ΔNp63α is an oncogene that targets chromatin remodeler Lsh to drive skin stem cell proliferation and tumorigenesis

Abstract

The p53 homolog p63 is essential for development, yet its role in cancer is not clear. We discovered that p63 deficiency evokes the tumor-suppressive mechanism of cellular senescence, causing a striking absence of stratified epithelia such as the skin. Here we identify the predominant p63 isoform, ΔNp63α, as a protein that bypasses oncogene-induced senescence to drive tumorigenesis in vivo. Interestingly, bypass of senescence promotes stem-like proliferation and maintains survival of the keratin 15-positive stem cell population. Furthermore, we identify the chromatin-remodeling protein Lsh as a new target of ΔNp63α that is an essential mediator of senescence bypass. These findings indicate that ΔNp63α is an oncogene that cooperates with Ras to promote tumor-initiating stem-like proliferation and suggest that Lsh-mediated chromatin-remodeling events are critical to this process.

Figure 1. Downregulation of ΔNp63α is required for oncogene-induced senescence

(A) Primary mouse keratinocytes were infected with retrovirus expressing GFP-vector (control) or oncogenic HRasV12 (Ras), with Ras-infected cells undergoing oncogene-induced senescence (left). QPCR (upper right) and western-blot (lower right) analysis 7 days post-infection indicates that ΔNp63 is downregulated at both the transcript and the protein level in senescent cells. DNA binding domain, DBD; ΔN, ΔNp63 isoforms; Uninfected, U; Control, C; Ras, R; cDNA. Quantitation ± S.D. Scale bar, 50μM. (B) Keratinocytes were infected with retroviral Ras (R), or were co-infected with Ras and GFP-expressing ΔNp63α (RΔN), TAp63α (RTA) or ΔNp63α^R279H (RΔN^R279H) constructs. Cells expressing Ras and ΔNp63α continue to proliferate. Scale bar, 50μM. (C) Senescence-associated-β Galactosidase (SA-β-Gal) assay and quantitation shows decreased senescence in cells expressing Ras+ΔNp63α. Scale bar, 50μM. (D) Western blotting showing expression of p63, Ras, and actin in infected cells.

Figure 2. ΔNp63α’s ability to bypass OIS leads to proliferation of cells with stem-like properties

(A) Primary keratinocytes infected with Ras+ΔNp63α-GFP form spheres in 3D-tissue culture, while cells infected with vector, Ras, ΔNp63α, or Ras and a short-hairpin specific for p53 (shp53) do not have this ability. (B) Immunofluorescence on 3D cultures shows elevated K14 expression in Ras+ΔNp63α spheres compared to Ras+shp53 cultures. (C) Primary keratinocytes expressing a GFP construct (vector) proliferate in low calcium media but terminally differentiate and stop proliferating in high calcium conditions, as shown with western blot for PCNA (right). Cells infected with Ras+ΔNp63α acquire the same morphological changes with calcium treatment, but do not stop proliferating, whereas cells expressing Ras+shp53 do not acquire the same differentiation features or stop proliferating. (D) QPCR for markers of basal epithelia (keratin 14), early differentiation (keratin 10) and late differentiation (involucrin) show cells expressing Ras+ΔNp63α differentiate in response to calcium, similar to early differentiation in vector expressing cells. Ras+shp53 cells have decreased keratin markers and do not differentiate.

Figure 3. Ras and ΔNp63α expression maintains survival of K15-expressing stem cells

(A) Whole mount immunofluorescence of the bulge region of adult mouse skin identifies keratin15 (green)- and Nestin (red)-positive stem cell populations in the bulge region of the hair follicle. The K15-positive population is visualized with an anti-K15 antibody; the Nestin-positive population is co-visualized by immunofluorescence using a GFP-specific antibody in Nestin-GFP transgenic mice. Sebaceous glands (sg) label non-specifically. (B) FACS analyses of primary keratinocyte cultures derived from K15-GFP transgenic mice after infection with vector or Ras+ΔNp63α. After 14 days, cells infected with vector are not maintained and GFP-positive stem cells are depleted. However, cells infected with Ras+ΔNp63α maintain the GFP-positive stem cells 14 days after infection. (C) Quantification of FACS experiments (K15, n=5; Nestin n=9) showing a loss of K15-positive stem cells infected with vector control after 14 days. In contrast, the GFP-positive stem cell population is maintained by Ras+ΔNp63α (top). Similar experiments with Nestin-GFP positive stem cells (bottom) are not maintained by either vector or Ras+ΔNp63α.

