Search

Post-translational modifications play a crucial role in regulation of the protein stability and pro-apoptotic function of p53 as well as its close relative p73. Using a yeast two-hybrid screening based on the Sos recruitment system, we identified protein kinase A catalytic subunit beta (PKA-Cbeta) as a novel binding partner of p73. Co-immunoprecipitation and glutathione S-transferase pull-down assays revealed that p73alpha associated with PKA-Cbeta in mammalian cells and that their interaction was mediated by both the N- and C-terminal regions of p73alpha. In contrast, p53 failed to bind to PKA-Cbeta. In vitro phosphorylation assay demonstrated that glutathione S-transferase-p73alpha-(1-130), which has one putative PKA phosphorylation site, was phosphorylated by PKA. Enforced expression of PKA-Cbeta resulted in significant inhibition of the transactivation function and pro-apoptotic activity of p73alpha, whereas a kinase-deficient mutant of PKA-Cbeta had no detectable effect. Consistent with this notion, treatment with H-89 (an ATP analog that functions as a PKA inhibitor) reversed the dibutyryl cAMP-mediated inhibition of p73alpha. Of particular interest, PKA-Cbeta facilitated the intramolecular interaction of p73alpha, thereby masking the N-terminal transactivation domain with the C-terminal inhibitory domain. Thus, our findings indicate a PKA-Cbeta-mediated inhibitory mechanism of p73 function.

Polo-like kinase 1 (Plk1) has an important role in the regulation of M phase of the cell cycle. In addition to its cell cycle-regulatory function, Plk1 has a potential role in tumorigenesis. Here we found for the first time that Plk1 physically binds to the tumor suppressor p53 in mammalian cultured cells, and inhibits its transactivation activity as well as its pro-apoptotic function. During the cisplatin-induced apoptosis in human neuroblastoma SH-SY5Y cells, the expression level of Plk1 was significantly decreased both at mRNA and protein levels, whereas cisplatin treatment caused a remarkable stabilization of p53. Systematic immunoprecipitation analyses using a series of deletion mutants of p53 revealed that a sequence-specific DNA-binding region of p53 is required and sufficient for the physical interaction with Plk1. The ectopically overexpressed Plk1 was co-localized with the endogenous p53 in mammalian cell nucleus, as shown by confocal laser microscopy. Expression of exogenous Plk1 and p53 in p53-deficient lung carcinoma H1299 cells greatly decreased the p53-mediated transcription from the p53-responsive p21(WAF1), MDM2, and BAX promoters, whereas the kinase-deficient mutant form of Plk1 failed to reduce the transcriptional activity of p53. Consistent with the luciferase reporter analysis, Plk1 had an ability to block the p53-dependent induction of the endogenous p21(WAF1). In addition, Plk1 inhibited the pro-apoptotic function of p53 in H1299 cells. Intriguingly, Plk1-mediated repression of p53 was attenuated with ATM. Thus, our present findings strongly suggest that p53 is a critical target of Plk1, and its function is abrogated through the physical interaction with Plk1.

p73 shares high sequence homology with the tumor suppressor p53. Like p53, ectopic overexpression of p73 induces cell cycle arrest and/or apoptosis, and these biological activities are linked to its sequence-specific transactivation function. The COOH-terminal region of p73 is unique and has a function to modulate DNA-binding ability and transactivation activity. To identify and characterize cellular proteins that interact with the COOH-terminal region of p73 alpha and regulate its activity, we employed a yeast-based two-hybrid screen with a human fetal brain cDNA library. We found MM1, a nuclear c-Myc-binding protein, was associated with p73 alpha in both yeast two-hybrid and in vitro pull-down assays. In mammalian cells, MM1 co-immunoprecipitated with p73 alpha, whereas p73 beta and tumor suppressor p53 did not interact with MM1. Overexpression of MM1 in p53-deficient osteosarcoma SAOS-2 cells enhanced the p73 alpha-dependent transcription from the p53/p73-responsive Bax and PG13 promoters, whereas p73 beta- and p53-mediated transcriptional activation was unaffected in the presence of MM1. MM1 also stimulated the p73 alpha-mediated growth suppression in SAOS-2 cells. More importantly, we found that c-Myc was physically associated with p73 alpha and significantly impaired the transcriptional activity of p73 alpha on Bax and p21(waf1) promoters. Expression of MM1 strongly reduced the c-Myc-mediated inhibitory activity on p73 alpha. These results suggest that MM1 may act as a molecular partner for p73 to prevent the c-Myc-mediated inhibitory effect on its activity.

This study describes the detection of cytokines, interleukin-1 alpha (IL-1α), tumor necrosis-factor alpha (TNFα) and interleukin-6 (IL-6) in the serum and brain homogenates of susceptible and resistant strains of mice infected with distinct species/strains of Borrelia . Significant elevation of inflammatory cytokines was found in the sera of susceptible C3H/HeN mice but not in the sera of resistant BALB/c mice on days 7 and 45 after inoculation. Brain cytokines were observed in C3H/HeN mice infected with B. garinii and B. afzelii and the production was associated with the presence of spirochetes. B. japonica was the only exception in disease-susceptibility and cytokine production of mice. These results suggest that there are differences in pathogenic potential amongst Borrelia strains, and that the susceptible mice demonstrate a stronger cytokine response and greater frequency of recovery of Borrelia from the brain than do the resistant mice.

