Objective To investigate the role and primary mechanism of a novel fusion gene RELCH-RET in driving the malignant transformation of normal human bronchial epithelial(HBE)cells.Methods Based on retrospective clinical data from 456 non-small cell lung cancer(NSCLC)patients admitted in the Second Affiliated Hospital of Army Medical University from January 2019 to June 2022,a fusion gene,RELCH-RET,was identified as a research target.Three cell models were established:negative control(HBE VC,transfected with empty lentiviral vector),RET control(HBE RET,transfected with lentiviral overexpression vector of Flag-RET),and experimental group(HBE RELCH-RET,transfected with lentiviral overexpression vector of Flag-RELCH-RET).MTS assay and Transwell assay were used to detect cell proliferation and migratory and invasive abilities.In vivo tumorigenicity of the 3 cell models was assessed in 15 female non-obese diabetic/severe combined immunodeficiency(NOD/SCID)mice(SPF grade,4 weeks old,weighing 15.1±0.4 g)via subcutaneous xenograft experiments,with 5 animals in each group.Western blotting was employed to detect the autophosphorylation of RET(Y905)and the phosphorylation of downstream signaling proteins ERK1/2,EGFR(Y845)and STAT3(Y705).Dimerization and multimerization status of RELCH-RET were analyzed by chemical cross-linking(DTME treatment)in combination with Western blotting,with the reversibility being confirmed through de-cross-linking experiments.Results There were 3 cases carrying RELCH-RET fusion gene screened out from the 469 NSCLC patients.Compared with the HBE VC and HBE RET groups,the HBE RELCH-RET group exhibited significantly enhanced cell proliferation(P<0.01),and acquired migratory and invasive abilities(P<0.01),while the control groups did not demonstrate the abilities.In the mouse xenograft tumor model,HBE cells stably expressing RELCH-RET developed significant tumor nodules(P<0.001),whereas the control groups(empty vector and wild-type RET)failed to exhibit detectable tumor growth.Western blotting revealed that RELCH-RET could induce the autophosphorylation of the RET tyrosine residue(Y905)and significantly up-regulate the phosphorylation levels of ERK1/2,EGFR(Y845),and STAT3(Y705)proteins.Chemical cross-linking combined with Western blot analysis demonstrated that RELCH-RET formed a dimer(～170 kDa)in HBE cells,which is reversibly dissociated into monomers upon decross-linking treatment.Conclusion The novel fusion gene RELCH-RET,promotes ligand-independent dimerization/oligomerization,thereby mediating RET autophosphorylation,subsequently activates the downstream typical RET signaling pathway and ultimately drives the malignant transformation of normal HBE cells.

Objective To investigate the effect of endothelial cells in the perivascular niche on the stem cell-like characteristics of osteosarcoma cells and primarily explore the possible molecular mechanism.Methods A co-culture model was established in vitro using SV40T-human umbilical vein endothelial cells(HUVEC-T1)and osteosarcoma stem cells(OSCs)derived from the human osteosarcoma cell line 143B.Thus,there were 2 groups of cells,OSCs and co-cultured OSCs.The self-renewal capacity of OSCs between the 2 groups was assessed using a limiting dilution forming sphere assay.Flow cytometry and Western blotting were used to detect the expression of stem cell marker CD133 and stemness transcription factors SOX2 and NANOG.Fourteen female nude mice(4～6 weeks old,weighing 18～20 g)were randomly and equally divided into 2 groups of subcutaneous xenograft models:the control group(OSCs suspension)and the experimental group(OSCs+HUVEC-T1 mixed suspension).The tumor volume and mass were compared between the 2 xenograft groups.Immunofluorescence(IF)staining was used to verify the spatial proximity between endothelial cells and OSCs in vivo,while immunohistochemistry was employed to compare microvessel density(MVD)and CD133 expression level between the 2 groups.RNA-seq was performed to identify potential signaling pathways of endothelial cells affecting the stemness of OSCs.PCR and Western blotting were applied to confirm the RNA-seq findings.Exogenous protein treatment,IF staining,Western blotting and sphere formation assay were utilized to preliminarily validate the role of the identified pathway in regulating the stemness phenotype of OSCs.Results The in vitro co-culture model of HUVEC-T1 and OSCs was successfully established.Compared with the control group,the co-culture group exhibited significantly enhanced self-renewal ability of OSCs,laeger proportion of the stemness marker CD133+[(8.20±1.64)％vs(4.32±1.34)％,P<0.05],enhanced expression of CD117,SOX2 and NANOG(P<0.05),along with more sphere formation(P<0.05)and elevated SOX2/NANOG protein levels.The xenograft mice from the experimental group showed larger tumor volume(643.10±413.50 vs 247.90±93.66 mm3,P<0.05)and heavier tumor weight(0.52±0.27 vs 0.24±0.10 g,P<0.05)when compared the control group,correspondingly showing increased MVD(22.57±11.84 vs 11.43±5.38,P<0.05)and elevated CD133 expression(P<0.05).IF staining confirmed the adjacency of CD31-labeled endothelial cells and CD133-labeled OSCs in vivo.RNA-seq and functional experiments demonstrated that CXCR4 was highly expressed in co-cultured OSCs,CXCL12 was highly expressed in co-cultured endothelial cells,and exogenous CXCL12 promoted the sphere formation and expression levels of SOX2 and NANOG of OSCs(P<0.05).Conclusion Endothelial cells within the perivascular niche may promote the stemness phenotype of osteosarcoma cells via the CXCL12/CXCR4 pathway.