Background and purpose: Artemisinin is an antimalarial drug exerting pleiotropic effects, such as the inhibition of the transcription factor nuclear factor-kappa B and of the sarcoplasmic/endoplasmic reticulum Ca(++)-ATPase (SERCA) of P. falciparum. As the sesquiterpene lactone thapsigargin, a known inhibitor of mammalian SERCA, enhances the expression of P-glycoprotein (Pgp) by increasing the intracellular Ca(++) ([Ca(++)](i)) level, we investigated whether artemisinin and its structural homologue parthenolide could inhibit SERCA in human colon carcinoma HT29 cells and induce a resistance to doxorubicin.
Experimental approach: HT29 cells were incubated with artemisinin or parthenolide and assessed for SERCA activity, [Ca(++)](i) levels, Pgp expression, doxorubicin accumulation and toxicity, and translocation of the hypoxia-inducible factor, HIF-1alpha.
Key results: Artemisinin and parthenolide, like the specific SERCA inhibitors thapsigargin and cyclopiazonic acid, reduced the activity of SERCA. They also increased intracellular calcium concentration ([Ca(++)](i)) and Pgp expression and decreased doxorubicin accumulation and cytotoxicity. The intracellular Ca(++) chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, and the inhibitor of calmodulin-dependent kinase II (CaMKII) KN93 prevented these effects. CaMKII is known to promote the phosphorylation and the activation of HIF-1alpha, which may induce Pgp. In HT29 cells, artemisinin and parthenolide induced the phosphorylation of HIF-1alpha, which was inhibited by KN93.
Conclusions and implications: Our results suggest that artemisinin and parthenolide may act as SERCA inhibitors and, like other SERCA inhibitors, induce resistance to doxorubicin in human colon cancer cells, via the CaMKII-dependent activation of HIF-1alpha and the induction of Pgp.