Extracellular signals often result in simultaneous activation of both the Raf-MEK-ERK and PI3K-Akt pathways (where ERK is extracellular-regulated kinase, MEK is mitogen-activated protein kinase or ERK kinase, and PI3K is phosphatidylinositol 3-kinase). However, these two signaling pathways were shown to exert opposing effects on muscle cell hypertrophy. Furthermore, the PI3K-Akt pathway was shown to inhibit the Raf-MEK-ERK pathway; this cross-regulation depended on the differentiation state of the cell: Akt activation inhibited the Raf-MEK-ERK pathway in differentiated myotubes, but not in their myoblast precursors. The stage-specific inhibitory action of Akt correlated with its stage-specific ability to form a complex with Raf, suggesting the existence of differentially expressed mediators of an inhibitory Akt-Raf complex.

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SUMMARY The ability for DNA mismatch repair, after oxidative stress-induced DNA damage, is critical for the persistence of Porphyromonas gingivalis in the inflammatory environment of the periodontal pocket. Our previous report demonstrated that, in contrast to other organisms, the repair of oxidative stress-induced DNA damage involving 8-oxo-7,8-dihydroguanine (8-oxoG) may occur by a yet-to-be described mechanism in P. gingivalis. 8-oxoG does not block DNA replication; rather, it mispairs with adenine, which can be repaired by the MutY glycosylase. To determine the function of the P. gingivalis MutY homologue in DNA repair, it was insertionally inactivated using the ermF-ermAM antibiotic cassette and used to create a mutY-deficient mutant (FLL147) by allelic exchange mutagenesis. FLL147 had an increased rate of spontaneous mutation and was more sensitive to hydrogen peroxide compared with the wild-type W83 strain. DNA oligomers containing a site-specific 8-oxoG:A mispair was repaired similarly in both the P. gingivalis mutY-defective mutant and wild-type strains. The P. gingivalis mutY homologue was shown to complement the mutY mutation in Escherichia coli. In a gel mobility shift assay, the purified recombinant MutY is able to bind an oligo containing an 8-oxoG:A mispair. Taken together, MutY may play the expected role in oxidative stress resistance in P. gingivalis. However, there may exist other redundant mechanism(s) for the removal of 8-oxoG:A mismatch in this organism.