Targeting mitochondrial ribosomal protein expression by andrographolide and melatonin for colon cancer treatment

Colospheroids contain colon cancer stem cells (CSCs) that cause colorectal cancer metastasis (mCRC). Colorectal cancer (CRC) is the second leading cause of cancer-related deaths in the U.S. Little is known about the role of mitochondria in the survival and metastatic ability of CSCs. In this study, we investigate the effect of andrographolide (AGP) and melatonin (MLT) on mitochondrial dynamics (including fusion and fission) and the expression of mitochondrial ribosomal proteins (MRPs). Our results show that AGP and MLT synergistically reduce the total active mitochondrial mass, downregulate fusion and fission proteins, reduce OXPHOS proteins, and lead to CSC growth inhibition via Nrf2 and KEAP1 signaling. Microarray revealed 4389 differentially expressed mRNAs in the AGP and MLT combination compared to the control. Results exhibiting a three-fold induction/reduction were validated by qRT-PCR and immunoblot. MRPS6, a mitochondrial ribosomal (Mitoribosome) small subunit protein, was dramatically downregulated by AGP + MLT treatment compared to control. MRPS6 inhibition by siRNA reduced mCRC cell viability. Molecular docking-based protein-ligand interactions showed that AGP has direct physical interaction with MRPS6 and increases the binding affinity of MLT to MRPS6. This drug combination downregulated genes in the NRF2 (NFE2L2) pathway in CSCs. MRPS6 may be directly linked to CSC proliferation and could be a therapeutic target for this population. Functionally, MRPS6 knockdown significantly reduced colony formation, with enhanced suppression in AGP + MLT-treated cells. In xenograft models, the AGP-MLT combination synergistically decreased MRPS6 expression and increased apoptosis, as evidenced by TUNEL assays, demonstrating the therapeutic potential of targeting MRPS6 in CRC.