ATF4 and hepatocellular carcinoma: However, it remains unclear whether reduced availability of extracellular l-serine elicits such responses in other cell types expressing Phgdh. Here, we show in the mouse hepatoma cell line Hepa1-6 that extracellular l-serine depletion transiently induced transcriptional activation of Atf4-target genes, including cation transport regulator-like protein 1 (Chac1).