PHGDH and hepatocellular carcinoma: We aimed to determine whether extracellular Ser depletion affects Atf4-target gene expression in other cell types expressing Phgdh. To see if Ser removal from extracellular environment alters the Atf4-mediated transcription, we measured Atf4-target genes Chac1 mRNA by qRT-PCR in the mouse hepatoma cell line Hepa1-6 cells cultured under Ser-supplemented or -depleted conditions for 6 h.