Elevated levels of MDM-2 and p53 expression are associated with high grade non-Hodgkin's lymphomas

Enhanced MDM2 Oncoprotein Expression in Soft Tissue Sarcoma: Several Possible Regulatory Mechanisms

Purpose. MDM2 is an oncogene whose protein product may promote tumorigenesis by blocking wild-type p53 tumor suppressor mediated G ₀/G₁ cell cycle arrest, thereby inhibiting repair of damaged DNA prior to cell division. While MDM2 DNA amplification is frequently observed in human sarcoma, the mechanisms linking this amplification to MDM2 oncoprotein over-production as well as its functional significance have not been well characterized in patients with soft tissue sarcoma.

Methods. A tissue bank of resected soft tissue sarcomas and autologous normal tissues was assembled; all specimens were snap frozen within 15 min of resection. DNA and RNA were extracted from tissues using isoamyl alcohol and phenol chloroform extraction methods, respectively; cell lysates were prepared using PBSTDS lysis buffer. DNA and mRNA were confirmed as being non-degraded and were then examined for MDM2 DNA amplification (Southern blots) and mRNA over-expression (Northern blots) using actin (DNA) and glyceraldehyde-3-phosphate dehydrogenase (mRNA) as loading controls. The MDM2 protein was examined on Western blots using the MDM2-specific monoclonal antibody IF2 (Oncogene Science, Inc). The presence of p53 DNA and expression of p53 mRNA was examined by rehybridizing the Southern and Northern filters using a p53-specific cDNA probe.

Results. Soft tissue sarcomas and autologous normal tissues were screened for MDM2 DNA amplification, which was detected in 10 of 30 tumors screened. After screening, there was sufficient biomaterials from six specimens for subsequent Northern and Western analysis to see whether MDM2 gene amplification correlated with over-expression of MDM2 mRNA and MDM2 protein. In addition, we examined whether other mechanisms may lead to over-expression of the MDM2 oncoprotein. Several possible mechanisms of MDM2 oncoprotein over-expression were identified. These most commonly included MDM2 DNA amplification, MDM2 mRNA over-expression and MDM2 oncoprotein over-expression. However, some soft tissue sarcoma patient specimens had no evidence of MDM2 mRNA over-expression yet had MDM2 oncoprotein over-production in the tumor relative to autologous normal tissue, implying possible post-transcriptional regulation. Of functional relevance, MDM2 oncoprotein over-production by tumors was associated with large decreases in the percentage of cells in the ₀/G₁ cell cycle interface compared with autologous normal tissue cells.

Discussion. It is likely that there are multiple mechanisms underlying human soft tissue sarcoma MDM2 oncoprotein over-production. Consequently, strategies that decrease MDM2 over-production, such as transcriptional repression to inhibit MDM2 promoter activity or RNA antisense approaches, may ultimately offer the best therapeutic efficacy.

Why target PIM1 for cancer diagnosis and treatment?

The highly conserved proto-oncogenic protein PIM1 is an unusual serine or threonine kinase, in part because it is constitutively active. Overexpression of PIM1 experimentally leads to tumor formation in mice, while complete knockout of the protein has no observable phenotype. It appears to contribute to cancer development in three major ways when it is overexpressed; by inhibiting apoptosis, by promoting cell proliferation and by promoting genomic instability. Expression in normal tissues is nearly undetectable. However, in hematopoietic malignancies and in a variety of solid tumors, increased PIM1 expression has been shown to correlate with the stage of disease. This characteristic suggests it can serve as a useful biomarker for cancer diagnosis and prognosis. Several specific and potent inhibitors of PIM1’s kinase activity have also been shown to induce apoptotic death of cancer cells, to sensitize cancer cells to chemotherapy and to synergize with other anti-tumor agents, thus making it an attractive therapeutic target.

Inhibition of the p53 E3 ligase HDM-2 induces apoptosis and DNA damage--independent p53 phosphorylation in mantle cell lymphoma

PURPOSE

The ubiquitin-proteasome pathway has been validated as a target in non-Hodgkin's lymphoma through demonstration of the activity of the proteasome inhibitor bortezomib.

EXPERIMENTAL DESIGN

Another potentially attractive target is the human homologue of the murine double minute-2 protein, HDM-2, which serves as the major p53 E3 ubiquitin ligase; we therefore evaluated the activity of a novel agent, MI-63, which disrupts the HDM-2/p53 interaction.

