NEURL1 acts as a candidate suppressor in bladder cancer by down-regulating PDE9A

Bladder cancer is a common malignancy in the genitourinary system with its incidence rate among the world's highest. Neuralized E3 ubiquitin protein ligase 1 (NEURL1) belongs to the RING E3 ubiquitin ligase family, and its role in bladder cancer has not been reported yet. We aimed to explore the expression and roles of NEURL1 in bladder cancer. The NEURL1 expression was determined in clinical samples of bladder cancer. We stably overexpressed NEURL1 and NEURL1 with the RING domain deletion in human bladder cancer cell lines 5637 and RT-112 to investigate its functions. NEURL1 was confirmed to be significantly down-regulated in clinical bladder tumor specimens. NEURL1 overexpression significantly inhibited the growth, colony formation, and Ki-67 protein expression in both bladder cancer cells. The overexpression of NEURL1 also increased the apoptosis rate and cleaved caspase-3 protein expression in 5637 and RT-112 cells. As expected, the RING-deleted NEURL1 had no such effect. Moreover, NEURL1 overexpression facilitated the apoptosis of 5637/RT-112 cells under cisplatin conditions. NEURL1 promotes ubiquitination and proteasomal degradation of PDE9A. PDE9A protein expression was notably increased in bladder tumors from clinical specimens. Overexpressed NEURL1 inhibited PDE9A protein expression in both cell lines, whereas NEURL1 with the RING domain deletion did not. The addition of proteasome inhibitors MG-132 reversed the decrease in PDE9A expression by NEURL1 overexpression. Cell viability was inhibited and apoptosis was increased in the 5637 and RT-112 cells with stable knockdown of PDE9A. NEURL1 may be involved in the bladder cancer progression by increasing apoptosis, thereby leading to tumor cell growth inhibition.