During development and regeneration, proliferation of tissue-specific stem cells is tightly controlled to produce organs of a predetermined size. The molecular determinants of this process remain poorly understood. Here, we investigate the function of Yap1, the transcriptional effector of the Hippo signaling pathway, in skin biology. Using gain- and loss-of-function studies we show that Yap1 is a critical modulator of epidermal stem cell proliferation and tissue expansion. Yap1 mediates this effect through interaction with TEAD transcription factors. Additionally, our studies reveal that α-catenin, a molecule previously implicated in tumor suppression and cell density sensing in the skin, is an upstream negative regulator of Yap1. α-catenin controls Yap1 activity and phosphorylation by modulating its interaction with 14-3-3 and the PP2A phosphatase. Together, these data identify Yap1 as a determinant of the proliferative capacity of epidermal stem cells and as an important effector of a ‘crowd control’ molecular circuitry in mammalian skin.

The Hippo pathway regulates contact inhibition of cell proliferation and, ultimately, organ size in diverse multicellular organisms. Inactivation of the Hippo pathway promotes nuclear localization of the transcriptional coactivator Yap1, a Hippo pathway effector, and can cause cancer. Here, we show that deletion of αE-catenin in the hair follicle stem cell compartment resulted in the development of skin squamous cell carcinoma in mice. Tumor formation was accelerated by simultaneous deletion of αE-catenin and the tumor suppressor-encoding gene p53. An siRNA screen revealed a functional connection between αE-catenin and Yap1. By interacting with Yap1, αE-catenin promoted its cytoplasmic localization, and Yap1 showed constitutive nuclear localization in αE-catenin-null cells. We also found an inverse correlation between αE-catenin abundance and Yap1 activation in human squamous cell carcinoma tumors. These findings identify αE-catenin as a tumor suppressor that inhibits Yap1 activity and sequesters it in the cytoplasm.

The generation and release of membrane-enclosed packets from cancer cells, called extracellular vesicles (EVs), play important roles in propagating transformed phenotypes, including promoting cell survival. EVs mediate their effects by transferring their contents, which include specific proteins and nucleic acids, to target cells. However, how the cargo and function of EVs change in response to different stimuli remains unclear. Here, we discovered that treating highly aggressive MDAMB231 breast cancer cells with paclitaxel (PTX), a chemotherapy that stabilizes microtubules, causes them to generate a specific class of EV, namely exosomes, that are highly enriched with the cell survival protein and cancer marker, Survivin. Treating MDAMB231 cells with a variety of other chemotherapeutic agents, and inhibitors that block cell growth and survival, did not have the same effect as PTX, with the exception of nocodazole, another inhibitor of microtubule dynamics. Exosomes isolated from PTX-treated MDAMB231 cells strongly promoted the survival of serum-starved and PTX-treated fibroblasts and SKBR3 breast cancer cells, an effect that was ablated when Survivin was knocked-down from these vesicles using siRNA. These findings underscore how the enrichment of a specific cargo in exosomes promotes cell survival, as well as can potentially serve as a marker of PTX resistance.

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Taken together, these results establish the involvement of miRNA in radiation response and may potentially help explain the mechanisms of gene regulation in the cellular response to ionising radiation exposure.