Subcellular redistribution of Rhein from whole cellular to single mitochondria for enhancing anti-hepatoma efficacy.

Background: RECQL4 is a critical factor in DNA repair and serves as a significant biomarker for liver cancer. Inhibiting RECQL4 induces apoptosis in liver cancer cells. Given the role of mitochondria in regulating apoptosis and the dual localization of RECQL4 in both the mitochondria and nucleus, targeting mitochondrial RECQL4 may induce mitochondrial dysfunction, thereby enhancing the therapeutic efficacy of liver cancer treatments.

Purpose: Unlike traditional drug structure optimization, which focuses solely on protein target interactions, our approach optimizes drug structure based on both organelle distribution and protein interactions.

Methods: As a proof of concept, we selected Rhein (4,5-dihydroxyanthraquinone-2-carboxylic acid), an auto-fluorescent, lipophilic anthraquinone found in a variety of different medicinal herbs used in TCM that is known to bind to RECQL4, a protein that is distributed throughout the cytosol, mitochondria, and other subcellular locations, albeit unevenly. Rhein has the tendency to accumulate in different subcellular organelles, which reduces the overall efficacy of the drug. To overcome this limitation, we conjugated a triphenylphosphonium group Rhein to direct the drug to the mitochondria, resulting in Rh-Mito.

Results: Our results demonstrated that this chemical modification retains the ability to intervene in the expression of RECQL4 protein while also improving the cellular uptake of Rh-Mito. Utilizing advanced imaging techniques, we show that Rh-Mito preferentially accumulated in the mitochondria, where it targeted RECQL4 to inhibit the repair of mtDNA, altered the morphology of mitochondrial cristae, and induced apoptosis. Notably, Rh-Mito displayed superior anti-tumor efficacy compared to unmodified Rhein.

Conclusion: Rh-Mito was designed to selectively target mitochondria, avoiding interactions with other organelles. This modification strengthens its binding to RECQL4, disrupts mitochondrial cristae, inhibits tumor cell migration, and promotes apoptosis, thereby improving its therapeutic efficacy in liver cancer treatment.