Functional Specificity of the Members of the Sos Family of Ras-GEF Activators: Novel Role of Sos2 in Control of Epidermal Stem Cell Homeostasis

SOS2 modulates the threshold of EGFR signaling to regulate osimertinib efficacy and resistance in lung adenocarcinoma

Son of sevenless 1 and 2 (SOS1 and SOS2) are RAS guanine nucleotide exchange factors (RasGEFs) that mediate physiologic and pathologic receptor tyrosine kinase (RTK)-dependent RAS activation. Here, we show that SOS2 modulates the threshold of epidermal growth factor receptor (EGFR) signaling to regulate the efficacy of and resistance to the EGFR tyrosine kinase inhibitor (EGFR-TKI) osimertinib in lung adenocarcinoma (LUAD). SOS2 deletion (SOS2KO ) sensitized EGFR-mutated cells to perturbations in EGFR signaling caused by reduced serum and/or osimertinib treatment to inhibit phosphatidylinositol 3-kinase (PI3K)/AKT pathway activation, oncogenic transformation, and survival. Bypassing RTK reactivation of PI3K/AKT signaling represents a common resistance mechanism to EGFR-TKIs; SOS2KO reduced PI3K/AKT reactivation to limit osimertinib resistance. In a forced HGF/MET-driven bypass model, SOS2KO inhibited hepatocyte growth factor (HGF)-stimulated PI3K signaling to block HGF-driven osimertinib resistance. Using a long-term in situ resistance assay, most osimertinib-resistant cultures exhibited a hybrid epithelial/mesenchymal phenotype associated with reactivated RTK/AKT signaling. In contrast, RTK/AKT-dependent osimertinib resistance was markedly reduced by SOS2 deletion; the few SOS2KO cultures that became osimertinib resistant primarily underwent non-RTK-dependent epithelial-mesenchymal transition (EMT). Since bypassing RTK reactivation and/or tertiary EGFR mutations represent most osimertinib-resistant cancers, these data suggest that targeting proximal RTK signaling, here exemplified by SOS2 deletion, has the potential to delay the development osimertinib resistance and enhance overall clinical responses for patients with EGFR-mutated LUAD.

Phosphoproteomics changes due to allograft-induced stress responses of Pinctada fucata martensii

Protein phosphorylation modifications are post-translational modifications (PTMs) that play important roles in signal transduction and immune regulation. Implanting a spherical nucleus into a recipient shellfish is critical in marine pearl aquaculture. Protein phosphorylation may be important in the immune responses of Pinctada fucata martensii after nucleus implantation, but their involvement in regulation remains unclear. Here, phosphoproteomics of P. f. martensii gill tissues was conducted 12 h after nuclear implantation using label-free data-independent acquisition (DIA) with LC-MS/MS. Among the 4024 phosphorylated peptides with quantitative information, 181 were up-regulated and 148 were down-regulated. Functional enrichment analysis of these differentially expressed phosphorylated proteins (DEPPs) revealed significant enrichment in functions related to membrane trafficking, exosomes, cytoskeleton, and signal transduction mechanisms. Further, 16 conserved motifs were identified among the DEPPs, including the RSphP, SphP, RSphA, RSphE, PTphP, and ATphP motifs that were significantly conserved, and which may be related to specific kinase recognition. Parallel response monitoring (PRM) analysis validated the abundances of 12 DEPPs from the proteomics, indicating that the phosphoproteomics analyses were robust. 12 DEPPs were selected from the proteomics results through Quantitative real-time PCR (qPCR) technology, and verification analysis was conducted at the gene level. The study suggests that kinases such as MAPKs, Akt, and CK2 may regulate the phosphorylation of related proteins following nuclear implantation. Furthermore, the important signaling pathways of Rap 1, IL-17A, and NF-κB, which are influenced by phosphorylated or dephosphorylated proteins, are found to be involved in this response. Overall, this study revealed the protein phosphorylation responses after nucleus implantation in P. f. martensii, helping to elucidate the characteristics and mechanisms of immune regulation responses in P. f. martensii, in addition to promoting a further understanding of protein phosphorylation modification functions in P. f. martensii.

SOS1 inhibition enhances the efficacy of and delays resistance to G12C inhibitors in lung adenocarcinoma

Clinical effectiveness of KRAS G12C inhibitors (G12Cis) is limited both by intrinsic and acquired resistance, necessitating the development of combination approaches. We found that targeting proximal receptor tyrosine kinase (RTK) signaling using the SOS1 inhibitor (SOS1i) BI-3406 both enhanced the potency of and delayed resistance to G12Ci treatment, but the extent of SOS1i effectiveness was modulated by both SOS2 expression and the specific mutational landscape. SOS1i enhanced the efficacy of G12Ci and limited rebound RTK/ERK signaling to overcome intrinsic/adaptive resistance, but this effect was modulated by SOS2 protein levels. Survival of drug-tolerant persister (DTP) cells within the heterogeneous tumor population and/or acquired mutations that reactivate RTK/RAS signaling can lead to outgrowth of tumor initiating cells (TICs) that drive therapeutic resistance. G12Ci drug tolerant persister cells showed a 2-3-fold enrichment of TICs, suggesting that these could be a sanctuary population of G12Ci resistant cells. SOS1i re-sensitized DTPs to G12Ci and inhibited G12C-induced TIC enrichment. Co-mutation of the tumor suppressor KEAP1 limits the clinical effectiveness of G12Cis, and KEAP1 and STK11 deletion increased TIC frequency and accelerated the development of acquired resistance to G12Ci in situ. SOS1i both delayed acquired G12Ci resistance and limited the total number of resistant colonies regardless of KEAP1 and STK11 mutational status. These data suggest that SOS1i could be an effective strategy to both enhance G12Ci efficacy and prevent G12Ci resistance regardless of co-mutations.

