Tissue specific expression of p53 target genes suggests a key role for KILLER/DR5 in p53-dependent apoptosis in vivo

The p53 tumor suppressor plays a key role in the cell's response to genotox ic stress and loss of this 'guardian of the genome' is an important step in carcinogenesis. The ability of p53 to induce apoptosis through transactiva tion of its target genes is critical for its function as tumor suppressor. We have found that overexpression of p53 in human cancer cell lines resulte d in apoptosis as measured by PARP cleavage. Furthermore we observed cleava ge of both caspase 9 and caspase 8 after overexpression of p53 and found th at p53-dependent apoptosis was inhibited by either cellular (c-Flip-s, BCI- X-L) or pharmacological inhibitors of caspase 8 or caspase 9 respectively. These results indicate that p53 is mediating apoptosis through both the mit ochondrial and death receptor pathways. To elucidate the relevant p53 targe t genes and examine the caspase pathways utilized in vivo, we treated p53+/ + and age matched p53-/- mice with 5 Gy ionizing radiation or 0.5 mg/animal dexamethasone and harvested tissues at 0, 6 and 24 h. We examined the mRNA expression of p21, bax, KILLER/DR5, FAS/APO1 and EI24/PIG8 using TaqMan re al time quantitative RT-PCR in the spleen, thymus and small intestine. Alth ough the basal mRNA levels of these genes did not depend on the presence of p53, we observed a p53-dependent induction of all these targets in respons e to gamma -radiation and a p53-independent regulation for p21 and KILLER/D R5 in response to dexamethasone. Furthermore, we have demonstrated that the relative induction of these p53 target genes is tissue specific. Despite o bserving otherwise similar levels of death in these tissues, our findings s uggest that in some cases apoptosis mediated through p53 occurs by redundan t pathways or by a 'group effect' while in other tissues one or few targets may play a key role in p53-dependent apoptosis. Surprisingly, KILLER/DR5 i s the dominantly induced transcript in both the spleen and small intestine suggesting a potentially important role for this p53 target gene in vivo.