Sonia Núñez-Vázquez

Mitophagy, the selective degradation of mitochondria by lysosomes, is a major mitochondrial quality-control pathway, yet its role in lung cancer initiation and therapeutic response remains poorly understood. We investigated PINK1-dependent mitophagy in human lung adenocarcinoma and directly monitored mitophagy in vivo using mito-QC reporter mice during lung tumor development. We found that PINK1 activity and mitophagy are increased in human tumors and are activated from the earliest stages of tumorigenesis. Genetic inhibition of PINK1 impaired oncogenic transformation but had little impact once tumor cells were established, identifying mitophagy as a critical early adaptation during lung cancer initiation. PINK1 deficiency markedly enhanced the efficacy of KRAS -targeted therapy in vitro and in vivo. Together, our findings identify PINK1-dependent mitophagy as an early vulnerability in KRAS -driven lung cancer and provide a rationale for targeting mitophagy to improve the efficacy of KRAS -targeted therapies.

Fluorizoline is a synthetic molecule that induces apoptosis, by selectively targeting prohibitins (PHBs), through induction of the BH3-only protein NOXA. This induction is transcriptionally regulated by the integrated stress response (ISR)-related transcription factors ATF3 and ATF4. Here, we evaluate the role of the four eIF2α kinases, to decipher which is responsible for the mechanism of ISR activation triggered by fluorizoline in HeLa and HAP1 cells. First, we demonstrated the involvement of the eIF2α kinases using ISR inhibitor (ISRIB) and by simultaneous downregulation of all four eIF2α kinases, as both approaches were able to increase cell resistance to fluorizoline-induced apoptosis. Furthermore, we confirmed that fluorizoline treatment results in endoplasmic reticulum (ER) stress, as evidenced by PERK activation. Despite PERK activation, this kinase was not directly involved in the ISR activation by fluorizoline. In this regard, we found that the eIF2α kinases are capable of compensating for each other’s loss of function. Importantly, we demonstrated that the mitochondrial-stress-related eIF2α kinase HRI mediates ISR activation after fluorizoline treatment.

The prohibitin (PHB)-binding compound fluorizoline as well as PHB-downregulation activate the integrated stress response (ISR) in HEK293T and U2OS human cell lines. This activation is denoted by phosphorylation of eIF2α and increases in ATF4, ATF3, and CHOP protein levels. The blockage of the activation of the ISR by overexpression of GRP78, as well as an increase in IRE1 activity, indicate the presence of ER stress after fluorizoline treatment. The inhibition of the ER stress response in HEK293T and U2OS led to increased sensitivity to fluorizoline-induced apoptosis, indicating a pro-survival role of this pathway after fluorizoline treatment in these cell lines. Fluorizoline induced an increase in calcium concentration in the cytosol and the mitochondria. Finally, two different calcium chelators reduced fluorizoline-induced apoptosis in U2OS cells. Thus, we have found that fluorizoline causes increased ER stress and activation of the integrated stress response, which in HEK293T and U2OS cells are protective against fluorizoline-induced apoptosis.