The role of laminin-integrin signaling in triggering MB formation. An in vivo and in vitro study.

It is still unclear as to how hepatocytes perceive external factors and transduce the signals which initiate MB formation. To investigate this phenomenon, the model of MB formation in liver in vivo and in primary culture of hepatocytes derived from drug-primed mice was used. Control mice were fed the control diet (group 1). MBs were induced in the livers of mice fed diethyl-1, 4-dihydro-2, 4, 6-trimethyl-3, 5-pyridinedicarboxylate (DDC) for 10 weeks (group 2). The induced MBs completely disappeared after the withdrawal of DDC for 4 weeks (group 3). Newly formed MBs were numerous after DDC was refed for 1 week (group 4). Relative mRNA abundance was determined by quantitative real-time RT-PCR in the liver from the mice. The expression of integrin alpha(6) and beta(2) was significantly increased in the livers of DDC-treated (group 2) and drug refed mice (group 4), when compared with the livers from controls (group 1) and DDC-withdrawn (group 3) mice. The increased mRNA of these two integrin genes was associated with the increased expression of laminin (a ligand for integrin alpha(6)beta(1) and alpha(6)beta(4)), Icam1 (a ligand of alphaLbeta2), Src, MEKK1, and ERK1. Primary cultures of isolated DDC-primed hepatocytes (group 4 mice were withdrawn from DDC-CMZ for 4-6 weeks) produced significantly more MBs on laminin-coated coverslips compared with plastic uncoated, fibronectin-, collagen-, or fibrinogen-coated coverslips. U0126, an inhibitor of MEK1 protein, significantly reduced the phosphorylated forms of ERK1/2 and MB formation in vitro. In conclusion, the current study revealed an association between MB formation and integrin-mediated signaling in vivo. The data indicate that laminin-integrin signaling which activates ERK, triggered MB formation in vitro, and an inhibitor of the signaling cascade reduced MB formation.