Aims: The mTOR/S6K1 signaling axis, known for cell growth regulation, is hyper-activated in multiple cancers. In this study, we have examined the mechanisms for ribosomal protein p70-S6 kinase 1 (S6K1) associated transformed human hepatocyte (THH) growth regulation.
Main methods: THH were treated with p70-S6K1 inhibitor and analyzed for cell viability, cell cycle distribution, specific marker protein expression by western blot, and tumor inhibition in a xenograft mouse model. We validated our results by knockdown of p70-S6K1 using specific siRNA.
Key findings: p70-S6K1 inhibitor treatment caused impairment of in vitro hepatocyte growth, and arrested cell cycle progression at the G1 phase. Further, p70-S6K1 inhibitor treatment exhibited a decrease in FAK and Erk activation, followed by altered integrin-β1 expression, caspase 8, and PARP cleavage appeared to be anoikis like growth inhibition. p70-S6K1 inhibitor also depolymerized actin microfilaments and diminished active Rac1/Cdc42 complex formation for loss of cellular attachment. Similar results were obtained with other transformed human hepatocyte cell lines. p70-S6K1 inhibition also resulted in a reduced phospho-EGFR, Slug and Twist; implicating an inhibition of epithelial-mesenchymal transition (EMT) state. A xenograft tumor model, generated from implanted THH in nude mice, following intraperitoneal injection of S6K1 inhibitor prevented further tumor growth.
Significance: Our results suggested that p70-S6K1 inhibition alters orchestration of cell cycle progression, induces cell detachment, and sensitizes hepatocyte growth impairment. Targeting p70 isoform of S6K1 by inhibitor may prove to be a promising approach together with other therapies for hepatocellular carcinoma (HCC) treatment.
Keywords: Actin depolymerization; Anoikis; Hepatocyte growth; S6K1 inhibitor; Transformed human hepatocytes.
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