Globally, hepatocellular carcinoma (HCC) is the fifth most common neoplasm and the third leading cause of cancer-related mortality, with a 5-year survival rate of about 18 % [[1], [2]]. Patents diagnosed at early stage may receive radical treatments, such as surgical resection, radiofrequency or microwave ablation and liver transplantation. However, most patients are diagnosed at intermediate or advanced stages. Even after radical treatments, the recurrence rates are relatively high (∼70 % at 5-year) [[3], [4]]. Therefore, systematic therapies still dominate the prognosis of advanced HCC [5].

Tyrosine kinase inhibitors (TKIs), such as sorafenib and Lenvatinib, are first-line therapies for HCC. Sorafenib was the first approved targeted drug for advanced HCC [6]. However, it was not until 2018 that the second systemic treatment drug, Lenvatinib was approved [7]. However, the efficacy of Lenvatinib is still limited. For example, the objective response rate (ORR) and time to progression (TTP) of Lenvatinib are only 40 % and 7.4 months, respectively [7]. Furthermore, most patients are unable to attain enduring clinical advantages as a result of acquired resistance. Therefore, there is an urgent requirement for the potential combination strategies to enhance the effectiveness of Lenvatinib.

Arsenic trioxide (ATO), a traditional Chinese medicine with a history of over 2000 years, has also demonstrated efficacy as a medication for advanced HCC [8]. ATO exerts anti-HCC effects through multiple mechanisms, inducing cell cycle arrest and apoptosis, suppressing tumor stem cells, as well as hampering tumor neovascularization [9]. Wang et al. [10] found that ATO can enhance the anti-HCC activity of sorafenib in some HCC cell lines; whereas the mechanisms have not been clearly elucidated.

Ferroptosis, induced by iron-dependent lipid peroxide damage, plays important roles in the development and treatment of HCC [11,12]. Interestingly, it has been reported that ATO is able to trigger ferroptosis in several malignant tumors, including lung adenocarcinoma, neuroblastoma and HCC, et al. [[13], [14], [15]]. ATO may upregulate acyl-CoA synthetase long-chain family member 4 (ACSL4) [13], downregulate glutathione peroxidase 4 (GPX4) [14], and inhibit N6-methyladenosine [15]. Increased ferroptosis sensitizes HCC to sorafenib, whereas inhibiting ferroptosis leads to sorafenib resistance [[16], [17]]. However, the mechanisms of ATO-induced ferroptosis are still largely uncovered and whether ATO may enhance the anti-HCC effect of Lenvatinib through inducing ferroptosis is still unknown. Therefore, we further examined the combined effect of ATO and Lenvatinib and explored the mechanisms of ATO through inducing ferroptosis.