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Many tumour cells show dependence on exogenous serine and dietary serine and glycine starvation can inhibit the growth of these cancers and extend survival in mice. However, numerous mechanisms promote resistance to this therapeutic approach, including enhanced expression of the de novo serine synthesis pathway (SSP) enzymes or activation of oncogenes that drive enhanced serine synthesis. Here we show that inhibition of PHGDH, the first step in the SSP, cooperates with serine and glycine depletion to inhibit one-carbon metabolism and cancer growth. In vitro, inhibition of PHGDH combined with serine starvation leads to a defect in global protein synthesis, which blocks the activation of an ATF-4 response and more broadly impacts the protective stress response to amino acid depletion. In vivo, the combination of diet and inhibitor shows therapeutic efficacy against tumours that are resistant to diet or drug alone, with evidence of reduced one-carbon availability. However, the defect in ATF4-response seen in vitro following complete depletion of available serine is not seen in mice, where dietary serine and glycine depletion and treatment with the PHGDH inhibitor lower but do not eliminate serine. Our results indicate that inhibition of PHGDH will augment the therapeutic efficacy of a serine depleted diet. Dietary serine and glycine starvation has emerged as a potential therapy for cancer. Here, the authors show that inhibition of PHGDH, which mediates the first step in the serine synthesis pathway, improves the therapeutic efficacy of serine depletion diet in mouse xenograft models.

Hepatic precursor cells (HPCs) are known to be bipotent and to give rise to both new hepatocytes and cholangiocytes upon acute liver injury. Stuart J. Forbes and his colleagues now show that interactions of HPCs with local macrophages and myofibroblasts potentiate Wnt and Notch signaling, respectively, to determine fate specification of the HPCs. Together, these mechanisms help determine proper organ regeneration after liver injury. During chronic injury a population of bipotent hepatic progenitor cells (HPCs) become activated to regenerate both cholangiocytes and hepatocytes. Here we show in human diseased liver and mouse models of the ductular reaction that Notch and Wnt signaling direct specification of HPCs via their interactions with activated myofibroblasts or macrophages. In particular, we found that during biliary regeneration, expression of Jagged 1 (a Notch ligand) by myofibroblasts promoted Notch signaling in HPCs and thus their biliary specification to cholangiocytes. Alternatively, during hepatocyte regeneration, macrophage engulfment of hepatocyte debris induced Wnt3a expression. This resulted in canonical Wnt signaling in nearby HPCs, thus maintaining expression of Numb (a cell fate determinant) within these cells and the promotion of their specification to hepatocytes. By these two pathways adult parenchymal regeneration during chronic liver injury is promoted.

Intestinal cancer: stem-cell destinies Inappropriate activation of the Wnt signalling pathway in intestinal stem cells causes them to become cancerous. Two papers in this issue help identify the cell type at the root of this cancer, which should in turn aid therapeutic design. Zhu et al . report that prominin 1, a surface protein found on both normal stem cells and cancer stem cells, is a marker for stem cells that are prone to neoplastic transformation. Barker et al . show that in cells expressing Lgr5 , previously identified as a marker for intestinal stem cells, activation of Wnt signalling is sufficient to initiate tumour formation. Intestinal tumours can originate from Lgr5 + intestinal stem cells after genetic activation of the Wnt signalling pathway. Intestinal cancer is initiated by Wnt-pathway-activating mutations in genes such as adenomatous polyposis coli ( APC ). As in most cancers, the cell of origin has remained elusive. In a previously established Lgr5 (leucine-rich-repeat containing G-protein-coupled receptor 5) knockin mouse model, a tamoxifen-inducible Cre recombinase is expressed in long-lived intestinal stem cells 1 . Here we show that deletion of Apc in these stem cells leads to their transformation within days. Transformed stem cells remain located at crypt bottoms, while fuelling a growing microadenoma. These microadenomas show unimpeded growth and develop into macroscopic adenomas within 3-5weeks. The distribution of Lgr5 + cells within stem-cell-derived adenomas indicates that a stem cell/progenitor cell hierarchy is maintained in early neoplastic lesions. When Apc is deleted in short-lived transit-amplifying cells using a different cre mouse, the growth of the induced microadenomas rapidly stalls. Even after 30weeks, large adenomas are very rare in these mice. We conclude that stem-cell-specific loss of Apc results in progressively growing neoplasia.

