Induction of fibrinogen biosynthesis and secretion from cultured pulmonary epithelial cells

Abstract

Although the liver is the primary site of fibrinogen (FBG) synthesis, epithelial cells from diverse tissues have been shown to express one or more of the FBG Aα, Bβ, and γ chain genes. In contrast, marrow megakaryocytes, which store FBG in the α-granules, are thought not to express the FBG genes. Our earlier studies have shown that epithelial cells in a variety of extrahepatic tissues express the γ chain gene ubiquitously, while the mRNAs for the Aα and Bβ chain genes are essentially undetectable. During systemic inflammation, the liver secretes increased levels of FBG into the circulation, end lung epithelium responds to local inflammation during pulmonary infection by increased transcription of the γ-FBG gene. Therefore, to determine whether extrahepatic epithelial cells express the Aα, Bβ, and γ chain genes in response to proinflammatory mediators, cultured lung epithelial cells were treated with interleukin-6 (IL-6) and dexamethasone (DEX). Northern blot analysis demonstrated that the levels of γ-FBG mRNA in cultured lung (A549) and liver (HepG2) epithelial cells increased 2-to 10- fold in response to treatment. Reverse-transcriptase-polymerase chain reaction amplification demonstrated increased accumulation of steady state levels of FBG Aα, Bβ, and γ chain mRNAs in lung epithelial cells after treatment. The basal level of lung cell γ-FBG gene transcription was not accompanied by appreciable levels of Aα and Bβ chain gene transcription; however, nuclear run-on analysis suggested that the increase in lung cell FBG mRNAs in response to DEX ± IL-6 was due to new transcription. Furthermore, stimulation of lung epithelial cells with IL-6 + DEX resulted in maximal secretion of intact FBG (340 kD) composed of the characteristic Aα, Bβ, and γ chain polypeptides. The data suggest that basal expression of the γ-FBG gene in extrahepatic tissue occurs ubiquitously in the absence of detectable levels of Aα- and Bβ-FBG gene expression, which are then upregulated on induction of an inflammatory response. This would result in local synthesis and secretion of FBG in the injured tissue, supporting the hypothesis that production of FBG by extrahepatic epithelial cells in response to inflammation plays a role in wound repair.