Showing papers by "Xinbin Chen published in 2006"

RNPC1, an RNA-binding protein and a target of the p53 family, is required for maintaining the stability of the basal and stress-induced p21 transcript

Abstract: p21, a cyclin-dependent kinase inhibitor, is transcriptionally regulated by the p53 family to induce cell cycle arrest. p21 is also regulated post-transcriptionally upon DNA damage in a p53-dependent manner, but the mechanism is uncertain. Here, we found that RNPC1, an RNA-binding protein and a target of the p53 family, is required for maintaining the stability of the basal and stress-induced p21 transcript. Specifically, we showed that RNPC1 is induced by the p53 family and DNA damage in a p53-dependent manner. The RNPC1 gene encodes at least two alternative spliced isoforms, RNPC1a and RNPC1b, both of which contain an intact RNA recognition motif. Interestingly, we found that RNPC1a, but not RNPC1b, induces cell cycle arrest in G1, although both isoforms are expressed in the nucleus and cytoplasm. In addition, we found that while both isoforms directly bind to the 3' untranslated region in p21 transcript, only RNPC1a is able to stabilize both the basal and stress-induced p21 transcripts. Conversely, RNPC1a knockdown destabilizes p21 transcript. Finally, we found that RNPC1a is required to maintain the stability of p21 transcript induced by p53.

DNA Polymerase η, the Product of the Xeroderma Pigmentosum Variant Gene and a Target of p53, Modulates the DNA Damage Checkpoint and p53 Activation

Abstract: DNA polymerase η (PolH) is the product of the xeroderma pigmentosum variant (XPV) gene and a well-characterized Y-family DNA polymerase for translesion synthesis. Cells derived from XPV patients are unable to faithfully bypass UV photoproducts and DNA adducts and thus acquire genetic mutations. Here, we found that PolH can be up-regulated by DNA breaks induced by ionizing radiation or chemotherapeutic agents, and knockdown of PolH gives cells resistance to apoptosis induced by DNA breaks in multiple cell lines and cell types in a p53-dependent manner. To explore the underlying mechanism, we examined p53 activation upon DNA breaks and found that p53 activation is impaired in PolH knockdown cells and PolH-null primary fibroblasts. Importantly, reconstitution of PolH into PolH knockdown cells restores p53 activation. Moreover, we provide evidence that, upon DNA breaks, PolH is partially colocalized with phosphorylated ATM at γ-H2AX foci and knockdown of PolH impairs ATM to phosphorylate Chk2 and p53. However, upon DNA damage by UV, PolH knockdown cells exhibit two opposing temporal responses: at the early stage, knockdown of PolH suppresses p53 activation and gives cells resistance to UV-induced apoptosis in a p53-dependent manner; at the late stage, knockdown of PolH suppresses DNA repair, leading to sustained activation of p53 and increased susceptibility to apoptosis in both a p53-dependent and a p53-independent manner. Taken together, we found that PolH has a novel role in the DNA damage checkpoint and that a p53 target can modulate the DNA damage response and subsequently regulate p53 activation.

Regulation of the p53 transcriptional activity.

Abstract: In response to various stresses, p53 is rapidly activated and transcriptionally regulates a number of target genes by which p53 modulates a variety of cellular activities. The transcriptional activity of p53 is delicately regulated by a plethora of cellular factors, independently or synergistically, in multiple ways in order to achieve a specific response. This article reviewed the role of the basal transcriptional machinery, co-activators, and co-repressors involved in p53-dependent transcription, and the underlying mechanism by which the p53 transcriptional activity is regulated. We also discussed some potentially interesting questions and future directions in the field.

The functional domains in p53 family proteins exhibit both common and distinct properties.

Abstract: p53 is a sequence-specific transcription factor that functions to transactivate genes that mediate cell cycle arrest, DNA repair, apoptosis, and other p53-dependent activities. In 1997 and 1998, p73 and p63, respectively, were identified and emerged as p53 homologues (reviewed by Yang et al.). The p53 family proteins share significant similarity at the aminoacid level within three domains: the transcriptional activation domain (AD), the sequence-specific DNA-binding domain (DBD), and the tetramerization domain (TD) (Figure 1a). Like p53, both p63 and p73 bind to the canonical p53-responsive element and transactivate p53 target genes (reviewed by Harms et al.). Unlike p53, the genes encoding p63 and p73 are rarely mutated in human cancer and knockout mice demonstrate developmental defects rather than a propensity for tumor formation (reviewed by Yang et al.). However, recent evidence suggests that p63 and p73 do indeed play a role in tumor suppression since heterozygous p63 and p73 mice are prone to tumor formation. Thus, the p53 family proteins possess both common as well as nonoverlapping functions. At the 12th International p53 Workshop, we and others presented data identifying the functional domains in the p53 family proteins required for transcriptional activity, cell cycle arrest, and apoptosis (reviewed by Braithwaite et al.). As each domain plays an integral role in facilitating the differential functions of these transcription factors, here, we discuss the common and distinct properties of the transcriptional ADs, the DBD, nuclear localization and nuclear export signals (nuclear localization signal (NLS) and nuclear export signals (NES)), the TD, the basic domain (BD) that is present in p53 but not in p63 or p73, and the sterile-a-motif (SAM) domain that is present in some p63 and p73 isoforms but is lacking in p53.

