The p53 isoform Δ133p53β promotes cancer stem cell potential

Abstract

Cancer stem cells (CSC) are responsible for cancer chemoresistance and metastasis formation. Here we report that Δ133p53β, a TP53 splice variant, enhanced cancer cell stemness in MCF-7 breast cancer cells, while its depletion reduced it. Δ133p53β stimulated the expression of the key pluripotency factors SOX2, OCT3/4, and NANOG. Similarly, in highly metastatic breast cancer cells, aggressiveness was coupled with enhanced CSC potential and Δ133p53β expression. Like in MCF-7 cells, SOX2, OCT3/4, and NANOG expression were positively regulated by Δ133p53β in these cells. Finally, treatment of MCF-7 cells with etoposide, a cytotoxic anti-cancer drug, increased CSC formation and SOX2, OCT3/4, and NANOG expression via Δ133p53, thus potentially increasing the risk of cancer recurrence. Our findings show that Δ133p53β supports CSC potential. Moreover, they indicate that the TP53 gene, which is considered a major tumor suppressor gene, also acts as an oncogene via the Δ133p53β isoform.

Figure 1

Selective Depletion of p53 Isoforms Affects the Sphere-Forming Ability of MCF-7 Cells (A) Schematic representation of p53 isoforms with the targets of the different shRNAs (Sh) used in this study. The calculated molecular weights of the different isoforms are indicated. (B) Mammosphere quantification in MCF-7 cells after transduction of Sh1, Sh2, Sh3, Sh4, and Sh5 (n = 3 independent experiments). (C–E) Western blot analysis of p53 isoform depletion in the corresponding cells. (F and G) The qRT-PCR quantification of the expression levels of C-MYC, SOX2, OCT3/4, and NANOG (F) as well as of Δ133p53 and p53 β isoforms (G) after transduction with Sh1 and Sh2 (n = 4 independent experiments).

Figure 2

The Isoform Δ133p53β Promotes CSC Potential in MCF-7 Cells (A and B) Mammosphere quantification in MCF-7 cells after silencing with Sh2 (shRNAs against the TA and Δ40 isoforms) or with Sh2 and Sh6 (against the 3′ end of the α isoforms) (A) and western blot analysis to confirm p53 depletion in the corresponding cell cultures (B) (n = 3 independent experiments). (C) Representative fluorescence-activated cell sorting (FACS) dot blots for the double labeling of CD24 and CD44 in MCF-7 transduced with Sh Luc (Control), Sh2, or Sh2 + 6. (D and E) Mammosphere quantification in MCF-7 cells after Δ133p53β or γ overexpression (D) and qRT-PCR analysis of C-MYC, SOX2, OCT3/4, and NANOG (E) expression in the corresponding cells (n = 4 independent experiments). (F) Mammosphere quantification in MCF-7 cells that overexpress Δ133p53β after harvesting and re-plating of the primary mammospheres (n = 4 independent experiments). (G) Mammosphere quantification in MCF-7 cells in which all p53 isoforms have been silenced with Sh1 and after expression in the same cells of Sh1-resistant Δ133p53β (n = 3 independent experiments).

Figure 3

Evaluation of the CSC Features of the MDA-MB-231 D3H2LN and C3LND Cell Lines (A) Mammosphere quantification in the modestly metastatic, parental MDA-MB-231 D3H2LN and the derived, highly metastatic C3LND cell lines (n = 3 independent experiments). (B) The qRT-PCR analysis of Δ133p53 isoform expression in MDA-MB-231 D3H2LN and C3LND cells (n = 4 independent experiments). (C) The qRT-PCR quantification of C-MYC, OCT3/4, NANOG, and SOX2 expression in MDA-MB-231 D3H2LN and C3LND cells (n = 4 independent experiments). (D) Western blot analysis of Δ133p53β-Flag transduced in MDA-MB-231 D3H2LN cells (Flag antibody). (E) The qRT-PCR analysis of C-MYC, OCT3/4, NANOG, and SOX2 expression in MDA-MB-231 D3H2LN cells after Δ133p53β overexpression (n = 4 independent experiments). (F) Mammosphere quantification in MDA-MB-231 D3H2LN cells that overexpress Δ133p53β (n = 3 independent experiments). (G) Mammosphere quantification in MDA-MB-231 C3LND transduced with Sh3 (n = 3 independent experiments). (H) The qRT-PCR analysis of Δ133p53 isoform expression in MDA-MB-231 C3LND transduced with Sh3 (n = 3 independent experiments). (I) The qRT-PCR quantification of C-MYC, OCT3/4, NANOG, and SOX2 expression in MDA-MB-231 C3LND cells transduced with Sh3 (n = 4 independent experiments). (J) Representative FACS dot plots for the double labeling of CD44 and CD24 in MDA-MB-231 C3LND transduced with Sh Luc (Control) or Sh3 (n = 3 independent experiments). (K) Bioluminescence ventral images of mice injected with MDA-MB-231 C3LND cells transduced with Sh3 or control. Pseudocolor scale bars show relative changes at metastatic sites. (L) Quantification of distant metastasis in brain and femur using bioluminescence imaging (n = 7/5) 25 days after the implantation. Bars represent mean ± SEM of biological replicates.

Figure 4

Chemotherapy Treatment of MCF-7 Breast Cancer Cells Upregulates Δ133p53 Isoform Expression and Activates Key Pluripotency Genes (A) Western blot analysis of p53, p21, and C-MYC expression in MCF-7 cells after treatment with increasing doses of etoposide for 16 hr (DO1 antibody). (B) The qRT-PCR analysis of C-MYC expression in MCF-7 cells upon treatment with increasing doses of etoposide (n = 4 independent experiments). (C) The qRT-PCR analysis of Δ133p53 isoform expression in MCF-7 cells after etoposide treatment (n = 4 independent experiments). (D) Western blot analysis of p53 isoform expression in MCF-7 cells after etoposide treatment (Sapu antibody). (E) The qRT-PCR analysis of SOX2, OCT3/4, and NANOG expression in MCF-7 cells upon treatment with increasing doses of etoposide (n = 4 independent experiments). (F) The qRT-PCR analysis of Δ133p53, SOX2, OCT3/4, and NANOG expression in control and MCF-7 cells transduced with Sh3 upon etoposide treatment (n = 4 independent experiments). (G) Mammosphere quantification in MCF-7 cells transduced with Sh2 and treated with 50 ng/ml/day etoposide for 7 days (n = 3 independent experiments). (H and I) The qRT-PCR analysis of C-MYC, NANOG, OCT3/4, and SOX2 (H) and Δ133p53 isoform (I) expression in MCF-7 cells transduced with Sh2 and treated with 50 ng/ml/day etoposide for 7 days (n = 4 independent experiments).