4. Carnitine and Cancer
Patients with cancer were found to be susceptible to carnitine deficiency. The caloric intake of cancer patients is often impaired, while their metabolic demands are increased. Aside from that, pharmacological therapy in cancer patients can interfere with carnitine synthesis, absorption, and excretion [89, 90]. Carnitine deficiency has been reported in chronic illnesses such as cancer [91]. Decreased serum levels of carnitine have been detected in multiple cancers, including endometrial cancer, breast cancer, CML, and pediatric cancer [92–95]. Numerous studies have reported beneficial effects of carnitine in patients with advanced cancer. In a literature review by Radkhouy et al. [96]
, the beneficial anticancer effect of carnitine was revealed in colon cancer, as evidenced by the prevention of tumor growth. Baci et al. showed that the administration of acetylcarnitine had an angiopreventive effect on prostate cancer cells. Aberrant expression of cytokines/chemokines in prostate cancer can govern progression, invasion, and angiogenesis [97]. High expression of chemokine receptor 4 (CXCR4), an angiogenic factor, is associated with metastatic behavior and poor survival. Acetylcarnitine exerts its anticancer effect by acting on the cytokine/chemokine axis of prostate cancer [98, 99].
Cachexia is a multifactorial syndrome characterized by loss of skeletal muscle mass with or without loss of fat mass. This condition cannot be fully compensated by conventional nutritional support, resulting in progressive functional defects in these patients. In a study done by Mitchell et al. [100], it was shown that pancreatic cancer patients exhibit cachexia at the time of diagnosis.
Patients with cancer cachexia are resistant to dietary interventions; however, carnitine supplementation could improve the quality of life and body mass. Impairment of FAO can be attributed to the reduced activity of CPTI and CPTII in the liver. CPTI and CPTII play a vital role in the development of cancer cachexia. Accumulating evidence has revealed the importance of carnitine molecules in fatty acid metabolism. In cancer cachectic mice, Liu et al. [101] found a decreased levels of serum-free carnitine and acetylcarnitine with downregulated mRNA levels of CPTI and CPTII. In addition, a hepatic reduction in CPTI activity was detected. According to their results, oral administration of carnitine at a dose of 18 mg/kg significantly restored CPT activity and downregulated the serum levels of interleukin-6 (IL-6) and TNF-
αin animal models. With this respect, it can be assumed that carnitine-mediated amelioration is associated with CPT regulation in the liver [101].
In a cachectic mouse model of colon cancer, Jiang et al. [102] showed that oral administration of carnitine at a dose of 9 mg/kg/day ameliorated the cachexia parameters. Carnitine can also decrease the elevated serum levels of IL-6 and TNF-
α in cancer cachectic mice [102]. Data from a similar recent study have indicated the potential benefits of carnitine in cancer therapy.
Their findings revealed that carnitine improved cancer cachexia in an animal model through the Akt/FOXO3/MaFbx and p70S6K pathways. Carnitine also decreased IL-1 and IL-6 serum levels, which are responsible for the progression of cancer-associated cachexia [63]. In addition, it has been shown that carnitine administration can alleviate disorders of lipid metabolism. Beyond this, carnitine can decrease the serum levels of hepatic enzymes, including aspartate aminotransferase (AST), alanine aminotransferase (ALT), and triglyceride (TG), which are significantly elevated during irregular feeding in cancer patients [103].
Metabolic reprogramming and increased ATP demand are well-established hallmarks of cancers [104]. FAO plays a crucial role in providing ATP, NADH, FADH2, and NADPH, thus providing survival benefits to cancer cells. CPTI is a rate-limiting FAO enzyme that contributes to cancer metabolic adaptation, and its overexpression can fuel tumor growth in numerous tumor types [105]. CPTI can crosstalk with various cellular signaling pathways involved in cancer pathogenesis. In this regard, inhibition of CPTI may suppress cancer development [106]. From these studies, it can be inferred that carnitine has a beneficial impact on the management of cancer symptoms.
As discussed earlier, cancer cells require more energy than normal cells. In other words, energy demand increases with tumor aggressiveness and malignancy [107]. Normal cells meet their energy requirements through TCA and oxidative phosphorylation in the mitochondria (Figure
3) [108]. Under aerobic conditions, normal cells meet their energy demands through glycolysis in the cytosol, followed by oxidative phosphorylation within the mitochondria. Cancer cells alter their metabolism to support growth, survival, proliferation, and long-term maintenance [109].
Indeed, cancer cells prefer to obtain energy from glycolysis even in the abundance of oxygen, a phenomenon referred to as the “Warburg effect.” Glycolysis is much faster (100 times) than oxidative phosphorylation, even though the energy production is much lower. These events occur in the cytosol, even in the presence of functional mitochondria and abundant oxygen. Cancer cells bypass the mitochondrial respiratory chain, which synthesizes ATP. Such metabolic reprogramming has been observed in various cancer types. [110–112]. In addition to aerobic glycolysis, cancer cells can also stimulate fatty acid biosynthesis and glutamine consumption. Glutamine is considered the second crucial growth-supporting substrate in cancer cells. During metabolic adaptation, most cancer cells utilize glucose and glutamine as their primary carbon sources [113]. In cancer cells, mitochondrial function is not entirely impaired, and oxidative phosphorylation and TCA are still functioning [114]. In addition, there is increasing evidence that
some cancers exhibit dual capacities for glycolysis and oxygen-consuming metabolism. Notably, metabolic flexibility exists in diverse cancers and cancers of the same type but at various stages. Metabolic plasticity can promote cancer cells growth, invasion, and metastatic behavior [16].