RESULTS

Treatment of wild-type p53 mantle cell lymphoma (MCL) cell lines with MI-63 resulted in a dose- and time-dependent inhibition of proliferation, with an IC(50) in the 0.5 to 5.0 micromol/L range. MI-63 induced p53 and HDM-2 accumulation, as well as other downstream p53 targets such as p53 up-regulated modulator of apoptosis and p21(Cip1). This was associated with cell cycle arrest at G(1)-S; activation of caspase-3, caspase-8, and caspase-9; cleavage of poly-(ADP-ribose) polymerase; and loss of E2F1. HDM-2 inhibition caused phosphorylation of p53 at multiple serine residues, including 15, 37, and 392, which coincided with low levels of DNA strand breaks. DNA damage occurred in a small percentage of cells and did not induce phosphorylation of the DNA damage marker H2A.X(Ser139). Combinations of MI-63 with the molecularly targeted agents bortezomib and rapamycin showed synergistic, sequence-dependent antiproliferative effects. Treatment of primary MCL patient samples resulted in apoptosis and induction of p53 and p21, which was not seen in normal controls.

CONCLUSIONS

These findings support the hypothesis that inhibition of the HDM-2/p53 interaction may be a promising approach both by itself and in combination with currently used chemotherapeutics against lymphoid malignancies.

MDM2 is recognized as a tumor-associated antigen in chronic lymphocytic leukemia by CD8+ autologous T lymphocytes

OBJECTIVE

Tumor-associated antigens (TAA) are the basis for antigen-specific immunotherapy. The human homolog of the murine double-minute 2 oncoprotein (MDM2) is a putative TAA because it is overexpressed in several malignancies, including chronic lymphocytic leukemia (CLL) cells compared with normal B lymphocytes.

PATIENTS AND METHODS

Autologous, MDM2-specific human leukocyte antigen (HLA)-A2-restricted T cells were identified using interferon (IFN)-gamma-ELISPOT assays and HLA-A2/dimer-peptide staining after 4 weeks of in vitro culture.

RESULTS

Using native CLL cells as antigen-presenting cells (APCs), we demonstrate the generation of MDM2-specific T cells in 7/12 CLL patients that recognized specifically the MDM2-derived peptide MDM2(81-88) bound to HLA-A2-dimers while they were inactive against an unrelated MAGE-3 peptide (p = 0.002). After 4 weeks, up to 32.3% of the expanded CD8(+) T cells specifically recognized MDM2(81-88) by HLA-A2-dimer/peptide staining and up to 0.9% of all T cells expanded reacted specifically against this MDM2-derived peptide in the IFN-gamma-ELISPOT assay. If T cells were not expandable using native CLL cells as APC, leukemic cells were stimulated with CD40-ligand to increase the antigen-presenting capacity. This resulted in successful generation of MDM2-specific T cells in three of five remaining cases tested. Additionally, the factor that correlated best with successful generation of antigen-specific T cells in CLL patients was the ability of APCs to secrete IFN-gamma upon stimulation.

CONCLUSION

In summary, MDM2(81-88) was shown for the first time in humans to elicit a functional autologous immune response. MDM2 was demonstrated to be naturally processed and presented as TAA in primary human CLL cells enabling expansion of functional autologous tumor-specific T cells.

MDM2 as MYCN transcriptional target: Implications for neuroblastoma pathogenesis

MYCN amplification is associated with an exceptionally poor prognosis in neuroblastoma. Furthermore, the crucial effectors of MYCN responsible for this aggressive subset of neuroblastoma await characterization. A critical negative regulator of the p53 tumor suppressor, MDM2, has been recently characterized in neuroblastoma cell lines as a transcriptional target of MYCN. Targeted inhibition of MYCN results in reduced MDM2 expression levels, with concomitant stabilization of p53 and stimulation of apoptosis in MYCN amplified neuroblastoma cell lines. These data suggest the possibility that MYCN-driven expression of MDM2 might play a role in counterbalancing the p53-dependent apoptotic pathways concurrently stimulated by over expression of MYC proteins. Mouse models of lymphoma have demonstrated that MDM2 expression, with decreased p53 activity, is critical for complete MYCC driven tumorigenesis. Our data suggest that a similar situation may apply for MYCN in neuroblastoma. Strategies for pharmacologic and genetic inhibition of MDM2 may prove to be an important new therapeutic approach in neuroblastoma.

