p73 is a p53 family member that plays a critical role in cell proliferation and differentiation.Molecular mechanisms of p73 regulation remain unclear.In this study, we investigated cell density mediated regulation of p73.We found that cell density is a sensitive factor that affects p73 levels in cell culture, and in particular, p73 and p73 are differentially regulated.While p73 protein levels were inversely correlated with cell densities, p73 protein levels behaved oppositely.We further show that density dependent changes in p73 follow the same patterns as E2F-1 and TAp73 mRNA levels, suggesting a transcriptional regulation.Results from cycloheximide (CHX) blocking experiments suggest that high levels of p73 at lower densities may be due to increased protein stability.As ubiquitin ligase AIP-4/Itch was reported to be a factor that regulates p73 protein stability, we also tested the role of AIP-4/Itch in this process.AIP-4/Itch knockdown resulted in the increase of p73*, but not p73*.The data suggest that Itch may not be involved in downregulation of p73 at high densities.Moreover, we also found that subcellular location of p73 isoforms varies as the culture density increases.While high levels of p73 at low density were mainly present in the nucleus, low levels of this protein at high density were mainly in the cytosol.Taken together, our findings reveal a novel mechanism that differentially regulates p73 expression, and underscores the role of cell-cell interaction in p73 regulation, which may advance our understanding of p73 expression and function in human cancers.