Creatine is an extensively researched molecule playing a critical role in the phosphocreatine (ATP-PCr) energy system. Historically lauded as an ergogenic aid, creatine’s benefits have since expanded to general health aspects, including bone metabolism, cognitive performance, and immune regulation. Creatine can be synthesized endogenously through the kidney-liver axis from arginine, glycine, and s-adenosyl methionine as a methyl donor, then taken up by cells expressing a creatine transporter. Alternatively, creatine consumed and absorbed from the diet can be similarly absorbed intracellularly via the creatine transporter. Some cells like adipocytes, skeletal muscle cells, and pancreatic acinar cells generate the molecule locally.
Creatine kinase is the enzyme responsible for catalyzing the reaction that transfers a phosphoryl group between ATP and creatine. It exists in four isoforms: two cytosolic (brain-type and muscle-type), and two mitochondrial (ubiquitous-type, and sarcomeric-type). These isoforms highlight where this energy system is most abundant: in mitochondrial dense muscle tissue and in the brain. Muscle and brain tissue exhibit both high and fluctuating energy demands. The phosphocreatine system is able to regenerate ATP at an accelerated pace, serving a critical function as an energy buffer. The elevated levels of ATP occur in parallel with elevated levels of cytosolic ADP, closing the concentration difference required for ADP diffusion back into the mitochondria for ATP regeneration. Herein lies the beauty of the creatine kinase-phosphocreatine circuit: in a multi-step process, the phosphocreatine shuttle acts to transfer high-energy phosphate groups between ADP and ATP for energy regeneration.
However, continued research has elucidated a more diverse role of creatine other than as an energy buffer. One fascinating area of creatine involvement is that of immunometabolism. Creatine kinase B has been discovered to participate in thymocyte selection and T cell proliferation via its interaction with the T cell receptor. T cells are a part of the adaptive immune system, originating in the bone marrow and maturing in the thymus. Within the thymus, T cell receptors undergo multiple rearrangements that determine their functional activity. T cell receptor restructuring must result in appropriate binding (or lack thereof) of the receptor to self-peptide-MHC complexes present on antigen-presenting cells. Reorganized T cell receptors that do not bind MHC alleles, or bind with very low affinity, are subject to apoptosis, known as death by neglect. Alternatively, reshuffled receptors that bind too strongly are also extinguished, protecting against over-reactivity and consequent autoimmunity. This process of positive selection ensures that only cells equipped with a receptor that weakly engages with MHC complexes can go on to differentiate, colonize the lymphoid organs, and be poised to recognize foreign antigens.
Creatine kinase enters the scene following the interaction of double positive (CD4+/CD8+) thymocytes with self-antigens and related selection. T cells that have undergone positive selection profoundly and proactively upregulate creatine kinase, creating an available pool of ATP ready to be used in cases of immune threat. Ample ATP facilitates full T cell activation, proliferative capacity, and cytotoxicity. Creatine kinase essentially behaves as a tuning knob, amplifying the T cell receptor signal and priming T cells to fight impending hazards. Young, developing T cells maintain low levels of creatine kinase to dampen T cell receptor strength and prevent perilous hyper-activation. The cooperation between T cells and creatine is also evident in the context of anticancer immunity, where T cells rely upon creatine availability to boost tumor defense. Tumor-infiltrating T cells have been noted to augment expression of the creatine transporter to absorb creatine from the local environment. Studies have demonstrated a positive correlation between dietary creatine and restrained tumor progression. Creatine supplementation additionally synergizes with immune checkpoint blockage therapies, thereby impeding tumor growth.
Moreover, creatine has been connected to macrophage polarization. Macrophages are masters of phagocytosis, gobbling up invading pathogens and assisting in the adaptive immune response. Their metabolic activity vacillates based on their polarization state, which is in turn dictated by environmental cues. The presence of lipopolysaccharide or IFN-γ causes macrophages to polarize to the M1-like pro-inflammatory, microbicidal state. IL-4, in contrast, induces the M2-like polarization state which is associated with debris clearance and tissue repair. The shift toward an M2-like state begins with creatine uptake, and this very influx of creatine enhances maintenance of M2-like polarization. Creatine additionally appears to inhibit the IFNγ-JAK-STAT1-iNOS axis and enhance the IL-4-STAT6-ARG1 axis to provoke M2-like polarization while suppressing M1-like polarization. It has been proposed that creatine improves chromatin accessibility and promotes STAT6-mediated transcription. Furthermore, when called to engage in phagocytosis, macrophages must generate substantial amounts of ATP, a process supported by creatine kinase.
The metabolic rewiring that occurs in cancer cells, in combination with the role of creatine in immunometabolism, illustrates a complex dynamic between creatine and cancer. Historical studies have utilized cyclocreatine, a creatine analogue, to show that the disruption of cancer energetics can block tumor growth. Modern evidence has additionally revealed creatine-facilitated tumor inhibition via energy-independent pathways or altered metabolic crosstalk with the surrounding microenvironment. Nevertheless, proliferating cancer cells must secure a continual energy source that enables their rapid multiplication, with creatine serving as an efficient energy buffer during such times of exceptional ATP need. Cancer cells demonstrate the capacity to exploit creatine, inducing creatine kinase expression as well as sequestering creatine from their surrounding to bolster their own energy availability. Exogenous creatine may also support cancel cell survival by mitigating lipid peroxidation and modulating membrane biology.
Creatine’s incredibly diverse and complex set of regulatory functions has led to a resurgence of interest in its therapeutic applications. Continued investigation will be essential to expand our understanding of creatine biology and determine how its metabolism influences pathological states.
Kazak L, Cohen P. Creatine metabolism: energy homeostasis, immunity and cancer biology. Nat Rev Endocrinol. 2020;16(8):421-436. doi:10.1038/s41574-020-0365-5