The serum antibody titers against oral streptococci were studied by enzyme-linked immunosorbent assay (ELISA) both in patients with Behçet's disease (BD) and control groups. The patients with BD showed significantly higher antibody titers to S. sanguis strains 113-20, 114-23, and 118-1 which were isolated from patients with BD, in comparison with control groups. Also, the reactions of high-titered sera to the crude cell wall and soluble (or membrane) fractions of the 113-20 strain were observed by western blot test. The sera of the patients with BD demonstrated strong bands of approximately 36 kDa, 82 kDa, and 87 kDa in the crude cell wall fractions, and many bands of 80 kDa to 150 kDa in the membrane fractions, indicating that these proteins are the ones leading the high antibody titers to this bacterium in the sera of patients with BD.

In response to DNA damage, p53 and its homolog p73 have a function antagonistic to NF-kappaB in deciding cell fate. Here, we show for the first time that p73, but not p53, is stabilized by physical interaction with nuclear IkappaB kinase (IKK)-alpha to enhance cisplatin (CDDP)-induced apoptosis. CDDP caused a significant increase in the amounts of nuclear IKK-alpha and p73alpha in human osteosarcoma-derived U2OS cells. Ectopic expression of IKK-alpha prolonged the half-life of p73 by inhibiting its ubiquitination and thereby enhancing its transactivation and pro-apoptotic activities. Consistent with these results, small interfering RNA-mediated knockdown of endogenous IKK-alpha inhibited the CDDP-mediated accumulation of p73alpha. The kinase-deficient mutant form of IKK-alpha interacted with p73alpha, but failed to stabilize it. Furthermore, CDDP-mediated accumulation of endogenous p73alpha was not detected in mouse embryonic fibroblasts (MEFs) prepared from IKK-alpha-deficient mice, and CDDP sensitivity was significantly decreased in IKK-alpha-deficient MEFs compared with wild-type MEFs. Thus, our results strongly suggest that the nuclear IKK-alpha-mediated accumulation of p73alpha is one of the novel molecular mechanisms to induce apoptotic cell death in response to CDDP, which may be particularly important in killing tumor cells with p53 mutation.

p73 responds to DNA damage and exerts its pro-apoptotic function. However, p73 might contribute to the development of drug-resistance in certain tumor cells. In this study, we found that p73 and MDM2 correlate with cisplatin-resistant phenotype of human epidermoid carcinoma-derived cells. p73 and MDM2 were kept at low levels in the cisplatin-sensitive KB-3-1 cells, whereas p53 was induced to be phosphorylated at Ser-15 in response to cisplatin. In contrast, p73 and MDM2 were expressed at higher levels, and cisplatin-mediated p53 phosphorylation was undetectable in the cisplatin-resistant KCP-4 cells. Enforced expression of p73 in KB-3-1 cells caused an accumulation of unphosphorylated form of p53 and MDM2, and conferred the cisplatin resistance. Collectively, our results suggest that a loss of the cisplatin sensitivity is at least in part due to a lack of cisplatin-induced p53 phosphorylation, and p73 might cooperate with MDM2 to be involved in this process.

p73 protein level is kept extremely low in mammalian cultured cells and its stability may be regulated by not only the ubiquitin/proteasome-dependent proteolysis but also through other unidentified mechanisms. Here, we found for the first time that p73 is physically as well as functionally associated with the U-box-type E3/E4 ubiquitin ligase UFD2a. The immunoprecipitation experiments demonstrated that this interaction is mediated by the COOH-terminal region of p73alpha containing SAM domain. During the cisplatin-induced apoptosis in SH-SY5Y neuroblastoma cells, p73alpha accumulated at a protein level, whereas the endogenous UFD2a was significantly reduced in response to cisplatin. Ectopic expression of UFD2a decreased the half-life of p73alpha in association with a significant inhibition of the p73alpha-mediated transactivation as well as proapoptotic activity. Downregulation of endogenous UFD2a by antisense strategy resulted in a remarkable accumulation of p73alpha. Unexpectedly, UFD2a-mediated degradation of p73alpha was sensitive to the proteasomal inhibitor, however, UFD2a did not affect the ubiquitination levels of p73alpha. Taken together, our present findings imply that UFD2a might promote the proteasomal turnover of p73 in a ubiquitination-independent manner, and also suggest that UFD2a might play an important role in the regulation of cisplatin-induced apoptosis mediated by p73.

p73, a newly identified member of p53 family, locates at human chromosome 1p36.2-3, a region which is frequently deleted in a wide variety of human tumors including neuroblastoma. p73 is induced to be accumulated in response to a subset of DNA damaging agents such as cisplatin, and thereby promoting G1/S cell cycle arrest and/or apoptosis. Since the expression levels of p73 are kept extremely low under normal conditions, stabilization of p73 is critical for its effects on cell growth inhibition and apoptosis. Indeed, p73 is induced at protein level in SH-SY5Y neuroblastoma cells exposed to cisplatin. Several lines of evidence indicate that stress-induced post-translational modifications of p73 such as phosphorylation and acetylation lead to a marked extension of its half-life. p73 stability is regulated at least in part by proteasome-dependent degradation pathway, however, MDM2 which mediates ubiquitination and subsequent degradation of p53 by the 26S proteasome, does not promote the proteolytic degradation of p73, implying that the protein stability of p73 is regulated through a pathway distinct from that of p53. Although little is known about the regulation of p73 turnover, we are now beginning to understand the regulatory mechanisms by which p73 is induced to be stabilized in response to apoptotic stimuli, and exerts its pro-apoptotic activity. In this review, we discuss about the cellular proteins implicated in the stability control of p73.