INTRODUCTION - Human epidermal growth factor receptor-2 (HER2) gene amplification (HER2+) drives tumor cell growth and survival in ~25% of breast cancers. HER2 signaling activates the type I phosphoinositide 3-kinase (PI3K), upon which these tumors rely. Consequently, inhibitors of HER2 and type I PI3K block growth and increase apoptosis in HER2+ breast cancers, especially when used in combination. However, the impact of type III PI3K inhibition, particularly in combination with HER2 blockade or type I PI3K inhibition, remains less clear.
METHODS - We utilized small molecule kinase inhibitors, locked nucleic acid antisense oligonucleotides (LNA-ASOs), and siRNA to assess proliferation, autophagy, apoptosis, and protein expression in cell culture models of HER2+ breast cancers.
RESULTS - Treatment of HER2+ breast cancer cells with HER2 inhibitors or type I PI3K kinase inhibitors, alone or in combination, blocked type I PI3K signaling, reduced tumor cell growth, and induced autophagy. Knockdown of the type I PI3K, p110α, using an LNA-ASO termed EZN4150 inhibited PI3K-mediated Akt phosphorylation. However, in contrast to catalytic inhibitors of type I PI3Ks, EZN4150 did not induce autophagy, and blocked autophagy in response to inhibitors of HER2 or type I PI3Ks in a dominant fashion. Sequence analysis of EZN4150 revealed significant homology to the gene encoding the type III PI3K, Vps34, a key component for autophagy induction. EZN4150 simultaneously reduced expression of both p110α and Vps34. Combined inhibition of PI3K signaling and autophagy using individual siRNAs against p110α and Vps34 or using pharmacological type I and type III PI3K inhibitors recapitulated what was seen with EZN4150, and robustly enhanced tumor cell killing.
CONCLUSIONS - These studies highlight the important role of Vps34-mediated autophagy in limiting the anti-tumor response to inhibitors of HER2 or type I PI3K in HER2+ breast cancers. The type III PI3K Vps34 represents a potential therapeutic target to block treatment-induced autophagy and enhance tumor cell killing.