A 60 kd MDM2 isoform is produced by caspase cleavage in non-apoptotic tumor cells

Exploitation of a non-apoptotic caspase to regulate the abundance of the cdkI p27(KIP1) in transformed lymphoid cells

Expression of the cyclin dependent kinase inhibitor p27(KIP1) is intimately linked to the control of proliferation, and is itself regulated by transcription, translation, phosphorylation, protein stability or sequestration. p27(KIP1) is also regulated during apoptosis; cleavage occurs at DPSD(139)S and ESQD(108)V, by a sub-set of Z-VAD-fmk-sensitive caspases. We have identified a related but distinct mechanism that regulates p27(KIP1) in proliferating lymphoid cell lines. In a B-lymphoid cell line (BJAB), the abundance of p27(KIP1) oscillates inversely to proliferation; loss of full-length p27(KIP1) correlates with the appearance of a truncated version corresponding to cleavage at DPSD(139)S. A direct correlation exists between the appearance of truncated p27(KIP1) and the presence of an activity able to cleave peptides representing DPSD(139)S and a caspase-8 substrate (Ac-IETD-AMC) in vitro. This activity is inhibited by Ac-IETD-CHO but not Z-VAD-fmk in vitro. Furthermore a requirement for caspase-8 has been excluded. The activity differs from the apoptosis related p27(KIP1)-cleaving activity; indeed few cells undergoing apoptosis are present in the population of proliferating cells. The activity is further distinguished by its inability to cleave a peptide based on ESQD(108)V in vitro, together with the lack of a corresponding cleavage product in vivo. Inhibition of the caspase activity in vivo promotes an accumulation of full length p27(KIP1), as well as a decrease in cell proliferation. Together these studies highlight the importance of non-apoptotic caspases in regulating p27(KIP1) in transformed lymphoid cells.

Identities of sequestered proteins in aggregates from cells with induced polyglutamine expression

Proteins with expanded polyglutamine (polyQ) tracts have been linked to neurodegenerative diseases. One common characteristic of expanded-polyQ expression is the formation of intracellular aggregates (IAs). IAs purified from polyQ-expressing cells were dissociated and studied by protein blot assay and mass spectrometry to determine the identity, condition, and relative level of several proteins sequestered within aggregates. Most of the sequestered proteins comigrated with bands from control extracts, indicating that the sequestered proteins were intact and not irreversibly bound to the polyQ polymer. Among the proteins found sequestered at relatively high levels in purified IAs were ubiquitin, the cell cycle-regulating proteins p53 and mdm-2, HSP70, the global transcriptional regulator Tata-binding protein/TFIID, cytoskeleton proteins actin and 68-kD neurofilament, and proteins of the nuclear pore complex. These data reveal that IAs are highly complex structures with a multiplicity of contributing proteins.

Additive effect of adenovirus-mediated E2F-1 gene transfer and topoisomerase II inhibitors on apoptosis in human osteosarcoma cells

Recently, it has been demonstrated that Etoposide, a topoisomerase II inhibitor, can induce apoptosis in MDM2-overexpressing tumor cells by inhibition of MDM2 synthesis. We have previously shown that E2F-1 overexpression induces apoptosis of MDM2-overexpressing sarcoma cells, which is related to the inhibition of MDM2 expression. Therefore, the present study was designed to investigate the in vitro and in vivo effect of combined treatment of adenovirus-mediated E2F-1 and topoisomerase II inhibitors on the growth inhibition and apoptosis in human sarcoma cells. Two human sarcoma cell lines, OsACL and U2OS, were treated with topoisomerase II inhibitors (Etoposide and Adriamycin), alone or in combination with adenoviral vectors expressing beta-galactosidase (Ad-LacZ) or E2F-1 (Ad-E2F-1). E2F-1 expression was confirmed by Western blot analysis. Ad-E2F-1 gene transfer at a low dose (multiplicity of infection, 2) markedly increased the sensitivity of human sarcoma cells to topoisomerase II inhibitor treatment. This cooperative effect of E2F-1 and topoisomerase II inhibitors was less marked in SAOS-2 cells (p53 and pRb null). Topoisomerase II inhibitors also cooperated with E2F-1 overexpression to enhance tumor cell killing in an in vivo model using xenografts in nude mice. When combined with Adriamycin or Etoposide, E2F-1 adenovirus therapy resulted in approximately 95% and 85% decrease in tumor size, respectively, compared to controls (P<.05). These results suggest a new chemosensitization strategy that is effective in MDM2-overexpressing tumors and may have clinical utility.

