Tissue, temporal and inducible expression pattern of haptoglobin in mice.

Haptoglobin (Hp) is a member of the acute phase plasma proteins previously thought to be synthesized solely by the adult liver. The present study analyzes the tissue and temporal expression pattern of endogenous haptoglobin in the mouse and acute phase inducibility in various tissues. The liver is found to be the major site of haptoglobin expression but significant expression levels were also observed in the lung and skin. Acute phase induction by bacterial lipopolysaccharide (LPS) demonstrated that haptoglobin was induced not only in the liver but also in other tissues, including lung, skin, spleen, and kidney. Temporal analyses demonstrated that haptoglobin is expressed during embryogenesis in the liver and is inducible in various tissues surveyed throughout development. Transgenic mice that harbored a 1.05-kilobase (kb) region of the human haptoglobin promoter linked to two different reporter genes gave rise to lung-specific expression in the majority of transgenic lines with minimal liver expression. However, when induced with lipopolysaccharide, the 1.05-kb fragment contained the necessary elements for a response comparable to endogenous expression levels. In conclusion, these studies demonstrate that haptoglobin is not an adult liver specific gene, and its role as an acute phase reactant may well be more diverse than previously suspected.