BMP4 and hepatocellular carcinoma: Importantly, treatment of EECs with either BMP4 or BMP7 induced phosphorylation of SMAD5 (Figure 4C, left panel), one of the central receptor‐regulated R‐SMAD proteins that act as transcription factors for a broad variety of target genes upon BMP‐receptor interaction.18, 19, 25, 26 As a control, treatment of the hepatocellular carcinoma cell line HepG2 resulted in comparable BMP‐mediated SMAD5 phosphorylation, confirming the results obtained with the primary EEC cultures (Figure 4C, right panel).