BMC Genomics 2007, 8:139 http://www.biomedcentral.com/1471-2164/8/139 cFAliedgaduritlryieodn8eatletramrgientesdwuistihngNaGnFti-dpe5p3esnhdReNntAinecxrperaesessingpc5e3llolcinceuspancy, but where p53-dependent transactivation could not be Additional targets with NGF-dependent increase in p53 occupancy, but where p53-dependent transactivation could not be clearly determined using anti-p53 shRNA expressing cell lines. Panels A-C show site-specific qPCR ChIP results on left with standard deviation, where significance at p ≤ 0.05 is designated by an asterisk (*). GAPDH controls in Figure 7 also apply to gene expression analyses in Figure 8. Note the p53-occupied sequence in Panel A lies between genes which were both exam- ined for changes in expression. Display and analyses are as described in Figure 4 legend. 10B) were not recapitulated in 5-FU and bleomycin treated cells. Results demonstrate an opposite transcrip- tional regulation for these p53-regulated targets depend- ing upon the treatment stimulus. Although the absolute levels varied, expression levels and direction of the remaining p53-regulated target genes were similar regard- less of the treatment (Figure 10C). Expression levels of all gene targets were similar to those found earlier using RT- PCR, with absolute differences attributed mainly to differ- ential assay sensitivities. These data demonstrate that the gene expression at chosen time points for some, but not all, p53 target genes is NGF-dependent. The remaining target genes studied, however, were induced or repressed both by NGF receptor-mediated or genotoxic p53-induc- ing stimuli. Discussion The p53 tumor suppressor is widely appreciated as a cen- tral node in the regulation of proliferation and apoptosis in response to various genotoxic insults. Its role within cellular differentiation, however, is uncertain. p53 has been considered non-essential to development since p53 mice are viable and succumb to tumors rather than developmental malformations [8]. The discovery of low frequency neural tube defects in p53-deficient mice [11] is an apparent paradox, suggesting the role of p53 in devel- opment may be more complex than initially believed. Interpretation of developmental effects in p53-deficient models at the organismal level are complicated by the overlapping transcriptional functions of the p53 family members p63 and p73 [58] which include such vital tar- gets as p21Waf1/Cip1, Bax and MDM2 and likely other unknown target genes. In addition, regulation of growth control and apoptosis can also be carried out through p53-independent cellular mechanisms which could cir- cumvent developmental defects in the majority of p53- deficient mice. The marked accumulation of p53 protein and serine 15 phosphorylation levels suggesting its activation during NGF exposure led us to investigate a possible transactiva- tional role for p53 in neuronal differentiation. Using the PC12 cell model of NGF-induced neuronal differentia- tion, we hypothesized that p53 is involved in differentia- tion through specific transcriptional regulation of target genes. A contribution of p53 to PC12 cell differentiation Page 10 of 17 (page number not for citation purposes)