Bile acid-induced EGFR activation in quiescent hepatic stellate cells can trigger both, proliferation and apoptosis

Quiescent HSC are fairly resistant to CD95-mediated apoptosis and CD95L [1] as well as hydrophobic bile acids [2] have been shown to activate EGFR in those cells. In addition, qHSC express CD133 and might represent a hepatic stem/progenitor cell compartment [3].

Rat HSC were cultured for 1–2 days. Activation of EGFR, Src and MAP kinases was measured by W.blot. CD95 was immunoprecipitated and detected for EGFR-association, Tyr-phosphorylation and DISC-formation. ROS-generation was measured by DCFDA-fluorescence. Proliferation and apoptosis were analyzed by BrdU-incorporation and TUNEL-staining, respectively.

In qHSC, hydrophobic bile acids (TLCS, TCDC, GCDC) induced an EGFR-phosphorylation on Y⁸⁴⁵ and Y¹¹⁷³, while TC and TUDC were ineffective. P-Y⁸⁴⁵ and P-Y¹¹⁷³ were sensitive to inhibition of Src kinases while only P-Y¹¹⁷³ was sensitive to inhibition of EGFR-Tyr-kinase-activity, suggestive for a Src kinase-mediated EGFR-activation. Hydrophobic bile acids activated Src family kinases Yes and Src, while no Fyn-activation occurred, and Yes but not Src was shown to associate with EGFR by IP-studies. TLCS induced a rapid ROS-formation which was sensitive to inhibition of ASM, atypical PKCs or NADPH oxidase. TLCS induced EGFR- and Erk-dependent proliferation, while no JNK-activation and apopotic cell death became detectable. If cyclohe ximide (CHX) and TLCS were coadministered, a JNK-activation occurred which led to EGFR/CD95-association, EGFR-mediated CD95-Tyr-P and subsequent apoptosis.

In qHSC, hydrophobic bile acids are a mitogen through a NADPH oxidase-dependent Yes-activation and subsequent EGFR-activation. If a 2^nd stimulus is coadministered which leads to JNK-activation, then EGFR/CD95-association, CD95-Tyr-P and subsequent apoptosis occurs. Thus depending on the respective signaling context, bile acid-induced EGFR-activation in qHSC can trigger both, proliferation and apoptosis which might contribute to their recently described stem/progenitor cell properties.

Literatur: [1] Reinehr et al. "CD95 ligand is a proliferative and antiapoptotic signal in quiescent hepatic stellate cells" Gastroenterology 2008 [2] Svegliati–Baroni et al. "Bile acids induce hepatic stellate cell proliferation via activation of the epidermal growth factor receptor" Gastroenterology 2005 [3] Kordes et al. "CD133+ hepatic stellate cells are progenitor cells" BBRC 2007