Curcumin analogues with potent and selective anti-proliferative activity on acute promyelocytic leukemia: involvement of accumulated misfolded nuclear receptor co-repressor (N-CoR) protein as a basis for selective activity.

Curcumin arrests the proliferation of acute promyelocytic leukemia (APL) cells by stabilizing the misfolded nuclear receptor co-repressor (N-CoR) protein, thereby sensitizing APL cells to apoptosis induced by the unfolded protein response. This phenomenon was attributed to inhibition of the proteasomal and protease-induced breakdown of misfolded N-CoR by curcumin. Curcumin is, however, a modest inhibitor and affected the viability of APL cells at micromolar concentrations. Modifying curcumin at its conjugated β-diketone linker and terminal phenyl rings yielded potent congeners with sub-micromolar growth inhibitory activities which selectively kill APL cells over non-APL leukemic and nonmalignant cells. Analogues with pronounced APL-selective anti-proliferative activities, as observed in representative dibenzylidenecyclohexanones and dibenzylidenecyclopentanones, strongly promoted the accumulation of misfolded and nonfunctional N-CoR at significantly lower concentrations than their growth inhibitory IC(50) values. These compounds also inhibited the human 20S proteasome in an enzyme-based assay, thus providing convincing support for the prevailing hypothesis that impeding the degradation of N-CoR is a key mechanistic event contributing to APL cell death.