Extracellular matrix modulates sensitivity of hepatocytes to fibroblastoid de-differentiation and TGF-β induced apoptosis

Hepatocytes in culture are a valuable tool to investigate mechanisms involved in the response of the liver to cytokines. However, it is well established that hepatocytes cultured on monolayers of monomeric collagen dedifferentiate, loosing specialized liver functions. Here we report that hepatocyte dedifferentiation is a reversible consequence of a specific signaling network constellation triggered by the extracellular matrix. Monomeric collagen activates focal adhesion kinase (FAK) via Src leading to activation of the Akt and ERK1/2 pathways. Akt causes resistance to TGF-β-induced apoptosis by antagonizing p38, whereas ERK1/2 signaling induces epithelial-mesenchymal transition (EMT). Apoptosis resistance is reversible by inhibiting Akt or Src and EMT can be abrogated by blocking the ERK1/2 pathway. In contrast, a collagen gel matrix does not activate FAK keeping hepatocytes sensitive to TGF-β-induced apoptosis and prevents EMT. In this culture system, inhibition of p38 as well as over-expression of constitutively active Akt causes apoptosis resistance, whereas constitutively active Ras induces EMT. Finally, we show that matrix-induced EMT is reversible by re-plating cells from monomeric to polymerized gel collagen. Our results demonstrate that hepatocyte dedifferentiation in vitro is an active process driven by FAK mediated Akt and ERK1/2 signaling. This leads to similar functional and morphological alterations as observed for regenerating hepatocytes in vivo and is reversible when Akt and/or ERK1/2 signaling pathways are antagonized. In conclusion, we have shown that hepatocytes can exist in a differentiated and a dedifferentiated state that are reversible and can be switched by the responsible key factors of the signaling network.

Extracellular matrix - TGFbeta - apoptosis - epithelial-mesenchymal transition - primary hepatocytes - signal transduction