MDM2 and Prognosis

The cellular stress response pathway regulated by the p53 tumor suppressor is critical to the maintenance of genomic integrity and to the prevention of oncogenic transformation. Intracellular levels of p53 are tightly regulated by an autoregulatory feedback loop comprised of p53 and MDM2. It might be predicted that disruption of this loop, either through p53 mutation or overexpression of MDM2, would be a negative prognostic marker for cancer development, likelihood of relapse, or response to therapy. In fact, although MDM2 overexpression is common in cancer, it can be both a positive and a negative predictor of outcome in different tumors, and its significance as a biomarker remains controversial. Data from a number of different tumor types are reviewed for the predictive significance of MDM2 expression, along with evidence for different mechanisms of MDM2 overexpression in these different tumors.

In light of the biological complexities underlying the p53-MDM2 loop, it is, perhaps, not surprising that no simple paradigm exists that is generally applicable. Much work remains to be done to elucidate the basic mechanisms underlying the physical interactions between the two proteins, the role of protein modifications in altering those interactions, and also the genetic and transcriptional deregulations by which protein levels are altered in human cancers. Only in this way will truly biologically relevant predictive factors emerge.

Tumor suppressor genes in normal and malignant hematopoiesis

Over the last decade, a growing number of tumor suppressor genes have been discovered to play a role in tumorigenesis. Mutations of p53 have been found in hematological malignant diseases, but the frequency of these alterations is much lower than in solid tumors. These mutations occur especially as hematopoietic abnormalities become more malignant such as going from the chronic phase to the blast crisis of chronic myeloid leukemia. A broad spectrum of tumor suppressor gene alterations do occur in hematological malignancies, especially structural alterations of p15(INK4A), p15(INK4B) and p14(ARF) in acute lymphoblastic leukemia as well as methylation of these genes in several myeloproliferative disorders. Tumor suppressor genes are altered via different mechanisms, including deletions and point mutations, which may result in an inactive or dominant negative protein. Methylation of the promoter of the tumor suppressor gene can blunt its expression. Chimeric proteins formed by chromosomal translocations (i.e. AML1-ETO, PML-RARalpha, PLZF-RARalpha) can produce a dominant negative transcription factor that can decrease expression of tumor suppressor genes. This review provides an overview of the current knowledge about the involvement of tumor suppressor genes in hematopoietic malignancies including those involved in cell cycle control, apoptosis and transcriptional control.

Germinal center phenotype and bcl-2 expression combined with the International Prognostic Index improves patient risk stratification in diffuse large B-cell lymphoma

The International Prognostic Index (IPI) identifies poor- and good-risk patients with diffuse large B cell lymphoma (DLBCL); however, the majority of patients have an intermediate IPI, with an uncertain prognosis. To determine whether cellular factors can be combined with the IPI to more accurately predict outcome, we have analyzed 177 presentation nodal DLBCLs for the expression of bcl-2 and a germinal center (GC) phenotype (defined by expression of bcl-6 and CD10). P53 gene band shifts were detected using single-stranded conformational polymorphism polymerase chain reaction analysis of exons 5-9 and were correlated with protein expression. In a Cox regression analysis, IPI (R = 0.22, P <.0001) and bcl-2 (R = 0.14, P =.0001) were independent poor prognostic factors and a GC phenotype predicted a favorable outcome (R = -0.025, P =.02). Neither p53 expression nor band shifts had a significant effect on survival. Using the IPI alone, 8% of patients were identified as high risk. Expression of bcl-2 in the intermediate IPI group identified a further 28% of patients with an overall survival comparable to the high IPI group. In the intermediate IPI, bcl-2(-) group, the presence of a GC phenotype improved overall survival to levels approaching the IPI low group. Following this analysis only 15% of patients failed to be assigned to a favorable- or poor-risk group. Sequential addition of bcl-2 expression and GC phenotype into the IPI significantly improves risk stratification in DLBCL. For the 36% of high-risk patients with a 2-year overall survival of 19%, alternative treatment strategies should be considered in future trials.

