An investigation into 53BP1 complex formation.

Loss of the control over cellular proliferation can lead to cell death or result in the abnormal proliferation characteristic of the cancerous state. Among the controls used to achieve normal cellular proliferation is the DNA damage checkpoint pathway that monitors genome integrity (Hartwell and Kastan 1994). 53BP1 was identified as a protein that interacts with the DNA-binding core domain of the tumor suppressor p53. The p53-binding region of 53BP1 maps to the C-terminal BRCT domains which are homologous to those found in the breast cancer protein BRCA1 and in other proteins involved in the DNA damage response, notably budding yeast Rad9. In addition to its recently reported role in sensing double strand breaks, 53BP1 is believed to have roles, currently ill understood, in many aspects of DNA metabolism ranging from transcription and class switch recombination to 'mediating' the DNA damage checkpoint response (Chai et al. 1999; Huyen et al. 2004; Sengupta et al. 2004; Ward et al. 2004). Here, we investigate 53BP1 complex formation. We investigate 53BP1 oligomerization and show that this is not dependent on the presence of disulfide bridges.

SNIP1 is a candidate modifier of the transcriptional activity of c-Myc on E box-dependent target genes.

Using a yeast two-hybrid screen, we found that SNIP1 (Smad nuclear-interacting protein 1) associates with c-Myc, a key regulator of cell proliferation and transformation. We demonstrate that SNIP1 functions as an important regulator of c-Myc activity, binding the N terminus of c-Myc through its own C terminus, and that SNIP1 enhances the transcriptional activity of c-Myc both by stabilizing it against proteosomal degradation and by bridging the c-Myc/p300 complex. These effects of SNIP1 on c-Myc likely contribute to synergistic effects of SNIP1, c-Myc, and H-Ras in inducing formation of foci in an in vitro transformation assay and also in supporting anchorage-independent growth. The significant association of SNIP1 and c-Myc staining in a non-small cell lung cancer tissue array is further evidence that their activities might be linked and suggests that SNIP1 might be an important modulator of c-Myc activity in carcinogenesis.

The FHA domain protein SNIP1 is a regulator of the cell cycle and cyclin D1 expression.

Smad nuclear interacting protein 1 (SNIP1) is an evolutionarily conserved protein containing a forkhead-associated (FHA) domain that regulates gene expression through interactions with multiple transcriptional regulators. Here, we have used short interfering RNAs (siRNAs) to knockdown SNIP1 expression in human cell lines. Surprisingly, we found that reduction in SNIP1 levels resulted in significantly reduced cell proliferation and accumulation of cells in the G1 phase of the cell cycle. Consistent with this result, we observed that cyclin D1 protein and mRNA levels were reduced. Moreover, SNIP1 depletion results in inhibition of cyclin D1 promoter activity in a manner dependent upon a previously characterized binding site for the AP-1 transcription factor family. SNIP1 itself is induced upon serum stimulation immediately prior to cyclin D1 expression. These effects were independent of the tumour suppressors p53 and retinoblastoma (Rb), but were consistent with an interaction with BRG1, a component of the ATP-dependent chromatin remodelling complex, Swi/Snf. These results define both a new function for SNIP1 and identify a previously unrecognized regulator of the cell cycle and cyclin D1 expression.

The p53-inhibitor pifithrin-alpha inhibits firefly luciferase activity in vivo and in vitro.

BACKGROUND: Pifithrin-alpha is a small molecule inhibitor of p53 transcriptional activity. It has been proposed that the use of pifithrin-alpha in conjunction with chemotherapeutic and radiation therapies for cancer will reduce the side effects of these treatments in normal tissue that still contains wild type p53. In addition, pifithrin-alpha provides a useful tool in the laboratory to investigate the function of p53 in model systems. RESULTS: While investigating the effects of pifithrin-alpha on the transcriptional activity of NF-kappaB, we observed a strong inhibition of reporter plasmids containing the firefly luciferase gene. Firefly luciferase is one of the most commonly used enzymes in reporter gene assays. In contrast, no inhibition of reporter plasmids containing Renilla luciferase or chloramphenicol acetyltransferase was observed. The inhibition of firefly luciferase activity by pifithrin-alpha was observed both in vivo and in vitro. Pifithrin-alpha did not inhibit firefly luciferase protein expression, but rather suppressed light production/emission, since addition of exogenous pifithrin-alpha to active extracts inhibited this activity. Furthermore, pifithrin-alpha also inhibited recombinant firefly luciferase protein activity. CONCLUSIONS: Among its other biological activities, pifithrin-alpha is an inhibitor of firefly luciferase activity. Caution must therefore be taken when using this compound, which has been characterised as an inhibitor of p53 transcriptional activity, to investigate effects on gene expression using transiently transfected reporter plasmids. Furthermore, these results demonstrate that when using novel compounds, the choice of vectors used in the experimental procedures might be of great importance for the correct conclusions to be made.