Cancer-associated fibroblasts (CAFs) are the predominant components of the tumor microenvironment (TME) and influence cancer hallmarks, but without systematic investigation on their ubiquitous characteristics across different cancer types. Here, we perform pan-cancer analysis on 226 samples across 10 solid cancer types to profile the TME at single-cell resolution, illustrating the commonalities/plasticity of heterogenous CAFs. Activation trajectory of the major CAF types is divided into three states, exhibiting distinct interactions with other cell components, and relating to prognosis of immunotherapy. Moreover, minor CAF components represent the alternative origin from other TME components (e.g., endothelia and macrophages). Particularly, the ubiquitous presentation of endothelial-to-mesenchymal transition CAF, which may interact with proximal SPP1⁺ tumor-associated macrophages, is implicated in endothelial-to-mesenchymal transition and survival stratifications. Our study comprehensively profiles the shared characteristics and dynamics of CAFs, and highlight their heterogeneity and plasticity across different cancer types. Browser of integrated pan-cancer single-cell information is available at https://gist-fgl.github.io/sc-caf-atlas/.

Targeting the SARS-CoV-2 main protease: Vaccines are an important tool in the fight against COVID-19, but developing antiviral drugs is also a high priority, especially with the rise of variants that may partially evade vaccines. The viral protein main protease is required for cleaving precursor polyproteins into functional viral proteins. This essential function makes it a key drug target. Qiao et al. designed 32 inhibitors based on either boceprevir or telaprevir, both of which are protease inhibitors approved to treat hepatitis C virus. Six compounds protected cells from viral infection with high potency, and two of these were selected for in vivo studies based on pharmokinetic experiments. Both showed strong antiviral activity in a mouse model. Science , this issue p. 1374

Multiple myeloma (MM) cells are characterized by high protein synthesis resulting in chronic endoplasmic reticulum (ER) stress, which is adaptively managed by the unfolded protein response. Inositol-requiring enzyme 1α (IRE1α) is activated to splice X-box binding protein 1 (XBP1) mRNA, thereby increasing XBP1s protein, which in turn regulates genes responsible for protein folding and degradation during the unfolded protein response. In this study, we examined whether IRE1α-XBP1 pathway is a potential therapeutic target in MM using a small-molecule IRE1α endoribonuclease domain inhibitor MKC-3946. MKC-3946 triggered modest growth inhibition in MM cell lines, without toxicity in normal mononuclear cells. Importantly, it significantly enhanced cytotoxicity induced by bortezomib or 17-AAG, even in the presence of bone marrow stromal cells or exogenous IL-6. Both bortezomib and 17-AAG induced ER stress, evidenced by induction of XBP1s, which was blocked by MKC-3946. Apoptosis induced by these agents was enhanced by MKC-3946, associated with increased CHOP. Finally, MKC-3946 inhibited XBP1 splicing in a model of ER stress in vivo, associated with significant growth inhibition of MM cells. Taken together, our results demonstrate that blockade of XBP1 splicing by inhibition of IRE1α endoribonuclease domain is a potential therapeutic option in MM.

Key Points: Deubiquitylating enzymes USP14 and UCHL5 are involved in the tumorigenesis of MM. b-AP15 is a specific USP14 and UCHL5 inhibitor, which blocks growth and induces apoptosis in MM cells.

Shaoguang Li and colleagues identify Alox5 as a key gene that regulates the function of leukemia stem cells but not normal hematopoietic stem cells in mice, highlighting how cancer and normal stem cells distinctly self-renew and differentiate. Targeting of cancer stem cells is believed to be essential for curative therapy of cancers, but supporting evidence is limited. Few selective target genes in cancer stem cells have been identified. Here we identify the arachidonate 5-lipoxygenase (5-LO) gene (Alox5) as a critical regulator for leukemia stem cells (LSCs) in BCR-ABL–induced chronic myeloid leukemia (CML). In the absence of Alox5, BCR-ABL failed to induce CML in mice. This Alox5 deficiency caused impairment of the function of LSCs but not normal hematopoietic stem cells (HSCs) through affecting differentiation, cell division and survival of long-term LSCs (LT-LSCs), consequently causing a depletion of LSCs and a failure of CML development. Treatment of CML mice with a 5-LO inhibitor also impaired the function of LSCs similarly by affecting LT-LSCs, and prolonged survival. These results demonstrate that a specific target gene can be found in cancer stem cells and its inhibition can completely inhibit the function of these stem cells.