Liver cancer patients are usually asymptomatic in the early stages of the disease and, therefore, the disease is often diagnosed in the advanced stages, making the treatment more challenging. In addition to chemotherapy, the most widely used systemic treatment for liver cancer is sorafenib, a multi-kinase inhibitor. However, despite sorafenib antitumor efficacy, cancer cells frequently develop resistance, resulting in disease progression. Predicting sorafenib efficacy and early identifying the emergence of sorafenib resistance is thus critical to adapt and optimize treatment, by personalizing therapeutic strategies.
In a recent paper by Silvia Pedretti et al., the authors headed by Nico Mitro -Group leader at the department of experimental oncology of IEO and professor at the University of Milan-, by exploring the mitochondria-related mechanisms underlying cancer cell response to sorafenib and emergence of therapy resistance, identified biomarkers allowing to monitor cancer cell response to therapy, predicting the emergence of therapy resistance.
Indeed, they demonstrated that sorafenib exerts its anticancer activity by interfering with mitochondrial function (specifically, interfering with oxidative phosphorylation by disrupting the assembly of the electron transport chain). Sorafenib-induced loss of mitochondrial function leads to cancer cell adaptation, through a deep metabolic reprogramming (towards glycolysis). However, the metabolic reprogramming leads to the accumulation of toxic metabolism byproducts, in turn inducing cancer cell death (by ferroptosis). Conversely, sorafenib-resistant cells deal with the accumulation of toxic metabolism byproducts by differently reprogramming their metabolism, successfully coping with oxidative stress, modulating lipid metabolism, ultimately preserving cell membrane integrity and avoiding cell death. The metabolic changes induced by sorafenib treatment alter the blood levels of molecules such as D-lactate and glycerol. These molecules can therefore serve as biomarkers of liver cancer cell response to sorafenib, indicating efficacy and resistance, respectively. Within a precision medicine frame, these results provide knowledge that, if applied in a clinical setting, may allow for a more personalized treatment approach of liver cancer patients.
Sorafenib interferes with mitochondrial activity. Mitochondrial function is known to be inhibited by sorafenib. By using sorafenib-resistant liver cell lines, the authors showed that sorafenib indeed inhibited electron transport chain activity, thus reducing mitochondrial function (despite slightly 20 increasing mitochondrial DNA content) and damaging the mitochondrial network, overall shifting mitochondrial metabolism from OxPhos to glycolysis.
Sorafenib-resistant cells change their metabolism to avoid ferroptosis-mediated cell death. Sorafenib-resistant cancer cells managed to survive sorafenib-elicited mitochondrial dysfunction, through a deep metabolic reprogramming/adaptation. Indeed, the authors showed that in sorafenib-sensitive cells, treatment-induced metabolism reprogramming (from Oxphos to glycolysis) led to the accumulation of byproducts of glycolysis, which became toxic. One of the byproducts of glycolysis is D-lactate, which is normally at low levels in cells but increased upon sorafenib treatment, inducing (ferroptosis-mediated) cell death. Conversely, sorafenib-resistant cells were able to cope with high D-lactate-related toxicity by redirecting the precursors of this toxic metabolite towards other metabolic routes, thus reducing its intracellular concentration and suggesting that higher extracellular D-lactate levels may represent a biomarker of sensitivity to sorafenib.
Consistently, in liver cancer patients treated with sorafenib, D-lactate levels in the blood were higher in responders, indicating that measuring D-lactate levels can be a way to monitor sensitivity to sorafenib.
Sorafenib-resistant cells efficiently manage oxidative stress and modify lipid metabolism to avoid ferroptosis-mediated cell death. The metabolic reprogramming of Sorafenib-resistant cells included an increased antioxidant activity, which contributed to reduce oxidative levels. Disrupting these sorafenib-induced antioxidant pathways sensitized sorafenib-resistant cells to sorafenib treatment. The increased antioxidant ability, along with the lipid metabolism reprogramming, contributed to sorafenib-resistant cancer cells ability to avoid ferroptosis-mediated cell death, survive and become resistant. Right after sorafenib treatment, cells underwent an increase in oxidative stress and membrane lipid peroxidation -a marker of ferroptosis; however, with the emergence of sorafenib resistance, the increase of antioxidant defense led to a reduction of overall oxidative stress, and cancer cell survival.
In-depth studies of the role of lipid metabolism in this mechanism showed that, upon sorafenib treatment, cancer cells significantly modified membrane lipid composition, by interfering with lipid metabolism, to avoid ferroptosis in order to survive and become resistant, suggesting that lipid metabolism may represent a therapeutically exploitable cancer cell vulnerability.
Interestingly, sorafenib-resistant cells failed to produce endogenous lipids required for the synthesis of lipid membranes, thus requiring exogenous lipids to survive. In particular, some specific lipids -such as triglycerides and free-fatty acids rather than cholesterol- contributed to increase cell resistance to sorafenib. Central to this process is the ability of sorafenib-resistant cells to divert glycolytic products toward glycerol rather than D-lactate, with glycerol serving as the backbone for membrane phospholipid synthesis.
Metabolic biomarkers of sorafenib resistance. Finally, they observed higher levels of glycerol in the culture medium of sorafenib-resistant cells and higher levels of glycerol in the blood of sorafenib-treated patients not responding to treatment, underlining the potential role of glycerol as biomarker of sorafenib resistance in liver cancer, underscoring the critical role of lipid metabolism in this phenotype.