Transforming growth factor beta blocks cystogenesis by MDCK epithelium in vitro by enhancing the paracellular flux: implication of collagen V.

Transforming growth factor beta (TGFbeta) determines a nearly complete inhibition of cystogenesis by MDCK cells grown in collagen I-enriched matrices in vitro. In order to elucidate the mechanism implicated in this phenomenon, we performed a series of experiments aimed at discovering a relevant role of extracellular matrix. TGFbeta (2 ng/ml) played a marked stimulatory effect on the expression of extracellular matrix by MDCK with a selective effect on collagen V (three to fourfold increase of protein and mRNA) and in parallel inhibited cystogenesis by 95%. Cotreatment with TGFbeta and anti-collagen V antibodies restored a normal cystogenesis. In analogy, when MDCK cells were grown in three-dimensional matrices containing collagen I and minor (10%) amounts of collagen V, cystogenesis was once again inhibited by 95%. To characterize the molecular mechanism activated by TGFbeta and collagen V, we looked at the electrophysiological characteristics of MDCK monolayers and found a drastic fall of transepithelial electrical resistance (TER) in both conditions. In parallel with the decrease in TER, TGFbeta and collagen V also induced the leakage of two high molecular weight tracers, i.e., [3H]-inulin and 150 kD FITC-Dextran, suggesting a perturbation of the paracellular permeability. Finally, TGFbeta at the relevant concentration did not stimulate apoptosis in our cellular model, as judged by propidium iodide staining and by in situ end labeling of DNA fragments. These observations suggest that TGFbeta inhibits cystogenesis by MDCK cells in vitro by altering the collagenic composition of the three-dimensional milieu where MDCK cells grow and form cysts. The molecular mechanism responsible for inhibition of cystogenesis is the increase of paracellular flux which overcomes the active transport of solutes and water inside cysts.