Metabolic Compartmentalization at the Leading Edge of Metastatic Cancer Cells

Abstract

Despite advances in targeted therapeutics and understanding in molecular mechanisms, metastasis remains a substantial obstacle for cancer treatment. Acquired genetic mutations and transcriptional changes can promote the spread of primary tumor cells to distant tissues. Additionally, recent studies have uncovered that metabolic reprogramming of cancer cells is tightly associated with cancer metastasis. However, whether intracellular metabolism is spatially and temporally regulated for cancer cell migration and invasion is understudied. In this review, we highlight the emergence of a concept, termed "membraneless metabolic compartmentalization," as one of the critical mechanisms that determines the metastatic capacity of cancer cells. In particular, we focus on the compartmentalization of purine nucleotide metabolism (e.g., ATP and GTP) at the leading edge of migrating cancer cells through the uniquely phase-separated microdomains where dynamic exchange of nucleotide metabolic enzymes takes place. We will discuss how future insights may usher in a novel class of therapeutics specifically targeting the metabolic compartmentalization that drives tumor metastasis.

Keywords: GTP-metabolism; cancer; leading edge; liquid-liquid phase separation; membraneless metabolic compartmentalization; metabolon; metastasis; purine biosynthesis.

Figure 1

Schematic models of the biological roles of compartmentalization, viscosity, and local protein concentrations. (A) Metabolic compartmentalization by membrane-bound organelles confines metabolites to organelles to increase reaction efficiency and protect cellular contents—analogous to potential prey that may be protected from sharks by confining the predators to a shark tank (left). Release of organellar contents into the cytoplasm can elicit changes in cell fate (e.g., induction of an apoptotic program by cytochrome C and dATP or cellular damage mediated by lysosomal enzymes)—analogous to sharks that can either attack prey or themselves die when there is a breach in a shark tank (middle). Generally, the transportation of molecules into membrane-bound organelles is highly selective and regulated—analogous to sharks that may be fed without opening oneself up to the possibility of bodily harm by introducing prey into a shark tank from a distance (right). (B) A schematic diagram of the negative correlation between the fluid viscosity of a medium and the diffusion rate of metabolites within it. (C). A schematic diagram of the effect of the local concentration or relative proximity of enzymes belonging to the same metabolic pathway. When Enzymes A and B are spatially separated, Enzyme B can receive only small amounts of its substrate b, which is generated by the distant Enzyme A. Thus, Enzyme B produces only small amounts of its enzymatic product c—analogous to a shark that can catch only a relatively small number of fish when the fish are sparse (left). However, when Enzymes A and B are in close proximity, Enzyme B can receive much more of its substrate b and thus produce much more of its product c—analogous to a shark that can capture more fish when the fish are schooling (right).

Figure 2

Nucleotide metabolic enzymes localize at the lamellipodial membrane. (A) Purine nucleotide biosynthesis schematic. The 9 enzymatic steps of de novo biosynthesis (orange), the ATP branch (green), GTP branch (blue), and selected salvage pathway enzymes (yellow). (B) Selected immunofluorescence staining images of purine biosynthetic enzymes (color scheme matches part A) localizing to the leading edge of migrating kidney cancer cells. Relative intensity map of immunofluorescence staining shown in bottom micrograph with intensity scale at the right. Immunofluorescence of GAPDH and fluorescence imaging of Cell Tracker dye, which stain intracellular proteins, show major signals in cytoplasm, which indicate that the localization of purine metabolic enzymes at the leading edge is specific. PRPS, phosphoribosyl pyrophosphate synthetase; FGAMS: phosphoribosyl formylglycinamidine synthase; PAICS, phosphoribosyl aminoimidazole succinocarboxamide synthetase; ADSL, adenylosuccinate lyase; ATIC, 5-amino-4-imidazolecarboxamide ribonucleotide transformylase/IMP cyclohydrolase; ADSS, adenylosuccinate synthase; AK, adenylate kinase; IMPDH, inosine-5′-monophosphate dehydrogenase; GMPS, GMP synthase; GUK1, guanylate kinase 1; NDPK, nucleoside-diphosphate kinase; APRT, adenine phosphoribosyltransferase; ADK, adenosine kinase; HPRT1, Hypoxanthine-guanine phosphoribosyltransferase; GAPDH, glyceraldehyde 3-phosphate dehydrogenase.