p73 loss triggers conversion to squamous cell carcinoma reversible upon reconstitution with TAp73alpha

The expression level of the p53 family member, p73, is frequently deregulated in human epithelial cancers, correlating with tumor invasiveness, therapeutic resistance, and poor patient prognosis. However, the question remains whether p73 contributes directly to the process of malignant conversion or whether aberrant p73 expression represents a later selective event to maintain tumor viability. We explored the role of p73 in malignant conversion in a clonal model of epidermal carcinogenesis. Whether sporadic or small interfering RNA (siRNA) induced, loss of p73 in initiated p53+/+ keratinocytes leads to loss of cellular responsiveness to DNA damage by ionizing radiation (IR) and conversion to squamous cell carcinoma (SCC). Reconstitution of TAp73alpha but not DeltaNp73alpha reduced tumorigenicity in vivo, but did not restore cellular sensitivity to IR, uncoupling p73-mediated DNA damage response from its tumor-suppressive role. These studies provide direct evidence that loss of p73 can contribute to malignant conversion and support a role for TAp73alpha in tumor suppression of SCC. The results support the activation of TAp73alpha as a rational mechanism for cancer therapy in solid tumors of the epithelium.     

p53 modulates base excision repair activity in a cell cycle-specific manner after genotoxic stress

To elucidate the nature of the cross-talk between the p53 protein and the DNA repair machinery, we have investigated the relationship between the two throughout the cell cycle. Base excision repair (BER) was analyzed in cell cycle phase-enriched populations of lymphoid cells expressing wild-type p53. Our study yielded the following novel findings: (a) BER exhibited two distinct peaks of activity, one associated with the G0-G1 checkpoint and the second with the G2-M checkpoint; (b) although the overall BER activity was reduced after exposure of cells to 400R, there was an augmentation of the G0-G1-associated BER activity and a reduction in the G2-M-associated BER activity; and (c) modulations in these patterns of BER after genotoxic stress were found to be p53 regulated. p53 protein levels induced after gamma-irradiation were distributed evenly in the various cell cycle populations (analyzed by the PAb-248 anti-p53 monoclonal antibody). However, both the dephosphorylation of serine 376 of p53 (contained in the PAb-421 epitope) and the specific DNA binding activity, as well as apoptosis, were enhanced toward the G2-M populations. Furthermore, inactivation of wild-type p53, mediated by mutant p53 expression, abolished the alterations in the BER pattern and showed no induction of a G2-M-associated apoptosis after gamma-irradiation. These results suggest that after genotoxic stress, stabilized p53 enhances the G0-G1-associated BER activity, whereas it predominantly reduces BER activity at the G2-M-enriched populations and instead induces apoptosis. After genotoxic stress, p53 functions as a modulator that determines the pattern of BER activity and apoptosis in a cell cycle-specific manner.     

p53-dependent cell cycle control: response to genotoxic stress

p53 protein is involved in key responses to genotoxic stress. These functions underlie the role of p53 as the 'guardian of the genome'. In a simplified manner, upon low or repairable levels of DNA damage, p53 mediates the delay or arrest at checkpoints preceding cell replication (the G1/S checkpoint), and is involved in delaying damaged cells prior premitotic chromosome condensation (the G2 and pre-meiotic check-points) and actual chromosome partition (the spindle check-point). During these delays, an opportunity is given to repair the DNA damage, before its fixation and propagation, that may lead to carcinogenesis. Upon high or irreparable DNA damage, p53 promotes the cells towards apoptosis. Here we review the known molecular pathways by which p53 controls the cell cycle, with a specific focus on the significance of p53-mediated checkpoint response for its 'tumor suppressor' function. The data reviewed is concerned with the in vivo mouse models including p53 knockout mice, transgenic mice harboring various mutant forms of p53 and mice knocked out for cell-cycle- and apoptosis-associated genes situated upstream or downstream from p53, that have been elaborated upon over the last few years.