Curcumin, a potential initiator of apoptosis via direct interactions with Bcl-xL and Bid

Apoptosis is a naturally occurring process during the growth and development of multicellular organisms and is increasingly active during times of cellular stress such as in response to intracellular DNA damage when removal of the host cell is paramount to prevent cancer. Unfortunately, once formed, cancer cells become impervious to apoptosis, creating a desperate need to identify an approach to induce apoptosis in these cells. An attractive option is to focus efforts on developing and locating compounds which activate apoptosis using natural compounds. Curcumin is a natural component in turmeric and is well-known for its pharmacological effects in preventing and combating many ailments and has been shown to decrease the rapid proliferation of a wide variety of tumor cells. However, to date, the apoptotic intermediates and interactions through which curcumin exerts its cytotoxic effects are unknown. Motivated by reports linking the intracellular modulation of the concentrations of Bid and Bcl-xL, following curcumin administration to cancer cells, we set out to probe for potential intermolecular interactions of these proteins with curcumin. Using several biophysical techniques, most notably, fluorescence, circular dichroism and nuclear magnetic resonance spectroscopy, we reveal binding interactions of curcumin with both Bcl-xL Delta C and full-length Bid (Bid-FL) and prove that this binding is hydrophobically driven and localized to well-known functional regions of each protein. Specifically, our NMR studies show that while Bid-FL interacts with curcumin through its hydrophobic and pore forming helices (alpha 6-alpha 7), Bcl-xL Delta C interacts with curcumin via its BH3 binding pocket (alpha 2-alpha 3-alpha 4-alpha 5), a critical region for mediating apoptosis.