Triggering of Suicidal Erythrocyte Death by Bexarotene

Abstract

Background/Aims: The retinoid X receptor agonist bexarotene is utilized for the treatment of cutaneous T-cell lymphoma and is effective in several further malignancies. The substance counteracts tumor growth in part by triggering suicidal death or apoptosis of tumor cells. Side effects of bexarotene treatment include anemia. Theoretically, bexarotene induced anemia could be secondary to stimulation of suicidal erythrocyte death or eryptosis, characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. Signaling potentially stimulating eryptosis include increase of cytosolic Ca²⁺ activity ([Ca²⁺]_i), induction of oxidative stress, increase of ceramide abundance, as well as activation of staurosporine sensitive protein kinase C, SB203580 sensitive p38 kinase, D4476 sensitive casein kinase 1, and zVAD sensitive caspases. The present study explored, whether bexarotene induces eryptosis and, if so, whether its effect involves Ca²⁺ entry, oxidative stress, ceramide, kinases and/or caspases. Methods: Flow cytometry was employed to quantify phosphatidylserine exposure at the cell surface from annexin-V-binding, cell volume from forward scatter, [Ca²⁺]_i from Fluo3-fluorescence, reactive oxygen species (ROS) abundance from DCFDA dependent fluorescence, and ceramide abundance utilizing specific antibodies. Hemolysis was estimated from hemoglobin concentration in the supernatant. Results: A 48 hours exposure of human erythrocytes to bexarotene (≥ 0.4 µg/ml) significantly increased the percentage of annexin-V-binding cells without significantly modifying forward scatter. Bexarotene significantly increased Fluo3-fluorescence and DCFDA fluorescence. Bexarotene tended to increase ceramide abundance, an effect, however, not reaching statistical significance. The effect of bexarotene on annexin-V-binding was significantly blunted by removal of extracellular Ca²⁺ and by addition of D4476 (10 µM), but not by addition of staurosporine (1 µM), SB203580 (2 µM), or zVAD (10 µM). Conclusions: Bexarotene triggers phospholipid scrambling of the erythrocyte cell membrane, an effect at least in part due to Ca²⁺ entry, oxidative stress, and activation of D4476 sensitive casein kinase.