SMAD4 and lung cancer: SMAD4 protein levels were checked by treating A549 lung cancer cells with inhibitors of O-GlcNAc editing enzymes which caused thiamet-G, an OGA inhibitor, to increase SMAD4 levels (Fig. 1B, left panel) whereas SMAD4 levels decreased when treated with OSMI-1, an OGT inhibitor (Fig. 1B, middle panel).