RecQ helicases maintain genome stability and suppress tumors in higher eukaryotes through roles in replication and DNA repair. The yeast RecQ homolog Sgs1 interacts with Top3 topoisomerase and Rmi1. In vitro, Sgs1 binds to and branch migrates Holliday junctions (HJs) and the human RecQ homolog BLM, with Top3␣, resolves synthetic double HJs in a noncrossover sense. Sgs1 suppresses crossovers during the homologous recombination (HR) repair of DNA double-strand breaks (DSBs). Crossovers are associated with long gene conversion tracts, suggesting a model in which Sgs1 helicase catalyzes reverse branch migration and convergence of double HJs for noncrossover resolution by Top3. Consistent with this model, we show that allelic crossovers and gene conversion tract lengths are increased in sgs1⌬. However, crossover and tract length suppression was independent of Sgs1 helicase activity, which argues against helicase-dependent HJ convergence. HJs may converge passively by a "random walk," and Sgs1 may play a structural role in stimulating Top3-dependent resolution. In addition to the new helicase-independent functions for Sgs1 in crossover and tract length control, we define three new helicase-dependent functions, including the suppression of chromosome loss, chromosome missegregation, and synthetic lethality in srs2⌬. We propose that Sgs1 has helicasedependent functions in replication and helicase-independent functions in DSB repair by HR.The repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) is critical for maintaining genome stability and cancer suppression. DSBs are produced by ionizing radiation, genotoxic chemicals, and nucleases and when replication forks encounter DNA damage. Broken ends are converted to Rad51 nucleoprotein filaments that search for and invade homologous duplex DNA, producing a Holliday junction (HJ) intermediate. Branch migration of HJs extends or eliminates heteroduplex DNA (hDNA), and mismatches in hDNA are repaired, resulting in a region of localized loss of heterozygosity termed a gene conversion tract.Crossovers accompany some gene conversions, posing risks of deletions, inversions, translocations, and large-scale loss of heterozygosity (31,45,54). The mechanisms that suppress tract lengths and crossovers are important to elucidate because they determine the extent and frequency of the loss of heterozygosity during DSB repair by HR and thereby regulate genome stability.In the yeast Saccharomyces cerevisiae, mitotic and meiotic crossovers are suppressed by Sgs1 (26, 55), a member of the RecQ helicase family that includes five human proteins, three of which (BLM, WRN, and RECQ4) suppress tumors (25). RecQ helicases have conserved structures and interactions with type I topoisomerases (e.g., yeast Top3). Yeast Rmi1 is in complex with Sgs1-Top3 and may promote binding to branched DNAs or Top3 strand passage (10, 43). Yeast sgs1, top3, and rmi1 mutants show DNA damage hypersensitivity, genome instability, slow growth, poor sporulation, and hyperrecombination (10, 43). Sgs1...

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In the field of osteoporosis, there has been growing interest in anabolic agents that enhance bone formation. Here, we examined the effects of betulinic acid on cell proliferation, differentiation, and mineralization of MC3T3-E1 osteoblasts. Then, the impact of betulinic acid on signaling pathways known to be implicated in osteoblastogenesis was explored. Betulinic acid (1-20 microM) markedly increased alkaline phosphatase (ALP) activity and calcium nodule formation, although without a notable effect on cell proliferation. Stimulation with betulinic acid not only increased the osteopontin level and osteocalcin mRNA expression but also upregulated the osteoprotegerin (OPG)/RANKL ratio. Noggin, but not ICI 182780, significantly repressed betulinic acid-induced ALP activity, suggesting a possible action of betulinic acid through the bone morphogenetic protein (BMP) pathway. This was strengthened by the induction of BMP-2 expression, increases in Smad1/5/8 phosphorylation, and Runx2 expression. Furthermore, betulinic acid increased the nuclear level of the active form beta-catenin. These results suggested that betulinic acid has the potential to enhance osteoblastogenesis probably through the activation of BMP/Smad/Runx2 and beta-catenin signal pathways. Furthermore, upregulation of the OPG/RANKL ratio to repress bone catabolism may also indirectly contribute to the bone anabolic effect of betulinic acid.

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