The Mechanism of Neurite Outgrowth Induction by Novel Synthetic Retinobenzoic Acids

Retinoids are a family of vitamin A-derived molecules and include the biologically active metabolite, retinoic acid (RA). RA acts as a specific modulator of neuronal differentiation and proliferation. However, teratogenicity and a large excess of RA have been found in animal studies. Thus, development of effective and stable retinoids is desirable. In this study, we showed that treatment with novel synthetic retinobenzoic acids promotes neurite outgrowth in a selected subpopulation of the human neuroblastoma cell line SK-N-SH. Furthermore, we found that, although acting via a different mechanism, retinobenzoic acids have the same neurite outgrowth-inducing effect as RA. Retinoids, including RA, bind to nuclear retinoic acid receptors (RARs). Therefore, we examined the expression of RARs in retinobenzoic acid-treated cells. Similar to already known retinoids, novel synthetic retinobenzoic acids promote the upregulation of RAR{beta} and have no effect on RAR or {gamma}. These results suggest that retinobenzoic acids act via RAR{beta} during neurite outgrowth. Moreover, stimulation with RA or retinobenzoic acids significantly increased the phosphorylation levels of both ERK1/2 and mTOR. ERK1/2 and mTOR inhibition blocked the retinobenzoic acid-induced increase in neurite outgrowth, suggesting that retinobenzoic acids promoted neurite outgrowth by activating the ERK1/2 and mTOR signaling pathways. Notably, the RA-induced increase in neurite outgrowth was blocked by the ERK1/2 inhibitor U0126, but not by the mTOR inhibitor rapamycin. In addition, ERK1/2 inhibition blocked the upregulation of RAR{beta} promoted by RA and retinobenzoic acids. In contrast, mTOR inhibition had no effect on upregulation of RAR{beta}. Our results show that novel synthetic retinobenzoic acids induce neurite outgrowth by a different mechanism than RA. These findings suggest that activation of both ERK1/2, which results in downstream regulation of RAR{beta}, and mTOR, are responsible for the novel synthetic retinobenzoic acid-induced neurite outgrowth in human neuroblastoma cells.