At least RASA4 and RASAL1 are known to be regulated by Ca2+; intracellular mobilization of Ca2+ drives a rapid C2 domain-dependent translocation of these two proteins to the plasma membrane, increasing RasGAP activity [141,142]. function of Ras inhibitors. Among Ras inhibitors, the GTPase-Activating Proteins (RasGAPs) are major players, given their ability to modulate multiple cancer-related pathways. In fact, most RasGAPs also have a multi-domain structure that allows them to act as scaffold or adaptor proteins, affecting additional oncogenic cascades. In cancer cells, various mechanisms can cause the loss of function of Ras inhibitors; here, we review the available evidence of RasGAP inactivation in cancer, with a specific focus on the mechanisms. We also consider extracellular inputs that can affect RasGAP levels and functions, implicating that specific conditions in the tumor microenvironment can foster or counteract Ras signaling through negative or positive modulation of RasGAPs. A better understanding of these conditions might have relevant clinical repercussions, M?89 since treatments to restore or enhance the function of RasGAPs in cancer would help circumvent the intrinsic difficulty of directly targeting the Ras protein. infection triggers activation of the TLR4/MYD88/NF-B axis that induces expression of miR-21, which inhibits RASA1 protein synthesis, fostering Ras activation and cell growth and proliferation [136]. Other miRNAs targeting RasGAPs are under the control of inflammatory stimuli, suggesting an indirect way to promote Ras signaling in response to inflammation. For example, NF-B stimulates expression of miR-223, a RASA1-targeting miRNA [16], via M?89 binding its promoter [137]. Similarly, transcription of the miR-149 gene, encoding miRNAs targeting DAB2IP [86], can be stimulated or counteracted, respectively, by fibroblast growth factor 2 (FGF2) or tumor necrosis factor- alpha (TNF-) [138,139]. Chronic inflammation linked to cigarette smoke is a common risk factor for pulmonary disorders, including Chronic Obstructive Pulmonary Disease (COPD) and lung cancer. SCA27 Interestingly, cigarette smoke and consequent chronic inflammation of the airways were shown to induce epigenetic silencing of DAB2IP via EZH2. This phenomenon can favor uncontrolled epithelial cell proliferation, possibly prompting the progression of inflammatory diseases of the airways towards lung cancer [140]. Most GAPs have one or more C2 domains, structural modules that can bind calcium ions (Ca2+) and mediate interaction with phospholipids. Therefore, extracellular inputs that trigger dynamic changes in cytosolic Ca2+ concentration can potentially modulate RasGAP functions. At least RASA4 and RASAL1 are known to be regulated by Ca2+; intracellular mobilization of Ca2+ drives a rapid C2 domain-dependent translocation of these two proteins to the plasma membrane, increasing RasGAP activity [141,142]. Interestingly, RASA4 is also a GAP for Rap1, and changes specificity by forming monomers (functional as RasGAP) or homodimers (functional as Rap1 GAP) via a calcium-regulated process; consequently, M?89 Ca2+ levels can also coordinate the activation of Ras and Rap1 signaling pathways [143]. There is also evidence that environmental metabolites can regulate RasGAPs, with implications for cancer. For example, glucose shortage in the tumor niche M?89 is an unfavorable condition experienced by cancer and stromal cells, which leads them to reprogram their metabolism. Intriguingly, DAB2IP expression may be sensitive to extracellular glucose concentration: in endothelial cells grown in low glucose, mRNA and protein levels of DAB2IP are reduced if compared with high glucose, leading to HIF1- (hypoxia inducible factor-alpha) activation and induction of VEGF (vascular endothelial growth factor) pro-angiogenic factor. The mechanism involved in glucose-dependent regulation of DAB2IP remains unknown [144]. Another common condition observed in the core of solid tumors is hypoxia, and there is evidence that low oxygen concentration can stimulate Ras activity by interfering with GAPs. For instance, hypoxia-activated TGF-1 can induce hypermethylation of the RASAL1 promoter via upregulation of DNMT1 [63]. Furthermore, hypoxia stimulates production of miR-182, which is able to target both RASA1 and DAB2IP [77]. Notably, hypoxic stress reprograms the expression of multiple miRNAs via activation of HIF1-, and several additional RasGAP-targeting miRNAs, such as miR-107, miR-130, miR-145, and miR-335, are upregulated by hypoxic conditions, potentially favoring Ras activation and tumor progression [145]. Finally, interaction with the extracellular matrix (ECM) affects the cytoskeleton and activates mechanosensory pathways that regulate crucial cell behaviors such as proliferation, EMT, chemoresistance, and.