Basal cell signaling by p63 controls luminal progenitor function and lactation via NRG1

Abstract

The mammary epithelium is organized as a bilayer of luminal and basal/myoepithelial cells. During pregnancy, the luminal compartment expands for milk production, while basal cells are thought to provide structural and contractile support. Here, we reveal a pregnancy-specific role of basal epithelia as a central coordinator of lactogenesis. We demonstrate that genetic deletion of the transcription factor p63 (Trp63) gene exclusively within basal cells of the adult gland during pregnancy leads to dramatic defects in luminal cell proliferation and differentiation, resulting in lactation failure. This phenotype is explained by direct transcriptional activation of the epidermal growth factor family ligand gene Nrg1 by p63 selectively in basal cells, which is required for luminal ERBB4/STAT5A activation and consequent luminal progenitor cell maturation. Thus, paracrine basal-to-luminal cell signaling, controlled by p63 via NRG1, orchestrates the entire lactation program. Collectively, these findings redefine the paradigm for cellular interactions specifying the functional maturation of the mammary gland.

Figure 1. Basal cell-specific expression of p63 increases during mammary gland maturation

(A) Sections of pubertal (6 weeks old), adult virgin (10 weeks old) and lactating mammary gland. Left, hematoxylin and eosin (H&E); right, immunohistochemistry (IHC) showing basal-specific staining (brown) for p63 and Keratin14. D, duct; Scale bars, 100 μm. (B–D) Representative flow cytometry dot plot showing basal/luminal cell selection by CD24 and CD29 expression in the viable, Lin⁻ (TER119⁻CD34⁻CD45⁻) population of pubertal (B), adult virgin (C) and lactating (D) gland. NE, non-epithelial. (E) Mean mRNA levels of Keratin14 (Krt14), Keratin18 (Krt18), p63 and ΔNp63 in lactating gland determined by quantitative RT-PCR relative to Gapdh, in basal (Bas.), luminal (Lum.) and non-epithelial (NE) populations sorted as in (D). Data represent mean of 3 animals. (F) Relative mRNA ratio of ΔNp63/TAp63 of 6-week-old pubertal (n=3) and L1 (n=9) control mice. (G) Mean p63 mRNA levels relative to Gapdh in sorted basal and luminal cells of pubertal (n=8), virgin (n=12) and lactation stage 1 (L1, n=7) animals. In (E–G) data represent mean ± SEM. * p<0.05; ** p<0.01; NS: not significant. See also Figure S1.

Figure 2. Basal cell p63 is required for lobuloalveolar development and lactation

(A) Schematic representation of experimental setup to excise p63 in basal epithelial cells of the mammary gland from female mice during pregnancy by Tamoxifen injection before mating. (B) Mean mRNA levels of transgenic K14-Cre recombinase relative to Gapdh in sorted basal and luminal cells of control non-transgenic (WT, n=3) and transgenic p63-cKO (Tg, n=4) mice at lactation stage 1. ND, not detectable. (C) Mean p63 DNA Cre/Lox-mediated excision determined by quantitative PCR relative to a control locus in total mammary tissue of control (Ctl, n=3) and p63-cKO (n=6) mice. (D) Absence of p63 DNA excision in sorted luminal cells of p63-cKO (n=3) relative to control (n=2) mice, determined as in (C). (E) Mean p63 mRNA levels relative to Gapdh in sorted basal cells from control (n=5) and p63-cKO (n=3) mice. (F) Left, Pictures of a pup of a control mother (top) with white stomach and a pup of a p63-cKO mother without milk in its stomach (bottom). Right, Quantification of pups born with white stomach of control (Ctl, n=14 pups) and p63-cKO mother (n=15 pups). p value calculated by Fisher’s exact test. (G) Representative whole mount staining of pregnant (L1) mammary glands of control and p63-cKO mice. Arrows point to lymph nodes. Squares in (G) are showing regions which are enlarged in (H). Scale bars, 5 mm (G) and 1 mm (H). (I) Representative H&E staining of L1 mammary glands from control (top) and p63-cKO (bottom) mice. Scale bars, 100 μm. D, ductal lumen; asterisk, alveolar lumen. (J) Mean filling of fat pad by alveoli (left) and relative area per alveolus (right) of control (n=6) and p63-cKO (n=6) mice. All graphs represent mean ± SEM. ** p<0.01; *** p<0.001; NS: not significant. See also Figure S2.

