Effects of inducible overexpression of DNp73α on cancer cell growth and response to treatment in vitro and in vivo

Abstract

The p73 gene has a complex regulation, which leads to the expression of different isoforms, often with opposite biological effects. We have generated in the human colocarcinoma cell line HCT116, expressing a wild-type p53, an inducible DNp73a expressing system. Two clones (HCT116/DN3 and HCT116/DN14), upon doxycycline addition, show a strong expression of DNp73a. In vitro the two DNp73a overexpressing clones grow at similar rate of the control transfected clone (HCT116/8a) and similarly respond to DNA damage. When injected in mice, HCT116/DN3, HCT116/DN14, and HCT116/8a cells grew similarly in the absence or presence of tetracycline. In HCT116/DN3 and HCT116/DN14 tumors, tetracycline induced a strong expression of DNp73a both as mRNA and protein. These results indicate that in this system the overexpression of the DNp73a does not induce a more aggressive phenotype and does not seem to be associated with a reduced response of the cells to treatment with anticancer agents.

Overexpressed TP73 induces apoptosis in medulloblastoma

“…Among different cell types, the apparent discrepancy between expression of TP73 variants at the RNA and protein levels suggests that post-translational modifications also contribute to observed levels. Like other transcription factors, TP73 variants are regulated by a variety of positive and negative networks involving: E2F1, MDM2, Sumo-1, c-Abl, NEDL2, and other DNA damage response factors [21,44]. Such complex regulatory influences probably account for the dynamic expression levels of TP73 , as seen with p53.…”

“…In contrast to normal brain tissue, primary tumor specimens and established medulloblastoma cell lines overexpress both TAp73 and ΔNp73 proteins, consistent with dysregulation of developmental expression and a possible contribution to their neoplastic phenotype. Functional studies of TAp73 and ΔNp73 have revealed differential effects in response to genotoxic stress [16,44-46]. TAp73 induces arrest and apoptosis, while in most systems ΔNp73 counteracts these pathways in response to ionizing radiation and genotoxic agents in vitro [as reviewed in [47]].…”

Questioning the oncogenic role of ΔNp73α in different cell lines expressing p53 or not

“…25 Our cellular models argued against a role of the ∆Np73α isoform as a potent pro-survival protein since ∆Np73α overexpression did not rescue cell lines from cell death or did not confer any growth advantage. These data were confirmed both in models with a p53 -/-or p53 +/+ genetic background and with a MIN +/+ (H1299) or MIN -/-(HCT116) phenotype.…”

“…24 On the other hand, recent results from our laboratory have shown that at least in cancer cells expressing p53 and p73 functional pathways, ∆Np73α is not associated, either in vitro or in vivo, with a more malignant phenotype or a more aggressive and resistant tumor. 25 To gain further knowledge of ∆Np73α's role in cancer cell growth and response to treatment, we investigated the effects of its overexpression in human cellular models with a p53 -/-and p73 +/+ genetic background.…”

ΔNp73: Misunderstood protein?

“…This might explain why ∆Np73α can transform primary fibroblasts in cooperation with oncogenes in vitro and in vivo, 5,21 but the overexpression of ∆Np73α in HCT116 and 1299 cells does not result in increased proliferation in vitro 2 or more aggressive xenografts in mice. 22 Like most tumor cell lines, H1299 and HCT116 cells have many alterations, and it is possible that these clones can tolerate ∆Np73α expression. Several oncogenes and tumor suppressor pathways are likely "abnormal" in these cells.…”