Retinoids, including all‐trans retinoic acid (tRA), have dose‐dependent pro‐fibrotic effects in experimental kidney diseases. To understand and eventually prevent such adverse effects, it is important to establish relevant in vitro models and unravel their mechanisms.

Fibrogenic effects of retinoids were assessed in NRK‐49F renal fibroblasts using picro‐Sirius red staining for collagens and quantified by spectrophotometric analysis of the eluted stain. Other methods included RT‐qPCR, immunoassays and matrix metalloproteinase (MMP) activity assays.

With or without TGF‐β1, tRA was dose‐dependently pro‐fibrotic, notably increasing collagen accumulation. tRA and TGF‐β1 additively suppressed expression of mRNA for MMP2, 3 and 13 and suppressed MMP activity. tRA, in the presence of TGF‐β1, induced plasminogen activator inhibitor‐1 (PAI‐1) mRNA and they additively induced PAI‐1 protein expression. A PAI‐1 inhibitor, a pan‐retinoic acid receptor (RAR) antagonist and a pan‐retinoid X receptor (RXR) antagonist each partially prevented the pro‐fibrotic effect of tRA. The dose‐dependent pro‐fibrotic effects of a pan‐RXR agonist were similar to those of tRA. A pan‐RAR agonist showed weaker, less dose‐dependent pro‐fibrotic effects and the pro‐fibrotic effects of RARα and RARβ‐selective agonists were even smaller. An RARγ‐selective agonist did not affect fibrogenesis.

An in vitro model for the pro‐fibrotic effects of retinoids was established in NRK‐49F cells. It was associated with reduced MMP activity and increased PAI‐1 expression, and was probably mediated by RXR and RAR. To avoid or antagonize the pro‐fibrotic activity of tRA, further studies on RAR isotype‐selective agonists and PAI‐1 inhibitors might be of value.