Abstract 5545: Molecular Dissection of Human Oncostatin M-Mediated Signal Transductions Through Site-Directed Mutagenesis
The binding of human oncostatin M (OM) to type I and type II receptor complexes elicits various biological responses by activating ERK and STAT signaling pathways. Some OM effects are clinically desirable such as reducing hyperlipidemia through the activation of hepatic LDL receptor (LDLR) transcription, a downstream event of ERK activation. The OM undesirable pro-inflammatory responses via induction of acute phase protein gene expression have been associated with STAT activation. We conducted a site-directed mutagenetic study to identify OM residues that are differentially involved in the activation of ERK or STAT signaling pathway. We used HepG2 cells as the assay system that naturally express both type I and type II receptor complexes. The activation of transcription of LDLR by OM was the readout for the regulation of lipid metabolism through ERK pathway and the induction of fibrinogen-beta expression was used as an indicator of inflammatory response. Now we have defined 4 OM residues that are differentially involved in the activation of ERK or STAT signaling pathway in HepG2 cells. We show that mutation of Lys163 to alanine totally abolished OM-mediated signaling. Gly120 mutation equally impaired activations of ERK and STAT signaling pathways by OM. In contrast, mutations of Gln20 and Asn123 differentially affected OM signaling through the ERK and the STAT pathways. Q20A and N123A retained strong activity in inducing ERK phosphorylation but they showed diminished abilities in activating STAT1 and STAT3. Consequently, mutations at Gln20 and Asn123 reduced OM induction of inflammatory gene fibrinogen-beta to a greater extent than that of LDL receptor. Molecular modeling has been used to compare the interactions between wild type or mutant OM with type I and type II receptors. Our calculations suggest that the binding affinity of Q20A and N123A to type II receptor is similar to that of wild type OM. However, they exhibit reduced binding affinity to type I receptor. This may account for the compromised signaling through the STAT pathway. These results together provide a structural basis for a better understanding of OM-mediated signaling and suggest a potential to improve OM therapeutic properties in treating hyperlipidemia via structural modification.