Siripan Limsirichaikul

OBJECTIVES: SMARCA4, a core component of the SWI/SNF chromatin remodeling complex, is frequently mutated in non-small cell lung cancer (NSCLC). SMARCA4-deficient cancer cells are associated with increased replication stress, one of the major causes of genomic instability, which may lead to cancer. SMARCAD1, a chromatin remodeler, is known as replication fork progressor, and SMARCAD1 dysregulation is also closely related to cancer development. This study aimed to investigate the role of the SMARCA4-SMARCAD1 axis in the toleration of replication stress in NSCLC, focusing on the regulatory relationship between SMARCA4 and SMARCAD1 during replication stress conditions. METHODS: Human NSCLC cell lines (Calu-6, NCI-H1975, Calu-1, and NCI-H460) were used for experiments. SMARCA4 and SMARCAD1 expression levels were analyzed by quantitative RT-PCR and immunoblotting. Transcriptional regulation of SMARCAD1 was analyzed by chromatin immunoprecipitation assay. Immunofluorescent analysis was performed to assess SMARCAD1 accumulation at stalled replication forks. Clonogenic assays were conducted to evaluate the roles of SMARCA4 and SMARCAD1 in cell survival. RESULTS: SMARCAD1 was highly expressed in SMARCA4-depleted cells under replication stress. Immunofluorescent analysis revealed significant accumulation of SMARCAD1 at stalled replication forks in SMARCA4-depleted cells. Chromatin immunoprecipitation assays demonstrated that SMARCA4 bound to the transcriptional regulatory region of SMARCAD1, and that this efficacy was decreased under replication stress, suggesting that SMARCA4 is a transcriptional suppressor of SMARCAD1. In a clonogenic analysis either SMARCA4 or SMARCAD1 is required for cell survival. CONCLUSIONS: The SMARCA4-SMARCAD1 axis is a novel mechanism that provides tolerance for replication stress.

CERS6 is associated with metastasis and poor prognosis in non-small cell lung cancer (NSCLC) patients through d18:1/C16:0 ceramide (C16 ceramide)-mediated cell migration, though the detailed mechanism has not been elucidated. In the present study, examinations including co-immunoprecipitation, liquid chromatography, and tandem mass spectrometry analysis were performed to identify a novel binding partner of CERS6. Among the examined candidates, LASP1 was a top-ranked binding partner, with the LIM domain possibly required for direct interaction. In accord with those findings, CERS6 and LASP1 were found to co-localize on lamellipodia in several lung cancer cell lines. Furthermore, silencing of CERS6 and/or LASP1 significantly suppressed cell migration and lamellipodia formation, whereas ectopic addition of C16 ceramide partially rescued those phenotypes. Both LASP1 and CERS6 showed co-immunoprecipitation with actin, with those interactions markedly reduced when the LASP1–CERS6 complex was abolished. Based on these findings, it is proposed that LASP1–CERS6 interaction promotes cancer cell migration.

ABSTRACT We isolated active mutants in Saccharomyces cerevisiae DNA polymerase α that were associated with a defect in error discrimination. Among them, L868F DNA polymerase α has a spontaneous error frequency of 3 in 100 nucleotides and 570-fold lower replication fidelity than wild-type (WT) polymerase α. In vivo, mutant DNA polymerases confer a mutator phenotype and are synergistic with msh2 or msh6 , suggesting that DNA polymerase α-dependent replication errors are recognized and repaired by mismatch repair. In vitro, L868F DNA polymerase α catalyzes efficient bypass of a cis-syn cyclobutane pyrimidine dimer, extending the 3′ T 26,000-fold more efficiently than the WT. Phe34 is equivalent to residue Leu868 in translesion DNA polymerase η, and the F34L mutant of S. cerevisiae DNA polymerase η has reduced translesion DNA synthesis activity in vitro. These data suggest that high-fidelity DNA synthesis by DNA polymerase α is required for genomic stability in yeast. The data also suggest that the phenylalanine and leucine residues in translesion and replicative DNA polymerases, respectively, might have played a role in the functional evolution of these enzyme classes.