Limonoid-rich fraction from Azadirachta indica A. Juss. (neem) stem bark triggers ROS-independent ER stress and induces apoptosis in 2D cultured cervical cancer cells and 3D cervical tumor spheroids.

Background: The existing anticancer drugs in clinical practice show poor efficacy in cervical cancer patients and are associated with multiple side effects. Our previous study demonstrated the strong antineoplastic activity of crude extract prepared from the stem bark of Azadirachta indica (Neem) against cervical cancer. However, the active phytoconstituents of neem stem bark extract and its underlying anticancer mechanism are yet to be investigated. Thus, the present study aimed to identify the active fraction from crude neem stem bark extract to further dissect its anticancer mechanism and determine the active components.

Methods: Dichloromethane (DCM) extract from neem stem bark was prepared and fractionated using thin-layer chromatography. The fractions obtained were screened against HeLa and ME-180 cervical cancer cell lines to identify the most active fraction, which was then selected for further studies. Clonogenic assay, cell cycle analysis, apoptosis assay, and reactive oxygen species (ROS) assay were performed to determine the cytotoxicity of the active fraction. Gene expression was analyzed using real-time PCR and western blot to determine the mechanism. Additionally, the HeLa cells-derived 3D spheroid model was used to determine the antitumor efficacy of the active fraction. Electrospray ionization-mass spectrometry, Fourier-transform infrared spectroscopy, and proton nuclear magnetic resonance were used to identify the phytoconstituents of the fraction.

Results: Initial screening revealed fraction 2 (F2) as the most active fraction. Additionally, F2 showed the least cytotoxic effect on normal human fibroblast cells. Mechanistically, F2 induced cell cycle arrest and apoptosis in cervical cancer cells. F2 increased ROS levels, induced ER stress, and activated cell survival pathway. Treatment with N-acetyl cysteine revealed that F2 induced ROS-independent ER stress and apoptosis. 3D spheroid viability and growth delay experiments demonstrated the strong antitumor potential of F2. Finally, six compounds, including one flavonoid (nicotiflorin) and five limonoids, were identified in the F2 fraction.

Conclusion: This is the first study to identify the active fraction and its phytoconstituents from neem stem bark and demonstrate the anticancer mechanism against cervical cancer. Our study highlights the importance of investigating neem stem bark-derived limonoids and nicotiflorin as a potential source to develop new anticancer therapeutic agents.