The two isoforms of the 90-kDalton nucleolus organizer region autoantigen (upstream binding factor) bind with different avidity to DNA modified by the antitumor drug cisplatin

Proteomic analysis of cisplatin- and oxaliplatin-induced phosphorylation in proteins bound to Pt-DNA adducts

Cisplatin and oxaliplatin are widely used anti-tumour chemotherapeutic agents with different spectra of activity. The therapeutic efficacy of such platinum-based drug is believed to, at least in part, result from formation of Pt-DNA adducts, followed by DNA damage response and ultimately apoptosis. However, it remains unclear whether these DNA lesions caused by cisplatin and oxaliplatin elicit distinct reactions in cellular signaling pathways. Here, a label-free comparative proteomic study was performed to profile the protein phosphorylation patterns using Pt-DNA probes with different ligand identities and geometries. Phosphorylated proteins recognizing different cisplatin- and oxaliplatin-DNA lesions were enriched and analyzed on LC-MS/MS. Proteomic analysis revealed that cisplatin mainly affected proteins involved in mRNA processing, while chromatin organization and rRNA processing are two major biological processes influenced by oxaliplatin. Changes to site-specific phosphorylation levels of two proteins YBX1 and UBF1 were also validated by Western blotting. In particular, platinum drug treatment in colon and liver cancer cell lines down-regulated S484 phosphorylation of UBF1, which is an essential transcription factor responsible for ribosomal DNA transcription activation, implying that inhibition of ribosome biogenesis might be involved in the cytotoxic mechanism of platinum drugs. Collectively, these results directly reflected distinct protein phosphorylation patterns triggered by cisplatin and oxaliplatin, and could also provide valuable resources for future mechanistic studies of platinum-based anti-tumour agents.

Depletion of the cisplatin targeted HMGB-box factor UBF selectively induces p53-independent apoptotic death in transformed cells

Cisplatin-DNA adducts act as strong decoys for the Upstream Binding Factor UBF (UBTF) and have been shown to inhibit transcription of the ribosomal RNA genes by RNA polymerase I. However, it is unclear if this plays a significant role in the chemotherapeutic activity of cis- or carboplatin. We find that cisplatin in fact induces a very rapid displacement of UBF from the ribosomal RNA genes and strong inhibition of ribosomal RNA synthesis, consistent with this being an important factor in its cytotoxicity. Using conditional gene deletion, we recently showed that UBF is an essential factor for transcription of the ribosomal RNA genes and for ribosome biogenesis. We now show that loss of UBF arrests cell proliferation and induces fully penetrant, rapid and synchronous apoptosis, as well as nuclear disruption and cell death, specifically in cells subjected to oncogenic stress. Apoptosis is not affected by homozygous deletion of the p53 gene and occurs equally in cells transformed by SV40 T antigens, by Myc or by a combination of Ras & Myc oncogenes. The data strongly argue that inhibition of UBF function is a major factor in the cytotoxicity of cisplatin. Hence, drug targeting of UBF may be a preferable approach to the use of the highly toxic platins in cancer therapy.

Concerted potent humoral immune responses to autoantigens are associated with tumor destruction and favorable clinical outcomes without autoimmunity

PURPOSE

The therapeutic importance of immune responses against single versus multiple antigens is poorly understood. There also remains insufficient understanding whether responses to one subset of antigens are more significant than another. Autoantibodies are frequent in cancer patients. They can pose no biological significance or lead to debilitating paraneoplastic syndromes. Autoreactivity has been associated with clinical benefits, but the magnitude necessary for meaningful results is unknown. Autologous tumor cells engineered to secrete granulocyte macrophage colony-stimulating factor generate immune infiltrates in preexisting metastases with associated tumor destruction. We sought to identify targets of responses from this vaccination strategy.

EXPERIMENTAL DESIGN

Postvaccination sera used in screening a cDNA expression library prepared from a densely infiltrated metastasis of a long-term surviving melanoma patient identified several autoantigens. Additional autoantigens were identified through similar screenings in non-small cell lung cancer and murine models, and proteins implicated in cancer propagation. ELISAs for several targets were established using recombinant proteins, whereas others were evaluated by petit serologies.

RESULTS

Eleven gene products were identified through serologic screening from two patients showing highly favorable clinical outcomes. A subset of antigens revealed significant changes in antibody titers compared with weak responses to other proteins. Time course analyses showed coordinated enhanced titers against several targets as a function of vaccination in responding patients.

CONCLUSIONS

This study shows the range of biologically significant antigens resulting from a whole-cell vaccine. Targets include autoantigens that are components of cell cycle regulation. Potent antibody responses against multiple autoantigens are associated with effective tumor destruction without clinical autoimmunity.

Cisplatin inhibits synthesis of ribosomal RNA in vivo

Cis -diammininedichloroplatinum(II) (cisplatin or cis -DDP) is a DNA-damaging agent that is widely used in cancer chemotherapy. Cisplatin crosslinks DNA and the resulting adducts interact with proteins that contain high-mobility-group (HMG) domains, such as UBF(upstream binding factor). UBF is a transcription factor that binds to the promoter of ribosomal RNA (rRNA) genes thereby supporting initiation of transcription by RNA polymerase I. Here we report that cisplatin causes a redistribution of UBF in the nucleolus of human cells, similar to that observed after inhibition of rRNA synthesis. A similar redistribution was observed for the major components of the rRNA transcription machinery, namely TBP, TAFIs and RNA polymerase I. Furthermore, we provide for the first time direct in vivo evidence that cisplatin blocks synthesis of rRNA, while activity of RNA polymerase II continues to be detected throughout the nucleus. The clinically ineffective trans isomer (trans -DDP) does not alter the localization of either UBF or other components of the RNA polymerase I transcription machinery. These results suggest that disruption of rRNA synthesis, which is stimulated in proliferating cells, plays an important role in the clinical success of cisplatin.