Mechanical strain induces constitutive and regulated secretion of glycosaminoglycans and proteoglycans in fetal lung cells

Abstract

We have previously shown that an intermittent strain regimen, which simulates fetal breathing movements, enhanced mixed fetal rat lung cell proliferation in organotypic culture. As glycosaminoglycans (GAGs) and proteoglycans (PGs) may modulate growth factor activities, we investigated the effect of intermittent strain on the formation and secretion of GAGs and PGs. Mechanical strain increased the incorporation of [3H]glucosamine and 35SO4 into GAGs and promoted the release of GAGs into the medium. The composition of the individual GAG molecules was not altered by strain. Mixed fetal lung cells subjected to strain secreted more [35S]biglycan into the medium than static controls but biglycan mRNA expression was not significantly altered. As mechanical strain primarily affected the secretion of GAGs and PGs, we then investigated which secretory pathways were stimulated by strain. Fetal lung cells secreted GAGs mainly through a constitutive (basal) pathway which was stimulated by strain. In contrast to static cultures, strain-induced constitutive secretion was partially blocked by the cytoskeletal disruptors colchicine and cytochalasin B, but not by the small G-protein inhibitors N-acetyl-S-farnesyl-L-cysteine and perillic acid. This result suggests that strain-induced constitutive export of GAGs depends on the functional integrity of the cytoskeleton. Strain also triggered the regulated secretion of GAGs. The strain-induced regulatory pathway in fetal lung cells was blocked by ionomycin, BAPTA/AM and gadolinium, suggesting that strain stimulated the regulatory pathway by inducing a rapid calcium influx via a stretch-activated ion channel. We conclude that mechanical strain of mixed fetal lung cells stimulates GAG and PG exocytosis via activation of both the regulated and constitutive pathways.