Generation of endoplasmic reticulum stress and inhibition of autophagy by plitidepsin induces proteotoxic apoptosis in cancer cells

Plitidepsin (PLD, Aplidin (R)), a cyclic depsipeptide originally isolated from the marine tunicate Aplidium albicans, has been recently approved by Australian regulatory authorities for the treatment of multiple myeloma patients. Plitidepsin binds to eEF1A2 and induces oxidative stress, Rac1 activation and JNK1 phosphorylation, triggering a rapid apoptotic program in tumor cells. Since oxidative stress is one of the known sources of endoplasmic reticulum stress, we investigated whether PLD was inducing a bona fide ER stress in HeLa cells and whether this process was essential in the mechanism of action of the compound. Indeed, PLD activated an ER stress-induced unfolded protein response (UPR), including the alternative splicing of XBP1, the proteolytic processing of ATF6 and the phosphorylation of eIF2 alpha and JNK. Interestingly, though PLD induced a strong phosphorylation of eIF2 alpha in all the analyzed cell lines, it did not elicit an increased expression of ATF4 and CHOP, a transcription factor involved in launching UPR-mediated apoptosis. On the contrary, a clear reduction of CHOP protein levels was observed after PLD treatment, most probably due to both the lack of transactivation by ATF4 and its rapid degradation by the ubiquitin/proteasome machinery. Using fibroblasts devoid of each one of the four possible kinases involved in eIF2 alpha phosphorylation, we observed that only PKR was involved in the response to PLD treatment and, accordingly, PKR-/- fibroblasts are shown to be resistant to the apoptogenic activity of the compound. Furthermore, eIF2 alpha phosphorylation itself was shown to be irrelevant for the induction of cell death by PLD. Instead, we reveal that PLD induces an increase in the levels of misfolded proteins while simultaneously inhibiting the autophagic flux. These two effects combined prevent PLD-treated cells from reducing proteotoxic stress and lead to apoptosis. Other anti-myeloma drugs like bortezomib, which target the proteasome, also inhibit the degradation of misfolded proteins through alternate pathways and a synergistic anticancer effect of the PLD plus bortezomib combination has been previously disclosed. The present results extend this synergy to in vivo experiments and provide a mechanistic rationale for this synergy.