Figure 6.

Naringenin prevents acute and chronic CCl₄-induced liver damage. Carbon tetrachloride (CCl₄) is activated by CYP2E1, CYP2B1, CYP2B2, and CYP3A (CYP) to form the trichloromethyl radical (CCl₃^•); then, it reacts with the oxygen-forming trichloromethylperoxy radical (CCl₃OO^•). The CCl₃OO^• initiates lipid peroxidation (LP), free radical (R^•) generation, and imbalance of the endogenous antioxidant system formed by superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione (GSH), heme oxygenase (OH-1), NADPH quinone oxidoreductase (NQO1) and nuclear factor-erythroid 2 related factor 2 (Nrf2). Naringenin prevents CCl₄ metabolism, LP and imbalance of the antioxidant system. Naringenin also prevents increased levels of alkaline phosphatase (AP), aspartate aminotransferase (AST), alanine aminotransaminase (ALT), and γ-glutamyl transferase (GGT). On the other hand, CCl₄ increases intrinsic and extrinsic apoptosis pathways in hepatocytes; however, naringenin prevents CYPc release, as well as BCL2-associated X protein (Bax), BCL2-antagonist/killer 1 (Bak), Caspase 3 (Cas3) and Caspase 9 (Cas9) elevation, a protein related with the intrinsic pathway. For the extrinsic apoptosis pathway, naringenin prevents Fas and Fas ligand (FasL) increases produced by CCl₄ administration. During CCl₄-induced fibrosis, there is a proinflammatory environment generated by Kupffer cells and HSCs. The NF-κB pathway starts when TLRs are activated; then, intermediates are activated leading to inhibitor κB (IKB) phosphorylation by IκB kinase (IKK) and NF-κB release. NF-κB regulates inflammatory protein expression, including tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), cyclooxygenase-2 (COX-2), interleukin-1 (IL-1) and inducible nitric oxide synthase (iNOS), while naringenin maintains normal levels of these proteins during CCl₄-induced liver damage. Transforming growth factor-β (TGF-β) activates receptor-activated Smad3 (Smad3), leading to transcriptional induction of α-smooth muscle actin (α-SMA), connective tissue growth factor (CTGF), and collagen-1 (Col-1). Moreover, Smad3 is also activated by JNK via linker domain phosphorylation. Naringenin prevents Smad3 activation and α-SMA, CTGF, and Col-1 elevation because it inhibits TGF-β elevation and JNK activation. Metalloproteases (MMPs) cleave extra cellular matrix (ECM) proteins, favoring TGF-β release as well as HSC migration to other sites, increasing fibrosis development; naringenin prevents MMPs elevation. On the other hand, CCl₄ decreases Smad7 protein levels; this protein inhibits the TGF-β signaling pathway by TGF-β receptor I (TβRI) ubiquitination, but nevertheless, naringenin maintains normal levels of Smad7 during CCl₄ treatment.