Figure 3. Basal p63 controls luminal cell proliferation, differentiation and luminal progenitor maturation during pregnancy

(A) Proliferation assessed by IHC for Ki67 in representative control and p63-cKO mammary glands at L1. Graph at right shows mean fraction of positive cells in glands from control (n=3) and p63-cKO (n=3) animals. Scale bars, 50 μm. (B) Absence of milk protein expression (brown) in p63-cKO compared to control mammary glands at L1, shown by IHC. Arrowheads, intraluminal milk protein. Scale bars, 50 μm. (C) Mean mRNA expression of whey acidic protein (Wap) and casein beta (Csn2) relative to Gapdh in control (n=3) and p63-cKO (n=3) mammary glands at L1. (D) Little or no difference in the proportion of luminal versus basal cells in control (n=9) versus p63-cKO (n=9) glands at L1 assessed by flow cytometry as in Figure 1. (E) Increased luminal progenitors in p63-cKO (n=7) versus control (n=8) glands at L1, assessed as CD61+ luminal (CD24⁺CD29^lo) cells. (F) Mean mRNA for Itgb3 (encoding CD61) among sorted control (n=8) versus p63-cKO (n=9) luminal cells at L1. (G) Increased colony-forming capacity of sorted luminal cells from p63-cKO versus control glands at L1, plated in matrigel plus complete medium. Scale bars, 200 μm. (H) Quantification of colony-forming capacity. Left, colony number per 1000 sorted luminal cells from control (n=5) versus p63-cKO (n=6) glands plated as in (G). Right, relative colony volume in cultures from control (n=4) versus p63-cKO (n=4). All graphs represent mean ± SEM. * p<0.05; ** p<0.01; *** p<0.001; NS: not significant. See also Figure S3.

Figure 4. Direct transcriptional regulation of Nrg1 expression by p63 during pregnancy

(A) Nrg1 is expressed in basal epithelia specifically during lactogenesis. Mean mRNA for Nrg1 relative to Gapdh in flow-sorted luminal and basal cells from pubertal (n=5), virgin (n=5), and L1 (n=7) mammary glands (left); and from pregnancy day 7–9 (P7–9, n=4), P12–14 (n=3), and L1 (n=4) glands (right). ND, not detectable. (B) P63-dependent expression of Nrg1 and Jag2, shown relative to Gapdh in control (n=2–4, each in duplicate) versus p63-cKO (n=2–6, each in duplicate) basal cells at L1. Data represent mean ± SEM. (C) Western blot of mammary gland lysates from two representative pregnant control (Ctl) and p63 cKO mice each at L1, showing NRG1 protein is decreased following conditional p63 excision. (D) Mean mRNA for p63 and NRG1 relative to beta-2-Microglobulin following p63 knockdown (72h) by lentiviral shRNA versus control vector in MCF10A. Shown is the mean of three independent experiments ± SEM. (E) Western blot of MCF10A cells showing decreased p63 and NRG1 proteins after transduction with shRNA against p63 in comparison to vector control cells (corresponds to C). β-Tubulin serves as a loading control. (F) Quantitative PCR (qPCR) analysis of ChIP for endogenous p63 or rabbit control IgG (rIgG), showing specific p63 binding upstream of the NRG1 transcription start site (TSS, −30 kb) but not to a control locus (−21 kB). Shown is the mean of two IP experiments analyzed in duplicate ± SEM. (G) qPCR analysis of ChIP for FLAG epitope in cells expressing a control vector or FLAG-tagged p63 wild-type (WT) or DNA binding-deficient point mutant (R304W). Shown is the mean of two ChIP analyzed in duplicate ± SEM. Right, western blot confirming tagged p63 protein expression. (H) Binding of endogenous p63 to the Nrg1 locus in vivo, shown by qPCR analysis of ChIP for p63 or control rIgG in primary mammary tissue. Shown is the mean of a representative ChIP analyzed in triplicate ± SD. * p<0.05; ** p<0.01; *** p<0.001; NS: not significant. See also Figure S4.

