Mixed signals at tumor margins: The Hippo signaling pathway has been implicated in tumor growth, sparking interest in the pathway as a potential therapeutic target. In a study of liver cancer in genetically manipulated mice, Moya et al. discovered that the role of this pathway in tumorigenesis is more complex than previously appreciated. They confirmed that activation of the Hippo pathway within tumor cells drives tumor growth; however, they also found that activation of the pathway in adjacent healthy cells has the opposite effect, suppressing tumor growth. Whether tumor cells survive or are eliminated thus appears to depend on competing signals produced by the tumor and surrounding tissue. Science , this issue p. 1029

The Hippo signaling pathway is widely considered a master regulator of organ growth because of the prominent overgrowth phenotypes caused by experimental manipulation of its activity. Contrary to this model, we show here that removing Hippo transcriptional output did not impair the ability of the mouse liver and Drosophila eyes to grow to their normal size. Moreover, the transcriptional activity of the Hippo pathway effectors Yap/Taz/Yki did not correlate with cell proliferation, and hyperactivation of these effectors induced gene expression programs that did not recapitulate normal development. Concordantly, a functional screen in Drosophila identified several Hippo pathway target genes that were required for ectopic overgrowth but not normal growth. Thus, Hippo signaling does not instruct normal growth, and the Hippo-induced overgrowth phenotypes are caused by the activation of abnormal genetic programs.

The Hippo pathway and its downstream effectors YAP and TAZ (YAP/TAZ) are heralded as important regulators of organ growth and regeneration. However, different studies provided contradictory conclusions about their role during regeneration of different organs ranging from promoting proliferation to inhibiting it. Here, we resolve the function of YAP/TAZ during regeneration of the liver, where Hippo's role in growth control has been studied most intensely. We evaluated liver regeneration after CCl4 toxic liver injury in mice with conditional deletion of Yap/Taz in hepatocytes and/or biliary epithelial cells and measured the behavior of different cell types during regeneration by histology, RNA-sequencing and flow cytometry. We found that YAP/TAZ were activated in hepatocytes in response to CCl4 toxic injury. However, their targeted deletion in adult hepatocytes did not noticeably impair liver regeneration. In contrast, Yap/Taz deletion in adult bile ducts caused severe defects and delay in liver regeneration. Mechanistically, we show that Yap/Taz mutant bile ducts degenerated, causing cholestasis which stalled the recruitment of phagocytic macrophages and the removal of cellular corpses from injury sites. Elevated bile acids activated PXR, which was sufficient to recapitulate the phenotype observed in mutant mice. Our data show that YAP/TAZ are practically dispensable in hepatocytes for liver development and regeneration. Rather, YAP/TAZ play an indirect role in liver regeneration by preserving bile duct integrity and securing immune cell recruitment and function.

The Hippo pathway and its downstream effectors, the YAP and TAZ transcriptional coactivators, are deregulated in multiple different types of human cancer and are required for cancer cell phenotypes in vitro and in vivo, while largely dispensable for tissue homeostasis in adult mice. YAP/TAZ and their main partner transcription factors, the TEAD1–4 factors, are therefore promising anticancer targets. Because of frequent YAP/TAZ hyperactivation caused by mutations in the Hippo pathway components NF2 and LATS2, mesothelioma is one of the prime cancer types predicted to be responsive to YAP/TAZ-TEAD inhibitor treatment. Mesothelioma is a devastating disease for which currently no effective treatment options exist. Here, we describe a novel covalent YAP/TAZ-TEAD inhibitor, SWTX-143, that binds to the palmitoylation pocket of all four TEAD isoforms. SWTX-143 caused irreversible and specific inhibition of the transcriptional activity of YAP/TAZ-TEAD in Hippo-mutant tumor cell lines. More importantly, YAP/TAZ-TEAD inhibitor treatment caused strong mesothelioma regression in subcutaneous xenograft models with human cells and in an orthotopic mesothelioma mouse model. Finally, SWTX-143 also selectively impaired the growth of NF2-mutant kidney cancer cell lines, suggesting that the sensitivity of mesothelioma models to these YAP/TAZ-TEAD inhibitors can be extended to other tumor types with aberrations in Hippo signaling. In brief, we describe a novel and specific YAP/TAZ-TEAD inhibitor that has potential to treat multiple Hippo-mutant solid tumor types.

Metabolic dysfunction-associated steatotic liver disease (MASLD) and its advanced form, metabolic dysfunction-associated steatohepatitis (MASH), are leading risk factors for liver cancer. While inflammation and fibrosis are known to promote tumorigenesis in MASH, the role of early MASLD in cancer initiation is unclear. Here, we investigated how steatosis alone, without significant fibrosis and inflammation, influences liver cancer development in mouse models. We fed mice a Western diet to induce early-stage MASLD, characterized by steatosis and mild hepatitis without fibrosis, and triggered liver tumorigenesis by hydrodynamic tail vein injection of plasmids encoding oncogenes. We combined this with genetic manipulation of Hippo signaling and assessed tumor development using histology and single-nucleus RNA sequencing. Western diet suppressed liver tumor development by promoting the elimination of tumor-initiating cells. This effect was not due to a direct impact on oncogene-expressing cells, but rather to increased competitiveness of the surrounding hepatocytes. Mechanistically, the Western diet activated the Hippo pathway effectors Yap and Taz in steatotic hepatocytes, enabling them to outcompete tumor-initiating cells. Progression to advanced MASH abrogated this tumor-suppressing effect, allowing tumor development. Steatotic hepatocytes in early MASLD activate a Yap/Taz-dependent cell competition program that can suppress tumor initiation. This tumor-suppressive effect is lost in advanced MASH, where inflammation, fibrosis and other defects undermine the competitiveness of non-mutant hepatocytes. The tumor-suppressive effect of early MASLD adds a new layer to our understanding of how fitness landscapes and cell interactions shape cancer risk during chronic liver disease. We found that early stages of steatotic liver disease (MASLD) can suppress tumor initiation in mouse models of liver cancer by triggering Yap/Taz-mediated cell competition. This reveals an unexpected tumor-suppressive role for early MASLD and highlights the importance of non-cell autonomous mechanisms during tumor initiation. Since Hippo-YAP/TAZ signaling is deregulated in many cancers, similar mechanisms may operate in other tissues.

Tracking cell death in vivo can enable a better understanding of the biological mechanisms underlying tissue homeostasis and disease. Unfortunately, existing cell death labeling methods lack compatibility with in vivo applications or suffer from low sensitivity, poor tissue penetration, and limited temporal resolution. Here, we fluorescently labeled dead cells in vivo with Trypan Blue (TBlue) to detect single scattered dead cells or to generate whole-mount three-dimensional maps of large areas of necrotic tissue during organ regeneration. TBlue effectively marked different types of cell death, including necrosis induced by CCl₄ intoxication in the liver, necrosis caused by ischemia-reperfusion in the skin, and apoptosis triggered by BAX overexpression in hepatocytes. Moreover, due to its short circulating lifespan in blood, TBlue labeling allowed in vivo “pulse and chase” tracking of two temporally spaced populations of dying hepatocytes in regenerating mouse livers. Additionally, upon treatment with cisplatin, TBlue labeled dead cancer cells in livers with cholangiocarcinoma and dead thymocytes due to chemotherapy-induced toxicity, showcasing its utility in assessing anticancer therapies in preclinical models. Thus, TBlue is a sensitive and selective cell death marker for in vivo applications, facilitating the understanding of the fundamental role of cell death in normal biological processes and its implications in disease.