Type VIII collagen modulates TGF-beta1-induced proliferation of mesangial cells.

摘要

Mesangial cells in diabetic mice and human kidneys with diabetic nephropathy exhibit increased type VIII collagen, a nonfibrillar protein that exists as a heterodimer composed of alpha1(VIII) and alpha2(VIII), encoded by Col8a1 and Col8a2, respectively. Because TGF-beta1 promotes the development of diabetic glomerulosclerosis, we studied whether type VIII collagen modulates the effects of TGF-beta1 in mesangial cells. We obtained primary cultures of mesangial cells from wild-type, doubly heterozygous (Col8a1(+/-)/Col8a2(+/-)), and double-knockout (Col8a1(-/-)/Col8a2(-/-)) mice. TGF-beta1 bound normally to double-knockout mesangial cells. In wild-type mesangial cells, TGF-beta1 inhibited proliferation, but in double-knockout cells, it stimulated proliferation, promoted cell cycle progression, and reduced apoptosis; we could reverse this effect by reconstituting alpha1(VIII). Furthermore, in wild-type cells, TGF-beta1 mainly stimulated the Smad pathways, whereas in double-knockout cells, it activated the MAPK and PI3K/Akt pathways and induced expression of fibroblast growth factor 21 (FGF21). Inhibiting FGF21 expression by either interfering with activation of the MAPK and PI3K/Akt pathways or by FGF21 siRNA attenuated the TGF-beta1-induced proliferation of double-knockout mesangial cells. In vivo, diabetic double-knockout mice had significantly higher expression of renal FGF21 mRNA and protein compared with diabetic wild-type mice. Immunohistochemistry revealed strong expression of FGF21 in both glomerular (mesangial) and tubular cells of diabetic mice. Taken together, these data suggest that type VIII collagen significantly modulates the effect of TGF-beta1 on mesangial cells and may therefore play a role in the pathogenesis of diabetic nephropathy.