SIHA2 Promotes Hepatocellular Carcinoma Progression by Mediating Ubiquitination and Degradation of DDIT4.

DNA Damage-Inducible Transcript 4 (DDIT4), a conserved stress-responsive protein with dual nuclear and cytoplasmic localization, has recently emerged as a critical regulator in multiple cancer types. However, its functional role and molecular mechanisms in hepatocellular carcinoma (HCC) remain poorly understood. In this study, we systematically investigated DDIT4's biological significance through comprehensive in vitro analyses. Clinical specimen analysis revealed significant downregulation of DDIT4 protein in HCC tumor tissues compared to adjacent non-tumor controls. Functional studies demonstrated that DDIT4 knockdown markedly enhanced HCC cell proliferation, whereas its overexpression exerted potent anti-proliferative effects. Mechanistically, our in vitro experiments revealed two key regulatory pathways: (1) DDIT4 overexpression suppressed AKT/mTOR signaling activation, and (2) DDIT4 underwent ubiquitin-mediated proteasomal degradation via specific interaction with the E3 ligase Seven in Absentia Homologue 2 (SIAH2). These findings establish DDIT4 as a tumor-suppressive protein in HCC pathogenesis, whose oncogenic downregulation is mediated through SIAH2-dependent ubiquitination. The resultant decrease in DDIT4 protein levels creates a permissive microenvironment for tumor growth by releasing AKT/mTOR pathway inhibition. Our results nominate DDIT4 as a promising therapeutic target for HCC intervention. Further preclinical and clinical investigations are warranted to validate DDIT4's translational potential and explore targeted strategies to stabilize its tumor-suppressive functions.