p27KIP1 is down-regulated by two different mechanisms in human lymphoid cells undergoing apoptosis

The cyclin-dependent kinase inhibitor p27KIP1 is a crucial component of the mammalian restriction point, and as such is subject to multiple regulatory mechanisms. It has recently been shown that the abundance of p27KIP1 is also regulated during apoptosis; p27KIP1 is cleaved by a Z-VAD-fmk-sensitive caspase during apoptosis induced by growth factor deprivation in endothelial cells, and also following exposure of myeloid leukaemia cells to etoposide. Here, we investigate p27KIP1 regulation in B- and T-lymphoid cells undergoing apoptosis. We observe that p27KIP1 is down-regulated following exposure to a variety of apoptotic stimuli including an agonistic anti-Fas antibody, cycloheximide and etoposide. Further investigation revealed the existence of two different routes of p27KIP1 regulation in lymphoid cells undergoing apoptosis. The first pathway is utilized by lymphoid cells stimulated through Fas, is abrogated in a caspase-8-deficient T-cell line, and is blocked by the caspase inhibitors Z-VAD-fmk and Boc-D-fmk. In contrast, the loss of p27KIP1 in cells exposed to cycloheximide and etoposide occurs in the absence of caspase-8 or any Z-VAD-fmk- or Boc-D-fmk-sensitive caspase activities. Thus the down-regulation of p27KIP1 is a common occurrence in lymphoid cells undergoing apoptosis but, depending on the apoptotic trigger, this can be affected by two different mechanisms.

p53 stabilization and functional impairment in the absence of genetic mutation or the alteration of the p14ARF–MDM2 loop in ex vivo and cultured adult T-cell leukemia/lymphoma cells

Human T-cell lymphotropic virus type I (HTLV-I) transforms T cells in vitro, and the viral transactivator Tax functionally impairs the tumor suppressor p53 protein, which is also stabilized in HTLV-I–infected T cells. Thus, the functional impairment of p53 is essential to maintain the viral-induced proliferation of CD4+ mature T cells. However, in the CD4+ leukemic cells of patients with adult T-cell leukemia/lymphoma (ATLL), the viral transactivator does not appear to be expressed, and p53 mutations have been found only in a fraction of patients. We sought to investigate whether p53 function is impaired, in ex vivo samples from patients with ATLL, in the absence of genetic mutations. Here we demonstrate that the p53 protein is stabilized also in ex vivo ATLL samples (10 of 10 studied) and that at least in 2 patients p53 stabilization was not associated with genetic mutation. Furthermore, the assessment of p53 function after ionizing radiation of ATLL cells indicated an abnormal induction of the p53-responsive genes GADD45 and p21WAF1 in 7 of 7 patients. In 2 of 2 patients, p53 regulation of cell-cycle progression appeared to be impaired as well. Because p53 is part of a regulatory loop that also involves MDM2 and p14ARF, the status of the latter proteins was also assessed in cultured or fresh ATLL cells. The p97 MDM2 protein was not detected by Western blot analysis in established HTLV-I–infected T-cell lines or ex vivo ATLL cell lysates. However, the MDM2 protein could be easily detected after treatment of cells with the specific proteasome inhibitor lactacystin, suggesting a normal regulation of the p53–MDM2 regulating loop. Similarly, p14ARF did not appear to be aberrantly expressed in ex vivo ATLL cells nor in any of the established HTLV-I–infected T-cell lines studied. Thus, p53 stabilization in HTLV-I infection occurs in the absence of genetic mutation and alteration of the physiologic degradation pathway of p53.

p53 stabilization and functional impairment in the absence of genetic mutation or the alteration of the p14ARF–MDM2 loop in ex vivo and cultured adult T-cell leukemia/lymphoma cells

