Gadd45 in the Liver: Signal Transduction and Transcriptional Mechanisms.

ABSTRACT

Injury and growth stimulation both remarkably increase the hepatic expression of Gadd45ß. This contrasts with expression in liver cancer, where promoter methylation frequently silences Gadd45ß, due to a suppressive function that is often proapoptotic. In normal hepatocytes, Gadd45ß facilitates cell survival, growth, and proliferation. Gadd45ß binds MKK7-downstream of TNFα and its receptors-to prevent this kinase from activating JNK2. Hence, the Gadd45ß-/- genotype increases cell injury and decreases cell proliferation during liver regeneration (compensatory growth and proliferation). Liver hyperplasia (de novo growth and proliferation) is an alternate form of growth, caused by drugs that activate the nuclear receptor, CAR. As in regeneration, the Gadd45ß-/- genotype considerably slows growth during hyperplasia. However, there is no injury and the slowing occurs because Gadd45ß normally binds to CAR and activates its transcriptional stimulation. Thus, Gadd45ß protects the liver through two entirely different processes Binding MKK7 to block damaging signal transduction, or binding CAR to coactivate anabolic transcription.