Molecular mechanisms of hyperthermia-induced apoptosis enhanced by docosahexaenoic acid: Implication for cancer therapy

• Hyperthermia-induced apoptosis was significantly enhanced by DHA.
• DHA synergizes hyperthermia to induce excess intracellular oxidative stress.
• Activation of PKC-δ plays important role in the enhancement of apoptosis.

To develop a non-toxic enhancer for hyperthermia-induced cell death as a potential cancer treatment, we studied the effect and mechanism of docosahexaenoic acid (DHA) on hyperthermia-induced apoptosis. Treatment with 20 μM DHA and 44 °C for 10 min induced significant apoptosis, increased intracellular reactive oxygen species (ROS), and caspase-3 activation in U937 cells, but heat or DHA alone did not induce notable apoptosis. Decreased mitochondrial transmembrane potentials were dramatically increased by the combined treatment, accompanied by increased pro-apoptotic Bcl-2 family protein tBid, and decreased anti-apoptotic Bcl-2 and Bcl-xL. Combined hyperthermia–DHA treatment induced significant phosphorylation of protein kinase C (PKC)-δ (p-PKC-δ), and apoptosis in a DHA dose-dependent manner. Using both 20 μM DHA and 44 °C for 10 min induced significant PKC-δ cleavage and its translocation to mitochondria. These results were also seen in HeLa cells. However, MAPKs and Akt were not affected by the treatment. In conclusion, DHA enhances hyperthermia-induced apoptosis significantly via a mitochondria–caspase-dependent pathway; its underlying mechanism involves elevated intracellular ROS, mitochondria dysfunction, and PKC-δ activation.