Human T-cell lymphotropic virus type I (HTLV-I) transforms T cells in vitro, and the viral transactivator Tax functionally impairs the tumor suppressor p53 protein, which is also stabilized in HTLV-I–infected T cells. Thus, the functional impairment of p53 is essential to maintain the viral-induced proliferation of CD4+ mature T cells. However, in the CD4+ leukemic cells of patients with adult T-cell leukemia/lymphoma (ATLL), the viral transactivator does not appear to be expressed, and p53 mutations have been found only in a fraction of patients. We sought to investigate whether p53 function is impaired, in ex vivo samples from patients with ATLL, in the absence of genetic mutations. Here we demonstrate that the p53 protein is stabilized also in ex vivo ATLL samples (10 of 10 studied) and that at least in 2 patients p53 stabilization was not associated with genetic mutation. Furthermore, the assessment of p53 function after ionizing radiation of ATLL cells indicated an abnormal induction of the p53-responsive genes GADD45 and p21WAF1 in 7 of 7 patients. In 2 of 2 patients, p53 regulation of cell-cycle progression appeared to be impaired as well. Because p53 is part of a regulatory loop that also involves MDM2 and p14ARF, the status of the latter proteins was also assessed in cultured or fresh ATLL cells. The p97 MDM2 protein was not detected by Western blot analysis in established HTLV-I–infected T-cell lines or ex vivo ATLL cell lysates. However, the MDM2 protein could be easily detected after treatment of cells with the specific proteasome inhibitor lactacystin, suggesting a normal regulation of the p53–MDM2 regulating loop. Similarly, p14ARF did not appear to be aberrantly expressed in ex vivo ATLL cells nor in any of the established HTLV-I–infected T-cell lines studied. Thus, p53 stabilization in HTLV-I infection occurs in the absence of genetic mutation and alteration of the physiologic degradation pathway of p53.

Regulation of p53 stability and activity in response to genotoxic stress

The p53 tumor suppressor is a universal sensor of genotoxic stress that regulates the transcription of genes required for cell-cycle arrest and apoptosis. In response to DNA damage, the p53 protein is phosphorylated at its amino-terminus and becomes stabilized upon disruption of an interaction with its negative regulator, MDM2. Subsequent phosphorylation and acetylation of p53 promote different interactions with other proteins and with target gene regulatory elements to facilitate cell-cycle arrest, apoptosis, or adaptation in response to DNA damage. Downstream of p53, p21 is responsible for growth arrest in G1, but other p53 target genes are responsible for G2 cell-cycle arrest. In response to genotoxic insult, p53-induced apoptosis results from overlapping downstream pathways that both suppress mitogenic and survival signaling and promote pro-apoptotic signaling. Adaptation to DNA damage is manifested by p53-mediated expression of its negative regulator, MDM2. The frequency of observed mutations in p53 predicts that its inactivation is a requisite step in tumorigenesis, as p53 is mutated in approximately 50% of human tumors. Thus, it is likely that in the remaining tumors, genetic aberrations will occur in pathways that regulate p53 or in pathways directly downstream of p53. The advances in the understanding of p53 signaling over the past few years point to many potential overlapping signaling pathways, where mutations may occur as alternative modes to p53 mutation.

Tissue-specific Bcl-2 protein partners in apoptosis: An ovarian paradigm

Apoptosis is an essential physiological process by which multicellular organisms eliminate superfluous cells. An expanding family of Bcl-2 proteins plays a pivotal role in the decision step of apoptosis, and the differential expression of Bcl-2 members and their binding proteins allows the regulation of apoptosis in a tissue-specific manner mediated by diverse extra- and intracellular signals. The Bcl-2 proteins can be divided into three subgroups: 1) antiapoptotic proteins with multiple Bcl-2 homology (BH) domains and a transmembrane region, 2) proapoptotic proteins with the same structure but missing the BH4 domain, and 3) proapoptotic ligands with only the BH3 domain. In the mammalian ovary, a high rate of follicular cell apoptosis continues during reproductive life. With the use of the yeast two-hybrid system, the characterization of ovarian Bcl-2 genes serves as a paradigm to understand apoptosis regulation in a tissue-specific manner. We identified Mcl-1 as the main ovarian antiapoptotic Bcl-2 protein, the novel Bok (Bcl-2-related ovarian killer) as the proapoptotic protein, as well as BOD (Bcl-2-related ovarian death agonist) and BAD as the proapoptotic ligands. The activity of the proapoptotic ligand BAD is regulated by upstream follicle survival factors through its binding to constitutively expressed 14-3-3 or hormone-induced P11. In contrast, the channel-forming Mcl-1 and Bok regulate cytochrome c release and, together with the recently discovered Diva/Boo, control downstream apoptosis-activating factor (Apaf)-1 homologs and caspases. Elucidation of the role of Bcl-2 members and their interacting proteins in the tissue-specific regulation of apoptosis could facilitate an understanding of normal physiology and allow the development of new therapeutic approaches for pathological states.

