Withaferin A inhibits lysosomal activity to block autophagic flux and induces apoptosis via energetic impairment in breast cancer cells.

Withaferin A (WFA), a steroidal lactone, negatively regulates breast cancer growth however, its mechanisms of action remain largely elusive. We found that WFA blocks autophagy flux and lysosomal proteolytic activity in breast cancer cells. WFA increases accumulation of autophagosomes, LC3B-II-conversion, expression of autophagy-related proteins and autophagosome/lysosome fusion. Autolysosomes display the characteristics of acidic compartments in WFA-treated cells; however, the protein degradation activity of lysosomes is inhibited. Blockade of autophagic flux reduces the recycling of cellular fuels leading to insufficient substrates for tricarboxylic acid (TCA) cycle and impaired oxidative phosphorylation. WFA decreases expression and phosphorylation of LDHA, the key enzyme that catalyzes pyruvate-to-lactate conversion, reduces ATP levels and increases AMPK activation. AMPK-inhibition abrogates while AMPK-activation potentiates WFA's effect. WFA and 2-deoxyglucose combination elicits synergistic inhibition of breast cancer cells. Genetic-knockout of BECN1 and ATG7 fails to rescue cells from WFA-treatment; in contrast, addition of methyl pyruvate to supplement TCA cycle protects WFA-treated cells. Together, these results implicate that WFA is a potent lysosomal inhibitor; energetic impairment is required for WFA-induced apoptosis and growth-inhibition and combining WFA and 2DG is a promising therapeutic strategy for breast cancer.