Tachpyridine activates cell-cycle checkpoint kinases CHK1 and CHK2 and induces phosphorylation of p53 on serine-15. (A) HeLa cells were treated with 7.5 μM tachpyridine and analyzed by Western blotting using phosphospecific antibodies against activated CHK1 (S-345) and CHK2 (T-68). β-Actin was included as a loading control. (B) HeLa cells were treated with 10 μM tachpyridine and analyzed by Western blotting using a phosphospecific antibody against phosphorylated p53 (S-15). Equivalent loading of protein was confirmed by Ponceau S staining of the transfer membrane (not shown).