Irradiation-induced expression of hyaluronan (HA) synthase 2 and hyaluronidase 2 genes in rat lung tissue accompanies active turnover of HA and induction of types I and III collagen gene expression

Hyaluronan (HA) is a linear glycosaminoglycan that accumulates in the inter stitium of injured lung and inhibits gas exchange between air and blood. in the present study we investigated the molecular mechanisms behind the loca l turnover of HA during the early phase of irradiation-evoked lung fibrosis in rats. Irradiation with a single dose of 30 Gy to the lower part of the right lung of rats induced an accumulation of HA in bronchoalveolar lavage fluid 6 wk after irradiation, followed by return to almost normal levels at 10 wk after irradiation. This was parallelled with a transient downregulat ion of HA receptors on alveolar macrophages (AMs); 4 and 6 wk after irradia tion the binding of [H-3]HA to AMs was decreased to about 50% of that of AM s from nonirradiated control rats, returning to almost normal level at 10 w k after irradiation. Analysis of the expression of rat HA synthase (HAS) is oforms (rHAS1, rHAS2, and rHAS3) and rat hyaluronidases (rHYAL1 and rHYAL2) by Northern blotting revealed an upregulation of rHAS2 messenger RNA at 4, 6, and 10 wk after irradiation, but a progressive decrease in the constitu tive expression of rHYAL2 at 6 and 10 wk after irradiation; rHAS1 was undet ectable, whereas rHAS3 and rHYAL1 were faintly detectable. Although transfo rming growth factor-pi stimulated HA production by normal lung fibroblasts, it inhibited HYAL activity in lysosomes and HYAL activity released into th e culture media. Another interesting observation was that HA fragments, whi ch likely result from the action of HYAL, induced expression of types I and III collagen genes. Our results indicate that rHAS2 and rHYAL2 are involve d in the turnover of HA during the early phase of lung injury and that rHAS 2 and rHYAL2 as well as HA fragments may play important roles in the pathog enesis of lung fibrosis.