The molecular basis of disease progression from UV-induced precancerous actinic keratosis (AK) to malignant invasive cutaneous squamous cell carcinoma (cSCC) and potentially lethal metastatic disease remains unclear. DNA sequencing studies have revealed a massive mutational burden but have yet to illuminate mechanisms of disease progression. Here we perform RNAseq transcriptomic profiling of 110 patient samples representing normal sun-exposed skin, AK, primary and metastatic cSCC and reveal a disease continuum from a differentiated to a progenitor-like state. This is accompanied by the orchestrated suppression of master regulators of epidermal differentiation, dynamic modulation of the epidermal differentiation complex, remodelling of the immune landscape and an increase in the preponderance of tumour specific keratinocytes. Comparative systems analysis of human cSCC coupled with the generation of genetically engineered murine models reveal that combinatorial sequential inactivation of the tumour suppressor genes Tgfbr2 , Trp53 , and Notch1 coupled with activation of Ras signalling progressively drives cSCC progression along a differentiated to progenitor axis. Taken together we provide a comprehensive map of the cSCC disease continuum and reveal potentially actionable events that promote and accompany disease progression. The process by which actinic keratosis differentiates to malignant invasive cutaneous squamous cell carcinoma is unclear. Here, the authors use RNA-seq to illustrate a disease continuum between the two states, and use in vivo models to confirm the role of Tgfbr2, Trp53, and Notch1 in this process.

Different thresholds of Wnt signalling are thought to drive stem cell maintenance, regeneration, differentiation and cancer. However, the principle that oncogenic Wnt signalling could be specifically targeted remains controversial. Here we examine the requirement of BCL9/9l, constituents of the Wnt-enhanceosome, for intestinal transformation following loss of the tumour suppressor APC. Although required for Lgr5+ intestinal stem cells and regeneration, Bcl9/9l deletion has no impact upon normal intestinal homeostasis. Loss of BCL9/9l suppressed many features of acute APC loss and subsequent Wnt pathway deregulation in vivo. This resulted in a level of Wnt pathway activation that favoured tumour initiation in the proximal small intestine (SI) and blocked tumour growth in the colon. Furthermore, Bcl9/9l deletion completely abrogated β-catenin driven intestinal and hepatocellular transformation. We speculate these results support the just-right hypothesis of Wnt–driven tumour formation. Importantly, loss of BCL9/9l is particularly effective at blocking colonic tumourigenesis and mutations that most resemble those that occur in human cancer. Whether the Wnt enhanceosome’ components BCL9/9l can affect intestinal homeostasis and tumorigenesis is still unclear. Using conditional Bcl9/9l KO mice, the authors of this study show that the BCL9/9l complex is required for intestinal stem cells to drive tissue regeneration and that loss of BCL9/9l suppresses Wnt-driven transformation.

The continuous renewal of the skin relies on stem and progenitor cells, yet their differential susceptibility to oncogenic mutations in cutaneous squamous cell carcinoma (cSCC) remains unclear. Rapid cSCC develops in melanoma patients on BRAF inhibitors due to paradoxical MAPK activation. To model this in mice, we use two complementary approaches: HRAS G12V with a BRAF inhibitor to mimic paradoxical MAPK activation, and BRAF V600E , which drives MAPK hyperactivation without further treatment. We target these mutations to the interfollicular stem and differentiation-committed progenitors of the basal epidermis. While stem cells rapidly form tumours, progenitors exhibit long-latency resistance despite retaining mutations and repopulating the basal layer. Ultimately, both populations produce similar tumours, showing a shared transformation process. However, SOX2 is uniquely upregulated in progenitor-derived tumours and is expressed in 20% of human cSCC, indicating it might mark tumours arising from committed progenitors. Here, we show that SOX2 overexpression, along with MAPK activation, in progenitors induces a stem-like state and renders this otherwise resistant population permissive to rapid transformation. The oncogenic potential of interfollicular stem and progenitor cells in cutaneous squamous cell carcinoma (cSCC) remains poorly understood. Here, the authors modelled rapidly growing cSCC driven by the hyperactivation of the MAPK signalling pathway in mice and showed that SOX2 overexpression renders progenitor cells prone to transformation.

TP53 mutation occurs in 50-75% of human pancreatic ductal adenocarcinomas (PDAC) following an initiating activating mutation in the KRAS gene. These p53 mutations frequently result in expression of a stable protein, p53R¹⁷⁵H, rather than complete loss of protein expression. In this study we elucidate the functions of mutant p53 (Trp53R¹⁷²H), compared to knockout p53 (Trp53fl), in a mouse model of PDAC. First we find that although KrasG¹²D is one of the major oncogenic drivers of PDAC, most KrasG¹²D-expressing pancreatic cells are selectively lost from the tissue, and those that remain form premalignant lesions. Loss, or mutation, of Trp53 allows retention of the KrasG¹²D-expressing cells and drives rapid progression of these premalignant lesions to PDAC. This progression is consistent with failed growth arrest and/or senescence of premalignant lesions, since a mutant of p53, p53R¹⁷²P, which can still induce p21 and cell cycle arrest, is resistant to PDAC formation. Second, we find that despite similar kinetics of primary tumor formation, mutant p53R¹⁷²H, as compared with genetic loss of p53, specifically promotes metastasis. Moreover, only mutant p53R¹⁷²H-expressing tumor cells exhibit invasive activity in an in vitro assay. Importantly, in human PDAC, p53 accumulation significantly correlates with lymph node metastasis. In summary, by using 'knock-in' mutations of Trp53 we have identified two critical acquired functions of a stably expressed mutant form of p53 that drive PDAC; first, an escape from KrasG¹²D-induced senescence/growth arrest and second, the promotion of metastasis.

