The nucleolus, a potential target of cardenolide-mediated anti-tumor activity

Mijatovic T, De Nève N, Gailly P, Mathieu V, Haibe-Kains B, Bontempi G,  Decaestecker C, Facchini V, Kiss R

The use of cardenolides like ouabain, digitoxin or oleandrin has been reported previously many times as a means of potentially combating human refractory prostate cancer by inducing apoptosis through an increase in intra-cellular calcium concentrations (a mechanism also commonly associated with the adverse effects of cardenolides). The structural variety of cardiotonic steroids seems to offer a multiplicity of compounds with undiscovered mechanisms of action. The present study indicates that the structurally novel new cardenolide: (1R,3aS,3bR,5aS,6aR,7aS, 9R,12aR,13aR,15aR)-3a,11a- dihydroxy-13a-(hydroxymethyl)-9,15a-dimethyl-1-(5-oxo-2,5-dihydrofuran-3-yl)icosahydro-1H,4'H-spiro[cyclopenta [7,8]phenanthro[2,3-b]pyrano[3,2-e][1,4]dioxine- 11,2'-[1,3]thiazolidin]-4'-one (coded UNBS1450), markedly decreases the in vitro viability/proliferation of human prostate cancer cell lines and in vivo the development of both subcutaneous and orthotopic xenografts of the PC-3 model in immunocompromized mice. ROS induction by several cardenolides is well documented and correlates with a
loss in cell viability, proliferation and defense mechanisms. A marked time and concentration dependent ROS increase was demonstrated following UNBS1450 treatment (10 and 100nM). However, the induced effects are not linked to an increase in intra-cellular calcium concentrations and subsequent induction of apoptosis, as observed with classic cardenolides. Rather, the induced effects appear to relate to the compound’s capacity to disorganize nucleolar structure and function through an impairment of CDK and c-Myc expression and related signaling pathways. The marked UNBS1450-induced c-Myc down-regulation could result, at least partly, from rapid compound-induced increases in ROS. The nucleolar disorganization at concentrations which markedly impair prostate cancer cell proliferation (paralleled by the disorganization of cancer cell-specific perinucleolar bodies as revealed by disruption of Sam68) is not observed in normal cells. This non-apoptotic cancer cell death mediated by severe nucleolar targeting is a completely new cardenolide-induced mechanism of anti-tumor action.