Metabolic Flexibility in Canine Mammary Tumors: Implications of the Carnitine System

SIMPLE SUMMARY: The process of cell transformation toward a malignant phenotype is generally due to genetic alterations and/or epigenetic changes, as well as rewiring of cellular signaling and reprogramming of metabolic pathways. In addition to glucose metabolism, cancer cells can derive fuel from fatty acid-β-oxidation (FAO), an important alternative bioenergetic pathway that is often dysregulated in cancer. Moreover, FAO enzymes (particularly components of the carnitine system) are overactivated in tumors, suggesting that they serve as metabolic signatures in various cancer cell types. Metabolic changes in carcinogenesis are a focus of current research and have been poorly studied in canine malignancies. We previously reported that CPT1A, the rate-limiting regulator of the FAO process, is deregulated in canine mammary tumor tissues and cells. In the present study, we examined the protein expression of the three remaining components of the carnitine system (CACT, CPT2, and CrAT) and confirmed their expression and deregulation in canine mammary tumor tissues and cells. We also found that low expression of carnitine system components was closely related to the malignancy grade of mammary tumors. Detailed studies to investigate the role of these components in canine mammary tumors are needed to also improve the therapeutic approach in dogs. ABSTRACT: Deregulation of fatty acid catabolism provides an alternative energy source to glycolysis for cancer cell survival and proliferation. The regulator enzymes of the carnitine system (CS), responsible for the transport of fatty acids across mitochondrial membranes for β-oxidation are deregulated in tumorigenesis. Recently, we found that Carnitine Palmitoyl Transferase 1 (CPT1), a crucial regulator of CS components, is expressed and dysregulated in canine mammary tumor (CMT) tissues and cells. In this study, we examined the protein expression of the three remaining enzymes of CS (Carnitine Acylcarnitine Translocase (CACT), Carnitine Palmitoyl Transferase 2 (CPT2), Carnitine O-acetyltransferase (CrAT), in canine mammary cells and tissues by Western blot and immunohistochemistry. Protein expression of the components of CS was found in normal mammary glands and a concomitant deregulation of expression in CMT tissues that inversely correlated with the degree of tumor differentiation. Moreover, the expression and a different deregulation of CS-related proteins was also observed in CF33, CMT-U27, CMT-U309, and P114 cell lines used as in vitro model. These results demonstrate for the first time the expression of CS components in CMT tissues and cancer cells; however, further studies are needed to elucidate their roles in dogs as well.