Autophagy is a cellular mechanism for removal of dysfunctional cellular organelles, large protein complexes. It drives a specialized form of program cell death termed as programmed cell death II. Recent studies have demonstrated a prominent role of the process of autophagy in cancer progression and drug sensitivity. Under conditions of stress, cancer cells try to utilize autophagy to leverage a survival advantage; however, excessive autophagy renders cells vulnerable.
Our previous work has demonstrated a pivotal role of N-myc interactor(NMI) in impeding breast cancer progression and metastasis. Loss of NMI is one of the key events that precedes and contributes to invasive progression of breast cancer. Here we provide evidence that NMI plays a crucial role in regulation of mTOR signaling ultimately impacting the autophagic response in breast cancer cells. Our initial observations showed that breast cancer cells restored for NMI expression contained more acidic vacuoles as determined by staining with Lysotracker. Further indication of autophagy was evidenced by the formation of distinct punctae in NMI cells upon transfection with GFP-LC3 which was ablated upon treatment with an autophagy inhibitor. Cells restored for expression of NMI showed decreased levels of p62 along with an increased LC3II processing. The mTOR signaling pathway plays a pivotal role in driving the autophagy. We found that p70S6K phosphorylation dramatically decreases in NMI expressing cells indicating an inhibition of mTOR signaling in these cells. Our observations show that active GSK3β, a negative modulator of mTOR, is responsible for mediating the effects of NMI. Conversely, NMI silenced cells showed indication of inactive GSK3β concomitant with activated mTOR signaling.
Functionally, NMI mediated sensitization of cells to autophagy rendered breast cancer cells incapable of sustaining stress of cisplatin treatment. The sensitivity of these cells to cisplatin was notably reduced upon inhibition of the autophagic process. This suggests a possible role for autophagy in NMI induced drug sensitivity. Collectively, our results suggest that NMI plays an important role in modulation of autophagy via activation of GSK3β and consequent inhibition of mTOR signaling.
Citation Format: Brandon J. Metge, Aparna Mitra, Lalita A. Shevde, Rajeev S. Samant. N-myc interactor impacts autophagy via modulation of GSK3β/mTOR signaling. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4046. doi:10.1158/1538-7445.AM2014-4046