Hsp72 mediates TAp73α anti-apoptotic effects in small cell lung carcinoma cells.
Bottom Line: However, the precise mechanism by which TAp73α exerts its pro-survival effect is yet unclear.Finally, we revealed that TAp73β counteracts the anti-apoptotic effect of TAp73α by preventing Hsp72 induction.Our results thus provide additional evidence for the potential oncogenic role of TAp73α, and extend the understanding of the mechanism for its anti-apoptotic effect.
Affiliation: Department of Oncology-Pathology, Cancer Centrum Karolinska, Karolinska Institutet, Stockholm, Sweden.Show MeSH
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Mentions: We have shown that full-length TAp73α represses drug-induced apoptosis in SCLC cells, whereas full-length TAp73β strengthens drug-induced apoptosis in the same settings . In addition, the anti-apoptotic actions of TAp73α upon drug-induced apoptosis was shown to be exerted upstream of the mitochondria, on the level of Bax . Previously, the capability of the ΔN forms of p73 and p63 (ΔNp73α and ΔNp63α) to regulate the expression of Hsp72 has been demonstrated [23, 28]. Moreover, similar to TAp73α, Hsp72 has been shown to exert many of its pro-survival functions upstream of the mitochondria . To test whether the anti-apoptotic effect of TAp73α in SCLC cells could be due to a direct regulation of Hsp72 protein expression, we performed luciferase gene reporter assay on the HSP72 promoter in SCLC H82 and HEK-293 cells. Full-length TAp73α was able to transactivate the HSP72 promoter in both H82 and HEK-293 cells (Fig. 1A), as well as in HeLa, HCT116 p53+/+ and HCT116 p53−/– cells (data not shown). However, no transcriptional activity of the full-length TAp73β isoform on the HSP72 promoter could be seen in any of the cell lines tested. In contrast, the transcriptional activity of ΔNp73α on the HSP72 promoter demonstrated a cell-type specificity, being significantly active in HEK-293, HeLa, HCT116 p53+/+ and HCT116 p53–/– cells, but not in H82 cells (Fig. 1A, and data not shown). An additional set of experiments was performed to confirm the induction of Hsp72 by TAp73α on both mRNA and protein levels. Indeed, a significant induction in Hsp72 mRNA levels could be seen after TAp73α overexpression, but not upon expression of TAp73β (Fig. 1B). Moreover, transient expression of TAp73α, but not TAp73β, was able to significantly induce Hsp72 protein level, as shown in single cells by immunofluorescent staining of H82 cells (Fig. 1C) and by immunoblot on HEK-293 cell extracts (Fig. 1D). However, no effect of either TAp73α or TAp73β expression could be detected on the levels of the Hsp72-related proteins HSF1, Hsp90/HSPC1, Hsc70/HSPA8 or Hsp27/HSPB1 (Fig. 1E). As control, Hsp72 protein expression during VP16 treatment and up to 24 hrs is also depicted in Figure S1A. Thus, based on these data we conclude that TAp73α is a selective and potent inducer of Hsp72 expression.
Affiliation: Department of Oncology-Pathology, Cancer Centrum Karolinska, Karolinska Institutet, Stockholm, Sweden.