Retinoic acid (Tretinoin), a metabolite of vitamin A, is a natural agonist of the retinoic acid receptor RAR and inhibits RARα/β/γ (IC50=14 nM). Retinoic acid induces cellular differentiation, reduces cellular proliferation, and inhibits tumorigenesis.

Tretinoin prevents skin atrophy induced by corticosteroids in hairless mice. When co-administered with miquimod in guinea pigs, tretinoin induces tattoo fading and moderate pigment clearance histopathologically. Applications of tretinoin on incisions in the skin of 45 CD-1 mice increase fibroblast differentiation and reduce collagen production. In aged male Fischer 344 rats treated with tretinoin, renal cortex protein content is 30% lower compared to controls, potentially due to suppressed expression of tumor necrosis factor-β1 and osteopontin.

In studies evaluating the impact on glutathione levels and catalase activity, Tretinoin increased both metrics in a time- and dose-dependent manner, offering protective and mitigating effects against H2O2 cytotoxicity in human renal mesangial cells. Treatment with Tretinoin resulted in elevated mRNA levels of catalase and γ-glutamylcysteine synthetase (the catalytic subunit responsible for the rate-limiting step in reduced glutathione synthesis) in cultured mesangial cells. Additionally, Tretinoin upregulated matrix metalloproteinase-8/13 in human keloid-derived fibroblasts.

Retinoic acid is dissolved in DMSO and stored, and then diluted with appropriate medium before use[3]. P19 cell are induced to undergo neuronal differentiation according to established procedures. Briefly, cells are cultured on 1% agarose-coated 10 cm dishes at 3×10 5 cells/mL in α-minimal essential medium supplemented with 10% FBS. Differentiation is induced by addition of Retinoic acid (1 μM) and medium containing Retinoic acid replaced 2 days later. On day 4, cell aggregates are collected by centrifugation, separated to single cells by trypsin/EDTA treatment, replated onto poly-L-lysine-coated plates, and cultured in α-minimal essential medium supplemented with 10% FBS. On day 6, medium is replaced with neurobasal medium containing B27 supplement and 2 mM GlutaMAX. Medium is replaced every 2 days for an additional week[3].