Hyaluronic acid‐degrading enzymes in rat alveolar macrophages and in alveolar fluid: stimulation of enzyme activity after oral treatment with the immunomodulator RU 41740

Abstract

RU 41740 (Biostim®) is an immunomodulator clinically used for the treatment of chronic bronchitis and recurrent pulmonary infections. In these diseases large amounts of mucus are produced which congest the bronchi. A major glycosaminoglycan constituent of this mucus is hyaluronic acid, one of the largest molecules in nature; its metabolic degradation is carried out by 3 acid hydrolases: hyaluronidase, β‐N‐acetylglucosaminidase, and β‐glucuronidase. In the lung these enzymes are especially synthesized and active in alveolar macrophages. It was thus interesting to study the effect of RU 41740 administration on the hyaluronic acid‐degrading activity of these cells. This compound was given by gastric gavage to rats and the activities of lung alveolar macrophage and alveolar fluid hyaluronidase, β‐N‐acetylglucosaminidase, β‐glucuronidase, and acid phosphatase as a lysosomal marker were determined. The effect on macrophage proliferation was also examined. The results obtained showed that: (1) unstimulated alveolar macrophages display the remarkable property, compared with other cell types, that hyaluronidase activity is about equally distributed between the inside and the outside of the cell; (2) RU 41740 administration increases the total activity of the 4 enzymes studied in the alveolar macrophages without inducing any increase in the number of macrophages; (3) the intracellular activities of β‐N‐acetylglucosaminidase and β‐glucuronidase are markedly increased, whereas intracellular hyaluronidase activity is not changed. However, in the extracellular fluid only hyaluronidase activity is highly increased; (4) even the lysosomal marker enzyme acid phosphatase has only its intracellular activity is increased. This would suggest the possibility that other lysosomal enzymes may also be increased by this immunomodulator. Most probably, the activation of the hyaluronic acid‐degrading enzymes by RU 41740 has the following consequences: extracellular hyaluronidase degrades hyaluronic acid to smaller molecules, which are then phagocytized by the macrophages; the intracellular hydrolytic enzymes, also strongly stimulated, complete the hydrolysis. Moreover, it was observed that the molecular form of rat macrophage hyaluronidase is the same as that of serum and other somatic cell hyaluronidase.