Hepatocytes Contribute to Soluble CD14 Production, and CD14 Expression Is Differentially Regulated in Hepatocytes and Monocytes

Abstract

CD14 presents as a glycosylphosphatidylinositollinked membrane protein on the surface of monocytes/ macrophages and as a soluble protein in the serum. Our previous studies have shown that an 80-kilobase pair (kb) genomic DNA fragment containing the human CD14 gene is sufficient to direct CD14 expression in a monocyte-specific manner in transgenic mice. In addition, we discovered that human CD14 is highly expressed in hepatocytes. Here, we report the generation of transgenic mice with either a 24-or 33-kb human CD14 genomic DNA fragment. Data from multiple transgenic lines show that neither the 24-nor the 33-kb transgenic mice express human CD14 in monocytes/macrophages. However, human CD14 is highly expressed in the liver of the 33-kb transgenic mice. These results demonstrate that human CD14 expression is regulated differently in monocytes and hepatocytes. Furthermore, we identified an upstream regulatory element beyond the 24-kb region, but within the 33-kb region of the human CD14 gene, which is critical for CD14 expression in hepatocytes, but not in monocytes/macrophages. Most importantly, the data demonstrate that the liver is one of the major organs for the production of soluble CD14. These transgenic mice provide an excellent system to further explore the functions of soluble CD14.Every year septic shock promoted by Gram-negative bacteria causes over 100,000 deaths in the United States (1, 2). Lipopolysaccharide (LPS), 1 an endotoxin of Gram-negative bacteria, is known to be responsible for initiating host responses leading to septic shock (3). LPS stimulates its response by inducing the host cells to produce and release endogenous mediators including the proinflammatory cytokines interleukin-1, interleukin-6, and tumor necrosis factor-␣ (4). Lipopolysaccharidebinding protein (LBP), CD14, and Toll-like receptors (TLRs) are mediators of LPS stimulation (5-10). There are two forms of CD14. One is the glycosylphosphatidylinositol-anchored membrane CD14 (mCD14) found mainly on the surface of cells of myeloid lineage. The other is soluble CD14 (sCD14), found in the serum and urine (11,12). LPS binds to CD14 in the presence of an acute phase response protein LBP. This complex mediates sepsis through TLRs. The role of CD14, LBP, and TLRs in LPS signaling has been well supported by studies in animal models (13)(14)(15)(16). Besides its function in endotoxin signaling, it has been proposed that CD14 is involved in transportation of other lipids (17, 18), cell-cell interaction during different immune responses (19 -21), and recognition of apoptotic cells (22,23). Therefore, CD14 becomes an interesting molecule to investigate in various biological processes.CD14 is highly expressed on the surface of monocytes/macrophages and strongly up-regulated during the differentiation of monocytic precursor cells into mature monocytes (24 -26). Therefore, CD14 has been commonly used as a differentiation marker for monocytes/macrophages. CD14 serves as an excellent model for the study of monocytic gene regulation and lineage dif...

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Effect of CD14 promoter polymorphism andH. pyloriinfection and its clinical outcomes on circulating CD14

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