonary fibrosis was significantly inhibited by SZL-P1-41. Moreover, p27 levels were increased by the SZL-P1-41 treatment, suggesting p27 may be an important Skp2 target for BLM-induced pulmonary fibrosis. Our study suggests that Skp2 is a potential molecular target for human pulmonary fibrosis including IPF.

Regulation of GATA-binding protein 2 levels via ubiquitin-dependent degradation by Fbw7: involvement of cyclin B-cyclin-dependent kinase 1-mediated phosphorylation of THR176 in GATA-binding protein 2.

A GATA family transcription factor, GATA-binding protein 2 (GATA2), participates in cell growth and differentiation of various cells, such as hematopoietic stem cells. Although its expression level is controlled by transcriptional induction and proteolytic degradation, the responsible E3 ligase has not been identified. Here, we demonstrate that F-box/WD repeat-containing protein 7 (Fbw7/Fbxw7), a component of Skp1, Cullin 1, F-box-containing complex (SCF)-type E3 ligase, is an E3 ligase for GATA2. GATA2 contains a cell division control protein 4 (Cdc4) phosphodegron (CPD), a consensus motif for ubiquitylation by Fbw7, which includes Thr(176). Ectopic expression of Fbw7 destabilized GATA2 and promoted its proteasomal degradation. Substitution of threonine 176 to alanine in GATA2 inhibited binding with Fbw7, and the ubiquitylation and degradation of GATA2 by Fbw7 was suppressed. The CPD kinase, which mediates the phosphorylation of Thr(176), was cyclin B-cyclin-dependent kinase 1 (CDK1). Moreover, depletion of endogenous Fbw7 stabilized endogenous GATA2 in K562 cells. Conditional Fbw7 depletion in mice increased GATA2 levels in hematopoietic stem cells and myeloid progenitors at the early stage. Increased GATA2 levels in Fbw7-conditional knock-out mice were correlated with a decrease in a c-Kit high expressing population of myeloid progenitor cells. Our results suggest that Fbw7 is a bona fide E3 ubiquitin ligase for GATA2 in vivo.

Interaction between RB protein and NuMA is required for proper alignment of spindle microtubules.

Retinoblastoma protein (pRB) controls cell cycle progression and cell cycle exit through interactions with cellular proteins. Many pRB-binding proteins, which function in gene transcription or modulation of chromatin structure, harbor LXCXE motifs in their binding domain for pRB. In this study, we found that nuclear mitotic apparatus protein (NuMA), a mitotic spindle organizer, interacts with pRB through LSCEE sequences located in its C-terminal region. siRNA-mediated down-regulation of pRB caused aberrant distribution of NuMA and alignment of spindle microtubules in mitotic cells. Abnormal organization of spindle microtubules was also accompanied by misalignment of an over-expressed NuMA mutant (mut-NuMA) with a defect in pRB binding caused by an LSGEK mutation. The mut-NuMA-over-expressing cells showed lower potency for survival than wild-type NuMA (wt-NuMA)-over-expressing cells during 2 weeks of culture. Interestingly, knockdown of pRB reduced the population of wt-NuMA-over-expressing cells to the same level as mut-NuMA cells after 2 weeks. Taken together, pRB may have a novel function in regulating the mitotic function of NuMA and spindle organization, which are required for proper cell cycle progression.

Coordinated regulation of differentiation and proliferation of embryonic cardiomyocytes by a jumonji (Jarid2)-cyclin D1 pathway.

In general, cell proliferation and differentiation show an inverse relationship, and are regulated in a coordinated manner during development. Embryonic cardiomyocytes must support embryonic life by functional differentiation such as beating, and proliferate actively to increase the size of the heart. Therefore, progression of both proliferation and differentiation is indispensable. It remains unknown whether proliferation and differentiation are related in these embryonic cardiomyocytes. We focused on abnormal phenotypes, such as hyperproliferation, inhibition of differentiation and enhanced expression of cyclin D1 in cardiomyocytes of mice with mutant jumonji (Jmj, Jarid2), which encodes the repressor of cyclin D1. Analysis of Jmj/cyclin D1 double mutant mice showed that Jmj was required for normal differentiation and normal expression of GATA4 protein through cyclin D1. Analysis of transgenic mice revealed that enhanced expression of cyclin D1 decreased GATA4 protein expression and inhibited the differentiation of cardiomyocytes in a CDK4/6-dependent manner, and that exogenous expression of GATA4 rescued the abnormal differentiation. Finally, CDK4 phosphorylated GATA4 directly, which promoted the degradation of GATA4 in cultured cells. These results suggest that CDK4 activated by cyclin D1 inhibits differentiation of cardiomyocytes by degradation of GATA4, and that initiation of Jmj expression unleashes the inhibition by repression of cyclin D1 expression and allows progression of differentiation, as well as repression of proliferation. Thus, a Jmj-cyclin D1 pathway coordinately regulates proliferation and differentiation of cardiomyocytes.

p130RB2 positively contributes to ATR activation in response to replication stress via the RPA32-ETAA1 axis.

