Natural variations of copper and sulfur stable isotopes in blood of hepatocellular carcinoma patients

Abstract

The widespread hypoxic conditions of the tumor microenvironment can impair the metabolism of bioessential elements such as copper and sulfur, notably by changing their redox state and, as a consequence, their ability to bind specific molecules. Because competing redox state is known to drive isotopic fractionation, we have used here the stable isotope compositions of copper ((65)Cu/(63)Cu) and sulfur ((34)S/(32)S) in the blood of patients with hepatocellular carcinoma (HCC) as a tool to explore the cancer-driven copper and sulfur imbalances. We report that copper is (63)Cu-enriched by ∼0.4‰ and sulfur is (32)S-enriched by ∼1.5‰ in the blood of patients compared with that of control subjects. As expected, HCC patients have more copper in red blood cells and serum compared with control subjects. However, the isotopic signature of this blood extra copper burden is not in favor of a dietary origin but rather suggests a reallocation in the body of copper bound to cysteine-rich proteins such as metallothioneins. The magnitude of the sulfur isotope effect is similar in red blood cells and serum of HCC patients, implying that sulfur fractionation is systemic. The (32)S-enrichment of sulfur in the blood of HCC patients is compatible with the notion that sulfur partly originates from tumor-derived sulfides. The measurement of natural variations of stable isotope compositions, using techniques developed in the field of Earth sciences, can provide new means to detect and quantify cancer metabolic changes and provide insights into underlying mechanisms.

Keywords: cancer; copper; liver; stable isotopes; sulfur.

Fig. 1.

Chemical and biochemical compositions of serum and RBC of HCC patients and controls. (A) Copper concentrations in serum. (B) Copper concentrations in RBC. (C) Cp concentrations in serum. (D) SOD1 concentrations in serum. (E) Copper isotope composition of serum. (F) Copper isotope composition of RBC. (G) Sulfur isotope composition of serum. (H) Sulfur isotope composition of RBC. For all panels, *P = 0.01–0.05, **P = 0.001–0.01, and ***P < 0.001.

Fig. 2.

Copper and sulfur isotope systematics in blood. (A) Copper isotope compositions in serum as a function of that of RBC. The black line stands for the least square correlation for all samples (y = 0.504(±0.115)x − 0.316(±0.088), R² = 0.335, P < 10⁻⁴). (B) Sulfur isotope compositions in serum as a function of that of RBC. The black line stands for the least square correlation for all samples (y = 1.051(±0.084)x − 0.161(±0.406), R² = 0.863, P*** < 10⁻⁴). (C) Sulfur isotope compositions as a function of that of copper in RBC. (D) Sulfur isotope compositions as a function of that of copper in serum.