Potential role of activated macrophages in acetaminophen hepatotoxicity. II. Mechanism of macrophage accumulation and activation.

Treatment of rats with acetaminophen (1.2 g/kg) results in the accumulation of activated macrophages in the centrilobular regions of the liver. To study the mechanism by which these cells accumulate and become activated, we examined the release of chemotactic and activating factors from cultured hepatocytes treated with acetaminophen (10-100 microM). We found a dose- and time-related generation of Kupffer cell and monocyte chemotactic activity by acetaminophen-treated hepatocytes. The maximum response was observed with a 25% dilution of medium collected 24 hr following treatment of hepatocytes with acetaminophen. Using a checkerboard assay, the factor in conditioned medium was determined to induce chemotaxis as well as chemokinesis in both Kupffer cells and monocytes. The hepatocyte-derived chemotactic factor was also found to be stable to freeze-thawing but to lose activity following heat or trypsin treatment. These results, together with our findings that chemotactic activity was eluted in the void volume following Sephadex G-25 size exclusion chromatography, suggested that the chemotactic factor released by hepatocytes is a large molecular weight protein. The release of Kupffer cell activating factors by acetaminophen-treated hepatocytes was also examined. Hepatocyte-conditioned medium was found to stimulate Kupffer cell phagocytosis and superoxide anion release, two characteristics of activated macrophages. These effects were maximal with conditioned medium collected from hepatocytes 24 hr following treatment with 50-100 microM acetaminophen. Acetaminophen alone had no effect on chemotaxis, phagocytosis, or superoxide anion production by Kupffer cells or monocytes. Taken together, these results suggest that macrophage accumulation and activation in the liver following acetaminophen treatment may be mediated, at least in part, by factors released from hepatocytes.