Copy number gain at 12q12-14 may be important in the transformation from follicular lymphoma to diffuse large B cell lymphoma

The purpose of this study was to identify novel areas of genomic copy number change associated with transformation from follicular lymphoma (FL) to diffuse large B cell lymphoma (DLBL). DNA was extracted from tumour cells micro-dissected from paraffin- embedded tissue sections in 24 patients with FL and subsequent transformation to DLBL and 18 patients with de novo DLBL. Tumour DNA was compared to reference DNA using comparative genomic hybridization. Abnormalities common to all 3 groups were gains on chromosomes 4q, 5q, 7q, 11q and X and losses on 3p, 8p and 10q. Copy number changes seen in both transformed and de novo DLBL and not seen in FL were gains on 2p and losses on 1q, 15q and Xq. Gains on 2q, 6p, 7p and 17q and losses on 5p and 8q were specific to transformed DLBL cases. Gain on 12q12-14 was found in 52% of the transformed DLBL cases and was never seen in its follicular counterpart. Patterns of genomic copy number change associated with specific clinical events in NHL have been demonstrated and suggest that gains on 2q, 6p, 7p, 12q and 17q and losses on 5p and 8q may be important in the transformation from low to high-grade disease.

MDM2 Protein Overexpression Promotes Proliferation and Survival of Multiple Myeloma Cells

The murine double minute 2 (MDM2) protein facilitates G1 to S phase transition by activation of E2F-1 and can enhance cell survival by suppressing wild-type p53 (wtp53) function. In this study, we examined MDM2 expression and function in multiple myeloma (MM) cells. MDM2 is strongly and constitutively expressed in MM cell lines (ARH-77, RPMI 8226, and OCI-My5) and in the cells of plasma cell leukemia (PCL) patients, but is not expressed in normal bone marrow mononuclear cells (BM MNCs). Treatment of MM cells with MDM2 antisense, but not sense, nonsense, or scrambled, oligodeoxyribonucleotides (ODNs) decreased DNA synthesis and cell viability; it also induced G1 growth arrest, as evidenced by propidium iodide (PI) staining and induction of retinoblastoma protein (pRB) to E2F-1 binding. Moreover, inhibition of MDM2 using antisense ODNs also triggered MM cell apoptosis as evidenced by acridine orange–ethidium bromide staining. We next studied the association of MDM2 with wtp53 and/or mutant p53 (mtp53), E2F-1, CDK4, and p21. MDM2 constitutively binds to E2F-1 in all MM cells, to both wtp53 and mtp53, and to p21 in tumor cells lacking p53. These data suggest that MDM2 may enhance cell-cycle progression in MM cells both by activating E2F-1 and by downregulating cell-cycle inhibitory proteins (wtp53 and p21). Overexpression of MDM2 may therefore contribute to both growth and survival of MM cells, suggesting the potential utility of treatment strategies targeting MDM2 in MM.

Analysis of the p53 and MDM-2 gene in acute myeloid leukemia

The MDM-2 (murine double minute 2) gene codes for a cellular protein that can bind to the p53 tumor suppressor gene product, thereby functioning as a negative regulator of p53. In order to define the role of the MDM-2 gene in the pathogenesis of human acute myeloid leukemia, the expression and the sequence of the MDM-2 gene were examined in samples of bone marrow and/or peripheral mononuclear cells of 38 patients by using immunostaining, polymerase chain reaction (PCR), single strand conformation polymorphism, and sequencing. Immunohistochemical staining detected a weak accumulation of the MDM-2 protein in AML patients of FAB classification M4 and M5. RT-PCR analysis revealed a heterogeneous expression pattern of MDM-2 mRNA in AML samples of different FAB classification. An increased level of MDM-2 mRNA expression was observed in 17 of 38 AML patients when compared to normal controls. No structural changes in a 488 bp region extending from nucleotide 890 to 1378 of the MDM-2 cDNA were detected using RT-SSCP and sequence analysis. In addition, heterogeneous expression of p53 transcripts was found with the highest p53 mRNA levels in AML M4 and M5. Interestingly, there seems to be a correlation between the relative ratios of p53 and MDM-2 mRNA levels in AML M4 and M5: in 15 of 23 cases high p53 mRNA expression was directly associated with high levels of MDM-2 transcripts. An exclusively intranuclear p53 immunostaining pattern was found in 10 of 16 (58%) AML FAB M4 and M5, whereas the remaining AML samples tested were negative for p53 (0/10). Using RT-SSCP analysis and direct sequencing of the RT-PCR amplification products of p53 exon 5-8, we observed that only 1 of 38 AML patients showed a point mutation in the p53 gene. This missense mutation occurred in the evolutionary highly conserved region of p53 at codon 255 (Ile to Phe). These data indicated that structural alterations of the p53 gene do not play an important role in the initiation and progression of AML. However, abrogation of p53 tumor suppressor function due to MDM-2 overexpression may be an alternative molecular mechanism by which a subset of AMLs may escape from p53-regulated growth control.