Masataka Sugimoto

Activation of cyclin D1-kinase in murine fibroblasts lacking both p21(Cip1) and p27(Kip1).

Deregulation of D-type cyclin-dependent kinases (CDK4 and 6) is widely observed in various human cancers, illustrating their importance in cell cycle control. Like other cyclin-dependent kinases (CDKs), assembly with cyclins is the most critical step for activation of CDK4/6. As previously reported elsewhere, we observed that the level of cyclinD1-CDK4 complex and its associated kinase activity were significantly low in asynchronously proliferating mouse embryo fibroblasts lacking both p21Cip1 and p27Kip1 (p21/p27-null MEFs). These evidences imply that p21Cip1 and p27Kip1 CDK inhibitors are ‘essential activators’ of cyclin D-kinases. We, however, discovered here that both the assembly and activation of cyclin D1-CDK4 complex occur when quiescent p21/p27-null MEFs were stimulated to re-enter the cell cycle. This mitogen-induced cyclin D1-kinase activity was blocked by overexpression of p16INK4a and resulted in the inhibition of S phase entry in p21/p27-null MEFs. Furthermore, ectopic expression of p34SEI-1, a mitogen-induced CDK4 binding protein, increased the levels of active cyclinD1-CDK4 complex in asynchronously proliferating p21/p27-null MEFs. Together, our results suggest that there are several independent ways to stimulate the assembly of cyclin D1-CDK4 kinases. Although p21Cip1 and p27Kip1 play a role in this process, our results demonstrate that additional mechanisms must occur in G0 to S phase transition.

Hzf, a p53-responsive gene, regulates maintenance of the G2 phase checkpoint induced by DNA damage.

ABSTRACT The hematopoietic zinc finger protein, Hzf, is induced in response to genotoxic and oncogenic stress. The Hzf protein is encoded by a p53-responsive gene, and its overexpression, either in cells retaining or lacking functional 53, halts their proliferation. Enforced expression of Hzf led to the appearance of tetraploid cells with supernumerary centrosomes and, ultimately, to cell death. Eliminating Hzf mRNA expression by use of short hairpin (sh) RNAs had no overt effect on unstressed cells but inhibited the maintenance of G2 phase arrest following ionizing radiation (IR), thereby sensitizing cells to DNA damage. Canonical p53-responsive gene products such as p21Cip1 and Mdm2 were induced by IR in cells treated with Hzf shRNA. However, the reduction in the level of Hzf protein was accompanied by increased polyubiquitination and turnover of p21Cip1, an inhibitor of cyclin-dependent kinases whose expression contributes to maintaining the duration of the G2 checkpoint in cells that have sustained DNA damage. Thus, two p53-inducible gene products, Hzf and p21Cip1, act concomitantly to enforce the G2 checkpoint.

Elimination of p19ARF-expressing cells enhances pulmonary function in mice

Senescent cells accumulate in many tissues as animals age and are considered to underlie several aging-associated pathologies. The tumor suppressors p19ARF and p16INK4a, both of which are encoded in the CDKN2A locus, play critical roles in inducing and maintaining permanent cell cycle arrest during cellular senescence. Although the elimination of p16INK4a-expressing cells extends the life span of the mouse, it is unclear whether tissue function is restored by the elimination of senescent cells in aged animals and whether and how p19ARF contributes to tissue aging. The aging-associated decline in lung function is characterized by an increase in compliance as well as pathogenic susceptibility to pulmonary diseases. We herein demonstrated that pulmonary function in 12-month-old mice was reversibly restored by the elimination of p19ARF-expressing cells. The ablation of p19ARF-expressing cells using a toxin receptor-mediated cell knockout system ameliorated aging-associated lung hypofunction. Furthermore, the aging-associated gene expression profile was reversed after the elimination of p19ARF. Our results indicate that the aging-associated decline in lung function was, at least partly, attributed to p19ARF and was recovered by eliminating p19ARF-expressing cells.

ARF Suppresses Tumor Angiogenesis through Translational Control of VEGFA mRNA

Vascular endothelial growth factor A (VEGFA) is a specific mitogen for vascular endothelial cells that plays a critical role in cancer neoangiogenesis. Here, we report that the nucleolar tumor suppressor p19(ARF) suppresses VEGFA expression, acting at the level of mRNA translation without affecting the transcription of the VEGFA gene. Translational repression of VEGFA mRNA by p19(ARF) does not require p53, a major target of the ARF tumor suppressor pathway, but instead correlates with binding to nucleophosmin/B23. Maintaining VEGFA expression relies on nucleophosmin/B23, and downregulating this protein by RNAi or p19(ARF) leads to translational repression of VEGFA. p19(ARF) inhibits VEGFA-dependent tumor angiogenesis in nude mice. Additionally, p14(ARF) expression and microvessel density are inversely correlated in human colon carcinomas. Taken together, our results define a mechanism by which the ARF tumor suppressor targets the translational repression of specific oncogenes during neoplastic transformation.