Figure 4. ΔNp63α is an oncogene that cooperates with oncogenic Ras in vivo

(A) Whole animal imaging of nude mice injected sub-cutaneously with primary keratinocytes expressing Ras+shp53-GFP or Ras+ΔNp63α-GFP shows the development of GFP-expressing tumors. (B) Pathological and immunohistological analyses demonstrate distinct tumor types. Ras+shp53 tumors present mainly as fibrosarcoma lacking expression of epithelial markers, whereas Ras+ΔNp63α tumors are squamous cell carcinomas expressing p63, K14 and β-catenin.

Figure 5. Downregulation of endogenous p63 occurs in senescent cells of premalignant lesions in vivo

(A) Papilloma and SCC were induced by treating mouse skin with DMBA. In papillomas, p63 expression is restricted to the basal layer, but decreases in the lesion, whereas p63 is expressed in all cells that progressed to squamous cell carcinoma (SCC). The schematic highlights areas of proliferation (P) and senescence (S). (B) Immunohistochemical analyses of premalignant papillomas indicates that Ki67 and p63 are co-expressed in the basal layer, but are absent in the non-proliferative region where the senescence markers γH2AX and p19 are expressed. The schematic highlights areas of proliferation (P) and senescence (S).

Figure 6. Lsh is a p63 target gene

(A) Timeline schematic of the experimental procedure. Phase I corresponds with initiating events leading to senescence bypass, whereas Phase II involves clonal proliferation of stem-like cells and tumor development. (B) Western blotting for p63 and senescence mediators in cells expressing Ras+ΔNp63α during phase 1 of senescence bypass indicates that expression of p53, p16, PML, or p21 are not reduced. (C) Chromatin Immunoprecipitation (ChIP) analysis shows p63 is bound to the Lsh promoter. p21 binding serves as a positive control. (Input, In; Immunoglobulin negative control, Ig). (D) Immunofluorescence for p63 and Lsh in E17.5 mouse skin shows overlap in the basal epithelia. (E) Primary mouse keratinocytes from 2 individual wildtype and p63^loxP/loxP mice were infected with retroviral vector or Cre. Expression of p63 DNA-binding domain (DBD) is lost in p63^loxP/loxP cells with Cre, and Lsh expression is decreased coordinately.

Figure 7. Lsh is required for ΔNp63α’s OIS-inhibiting activity

(A) Western blotting for Lsh and p63 in primary mouse keratinocytes expressing vector (V), Ras (R) ΔNp63α (ΔN) and Ras+ΔNp63α (RΔN) during phase I senescence bypass indicates that Lsh expression is increased in ΔNp63α-expressing cells. (B) Western blotting for Lsh and p63 in primary mouse keratinocytes expressing vector (V), ΔNp63α (ΔN) and Ras+ΔNp63α (RΔN) co-infected with retroviral vector (LMP) or shRNA-Lsh. (C) Cell morphology by phase contrast and GFP-fluorescence shows fewer cells and senescent-like morphology in Lsh-knockdown cells. Immunostaining for BrdU incorporation and quantitation shows that Lsh-knockdown cells expressing ΔNp63α proliferate less. (BrdU quantitation, ± S.E.M: one way ANOVA/Tukey’s t-test). (D) Immunostaining indicates that p63 and Lsh are coexpressed in DMBA-induced premalignant papillomas in situ. (E) Immunostaining indicates that Lsh is expressed robustly in tumors arising from sub-cutaneous injection of RΔN keratinocytes, but not those arising from sub-cutaneous injection of Rshp53 keratinocytes. (F) Western blotting shows increased Lsh expression in 2 squamous cell carcinoma cell lines (029, 011) that have increased p63 expression, compared to human primary keratinocytes (HPK).