Oncogenic KRAS mutations are well-described functionally and are known to drive tumorigenesis. Recent reports describe a significant prevalence of KRAS allelic imbalances or gene dosage changes in human cancers, including loss of the wild-type allele in KRAS mutant cancers. However, the role of wild-type KRAS in tumorigenesis and therapeutic response remains elusive. We report an in vivo murine model of colorectal cancer featuring deletion of wild-type Kras in the context of oncogenic Kras . Deletion of wild-type Kras exacerbates oncogenic KRAS signalling through MAPK and thus drives tumour initiation. Absence of wild-type Kras potentiates the oncogenic effect of KRASG12D, while incidentally inducing sensitivity to inhibition of MEK1/2. Importantly, loss of the wild-type allele in aggressive models of KRASG12D-driven CRC significantly alters tumour progression, and suppresses metastasis through modulation of the immune microenvironment. This study highlights the critical role for wild-type Kras upon tumour initiation, progression and therapeutic response in Kras mutant CRC. The function of wild-type KRAS in KRAS mutant cancers remains to be explored. Here, the authors show that deletion of the tumour-suppressive wild-type Kras in a KRASG12D driven colon cancer model exacerbates tumour initiation in a MAPK dependent manner, while acting to suppress metastasis through impaired immune suppression.

Tissue progenitor cells are an attractive target for regenerative therapy. In various organs, bone marrow cell (BMC) therapy has shown promising preliminary results, but to date no definite mechanism has been demonstrated to account for the observed benefit in organ regeneration. Tissue injury and regeneration is invariably accompanied by macrophage infiltration, but their influence upon the progenitor cells is incompletely understood, and direct signaling pathways may be obscured by the multiple roles of macrophages during organ injury. We therefore examined a model without injury; a single i.v. injection of unfractionated BMCs in healthy mice. This induced ductular reactions (DRs) in healthy mice. We demonstrate that macrophages within the unfractionated BMCs are responsible for the production of DRs, engrafting in the recipient liver and localizing to the DRs. Engrafted macrophages produce the cytokine TWEAK (TNF-like weak inducer of apoptosis) in situ. We go on to show that recombinant TWEAK activates DRs and that BMC mediated DRs are TWEAK dependent. DRs are accompanied by liver growth, occur in the absence of liver tissue injury and hepatic progenitor cells can be isolated from the livers of mice with DRs. Overall these results reveal a hitherto undescribed mechanism linking macrophage infiltration to DRs in the liver and highlight a rationale for macrophage derived cell therapy in regenerative medicine.

Melanoma patients treated with oncogenic BRAF inhibitors can develop cutaneous squamous cell carcinoma (cSCC) within weeks of treatment, driven by paradoxical RAS/RAF/MAPK pathway activation. Here we identify frequent TGFBR1 and TGFBR2 mutations in human vemurafenib-induced skin lesions and in sporadic cSCC. Functional analysis reveals these mutations ablate canonical TGFβ Smad signalling, which is localized to bulge stem cells in both normal human and murine skin. MAPK pathway hyperactivation (through Braf V600E or Kras G12D knockin) and TGFβ signalling ablation (through Tgfbr1 deletion) in LGR5 +ve stem cells enables rapid cSCC development in the mouse. Mutation of Tp53 (which is commonly mutated in sporadic cSCC) coupled with Tgfbr1 deletion in LGR5 +ve cells also results in cSCC development. These findings indicate that LGR5 +ve stem cells may act as cells of origin for cSCC, and that RAS/RAF/MAPK pathway hyperactivation or Tp53 mutation, coupled with loss of TGFβ signalling, are driving events of skin tumorigenesis. Cutaneous squamous cell carcinomas is a growing problem but the driver genes causing this remain poorly defined. Here, the authors demonstrate that inactivating driver mutations in TGFBR1 and TGFBR2 occur in vemurafenib-induced and sporadic cutaneous squamous cell carcinomas.