Hzf regulates adipogenesis through translational control of C/EBPα

Adipocyte differentiation requires a well‐defined programme of gene expression in which the transcription factor C/EBPα (CCAAT/enhancer‐binding protein) has a central function. Here, we show that Hzf (haematopoietic zinc‐finger), a previously identified p53 transcriptional target, regulates C/EBPα expression. Hzf is induced during differentiation of preadipocyte cell lines, and its suppression by short hairpin RNA disrupts adipogenesis. In Hzfs absence, expression of C/EBPα is severely impaired because of reduced translation of its mRNA. Hzf physically interacts with the 3′ untranslated region of C/EBPα mRNA to enhance its translation. Taken together, these findings underscore a critical role of Hzf in the adipogenesis regulatory cascade.

Cooperative role of the RNA-binding proteins Hzf and HuR in p53 activation

ABSTRACT The RNA-binding protein Hzf (hematopoietic zinc finger) plays important roles in mRNA translation in cerebellar Purkinje cells and adipocytes. We along with others have reported that the expression of the Hzf gene is transcriptionally regulated by the p53 tumor suppressor protein. We show here that Hzf regulates p53 expression in cooperation with HuR. Hzf and HuR independently interact with the 3′ untranslated region (UTR) of p53 mRNA, which facilitates the cytoplasmic localization of p53 mRNA in the presence of the ARF tumor suppressor protein. In the absence of Hzf and HuR, p53 induction by p19ARF is significantly attenuated, and the cells consequently acquire resistance to p19ARF. Thus, these findings demonstrate that in addition to Mdm2 inhibition, p19ARF increases the concentration of p53 through posttranscriptional control of p53 mRNA and suggest critical roles for the RNA-binding proteins Hzf and HuR in p53 induction.

Differential activity of a variant form of the human Id‐1 protein generated by alternative splicing

Members of the Id family of helix‐loop‐helix proteins are ubiquitously expressed and dimerize with members of the class A and class B basic helix‐loop‐helix proteins. Due to the absence of a basic region, Id proteins act as dominant‐negative antagonists of basic helix‐loop‐helix transcription factors, which regulate cell growth and differentiation in diverse cell types. Recent findings suggest that the functions of Id proteins are well regulated at both the transcriptional level and the post‐transcriptional level. We show here that the alternative splicing variant of human Id‐1 protein possesses a different binding specificity for basic helix‐loop‐helix transcription factors and is expressed in a cell cycle‐dependent manner. Therefore, alternative splicing of Id‐1 could provide a post‐transcriptional mechanism to regulate Id‐1 function.

Artemis-dependent DNA double-strand break formation at stalled replication forks.

Stalled replication forks undergo DNA double‐strand breaks (DSBs) under certain conditions. However, the precise mechanism underlying DSB induction and the cellular response to persistent replication fork stalling are not fully understood. Here we show that, in response to hydroxyurea exposure, DSBs are generated in an Artemis nuclease‐dependent manner following prolonged stalling with subsequent activation of the ataxia–telangiectasia mutated (ATM) signaling pathway. The kinase activity of the catalytic subunit of the DNA‐dependent protein kinase, a prerequisite for stimulation of the endonuclease activity of Artemis, is also required for DSB generation and subsequent ATM activation. Our findings indicate a novel function of Artemis as a molecular switch that converts stalled replication forks harboring single‐stranded gap DNA lesions into DSBs, thereby activating the ATM signaling pathway following prolonged replication fork stalling.

Implication of p53-dependent cellular senescence related gene, TARSH in tumor suppression

A novel target of NESH-SH3 (TARSH) was identified as a cellular senescence related gene in mouse embryonic fibroblasts (MEFs) replicative senescence, the expression of which has been suppressed in primary clinical lung cancer specimens. However, the molecular mechanism underlying the regulation of TARSH involved in pulmonary tumorigenesis remains unclear. Here we demonstrate that the reduction of TARSH gene expression by short hairpin RNA (shRNA) system robustly inhibited the MEFs proliferation with increase in senescence-associated beta-galactosidase (SA-beta-gal) activity. Using p53-/- MEFs, we further suggest that this growth arrest by loss of TARSH is evoked by p53-dependent p21(Cip1) accumulation. Moreover, we also reveal that TARSH reduction induces multicentrosome in MEFs, which is linked in chromosome instability and tumor development. These results suggest that TARSH plays an important role in proliferation of replicative senescence and may serve as a trigger of tumor development.

Death-associated protein 3 regulates cellular senescence through oxidative stress response

Death‐associated protein 3 (DAP3) has been originally identified as a positive mediator of apoptosis. It has been revealed recently that the predominant localization of DAP3 to mitochondria implies its functional involvement in mitochondrial metabolism in addition to apoptosis. However, little is known about the molecular basis of these physiological functions of DAP3. Here, we demonstrate that DAP3 is reduced in both replicative and premature senescence induced by oxidative stress, and the DAP3 reduction induced by oxidative stress is observed mostly in a mitochondrial fraction. Using DAP3‐specific short hairpin RNA (shRNA) in a clonogenic survival assay, we reveal that reduction of DAP3 induces resistance to oxidative stress and decreases intracellular reactive oxygen species (ROS) production. Furthermore, this strategy allows us to show that loss of DAP3 is involved in the avoidance of replicative senescence in mouse embryonic fibroblasts (MEFs). Thus, our study offers an insight into the potential regulatory function of mitochondrial DAP3 involved in cellular senescence.