The activation of mTOR independent autophagy in kidney carcinoma cells by the upregulation of miR501-5p occurs through the decrease of mitochondrial calcium uptake

INTRODUCTION & OBJECTIVES: The most common subtype of kidney cancer is the clear cell renal cell carcinoma (ccRCC) that accounts for 70-80% of cases. Many factors may contribute to the development and progression of ccRCC including autophagy. This process may provide energy for cancer cells under unfavorable conditions, therefore it could be associated with cancer expansion in ccRCC. We have observed that miR501-5p is upregulated in ccRCC cells and tissues, and contributes to cancer progression. Moreover, kidney cancer cells that overexpress the microRNA501-5p shows increased levels of autophagy. Thus, the study of autophagy could clarify the role of this process in kidney cancer. MATERIAL & METHODS: Autophagy was analysed in normal and ccRCC cells by Western blotting evaluating the expression of the microtubule-associated protein light chain 3 (LC3), usually used as autophagic marker. The overexpression of miR501-5p was obtained by cell transfection with a plasmid expressing specific sequences of this miR (PL501). Mitochondrial calcium uniporter (MCU) gene expression in kidney cells overexpressing miR501-5p was analyzed by Western blotting. The activity of AMP kinase (AMPK) was evaluated by Western blotting and mitochondrial calcium uptake was measured by a luminometer using specific mito-aequorin probes. RESULTS: Generally, cancer cells show higher levels of autophagy than normal cells. Consistently, we have found that different ccRCC cell lines that express high levels of miR501-5p, have a greater activation of autophagy than control kidney cells. Moreover, the upregulation of this miR increases the mTOR independent autophagy by the stimulation of AMP kinase. This process could be associated with a low energy availability or due to the reduction of mitochondrial activity caused by the reduction of calcium uptake. In fact, miR501-5p that targets MCU mRNA, induces a strong reduction of MCU expression in kidney cells transfected with the PL501plasmid. Consistently, a reduction of mitochondrial calcium levels after ATP stimulation in kidney cells transfected with PL501 was found. CONCLUSIONS: The upregulation of miR501-5p that is associated with kidney cancer progression, stimulates the mTOR independent autophagy by the reduction of mitochondrial calcium uptake. Autophagy may contribute to poor prognosis in ccRCC and its inhibition could open new opportunities for the treatment of this cancer.