Evaluation of Injury-induced Senescence and <em>In Vivo</em> Reprogramming in the Skeletal Muscle

Cellular senescence is a stress response that is characterized by a stable cellular growth arrest, which is important for many physiological and ...

Institut Pasteur

Over the last few years, the discovery of basal-type mammary carcinomas and the association of the regenerative potential of the mammary epithelium with the basal myoepithelial cell population have attracted considerable attention to this second major mammary lineage. However, many questions concerning the role of basal myoepithelial cells in mammary morphogenesis, functional differentiation and disease remain unanswered. Here, we discuss the mechanisms that control the myoepithelial cell differentiation essential for their contractile function, summarize new data concerning the roles played by cell-extracellular matrix (ECM), intercellular and paracrine interactions in the regulation of various aspects of the mammary basal myoepithelial cell functional activity. Finally, we analyze the contribution of the basal myoepithelial cells to the regenerative potential of the mammary epithelium and tumorigenesis.

The mammary myoepithelial cell.

The mammary epithelium comprises two major cell lineages: basal and luminal. Basal cells (BCs) isolated from the mammary epithelium and transplanted into the mouse mammary fat pad cleared from the endogenous epithelium regenerate the mammary gland, strongly suggesting that the basal epithelial compartment harbors a long-lived cell population with multipotent stem cell potential. The luminal cell layer is devoid of the regenerative potential, but it contains cells with clonogenic capacity, the luminal progenitors. Mammary BCs and luminal progenitors express high levels of the transcription factor Myc. Here, we show that deletion of Myc from mammary basal epithelial cells led to impaired stem cell self-renewal as evaluated by limiting dilution and serial transplantation assays. Luminal progenitor population was significantly diminished in mutant epithelium suggesting control by the BC layer. Colony formation assay performed with isolated BCs showed that clonogenic capacity was abolished by Myc deletion. Moreover, transplanted BCs depleted of Myc failed to produce epithelial outgrowths. Stimulation with ovarian hormones estrogen (E) and progesterone (P) partially rescued the repopulation capacity of Myc-depleted BCs; however, the Myc-deficient mammary epithelium developed in response to E/P treatment lacked stem and progenitor cells. This study provides the first evidence that in the mammary gland, Myc has an essential nonredundant function in the maintenance of the self-renewing multipotent stem cell population responsible for the regenerative capacity of the mammary epithelium and is required downstream from ovarian hormones, for the control of mammary stem and progenitor cell functions.

The proto-oncogene Myc is essential for mammary stem cell function.

Mammary epithelium comprises a layer of luminal cells and a basal myoepithelial cell layer. Both mammary epithelial compartments, basal and luminal, contain stem and progenitor cells, but only basal cells are capable of gland regeneration upon transplantation. Aberrant expansion of stem/progenitor cell populations is considered to contribute to breast tumorigenesis. Germline deletions of p53 in humans and mice confer a predisposition to tumors, and stem cell frequency is abnormally high in the mammary epithelium of p53-deficient mice. However, it is unknown whether stem/progenitor cell amplification occurs in both, basal and luminal cell populations in p53-deficient mammary tissue. We used a conditional gene deletion approach to study the role of p53 in stem/progenitor cells residing in the mammary luminal and basal layers. Using two- and three-dimensional cell culture assays, we showed that p53 loss led to the expansion of clonogenic stem/progenitor cells in both mammary epithelial cell layers. Moreover, following p53 deletion, luminal and basal stem/progenitor cells acquired a capacity for unlimited propagation in mammosphere culture. Furthermore, limiting dilution and serial transplantation assays revealed amplification and enhanced self-renewal in the basal regenerating cell population of p53-deficient mammary epithelium. Our data suggest that the increase in stem/progenitor cell activity may be, at least, partially mediated by the Notch pathway. Taken together, these results strongly indicate that p53 restricts the propagation and self-renewal of stem/progenitor cells in both layers of the mammary epithelium providing further insight into the impact of p53 loss in breast cancerogenesis.

Somatic loss of p53 leads to stem/progenitor cell amplification in both mammary epithelial compartments, basal and luminal.

Basal-like breast cancer is a heterogeneous disease characterized by the expression of basal cell markers, no estrogen or progesterone receptor expression and a lack of HER2 overexpression. Recent studies have linked activation of the Wnt/β-catenin pathway, and its downstream target, Myc, to basal-like breast cancer. Transgenic mice K5ΔNβcat previously generated by our team present a constitutive activation of Wnt/β-catenin signaling in the basal myoepithelial cell layer, resulting in focal mammary hyperplasias that progress to invasive carcinomas. Mammary lesions developed by K5ΔNβcat mice consist essentially of basal epithelial cells that, in contrast to mammary myoepithelium, do not express smooth muscle markers.

