Let-7d microRNA Attenuates 6-OHDA-Induced Injury by Targeting Caspase-3 in MN9D Cells

Parkinson's disease (PD) mainly results from the progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc), and the exact underlying mechanisms of the loss of DA neurons in PD remains largely unclear. The results of our previous work showed that let-7d was significantly downregulated in a 6-OHDA-induced cellular model of PD. However, the exact effect of let-7d on DA neural cells was unclear. In MN9D dopaminergic neuronal cells, we used a let-7d mimic and inhibitor to upregulate and downregulate the expression of let-7d, respectively, a cell counting kit to assess cell viability, and a TUNEL staining assay and flow cytometry to examine the cell death rate, and we found that let-7d could negatively regulate 6-OHDA-induced cell injury. Then, we verified that caspase-3 was a target gene of let-7d by using a dual-luciferase reporter system. Furthermore, using caspase-3 siRNA and a caspase-3-overexpression vector (without the 3'UTR) to respectively inhibit and increase the expression of caspase-3, we found that caspase-3 siRNA could reverse the cell injury induced by the let-7d inhibitor and that caspase-3 overexpression could reverse the protective effects of the let-7d mimic on 6-OHDA-induced cell injury. Taken together, these findings strongly suggest that let-7d plays an important role in DA neuronal cell injury and that the effects of let-7d are, at least in part, via the suppression of caspase-3 expression.