Ataxia-telangiectasia mutated and Rad3-related (ATR) kinase is a crucial regulator of the cell cycle checkpoint and activated in response to DNA replication stress by two independent pathways via RPA32-ETAA1 and TopBP1. However, the precise activation mechanism of ATR by the RPA32-ETAA1 pathway remains unclear. Here, we show that p130RB2, a member of the retinoblastoma protein family, participates in the pathway under hydroxyurea-induced DNA replication stress. p130RB2 binds to ETAA1, but not TopBP1, and depletion of p130RB2 inhibits the RPA32-ETAA1 interaction under replication stress. Moreover, p130RB2 depletion reduces ATR activation accompanied by phosphorylation of its targets RPA32, Chk1, and ATR itself. It also causes improper re-progression of S phase with retaining single-stranded DNA after cancelation of the stress, which leads to an increase in the anaphase bridge phenotype and a decrease in cell survival. Importantly, restoration of p130RB2 rescued the disrupted phenotypes of p130RB2 knockdown cells. These results suggest positive involvement of p130RB2 in the RPA32-ETAA1-ATR axis and proper re-progression of the cell cycle to maintain genome integrity.

CCIVR2 facilitates comprehensive identification of both overlapping and non-overlapping antisense transcripts within specified regions.

Pairs of sense and antisense transcriptions that are adjacent at their 5' and 3' regions are called divergent and convergent transcription, respectively. However, the structural properties of divergent/convergent transcription in different species or RNA biotypes are poorly characterized. Here, we developed CCIVR2, a program that facilitates identification of both overlapping and non-overlapping antisense transcripts produced from divergent/convergent transcription whose transcription start sites (TSS) or transcript end sites (TES) are located within a specified region. We used CCIVR2 to analyze antisense transcripts starting around the sense TSS (from divergent transcription) or ending around the sense TES (from convergent transcription) in 11 different species and found species- and RNA biotype-specific features of divergent/convergent transcription. Furthermore, we confirmed that CCIVR2 enables the identification of multiple sense/antisense transcript pairs from divergent transcription, including those with known functions in processes such as embryonic stem cell differentiation and TGFß stimulation. CCIVR2 is therefore a valuable bioinformatics tool that facilitates the characterization of divergent/convergent transcription in different species and aids the identification of functional sense/antisense transcript pairs from divergent transcription in specified biological processes.

Activity-based protein profiling of a surfactin-producing nonribosomal peptide synthetase in Bacillus subtilis.

We present an in vitro and in-cell activity-based protein profiling (ABPP) protocol for endogenous nonribosomal peptide synthetases (NRPSs). This protocol enables the fluorescence labeling and imaging of an endogenous SrfAB-NRPS with high selectivity and sensitivity in the surfactin producer Bacillus subtilis. While we optimized this protocol for use with B. subtilis, the protocol can be applied to Aneurinibacillus migulanus and Escherichia coli. For complete details on the use and execution of this protocol, please refer to Ishikawa et al. (2022).

Chemoproteomics profiling of surfactin-producing nonribosomal peptide synthetases in living bacterial cells.

Much of our current knowledge on nonribosomal peptide synthetases (NRPSs) is based on studies in which the full NRPS system or each protein domain is expressed in heterologous hosts. Consequently, methods to detect the endogenous activity of NRPSs, under natural cellular conditions, are needed for the study of NRPS cell biology. Here, we describe the in vivo activity-based protein profiling (ABPP) for endogenous NRPSs and its applications to the study of their activities in bacteria. Remarkably, in vitro and in vivo ABPP in the context of the surfactin producer Bacillus subtilis enabled the visualization, tracking, and imaging of an endogenous SrfAB-NRPS with remarkable selectivity and sensitivity. Furthermore, in vivo, ABPP allowed the discovery of the degradation processes of the endogenous SrfAB-NRPS in the context of its native producer bacteria. Overall, this study deepens our understanding of the properties of NRPSs that cannot be addressed by conventional methods.