Figure 5. P63 is essential for luminal STAT5A signaling and transcription during pregnancy

(A) Mean Erbb4 mRNA levels relative to Gapdh in sorted basal and luminal cells of pubertal (n=8), virgin (n=9) and lactation stage 1 (L1, n=6) animals. (B) Luminal STAT5A activation shown by immunofluorescence for phosphorylated STAT5A (pSTAT5A) in normal mammary gland at L1. Scale bars, 20 μm. (C) STAT5A activation assessed by IHC for pSTAT5A (brown) in representative control and p63-cKO mammary glands at L1. Scale bars, 50 μm. (D) Mean percentage of cells with activated STAT5A in control (n=5) and p63-cKO (n=6) glands at L1. (E) Mean mRNA for STAT5A target genes Elf5 and Ccnd1 relative to Gapdh in flow-sorted luminal and basal cells from control (n=5–6) and p63-cKO (n=7–8) glands at L1. All graphs represent mean ± SEM.; ** p<0.01; *** p<0.001; NS: not significant. (F) IHC showing absent pSTAT5A in the post-partum Nrg1α−/− mammary gland, which phenocopies failed STAT5A activation in the p63 cKO gland. See also Figure S5.

Figure 6. P63 and NRG1 control ERBB4/STAT5A signaling to mediate luminal progenitor function and lactogenesis

(A) Western blot showing p63 is required for phosphorylation of STAT5A in HC11 cells following lentiviral p63 shRNA (shp63) or control vector expression. (B) RNA for milk genes Csn2 and Wap relative to Gapdh in cells treated as in (A). Shown is the mean of 2–4 experiments analyzed in duplicate. (C) P63 regulates prolactin (Prl)-dependent HC11 differentiation, shown by western blot for p63 and milk protein β-Casein treated as in (A). β-Tubulin, loading control. (D) Left, representative colony-forming assays of luminal cells from p63-cKO glands as in Figure 3, in the presence or absence of prolactin (Prl) and the ERBB4 inhibitors JNJ and HDS (see text). Scale bars, 500 μm. Right, graphs showing dose-dependent inhibition of luminal progenitor proliferation by ERBB4 inhibitors. The mean of 3 experiments is shown. (E) Mean RNA for Csn2 relative to Gapdh in luminal colonies from (D) in the presence and absence of prolactin and ERBB4 inhibitors. ND, not detectable. (F) qRT-PCR of ERBB4/STAT5 target gene Elf5 in luminal cells (n=3–4 each) in matrigel in presence or absence of ERBB4 inhibitor, 200 nM JNJ 28871063 hydrochloride (JNJ) or 0.5 nM HDS 029 (HDS), with or without lactogenic complete media (Prl) to show that ERBB4 inhibitor blocked its target gene expression. All graphs represent mean ± SEM. * p<0.05; ** p<0.01; *** p<0.001; NS: not significant. See also Figure S6.

Figure 7. Basal epithelial p63 controls luminal progenitor maturation and lactogenesis through activation of paracrine NRG1/ERBB4/STAT5A signaling

Left, basal p63 is a direct transcriptional regulator of Nrg1, encoding a specific ligand for luminal-specific ERBB4. Right top, p63- and NRG1-mediated ERBB4/STAT5A activation, evidenced by nuclear phosphorylated STAT5A (pS5), induces luminal progenitor cell proliferation and differentiation resulting in milk production. Bottom, loss of p63 results in luminal progenitors that are reversibly blocked in development. Cooperating genetic events may ultimately result in malignant transformation of such progenitors, which are known to be the precursors of a particularly aggressive form of breast cancer (see Discussion).