Induction of Egr-1 by oncostatin M precedes up-regulation of low density lipoprotein receptors in HepG2 cells.

Oncostatin M (OM), a 28 kilodalton glycoprotein cytokine, is structurally and functionally related to interleukin 6 and leukemia-inhibitory factor. We reported previously that OM strongly up-regulated low density lipoprotein (LDL) receptors in human liver cells by a tyrosine kinase-mediated mechanism. Now, we demonstrate that the transcription factor Egr-1 is induced by OM. The induction of Egr-1 was time and concentration dependent; maximal inductions of 10-fold occurred by 30 min at concentrations of 10-25 ng/ml and higher. This concentration dependency was identical to those for OM-mediated tyrosine phosphorylation and LDL receptor up-regulation. The Egr-1, tyrosine kinase, and LDL receptor responses were inhibited at similar concentrations of genistein, suggesting that induction of Egr-1 and up-regulation of LDL receptors depended on activation of tyrosine kinase by OM. In contrast, depletion of protein kinase C by preincubation with 4 beta-phorbol 12-myristate 13 alpha-acetate did not affect OM-mediated induction of Egr-1 or up-regulation of LDL receptors, indicating that protein kinase C is not required for the OM action. Other similar cytokines were investigated, and, of these, only interleukin 1 could increase both Egr-1 and LDL receptor activity. The correlation among tyrosine kinase phosphorylation, Egr-1 induction, and LDL receptor regulation suggests that Egr-1 may be a nuclear signal transducer utilized by OM to induce transcription of the LDL receptor gene. In support of this possibility is the discovery of an Egr-1 consensus sequence (GAGGGGGCG) at approximately 330 base pairs upstream from the transcription initiation site of the LDL receptor promoter region.