PARENT SESSION
1:30 PM to 3:30 PM
Wednesday, April 24, 2002
Poster Session 26 Apoptosis
Room: Nevada Exhibition Center

(P31-307) Ionizing irradiation differential regulates p73 isoforms in some heamatopiotic malignant cell lines; role of p73 in apoptosis.

Mahmoud-Ahmed, Ashraf^*,1, Dupree, Erica², Almasan, Alex^1,2, ¹ Dept. Radiation Oncology, Cleveland, Ohio² Dept. Cancer Biology, 9500 Euclid Avenue, Ohio

ABSTRACT-
The newly discovered p73 gene encodes a nuclear protein that has high homology to p53. We examined p73 expression in hematopioietic cancer cell lines treated with ionizing irradiation and the chemotherapeutic agent, ara-C. P73 mRNA and protein were determined by quantitative reverse transcription-polymerase chain reaction (QRT- PCR), Northern, and Western blotting, respectively. The expression of different isoforms of p73 was detected using the nested PCR using specially designed primers. Ionizing irradiation treatment resulted in a decline in p73 at both the protein and mRNA levels in several the cell lines examined (e.g. IM-9, MOLT-4, Jurkat, but not U266). However, in striking contrast to the alterations in p73, a significant accumulation of p53 protein was detected. Moreover, these changes in expression levels of p73 were observed in cells containing wild-type, mutant, or no p53. Further studies indicated that this p53-independent down regulation of p73 following radiation treatment was dose- and time-dependent. Moreover, the p73 protein decline was specific to radiation, as it was not detected after ara-C treatment. Five (, ,,, and ) of the known p73 isoforms were diagnosed by nested PCR and DNA sequencing. In comparison to the down regulation of the 4 p73 isoforms, there was a differential upregulation of p73 following irradiation. Consistent with a possible anti-apoptotic role of p73, stable overexpression of p73 (the full-length form of p73) in Jurkat cells was associated with a higher resistance to radiation-induced apoptosis, as cdetermined by flowcytometry using Annexin V -PE . As expected, P73 expression had also an effect on cell cycle progression. We conclude that although p73 shares a similar structural and functional composition with p53, there are likely to be significant differences in the mechanisms that govern the responses of p53 and p73 to ionizing irradiation-induced DNA damage. Some isoforms of p73, such as p73, may play a different role than p53 in irradiation-induced apoptosis.

KEYWORDS: tumor supressor, apoptosis, cell cycle