Besides its classical function of bone metabolism regulation, 1alpha, 25-dihydroxyvitamin D3 (1,25(OH)2D3), acts on a variety of tissues including the nervous system, where the hormone plays an important role as neuroprotective, antiproliferating and differentiating agent. Sphingolipids are bioactive lipids that play critical and complex roles in regulating cell fate. In the present paper we have investigated whether sphingolipids are involved in the protective action of 1,25(OH)2D3. We have found that 1,25(OH)2D3 prevents amyloid-β peptide (Aβ(1−42)) cytotoxicity both in differentiated SH-SY5Y human neuroblastoma cells and in vivo. In differentiated SH-SY5Y cells, Aβ(1−42) strongly reduces the sphingosine-1-phosphate (S1P)/ceramide (Cer) ratio while 1,25(OH)2D3 partially reverts this effect. 1,25(OH)2D3 reverts also the Aβ(1−42)-induced reduction of sphingosine kinase activity. We have also studied the crosstalk between 1,25(OH)2D3 and S1P signaling pathways downstream to the activation of S1P receptor subtype S1P1. Notably, we found that 1,25(OH)2D3 prevents the reduction of S1P1 expression promoted by Aβ(1−42) and thereby it modulates the downstream signaling leading to ER stress damage (p38MAPK/ATF4). Similar effects were observed by using ZK191784. In addition, chronic treatment with 1,25(OH)2D3 protects from aggregated Aβ(1−42)-induced damage in the CA1 region of the rat hippocampus and promotes cell proliferation in the hippocampal dentate gyrus of adult mice.
In conclusion, these results represent the first evidence of the role of 1,25(OH)2D3 and its structural analogue ZK191784 in counteracting the Aβ(1−42) peptide-induced toxicity through the modulation of S1P/S1P1/p38MAPK/ATF4 pathway in differentiated SH-SY5Y cells.