Hispidulin suppresses tumor growth and metastasis in renal cell carcinoma by modulating ceramide-sphingosine 1-phosphate rheostat

Sphingosine 1-phosphate (S1P) rheostat is considered as a key signal that determines cell fate. This study aimed to report that hispidulin, a polyphenolic flavonoid, exerted anti-growth and anti-metastasis effects against renal cell carcinoma (RCC) by modulating the balance of ceramide-S1P. In vitro studies showed that hispidulin could effectively inhibit cell proliferation, cell migration, cell invasion, and epithelial-mesenchymal transition, and promote cell apoptosis in Caki-2 and A498 cell lines. Moreover, it also increased the ceramide/S1P ratio. Consistent with the in vitro findings, the efficacy of hispidulin in vivo showed that it effectively suppressed tumor growth and lung metastasis. Furthermore, the results revealed that hispidulin significantly suppressed the activity of sphingosine kinase 1 (Sphk1) in RCC cells; however, no significant change was observed in the mRNA or protein expression of Sphk1. The overexpression of Sphk1 could significantly abrogate the anti-growth and anti-metastasis effects of hispidulin, whereas the siRNA-targeting Sphk1 or Sphk1 inhibitor was able to augment the anticancer effects of hispidulin against RCC. Moreover, hispidulin interfered with the phosphorylation and translocation of Sphk1, leading to inhibitory effects of Sphk1 activity. In summary, the findings suggested that hispidulin suppressed tumor growth and metastasis by inhibiting the Sphk1 activity and consequently modulating ceramide-S1P rheostat. It also presented a new explanation for the antitumor mechanisms of hispidulin against RCC.