Bladder cancer is one of the ten most common malignant tumors worldwide and the fourth most common malignancy in men . Currently, there are 1.65 million cases of bladder cancer, with 550,000 new cases diagnosed each year . The highest incidence of bladder cancer is observed in Europe, North America, Australia, New Zealand, and other developed countries and regions . With the rise in the general and aging populations, the number of diagnosed bladder cancers will continue to increase in the future . Depending on the severity of the stage and grade of bladder cancer, treatment options may include surgery (e.g., transurethral resection of bladder tumor; TURBT), Bacillus Calmette-Guerin (BCG), chemotherapy, or radiotherapy . However, current treatment methods for bladder cancer still have several disadvantages. For example, after radical resection of bladder cancer, the quality of life and sexual function of patients are often seriously affected . The combination of TURBT with BCG treatment has been used in clinical application for 40 years, but the mechanism of BCG treatment is unclear. This treatment fails in some patients due to adaptive immune resistance, and the recurrence rate within two years is 40–60% [3, 4]. Besides, BCG also has the potential risk of local or systemic infection complications and multi-system diseases [5–7]. Existing chemotherapy and radiotherapy methods also have drawbacks such as high toxicity, high side effects, poor efficacy, and chemoresistance . Therefore, there is an urgent need to develop new anti-cancer drugs with low toxicity and high efficiency for bladder cancer therapy and the prevention of recurrence.
Anti-cancer phytochemicals, a type of medicinal and food homologous substances, have attracted the attention of scientists in the fields of pharmacology and oncology . Many anti-cancer natural compounds derived from plant sources, such as resveratrol, camptothecin, capsaicin, and catechin, have been extensively studied [10, 11]. Moreover, paclitaxel and vincristine have been clinically applied as first-line drugs in cancer therapy [12, 13]. Corosolic acid (CA) is a pentacyclic triterpenoid that is found in kiwi, crape myrtle, loquat, and other plants. It has been discovered that CA has a variety of biological activities, including anti-oxidation, regulation of blood sugar levels, and anti-fungal and anti-tumor properties . Owing to these benefits, CA has been widely used as a health food supplement [15, 16]. More importantly, due to its significant role in diabetes treatment, CA is known as "plant insulin" . CA has been reported to inhibit certain protein tyrosine phosphatases to enhance insulin receptor β phosphorylation and stimulate glucose metabolism, which in turn decreases blood sugar levels .
In addition, recent progress has been made in anti-tumor studies involving CA. For instance, CA inhibits colorectal cancer cell growth by suppressing the PI3K/Akt/PKA signaling pathway through binding to the outer domain of HER3 and forming stable hydrogen bonds with Gly515, Arg444, Ser412, and Pro512 . Moreover, Wang et al. reported that CA dose-dependently suppressed Y-79 retinoblastoma cell growth by blocking the cell cycle and inducing apoptosis by targeting maternal embryonic leucine zipper kinase (MELK) and forkhead box M1 (FoxM1) . The anti-cancer function of CA in urinary system tumors has also been studied. CA has been found to inhibit TRAMP-C1 prostate cancer cell growth by decreasing the methylation level of nuclear factor erythroid 2-related factor 2 (Nrf2) promoter CpG sites, resulting in the upregulation of both mRNA and protein levels of Nrf2. In addition, CA can induce the expression of heme oxygenase-1 (HO-1) and NADH quinone oxidoreductase 1 (NQO1), suppressing the transformation of TRAMP-C1 cells . However, the inhibitory effect of CA on bladder cancer and its underlying molecular mechanism remains unknown. In the present study, using cell proliferation, colony formation, and DNA synthesis assays, we found that CA significantly inhibited bladder cancer cell growth and verified the anti-cancer efficacy of CA. Using transcriptome and proteome analysis, we revealed a panoramic view of the mechanism by which CA inhibits bladder cancer: CA does not induce apoptosis but represses DNA replication and mitosis via significant downregulation of cell multiplication-related molecules TOP2A, LIG1, CCNA2, CCNB1, and CDC20. Interestingly, high concentrations of CA can lead to cell death in bladder cancer cells through induction of mitophagy by upregulating NBR1, SQSTM1/P62, UBB, and LC3. This study provides a comprehensive insight into the anti-cancer effects, toxicology, and pharmacological mechanisms of CA, which would provide compelling support for the development of anti-tumor drugs based on CA.