Rapamycin enhances growth inhibition on urothelial carcinoma cells through LKB1 deficiency‐mediated mitochondrial dysregulation

Abstract

Rapamycin, a mammalian target of rapamycin (mTOR) inhibitor, has significant potential for application in the treatment of urothelial carcinoma (URCa) of the bladder. Previous studies have shown that regulation of the AMP‐activated serine/threonine protein kinase (AMPK)–mTOR signaling pathway enhances apoptosis by inducing autophagy or mitophagy in bladder cancer. Alteration of liver kinase B1 (LKB1)‐AMPK signaling leads to mitochondrial dysfunction and the accumulation of autophagy‐related proteins as a result of mitophagy, resulting in enhanced cell sensitivity to drug treatments. Therefore, we hypothesized that LKB1 deficiency in URCa cells could lead to increased sensitivity to rapamycin by inducing mitochondrial defect‐mediated mitophagy. To test this, we established stable LKBI‐knockdown URCa cells and analyzed the effects of rapamycin on their growth. Rapamycin enhanced growth inhibition and apoptosis in stable LKB1‐knockdown URCa cells and in a xenograft mouse model. In spite of the stable downregulation of LKB1 expression, rapamycin induced AMPK activation in URCa cells, causing loss of the mitochondrial membrane potential, ATP depletion, and ROS accumulation, indicating an alteration of mitochondrial biogenesis. Our findings suggest that the absence of LKB1 can be targeted to induce dysregulated mitochondrial biogenesis by rapamycin treatment in the design of novel therapeutic strategies for bladder cancer.

Fine-tuning of AMPK–ULK1–mTORC1 regulatory triangle is crucial for autophagy oscillation

“…AMPK-P is able to phosphorylate ULK1 at Ser-555 followed by an intensive autophagic process 23 , 28 . Parallel with our results, in other studies an increased amount of the phosphorylated AMPK was observed upon rapamycin treatment 31 . However, another experimental data suggested that ULK1 and autophagy activation occurred in an AMPK independent manner 14 and AMPK activation was not detected at all 32 , 33 .…”

“…Interestingly, in some studies AMPK phosphorylation was detected upon rapamycin-dependent down-regulation of mTORC1 23 , 31 , while other scientists were not able to detect AMPK activation during rapamycin treatment 32 , 33 . However, in these cases samples were taken only at the end of treatment.…”

mTORC1 as a Regulator of Mitochondrial Functions and a Therapeutic Target in Cancer

“…The inhibition of mTORC1 with rapamycin, neutralized the roles of BCAT1 in mitochondrial function and breast cancer cell growth (143). Recently, it was shown that rapamycin, enhanced the processes of apoptosis and initiation of autophagy in LKB1 deficient urothelial carcinoma of the bladder both in vitro and in vivo, which was associated with deregulated mitochondrial biogenesis and AMPK activation (144). These results are relevant because AMPK is an important regulator of mitochondrial biogenesis via PGC1-α (145), which also inhibits the mTORC1 pathway.…”

Role of mitochondrial quality control in the pathogenesis of nonalcoholic fatty liver disease

“…Tight regulation of mitochondrial fission and fusion is required to constantly adapt to altering physiological needs [18][19][20]. When individual mitochondria or their constituents such as OXPHOS protein complexes and lipids are irreversibly damaged, they are degraded through mitophagy, a lysosome-dependent proteolytic system in order to maintain cellular homeostasis [21][22][23][24]. When sustained oxidative insults overwhelm the MQC mechanisms, it will result in significant mitochondrial injury that will detrimental to the function and survival of the hepatocytes.…”