Mechanisms of cytotoxicity of anticancer titanocenes

The organometallic compound titanocene (bis(cyclopentadienyl)titanium(IV) dichloride) showed promising preclinical anticancer activity in generally resistant tumors in vitro and in vivo but failed in clinical trials. A broad range of analogs with modified cyclopentadienyl ligands conferring increased stability and higher cytotoxicity were developed. Titanium was found to accumulate in the nucleus and inhibit DNA replication and transcription. The active species causing irreversible damage and exact mechanisms of action resulting in cell cycle arrest and apoptosis have not been identified to date. Our group investigated changes in global gene expression of NCI-H526 small cell lung cancer cells in response to the novel analog titanocene C (bis-N,N-dimethylamino-2-(N-methylpyrrolyl) methyl) titanium (IV)). Differences observed in transcript levels indicated downregulation of DNA unwinding by topoisomerases I and II alpha and activation of responses to DNA damage and cellular stress as well as shutdown of energy metabolism and, finally, apoptosis. Besides direct interaction of Ti(2+) with DNA, induction of the MT1 family of metallothionein genes and downregulation of cellular Zn(2+) uptake in response to titanocene C pointed to disturbed Zn(2+) homeostasis, which triggers cell cycle arrest and apoptosis due to defective transcription factors and metalloenzymes. In particular, histone H4 genes dependent on Zn(2+)-containing transcription factors H4TF-1/2 were specifically downregulated, and accumulation of defective metalloproteins in the endoplasmatic reticulum seemed to activate unfolded protein response. In conclusion, according to these results, we propose a model of titanocene-induced cytotoxicity, comprising direct DNA damage and perturbation of Zn(2+) homeostasis with impairment of the functions of cellular metalloproteome.