Depletion of NBR1 in urothelial carcinoma cells enhances rapamycin‐induced apoptosis through impaired autophagy and mitochondrial dysfunction

Abstract

Rapamycin is well‐recognized in the clinical therapeutic intervention for patients with cancer by specifically targeting mammalian target of rapamycin (mTOR) kinase. Rapamycin regulates general autophagy to clear damaged cells. Previously, we identified increased expression of messenger RNA levels of NBR1 (the neighbor of BRCA1 gene; autophagy cargo receptor) in human urothelial cancer (URCa) cells, which were not exhibited in response to rapamycin treatment for cell growth inhibition. Autophagy plays an important role in cellular physiology and offers protection against chemotherapeutic agents as an adaptive response required for maintaining cellular energy. Here, we hypothesized that loss of NBR1 sensitizes human URCa cells to growth inhibition induced by rapamycin treatment, leading to interruption of protective autophagic activation. Also, the potential role of mitochondria in regulating autophagy was tested to clarify the mechanism by which rapamycin induces apoptosis in NBR1‐knockdown URCa cells. NBR1‐knockdown URCa cells exhibited enhanced sensitivity to rapamycin associated with the suppression of autophagosomal elongation and mitochondrial defects. Loss of NBR1 expression altered the cellular responses to rapamycin treatment, resulting in impaired ATP homeostasis and an increase in reactive oxygen species (ROS). Although rapamycin treatment‐induced autophagy by adenosine monophosphate‐activated protein kinase (AMPK) phosphorylation in NBR1‐knockdown cells, it did not process the conjugated form of LC3B‐II after activation by unc‐51 like autophagy‐activating kinase 1 (ULK1). NBR1‐knockdown URCa cells exhibited rather profound mitochondrial dysfunctions in response to rapamycin treatment as evidenced by Δψm collapse, ATP depletion, ROS accumulation, and apoptosis activation. Therefore, our findings provide a rationale for rapamycin treatment of NBR1‐knockdown human urothelial cancer through the regulation of autophagy and mitochondrial dysfunction by regulating the AMPK/mTOR signaling pathway, indicating that NBR1 can be a potential therapeutic target of human urothelial cancer.

LncRNA OTUD6B-AS1 promotes paclitaxel resistance in triple negative breast cancer by regulation of miR-26a-5p/MTDH pathway-mediated autophagy and genomic instability

“…Finally, we identified all of those three miRNAs were GIN-related miRNAs. As showed in Figure 2A and 2B , let-7b-3p was positive co-expression with BCL2 (R = 0.327, P < 0.001) and NBR1 (R = 0.318, P < 0.001), among which the BCL2 was reported to inhibit autophagy [ 14 , 15 ], while NBR1 was reported to promote autophagy [ 16 , 17 ]. miR-26a-5p and HSP-A8 were negative co-expression (R = −0.329, P < 0.0001), which was an autophagy inhibitor [ 18 ]; miR-151a-5p and GRID1 (R = −0.291, P < 0.0001) and FOXO1 (R = −0.291, p < 0.0001) were negative co-expression, amongst which FOXO1 was reported as an autophagy promotor [ 19 ]; while miR-151a-5p was positive co-expression with RHEB (R = 0.358, p < 0.0001), BIRC5 (R = 0.316, p < 0.0001) and HSP90AB1 (R = 0.391, p < 0.0001), among which RHEB was reported as an autophagy promotor [ 20 ], while BIRC5 and HSP90AB1 were reported as autophagy inhibitor [ 21 , 22 ].…”

Cucurbitacin B and cisplatin induce the cell death pathways in MB49 mouse bladder cancer model

“…32 AMPKa also stimulates autophagy by inhibiting mTOR kinase. 32,33 Low AMPKa phosphorylation is also seen in bladder cancer. 34 AKT activates mTOR by phosphorylating it, and then activated mTOR suppresses autophagy.…”

Contrast effects of autophagy in the treatment of bladder cancer

“…Kim et al. 46 stated that the deterioration in NRB1 functioning restrained autophagosome formation and disrupted the level of p62 protein and autophagosome turnover. They reported that rapamycin activated protective autophagy by increasing the amount of NRB1 in bladder cancer cell lines.…”