Transferrin receptor gene expression during rat liver regeneration. Evidence for post-transcriptional regulation by iron regulatory factorB, a second iron-responsive element-binding protein.

Transferrin receptor (TfR) expression is regulated by iron at the level of mRNA stability through a factor (IRF/ IRE-BP) which binds to specific iron-responsive elements (IRE). On the other hand, growth-dependent regulation of TfR expression is generally believed to be transcriptionally controlled. We analyzed the molecular mechanisms that control TfR gene expression at the onset of cell proliferation in vivo during liver regeneration after partial hepatectomy. The amount of TfR mRNA increased considerably after partial hepatectomy while run-on assays did not show significant changes in TfR gene transcription. RNA band-shift assays documented a significant activation of IRF/IRE-BP specific for the faster migrating IRE-protein complex (IRF(B)). These changes occurred in the absence of modifications of total liver iron concentration but together with a significant decrease of ferritin content. Moreover, when extreme variations of liver iron content were achieved by either chronic iron overload or severe iron deficiency, liver regeneration was unable to influence IRE-binding activity. We conclude that IRF/IRE-BP-mediated post-transcriptional control can fully account for TfR mRNA induction during liver cell proliferation in vivo. IRF/ IRE-RE activation in the absence of changes in total tissue iron content might depend either on a drop of iron levels into the regulatory pool or on a relatively iron-independent mechanism specific for the faster migrating complex.