Myc is required for β-catenin-mediated mammary stem cell amplification and tumorigenesis.

Current androgen ablation therapies for prostate cancer are initially successful, but the frequent development of castration resistance urges the generation of alternative therapies and represents an important health concern. Prolactin/signal transducer and activator of transcription 5 (STAT5) signaling is emerging as a putative target for alternative treatment for prostate cancer. However, mechanistic data for its role in development or progression of prostate tumors are scarce. In vivo mouse studies found that local prolactin induced the amplification of prostate epithelial basal/stem cells. Because these cells are proposed cells of origin for prostate cancer and disease recurrence, we looked further into this amplification. Our results indicated that sustained Stat5 activation was associated with the occurrence of abnormal basal/stem cell clusters in prostate epithelium of prostate-specific prolactin-transgenic mice. Analysis of epithelial areas containing these clusters found high proliferation, Stat5 activation, and expression of stem cell antigen 1. Furthermore, enhanced prolactin signaling also led to amplification of a luminal cell population that was positive for stem cell antigen 1. These cells may originate from amplified basal/stem cells and might represent important progenitors for tumor development in prostate epithelium. These data provide a deeper understanding of the initial stages of prostate tumorigenesis induced by prolactin to help determine whether this hormone or its downstream messengers could be useful targets for prostate cancer treatment in the future.

Prolactin-induced prostate tumorigenesis links sustained Stat5 signaling with the amplification of basal/stem cells and emergence of putative luminal progenitors.

HGF/Met signaling has recently been associated with basal-type breast cancers, which are thought to originate from progenitor cells residing in the luminal compartment of the mammary epithelium. We found that ICAM-1 efficiently marks mammary luminal progenitors comprising hormone receptor-positive and receptor-negative cells, presumably ductal and alveolar progenitors. Both cell populations strongly express Met, while HGF is produced by stromal and basal myoepithelial cells. We show that persistent HGF treatment stimulates the clonogenic activity of ICAM1-positive luminal progenitors, controlling their survival and proliferation, and leads to the expression of basal cell characteristics, including stem cell potential. This is accompanied by the induction of Snai1 and Snai2, two major transcription factors triggering epithelial-mesenchymal transition, the repression of the luminal-regulatory genes Elf5 and Hey1, and claudin down-regulation. Our data strongly indicate that paracrine Met signaling can control the function of luminal progenitors and modulate their fate during mammary development and tumorigenesis.

Paracrine Met signaling triggers epithelial-mesenchymal transition in mammary luminal progenitors, affecting their fate.

In a cohort of 95 women with multiple breast fibroadenomas (MFAs), we recently identified patients harboring germline heterozygous variants of the prolactin receptor (PRLR) exhibiting constitutive activity (PRLRI146L and PRLRI176V).

Gain-of-function Prolactin Receptor Variants Are Not Associated With Breast Cancer and Multiple Fibroadenoma Risk.

In vivo reprogramming is a promising approach for tissue regeneration in response to injury. Several examples of in vivo reprogramming have been reported in a variety of lineages, but some including skeletal muscle have so far proven refractory. Here, we show that acute and chronic injury enables transcription-factor-mediated reprogramming in skeletal muscle. Lineage tracing indicates that this response frequently originates from Pax7+ muscle stem cells. Injury is associated with accumulation of senescent cells, and advanced aging or local irradiation further enhanced in vivo reprogramming, while selective elimination of senescent cells reduced reprogramming efficiency. The effect of senescence appears to be, at least in part, due to the release of interleukin 6 (IL-6), suggesting a potential link with the senescence-associated secretory phenotype. Collectively, our findings highlight a beneficial paracrine effect of injury-induced senescence on cellular plasticity, which will be important for devising strategies for reprogramming-based tissue repair.

Aurelie Chiche

Cellular Plasticity & Disease Modelling,
Department of Developmental & Stem Cell Biology,
CNRS UMR 3738,
Cellular Plasticity & Disease Modelling, Department of Developmental & Stem Cell Biology, CNRS UMR 3738

Institut Pasteur

Evaluation of Injury-induced Senescence and In Vivo Reprogramming in the Skeletal Muscle

Aurelie Chiche

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Institut Pasteur

Evaluation of Injury-induced Senescence and In Vivo Reprogramming in the Skeletal Muscle

Cellular Plasticity & Disease Modelling,
Department of Developmental & Stem Cell Biology,
CNRS UMR 3738,
Cellular Plasticity & Disease Modelling, Department of Developmental & Stem Cell Biology, CNRS UMR 3738