Myosin VI Is a Mediator of the p53-Dependent Cell Survival Pathway

Abstract: Myosin VI is an unconventional motor protein, and its mutation is responsible for the familiar conditions sensorineural deafness and hypertrophic cardiomyopathy. Myosin VI is found to play a key role in the protein trafficking and homeostasis of the Golgi complex. However, very little is known about how myosin VI is regulated and whether myosin VI has a function in the DNA damage response. Here, we found that myosin VI is regulated by DNA damage in a p53-dependent manner and possesses a novel function in the p53-dependent prosurvival pathway. Specifically, we show that myosin VI is induced by p53 and DNA damage in a p53-dependent manner. We found that p53 directly binds to, and activates, the promoter of the myosin VI gene. We also show that the intracellular localization of myosin VI is substantially altered by p53 and DNA damage in a p53-dependent manner such that the pool of myosin VI in endocytic vesicles, membrane ruffles, and cytosol migrates to the Golgi complex, perinuclear membrane, and nucleus. Furthermore, we show that knockdown of myosin VI attenuates activation of p53 and impairs Golgi complex integrity, which makes myosin VI-deficient cells susceptible to apoptosis upon DNA damage. Taken together, we found a novel function for p53 in the maintenance of Golgi complex integrity and for myosin VI in the p53-dependent prosurvival pathway.

The epithelial cell transforming sequence 2, a guanine nucleotide exchange factor for Rho GTPases, is repressed by p53 via protein methyltransferases and is required for G1-S transition

Abstract: The epithelial cell transforming sequence 2 (ECT2), a member of the Dbl family of guanine nucleotide exchange factor for Rho GTPases, is required for cytokinesis. The tumor suppressor p53 plays a crucial role in coordinating cellular processes, such as cell cycle arrest and apoptosis, in response to stress signals. Here, we showed that ECT2 is negatively regulated by wild-type p53 but not tumor-derived mutant p53 or other p53 family members. In addition, ECT2 is down-regulated in multiple cell lines by DNA damage agents and Nutlin-3, an MDM2 antagonist, in a p53-dependent manner. We also showed that the activity of the ECT2 promoter is repressed by wild-type p53, and to a lesser extent, by p21. In addition, the second activation domain in p53 is necessary for the efficient repression of ECT2. Importantly, we found that the ECT2 gene is bound by p53 in vivo in response to DNA damage and Nutlin-3 treatment. Furthermore, we provided evidence that inhibition of protein methyltransferases, especially arginine methyltransferases, relieve the repression of ECT2 induced by DNA damage or Nutlin-3 in a p53-dependent manner. Finally, we generated multiple cell lines in which ECT2 is inducibly knocked down and found that ECT2 knockdown triggers cell cycle arrest in G1. Taken together, we uncovered a novel function for ECT2 and provided a novel mechanism by which p53 represses gene expression via protein methyltransferases.

p53 is required for nerve growth factor-mediated differentiation of PC12 cells via regulation of TrkA levels.

Abstract: p53 is necessary for the elimination of neural cells inappropriately differentiated or in response to stimuli. However, the role of p53 in neuronal differentiation is not certain. Here, we showed that nerve growth factor (NGF)-mediated differentiation in PC12 cells is enhanced by overexpression of wild-type p53 but inhibited by mutant p53 or knockdown of endogenous wild-type p53, the latter of which can be rescued by expression of exogenous wild-type p53. Interestingly, p53 knockdown or overexpression of mutant p53 attenuates NGF-mediated activation of TrkA, the high-affinity receptor for NGF and a tyrosine kinase, and activation of the mitogen-activated protein kinase pathway. In addition, p53 knockdown reduces the constitutive levels of TrkA, which renders PC12 cells inert to NGF. And finally, we showed that both constitutive and stimuli-induced expressions of TrkA are regulated by p53 and that induction of TrkA by activated endogenous p53 enhances NGF-mediated differentiation. Taken together, our data demonstrate that p53 plays a critical role in NGF-mediated neuronal differentiation in PC12 cells at least in part via regulation of TrkA levels.

Transcriptional responses to ionizing radiation reveal that p53R2 protects against radiation-induced mutagenesis in human lymphoblastoid cells

Abstract: Transcriptional responses to ionizing radiation reveal that p53R2 protects against radiation-induced mutagenesis in human lymphoblastoid cells

p19ras Brings a New Twist to the Regulation of p73 by Mdm2

Abstract: p53 is the most commonly mutated gene in human cancer. It is now known that the p53 family proteins p63 and p73 play important roles in tumor suppression as well as in development. Because p63 and p73 are rarely mutated in human cancer, understanding the signaling pathways that activate p63 and p73 will not only shed light on the developmental processes regulated by p63 and p73 but may also yield insight into ways to harness p63 and p73 activity for cancer therapy. Recent research has shown that an alternative splice form of c-H- ras , called p19 ras , is a positive regulator of p73β through a mechanism that involves the E3 ubiquitin ligase Mdm2. Implications for this previously unidentified means of regulation are discussed in light of tumor suppression and are extended to p53 and p63.