The injury response is a complex set of events, which represents the reaction of a biological system to a perceived change in its environment in an attempt to maintain system integrity. Isolation of individual events or components of this response cannot describe the overall process, but may reflect general mechanisms that have evolved over time to solve the complex requirements of the injury response. The process, generally termed the acute phase response, is a series of organ-specific responses that begin shortly after a systemic injury. In the liver, this response involves both dramatic inductions and reductions in specific sets of genes, and an overall widespread global change in proteins produced. This can be thought of as a phenotypic change or ‘reprogramming’ of the liver. These changes in protein production are modulated and regulated at the level of transcription and involve significant manipulations of transcriptional regulatory mechanisms. Hepatocyte nuclear factor 4 (HNF-4) is a liver enriched transcription factor that regulates a large number of liver-specific genes, which play important roles in the critical pathways modulated by the response to injury. HNF-4 also performs an essential role in overall development and is critical for the normal expression of multiple genes in the developed liver, as well as being upstream of HNF-1 in a transcriptional hierarchy that drives hepatocyte differentiation. The role of HNF-4 in regulating liver-specific transcriptional changes directed by injury remains to be defined. In our cell-culture and whole-animal models, we demonstrate that the binding activity of HNF-4 decreases quickly after injury due to post-translational modification by phosphorylation. The mechanisms by which HNF-4 is modified after injury involve the activation of Janus kinase 2 (JAK2) signal transduction pathways, but the direct or indirect interaction of JAK2 with HNF-4 remains to be defined.
- acute phase
- electrophoretic mobility-shift assay (EMSA)
- tyrosine kinase
Abbreviations used: APR, acute phase response; ATF-2, activating transcription factor-2; CAT, chloramphenicol acetyltransferase; C/EBP, CAAAT/enhancer binding protein; CIP, calf intestinal alkaline phosphatase; CRE, cAMP responsive element; CREB, CRE binding protein; DTT, dithiothreitol; EMSA, electrophoretic mobility-shift assays; GH, growth hormone; HNF-4, hepatocyte nuclear factor 4; IL-1, interleukin-1; JAK2, janus kinase 2; L-PK, L-pyruvate kinase; MAP kinase, mitogen-activated protein kinase; MEM, minimal essential medium; NE, nuclear extract(s); NP40, Nonidet P40; PEPCK, phosphoenolpyruvate carboxykinase; PKA, protein kinase A; PKC, protein kinase C; TK, thymidine kinase; WT, wild-type.
- The Biochemical Society, London ©2002