Characterization of bone morphogenetic protein-9 expression, binding, and biological activity in the adult rat liver

Originally characterized by their ability to induce the growth of ectopic bone, bone morphogenetic proteins (BMPs) have emerged as critical cell-to-cell communication factors. As members of the Transforming growth factor-beta (TGF-beta) superfamily, these multipotent cytokines generally evoke growth and differentiation responses in both skeletal and soft tissues and have also been shown to regulate more specific cellular functions including, gene expression and chemotaxis. BMP-9 was initially cloned from a fetal mouse liver cDNA library. It was shown to have receptor binding and growth promoting activity in human hepatocarcinoma cells and primary rat hepatocytes. Here, BMP-9 expression, receptor biding and biological activity have been characterized in the cellular context of the adult rat liver. Among the major organs, expression of BMP-9 mRNA occurs most abundantly in the liver. Specifically, it is the hepatic non-parenchymal cells, the Kupffer cells (KC), liver endothelial cells (LEC) and hepatic stellate cells (HSC) that are primarily responsible for BMP-9 synthesis. Cellular receptors, highly specific for BMP-9 binding and having a sub-nanomolar binding constant have also been characterized in KC and LEC. This receptor was further characterized by affinity-label crosslinking and found to be of similar molecular mass to other known BMP type I receptors. [125I]BMP-9 binding was not displaced by excess unlabeled BMP-2, -4, -6, -7, and -12 or TGF-beta isotypes 1 or 3. Interestingly, neither BMP-9 binding nor crosslinking could be detected in rat hepatocytes. BMP-9 receptor binding is followed by receptor internalization and signal transduction. Signaling activity downstream of this receptor regulates the activity of the mitogen activated protein kinase (MAP kinase) pathway enzymes termed extracellular regulated kinase-1 and -2 (ERK-1, and -2). These cytosolic serine/threonine kinases are activated when KCs, the resident macrophages of the liver, are treated with the inflammation-promoting lipid platelet activating factor (PAF). However, when KC are pre-treated with BMP-9, the level of PAF regulated ERK activity is strongly attenuated, suggesting that BMP-9 may be a locally available immunomodulating cytokine in the liver.