Identification of a CK2 phosphorylation site in mdm2

Mdm2 is a cellular oncoprotein the most obvious function of which is the down-regulation of the growth suppressor protein p53. It represents a highly phosphorylated protein but only little is yet known about the sites phosphorylated in vivo, the kinases that are responsible for the phosphorylation or the functional relevance of the phosphorylation status. Recently, we have shown that mdm2 is a good substrate for protein kinase CK2 at least in vitro. Computer analysis of the primary amino acid sequence of mdm2 revealed 19 putative CK2 phosphorylation sites. By using deletion mutants of mdm2 and a peptide library we identified the serine residue at position 269 which lies within a canonical CK2 consensus sequence (EGQELSDEDDE) as the most important CK2 phosphorylation site. Moreover, by using the mdm2 S269A mutant for in vitro phosphorylation assays this site was shown to be phosphorylated by CK2. Binding studies revealed that phosphorylation of mdm2 at S269 does not have any influence on the binding of p53 to mdm2.

Biochemical uncovering of mdm2/p53 complexes in liposarcomas parallels their immunohistochemical detection

Recent observations indicate the existence of pathogenetically distinct groups of well-differentiated (WD) dedifferentiated (DD) liposarcomas. In the retroperitoneal WD-DD liposarcomas, the predominant phenotype is represented by the aberrant (overexpressed) mdm2+/p53+ wild-type profile. At the nonretroperitoneal site, the WD liposarcomas present a wider association of MDM2/P53 gene expression; i.e., mdm2+/p53+, mdm2+/p53-, mdm2-/p53+ and mdm2-/p53-, and TP53 mutations seem to correlate with the dedifferentiation process. A biochemical study of mdm2-p53 association in 11 tumor samples characterized by the presence of different mdm2 and p53 immunophenotypes was performed. Immunoprecipitation assays using a p53-specific antibody were performed on tumor tissue and surrounding normal tissue; the immunoprecipitated material was then investigated for the presence of p53 (control) and of coimmunoprecipitated mdm2. This biochemical analysis showed that, in mdm2+/p53+/wild-type retroperitoneal liposarcomas, a band corresponded to mdm2 protein in the cellular lysates immunoprecipitated with a p53-directed antibody. In contrast, the mdm2+/p53- liposarcoma did not evidence the presence of mdm2 protein nor was p53 protein available to direct immunoprecipitation, as in the p53 mutant tumor samples with mdm2-/p53+ and mdm2-/p53- phenotypes. From the normal counterpart of retroperitoneal liposarcoma lysates, no p53 protein was immunoprecipitated. The findings in this study agree with the molecular data and they show the physical association of mdm2 and p53 in fresh liposarcoma surgical specimens.

Activation of an MDM2-specific caspase by p53 in the absence of apoptosis

Cells undergoing p53-mediated apoptosis activate caspase 3-like activities, resulting in the cleavage of the MDM2 oncoprotein and other apoptotic substrates such as poly(ADP-ribose) polymerase. To investigate the mechanism of p53-mediated apoptosis and to determine whether cleavage of MDM2 has a potential role in regulating p53, we examined caspase activation and cleavage of MDM2 in a cell line undergoing p53-mediated growth arrest and delayed apoptosis. We found that in H1299 cells expressing a temperature-sensitive human p53, a distinct caspase activity specific for the MDM2 cleavage site DVPD is induced by p53 prior to the onset of apoptosis and loss of viability. This is accompanied by the cleavage of MDM2 but not the apoptotic substrate poly(ADP-ribose) polymerase. The cleaved MDM2 loses the ability to promote p53 degradation and may potentially function in a dominant-negative fashion to stabilize p53. These results suggest that p53 activation may induce a positive feedback effect by cleavage of MDM2 through a unique caspase.