Quiescent cancer cells (QCCs), also known as dormant cancer cells, resist and survive chemo- and radiotherapy, resulting in treatment failure and later cancer recurrence when QCCs resume cell cycle progression. However, drugs selectively targeting QCCs are lacking. Saikosaponin A (SSA) derived from Bupleurum DC., is highly potent in eradicating multidrug-resistant prostate QCCs compared with proliferative prostate cancer cells. By further exacerbating the already increased autophagy through inactivation of Akt-mTOR signaling, SSA triggered cell death in QCCs. Contrarily, inhibition of autophagy or activation of Akt signaling pathway prevented SSA-induced cell death. The multicycle of Docetaxel treatments increased the proportion of QCCs, whereas administering SSA at intervals of Docetaxel treatments aggravated cell death in vitro and led to tumor growth arrest and cell death in vivo. In conclusion, SSA is posed as a novel QCCs-eradicating agent by aggravating autophagy in QCCs. In combination with the current therapy, SSA has potential to improve treatment effectiveness and to prevent cancer recurrence.

Background: Gemcitabine (GEM) is the first-line chemotherapeutic drug used to treat pancreatic ductal adenocarcinoma carcinoma (PDAC), but chemoresistance is often encountered clinically. Nrf2, an oxidative stress responsive transcription factor, is an important contributor to chemoresistance and poor prognosis of PDAC. Brucein D (BD), a naturally occurring quassinoid, has been reported to exert anti-tumor effect in several cancers including PDAC. In this study, we aimed to investigate the efficacy of BD and the role of Nrf2 axes on the chemosensitivity of GEM and elucidate the underlying molecular mechanisms. Methods: Analyses of clinical samples of PDAC and GEPIA database were first conducted to identify the expression of Nrf2 in PDAC. We then established cell lines with stable deletion of Nrf2 through transfecting lentivirus into PDAC cells. Quantitative real-time PCR (qRT-PCR) and Western blotting were performed to determine the expression of Nrf2 in these cell lines. The effects of BD and Nrf2 axes on PDAC cell proliferation, colony-formation, tumor growth and chemosensitivity were determined both in vitro and in vivo. Orthotopic xenograft and genetically engineered KPC mouse models of PDAC were used to evaluate the anti-pancreatic cancer effects of BD and GEM. Results: Nrf2 was highly expressed in PDAC in the clinical samples and GEPIA analysis. Gain- and lost-function study demonstrated that Nrf2 affected the chemosensitivity of GEM on PDAC cells both in vitro and in vivo. We further found that BD effectively inhibited PDAC cell proliferation and enhanced the chemosensitivity of GEM. Mechanistic studies revealed that BD sensitized GEM in PDAC cells through the ubiquitin–proteasome-dependent degradation of Nrf2, and downregulating the Nrf2 pathway. Silencing of Nrf2 plus BD treatment resulted in more potent inhibitory effects of GEM. In contrast, Nrf2 activation attenuated the chemosensitivity of GEM, indicating that the action of BD was Nrf2 dependent. Finally, the efficacy of BD alone and in combination with GEM on PDAC was validated on both orthotopic xenograft and genetically engineered KPC mouse models. Conclusions: BD was able to enhance the chemosensitivity of GEM in PDAC through inhibition of the Nrf2 pathway. Our experimental findings indicate that BD, a potent Nrf2 inhibitor, holds promise for further development into a novel adjuvant therapy for PDAC.

Quiescent cancer cells (QCCs) are known to resist chemoradiotherapy, evade immune surveillance and have the potential to drive recurrence years after initial treatment. However, the key regulators of QCC survival during reactivation remain unclear. This study revealed that superoxide dismutase 2 (SOD2) levels are significantly greater in quiescent prostate cancer (PCa) cells than in proliferative cells. SOD2 overexpression induces apoptosis in awakening quiescent PCa cells, whereas its knockdown promotes reactivation. Elevated SOD2 also suppresses recurrent tumor growth by quiescent PCa cells and prolongs survival. Pterostilbene (PTE), a natural compound, preferentially induces apoptosis in quiescent PCa cells during awakening and reduces their long-term proliferative capacity by upregulating SOD2. Additionally, PTE inhibits tumorigenesis and significantly reduces the growth of quiescent PCa cells without apparent toxicity. Further mechanistic studies revealed that CCAAT/enhancer-binding protein beta (C/EBP-β) is critical for PTE-mediated SOD2 upregulation by enhancing SOD2 transcription. C/EBP-β knockdown significantly reduces PTE-induced apoptosis in awakening quiescent PCa cells. Clinical analysis revealed a positive correlation between CEBPB and SOD2, with low C/EBP-β expression linked to poor prognosis. Overall, the C/EBP-β-SOD2 pathway is crucial for eliminating awakening quiescent PCa cells and highlights PTE as a promising agent for preventing PCa recurrence.