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Microtubule depolymerization inhibits the regulation of alpha 1-acid glycoprotein mRNA by hepatocyte stimulating
K C Carter1, R Cooper, J Papaconstantinou
1Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch, Galveston 77550.
Abstract:
Hepatocyte stimulating factor (HSF, a polypeptide cytokine) is a major regulatory hormone responsible for hepatic acute-phase reactant (APR) induction following acute systemic injury. The mechanisms by which HSF regulates APR synthesis in the liver are unknown. Microtubules are involved in a number of polypeptide hormone-mediated events which can be modified, either positively or negatively, by microtubule depolymerizing agents. In this study we have used colchicine (a microtubule depolymerizing drug) to assess whether or not HSF-mediated changes in rat hepatic alpha 1-acid glycoprotein (AGP) or albumin mRNA levels require an intact microtubule cytoskeletal system. Cultured rat hepatocytes were pretreated for 30 min with either colchicine (10(-6) M), or the inactive isomer lumicolchicine (10(-6) M), or fresh medium. Following pretreatment, purified murine macrophage HSF (10 units/ml) was added and the cells were incubated for an additional 12 h. Colchicine, but not lumicolchicine, significantly inhibited the HSF-dependent regulation of mRNA for the positive APR, AGP, but had no effect on the mRNA levels of albumin, a negative APR. Furthermore, removal of colchicine from previously inhibited cultures allowed HSF to restimulate AGP mRNA expression. These data suggest that microtubules may play a regulatory role in controlling the expression of the genes for positive acute-phase proteins and may explain the temporal differences found in vivo between positive and negative APR expression.
Insights
Hepatocyte stimulating factor (HSF) regulates acute-phase protein synthesis. Microtubules are essential for HSF
Area of Science:
- Cell Biology
- Molecular Biology
- Hepatology
Background:
- Hepatocyte stimulating factor (HSF) is a key cytokine for hepatic acute-phase reactant (APR) induction after injury.
- The molecular mechanisms underlying HSF's regulation of APR synthesis remain unclear.
- Microtubules are implicated in polypeptide hormone signaling pathways.
Purpose of the Study:
- To investigate the role of the microtubule cytoskeleton in HSF-mediated regulation of alpha 1-acid glycoprotein (AGP) and albumin mRNA levels in rat hepatocytes.
Main Methods:
- Primary rat hepatocytes were cultured and pretreated with colchicine (a microtubule depolymerizing agent) or lumicolchicine.
- Cells were subsequently treated with HSF, and mRNA levels for AGP and albumin were quantified.
- The effect of colchicine removal on HSF-induced AGP mRNA expression was also assessed.
Main Results:
- Colchicine significantly inhibited HSF-induced expression of AGP mRNA (a positive APR) but did not affect albumin mRNA (a negative APR).
- Lumicolchicine, the inactive isomer, had no effect on HSF-mediated mRNA regulation.
- Removal of colchicine restored the ability of HSF to induce AGP mRNA expression.
Conclusions:
- Microtubules play a regulatory role in controlling the expression of genes for positive acute-phase proteins.
- These findings may explain the differential timing of positive and negative APR expression observed in vivo.
- The microtubule cytoskeleton is a critical component of HSF signaling pathways in hepatocytes.