CCIVR facilitates comprehensive identification of cis-natural antisense transcripts with their structural characteristics and expression profiles.

Cis-natural antisense transcripts (cis-NATs) are transcribed from the same genomic locus as their partner gene but from the opposite DNA strand and overlap with the partner gene transcript. Here, we developed a simple and convenient program termed CCIVR (comprehensive cis-NATs identifier via RNA-seq data) that comprehensively identifies all kinds of cis-NATs based on genome annotation with expression data obtained from RNA-seq. Using CCIVR with genome databases, we demonstrated total cis-NAT pairs from 11 model organisms. CCIVR analysis with RNA-seq data from parthenogenetic and androgenetic embryonic stem cells identified well-known imprinted cis-NAT pair, KCNQ1/KCNQ1OT1, ensuring the availability of CCIVR. Finally, CCIVR identified cis-NAT pairs that demonstrate inversely correlated expression upon TGFß stimulation including cis-NATs that functionally repress their partner genes by introducing epigenetic alteration in the promoters of partner genes. Thus, CCIVR facilitates the investigation of structural characteristics and functions of cis-NATs in numerous processes in various species.

Chromatin-remodeling factor BAZ1A/ACF1 targets UV damage sites in an MLL1-dependent manner to facilitate nucleotide excision repair.

Ultraviolet (UV) light irradiation generates pyrimidine dimers on DNA, such as cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts. Such dimers distort the high-order DNA structure and prevent transcription and replication. The nucleotide excision repair (NER) system contributes to resolving this type of DNA lesion. There are two pathways that recognize pyrimidine dimers. One acts on transcribed strands of DNA (transcription-coupled NER), and the other acts on the whole genome (global genome-NER; GG-NER). In the latter case, DNA damage-binding protein 2 (DDB2) senses pyrimidine dimers with several histone modification enzymes. We previously reported that histone acetyltransferase binding to ORC1 (HBO1) interacts with DDB2 and facilitates recruitment of the imitation switch chromatin remodeler at UV-irradiated sites via an unknown methyltransferase. Here, we found that the phosphorylated histone methyltransferase mixed lineage leukemia 1 (MLL1) was maintained at UV-irradiated sites in an HBO1-dependent manner. Furthermore, MLL1 catalyzed histone H3K4 methylation and recruited the chromatin remodeler bromodomain adjacent to zinc finger domain 1A (BAZ1A)/ATP-utilizing chromatin assembly and remodeling factor 1 (ACF1). Depletion of MLL1 suppressed BAZ1A accumulation at UV-irradiated sites and inhibited the removal of CPDs. These data indicate that the DDB2-HBO1-MLL1 axis is essential for the recruitment of BAZ1A to facilitate GG-NER.

Dynamics of transcription-mediated conversion from euchromatin to facultative heterochromatin at the Xist promoter by Tsix.

The fine-scale dynamics from euchromatin (EC) to facultative heterochromatin (fHC) has remained largely unclear. Here, we focus on Xist and its silencing initiator Tsix as a paradigm of transcription-mediated conversion from EC to fHC. In mouse epiblast stem cells, induction of Tsix recapitulates the conversion at the Xist promoter. Investigating the dynamics reveals that the conversion proceeds in a stepwise manner. Initially, a transient opened chromatin structure is observed. In the second step, gene silencing is initiated and dependent on Tsix, which is reversible and accompanied by simultaneous changes in multiple histone modifications. At the last step, maintenance of silencing becomes independent of Tsix and irreversible, which correlates with occupation of the -1 position of the transcription start site by a nucleosome and initiation of DNA methylation introduction. This study highlights the hierarchy of multiple chromatin events upon stepwise gene silencing establishment.

Adenovirus E1A inhibits SCF(Fbw7) ubiquitin ligase.

