Combinations of anti-cancer drugs can overcome resistance to therapy and provide new more effective treatments. In this work we have analyzed the effect of the polyphenol quercetin and the anti-cancer sphingosine analog fingolimod on the sphingolipid metabolism in HepG2 cells, since sphingolipids are recognized as mediators of cell proliferation and apoptosis in cancer cells. Treatment of hepatocellular carcinoma HepG2 cells with quercetin and fingolimod, alone or in combination, induced different degrees of sphingomyelin (SM) reduction and a corresponding activation of neutral sphingomyelinase (nSMase). Western blot analysis showed that only treatments containing quercetin induced up-regulation of nSMase expression. The same treatment caused elevation of ceramide (CER) levels, whereas the observed alterations in sphingosine (SPH) content were not statistically significant. The two tested drugs induced a reduction of the pro-proliferative sphingolipid, sphingosine 1 phosphate (S1P), in the following order: quercetin, fingolimod, quercetin + fingolimod. The activity of the enzyme responsible for CER hydrolysis, alkaline ceramidase (ALCER) was down-regulated only in the incubations involving quercetin and fingolimod did not affect this activity. The enzyme, maintaining the balance between apoptosis and proliferation, sphingosine kinase 1 (SK1), was down-regulated by incubations in the following order: quercetin, fingolimod, quercetin + fingolimod. Western blot analysis showed down-regulation in SK1 expression upon quercetin but not upon fingolimod treatment. Studies on the effect of quercetin and fingolimod on the two proteins associated with apoptotic events, AKT and Bcl-2, showed that only quercetin, alone or in combination, down-regulated the activity of the two proteins. The reported observations provide information which can be useful in the search of novel anti-tumor approaches, aiming at optimization of the therapeutic effect and maximal preservation of healthy tissues.
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