Critical requirement of SOS1 for tumor development and microenvironment modulation in KRASG12D-driven lung adenocarcinoma

The impact of genetic ablation of SOS1 or SOS2 is evaluated in a murine model of KRASG12D-driven lung adenocarcinoma (LUAD). SOS2 ablation shows some protection during early stages but only SOS1 ablation causes significant, specific long term increase of survival/lifespan of the KRASG12D mice associated to markedly reduced tumor burden and reduced populations of cancer-associated fibroblasts, macrophages and T-lymphocytes in the lung tumor microenvironment (TME). SOS1 ablation also causes specific shrinkage and regression of LUAD tumoral masses and components of the TME in pre-established KRASG12D LUAD tumors. The critical requirement of SOS1 for KRASG12D-driven LUAD is further confirmed by means of intravenous tail injection of KRASG12D tumor cells into SOS1KO/KRASWT mice, or of SOS1-less, KRASG12D tumor cells into wildtype mice. In silico analyses of human lung cancer databases support also the dominant role of SOS1 regarding tumor development and survival in LUAD patients. Our data indicate that SOS1 is critically required for development of KRASG12D-driven LUAD and confirm the validity of this RAS-GEF activator as an actionable therapeutic target in KRAS mutant LUAD.

Critical Requirement of SOS1 for Development of BCR/ABL-Driven Chronic Myelogenous Leukemia

Simple Summary

The p210^BCR/ABL oncoprotein is necessary and sufficient to trigger chronic myelogenous leukemia (CML) in mice. Our prior in vitro studies showing that the ABL-mediated phosphorylation of SOS1 promotes RAC activation and contributes to BCR-ABL leukemogenesis suggested the significant role of SOS1 in the development of CML. To provide direct in vivo experimental evidence of the specific contribution of SOS1 to the development of CML, here, we analyzed the effect of the direct genetic ablation of SOS1 or SOS2 on the genesis of p210^BCR/ABL -driven CML in mice. Our data showed that direct SOS1 genetic ablation causes the significant suppression of all the pathological hallmarks typical of CML, demonstrating that SOS1 deficiency is protective against CML development and identifying this cellular GEF as a relevant, novel therapeutic target for the clinical treatment of this hematological malignancy.

Abstract

We showed previously that the ABL-mediated phosphorylation of SOS1 promotes RAC activation and contributes to BCR-ABL leukemogenesis, suggesting the relevant role of SOS1 in the pathogenesis of CML. To try and obtain direct experimental evidence of the specific mechanistic implication of SOS1 in CML development, here, we combined a murine model of CML driven by a p210^BCR/ABL transgene with our tamoxifen-inducible SOS1/2-KO system in order to investigate the phenotypic impact of the direct genetic ablation of SOS1 or SOS2 on the pathogenesis of CML. Our observations showed that, in contrast to control animals expressing normal levels of SOS1 and SOS2 or to single SOS2-KO mice, p210^BCR/ABL transgenic mice devoid of SOS1 presented significantly extended survival curves and also displayed an almost complete disappearance of the typical hematological alterations and splenomegaly constituting the hallmarks of CML. SOS1 ablation also resulted in a specific reduction in the proliferation and the total number of colony-forming units arising from the population of bone marrow stem/progenitor cells from p210^BCR/ABL transgenic mice. The specific blockade of CML development caused by SOS1 ablation in p210^BCR/ABL mice indicates that SOS1 is critically required for CML pathogenesis and supports the consideration of this cellular GEF as a novel, alternative bona fide therapeutic target for CML treatment in the clinic.

SOS2 Comes to the Fore: Differential Functionalities in Physiology and Pathology

The SOS family of Ras-GEFs encompasses two highly homologous and widely expressed members, SOS1 and SOS2. Despite their similar structures and expression patterns, early studies of constitutive KO mice showing that SOS1-KO mutants were embryonic lethal while SOS2-KO mice were viable led to initially viewing SOS1 as the main Ras-GEF linking external stimuli to downstream RAS signaling, while obviating the functional significance of SOS2. Subsequently, different genetic and/or pharmacological ablation tools defined more precisely the functional specificity/redundancy of the SOS1/2 GEFs. Interestingly, the defective phenotypes observed in concomitantly ablated SOS1/2-DKO contexts are frequently much stronger than in single SOS1-KO scenarios and undetectable in single SOS2-KO cells, demonstrating functional redundancy between them and suggesting an ancillary role of SOS2 in the absence of SOS1. Preferential SOS1 role was also demonstrated in different RASopathies and tumors. Conversely, specific SOS2 functions, including a critical role in regulation of the RAS-PI3K/AKT signaling axis in keratinocytes and KRAS-driven tumor lines or in control of epidermal stem cell homeostasis, were also reported. Specific SOS2 mutations were also identified in some RASopathies and cancer forms. The relevance/specificity of the newly uncovered functional roles suggests that SOS2 should join SOS1 for consideration as a relevant biomarker/therapy target.