The SCF(Fbw7) ubiquitin ligase complex plays important roles in cell growth, survival, and differentiation via the ubiquitin-proteasome-mediated regulation of protein stability. Fbw7 (also known as Fbxw7, Sel-10, hCdc4, or hAgo), a substrate recognition subunit of SCF(Fbw7) ubiquitin ligase, facilitates the degradation of several proto-oncogene products by the proteasome. Given that mutations in Fbw7 are found in various types of human cancers, Fbw7 is considered to be a potent tumor suppressor. In the present study, we show that E1A, an oncogene product derived from adenovirus, interferes with the activity of the SCF(Fbw7) ubiquitin ligase. E1A interacted with SCF(Fbw7) and attenuated the ubiquitylation of its target proteins in vivo. Furthermore, using in vitro purified SCF(Fbw7) component proteins, we found that E1A directly bound to Roc1/Rbx1 and CUL1 and that E1A inhibited the ubiquitin ligase activity of the Roc1/Rbx1-CUL1 complex but not that of another RING-type ubiquitin ligase, Mdm2. Ectopically expressed E1A interacted with cellular endogenous Roc1/Rbx1 and CUL1 and decelerated the degradation of several protooncogene products that were degraded by SCF(Fbw7) ubiquitin ligase. Moreover, after wild-type adenovirus infection, adenovirus-derived E1A interacted with endogenous Roc1/Rbx1 and decelerated degradation of the endogenous target protein of SCF(Fbw7). These observations demonstrated that E1A perturbs protein turnover regulated by SCF(Fbw7) through the inhibition of SCF(Fbw7) ubiquitin ligase. Our findings may help to explain the mechanism whereby adenovirus infection induces unregulated proliferation.

Reduction of transforming growth factor-beta type II receptor is caused by the enhanced ubiquitin-dependent degradation in human renal cell carcinoma.

Although dysregulation of transforming growth factor-beta (TGF-beta) signaling is implicated in renal carcinogenesis, its precise mechanism is unknown in renal cell carcinoma (RCC). In our study, we investigated Smad-mediated TGF-beta signaling pathway and its regulatory mechanisms in surgical samples from patients with RCC. We found that immunoreactivity for nuclear phosphorylated Smad2 was significantly decreased in RCC compared to normal renal tissues, thereby TGF-beta signaling was suggested to be attenuated in RCC tissues. In accordance with the result, transcriptional downregulation of Smad4 and post-transcriptional downregulation of TGF-beta type II receptor (TbetaR-II) were frequently found in RCC tissues compared to normal renal tissues. Next, to clarify the reason why the protein level of TbetaR-II was decreased in RCC, we investigated the activities of degradation and ubiquitination of TbetaR-II. We found that both proteasome-mediated degradation and ubiquitination of TbetaR-II were markedly enhanced in RCC tissues. Moreover, we found that the level of Smad-ubiquitination regulatory factor 2 (Smurf2), the E3 ligase for TbetaR-II, was increased in RCC tissues of the patients with higher clinical stages compared to the normal tissues and was inversely correlated with the level of TbetaR-II. Our results suggest that the low TbetaR-II protein level is due to augmented ubiquitin-dependent degradation via Smurf2 and might be involved in the attenuation of TGF-beta signaling pathway in RCC.

Decrease in tumor necrosis factor-alpha receptor-associated death domain results from ubiquitin-dependent degradation in obstructive renal injury in rats.

Increased expression levels of tumor necrosis factor-alpha (TNFalpha) is involved in tubulointerstitial cell proliferation and apoptosis in obstructive renal injury. Two TNFalpha receptors (TNFRs), TNFR1 and TNFR2, are known to exist. On TNFalpha binding, TNFR1 recruits TNFR-associated death domain (TRADD), an assembly platform to mediate TNFR1 signaling. We investigated postreceptor TRADD regulation in rat kidneys with unilateral ureteral obstruction (UUO). Whereas UUO was associated with increased expression levels of TNFalpha, TNFR1, TNFR2, and TRADD mRNAs, it resulted in the marked decrease of TRADD protein levels (which appeared at day 1 and persisted thereafter) and a slight decrease in TNFR1 protein levels at days 7 and 14. Both ubiquitination and degradation of TRADD were increased in UUO kidneys, degradation of TRADD was stimulated by TNFalpha in HK-2 cells, and TRADD degradation was suppressed by proteasome inhibitor. Inhibition of TNFalpha by soluble TNFR2, etanercept, reduced significantly, although transiently, tubular and interstitial cell proliferation, fibronectin expression, and apoptosis in UUO kidneys, and also suppressed TRADD degradation. These data suggest that the decrease in TRADD resulting from enhanced ubiquitin-dependent degradation is involved in obstructive renal injury. Since TRADD is not incorporated into TNFR2-mediated TNFalpha signaling, the persistent decrease in TRADD, associated with a mild decrease in TNFR1 levels, may function, at least in part, to divert TNFalpha signals toward a TNFR2-mediated pathway in UUO kidneys.

