Copper-induced pro-apoptotic response and its alleviation by Quercetin through autophagic modulation in HEPG2 cells

Recent studies indicated that the liver is susceptible to Cu-induced stress as it stores and distributes the metal to other cellular organelles. To counteract the hepatocytic damage, a known polyphenol, quercetin, was employed for its remarkable antioxidant properties. Thus, the study aimed to assess quercetin's potency against Cu-induced toxicity in HEPG2 cells.

The cellular viability of HEPG2 cells was carried out by MTT assay. All the cellular experiments were divided into control, Cu 100 µM, Cu 100 µM (with Q μM), Cu 300 µM, Cu 300 µM (with Q 50 nM), and only quercetin (50 nM). Following this, reactive oxygen species (ROS) levels and mitochondrial membrane potential (MMP) were evaluated in co-exposure studies. Moreover, rhodamine-123, Hoechst stain, monodansylcadaverine (MDC), and acridine orange (AO) stain were used to visualize the morphological changes under bright field and fluorescent microscopy. Subsequently, western blotting was adopted to determine the expression level of apoptotic and autophagic marker proteins.

Copper increased intracellular ROS, resulted in morphological abnormalities, nuclear condensation, and disrupted MMP. Moreover, Cu caused apoptotic cell deaths characterized by overexpressed pro-apoptotic proteins such as poly (ADP-ribose) polymerase (PARP), cysteine-dependent aspartate-specific proteases 3 (Caspase 3), and Bcl-2-associated X protein (Bax) and downregulated anti-apoptotic proteins such as B-cell lymphoma 2 (Bcl-2), respectively. However, quercetin restored overexpressed pro-apoptotic proteins and induced autophagosome-bound microtubule-associated protein light chain-3 (LC3II) conversion from LC3I. Furthermore, Cu-modulated autophagy marker proteins, including sequestosome-1 (p62), heat shock cognate proteins (Hsc 70, Hsc 90), lysosome-associated membrane protein (LAMP-2A), and AMP-associated protein kinase (AMPK).

This study promotes the nutraceutical ability of quercetin to combat Cu-induced hepatotoxicity by understanding the intricate biological signaling pathways within cells.