Studies were carried out in human lung fibroblasts (IMR-90) to investigate 1) the relative contribution of two extracellular pools, inorganic sulfate and sulfur-containing amino acids, to the intracellular fraction precipitable by trichloroacetic acid and 2) the possibility that the transport of these sulfur-containing substrates at the plasma membrane may be a limiting step for macromolecular sulfation. Our studies indicate that the ability to use SO4(2-) released by intracellular catabolism of the sulfur-containing amino acid L-cysteine differs from one cell system to another. In contrast to smooth muscle cells, in the human lung fibroblast, L-cysteine contributes significantly to the intercellular pool of SO4(2-) used for sulfation at extracellular [SO4(2-)] less than 100 microM. However, under physiological conditions with respect to SO4(2-) ([SO4(2-)]0 = 300 microM), L-cysteine does not contribute greater than 30% of the sulfate incorporated into the cellular fraction. Taurine (2-aminoethanesulfonic acid) inhibits SO4(2-) incorporation into the cell-associated macromolecular fraction. However, results suggest that the effect is not due to either SO4(2-) released by its catabolism or to an effect on SO4(2-) transport into the cell. The fact that the transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid inhibits sulfate incorporation indicates that carrier-mediated sulfate transport at the cellular plasma membrane may be a limiting step for sulfate incorporation. In conclusion, under physiological conditions with respect to SO4(2-), inorganic sulfate is a major source of sulfate for sulfation in human lung fibroblasts and macromolecular sulfation may be limited by its transport into the cells.
Transforming growth factor beta stimulates the expression of fibronectin and of both subunits of the human fibronectin receptor by cultured human lung fibroblasts.