HDAC3 Is Required for XPC Recruitment and Nucleotide Excision Repair of DNA Damage Induced by UV Irradiation.

Recent studies have demonstrated that lysine acetylation of histones is crucial for nucleotide excision repair (NER) by relaxing the chromatin structure, which facilitates the recruitment of repair factors. However, few studies have focused on the contribution of histone deacetylases (HDAC) to NER. Here, we found that histone H3 Lys14 (H3K14) was deacetylated by HDAC3 after UV irradiation. Depletion of HDAC3 caused defects in cyclobutene pyrimidine dimer excision and sensitized cells to UV irradiation. HDAC3-depleted cells had impaired unscheduled DNA synthesis, but not recovery of RNA synthesis, which indicates that HDAC3 was required for global genome NER. Moreover, xeroderma pigmentosum, complementation group C (XPC) accumulation at the local UV-irradiated area was attenuated in HDAC3-depleted cells. In addition to the delay of XPC accumulation at DNA damage sites, XPC ubiquitylation was inhibited in HDAC3-depleted cells. These results suggest that the deacetylation of histone H3K14 by HDAC3 after UV irradiation contributes to XPC recruitment to DNA lesions to promote global genome NER. IMPLICATIONS: Involvement of histone deacetylation for XPC accumulation after UV irradiation indicates conversion of chromatin structure is essential for nucleotide excision repair in human cancer cells.

Substitution of Thr572 to Ala in mouse c-Myb attenuates progression of early erythroid differentiation.

The expression level of transcription factor c-Myb oscillates during hematopoiesis. Fbw7 promotes ubiquitin-mediated degradation of c-Myb, which is dependent on phosphorylation of Thr572. To investigate the physiological relevance of Fbw7-mediated c-Myb degradation, we generated mutant mice carrying c-Myb-T572A (TA). Homozygous mutant (TA/TA) mice exhibited a reduction in the number of peripheral red blood cells and diminished erythroblasts in bone marrow, presumably as a result of failure during erythroblast differentiation. We found that c-Myb high-expressing cells converged in the Lin-CD71+ fraction, and the expression of c-Myb was higher in TA/TA mice than in wild-type mice. Moreover, TA/TA mice had an increased proportion of the CD71+ subset in Lin- cells. The c-Myb level in the Lin-CD71+ subset showed three peaks, and the individual c-Myb level was positively correlated with that of c-Kit, a marker of undifferentiated cells. Ultimately, the proportion of c-Mybhi subgroup was significantly increased in TA/TA mice compared with wild-type mice. These results indicate that a delay in reduction of c-Myb protein during an early stage of erythroid differentiation creates its obstacle in TA/TA mice. In this study, we showed the T572-dependent downregulation of c-Myb protein is required for proper differentiation in early-stage erythroblasts, suggesting the in vivo significance of Fbw7-mediated c-Myb degradation.

High expression of Pirh2, an E3 ligase for p27, is associated with low expression of p27 and poor prognosis in head and neck cancers.

Downregulation of the cyclin-dependent kinase inhibitory protein p27 is frequently observed in various cancers due to enhancement of its degradation. We recently reported that p53-inducible protein with RING-H2 domain (Pirh2) is a novel ubiquitin ligase for p27, required for the ubiquitylation and consequent degradation of p27 protein. However, there is no reports about the involvement of Pirh2 in both p27 downregulation and pathogenesis in human cancers. In the present study, we investigated them using cultured cell lines and surgical specimens derived from human head and neck squamous cell carcinoma (HNSCC). Depletion of Pirh2 by short interfering RNA induced accumulation of p27 and inhibited the growth of cultured HNSCC cells. By immunohistochemical analysis in 57 cases of HNSCC specimens, higher levels of Pirh2 expression (labeling index > or = 60%) were found in 61.4% of HNSCC in comparison with 0% of normal mucosa. In addition, 83.3% of HNSCC with lower p27 expression (labeling index < 20%) displayed high Pirh2 levels. Therefore, Pirh2 expression was inversely correlated with p27 expression. Finally, Pirh2 expression was well correlated with poor prognosis. These findings suggest that Pirh2 overexpression may have an important role in the development and maintenance of HNSCC at least partially through p27 degradation, and that Pirh2 may be a potential molecular target for human HNSCC.