Metabolic control of collagen synthesis

Abstract

The extracellular matrix (ECM) is present in all tissues and crucial in maintaining normal tissue homeostasis and function. Defects in ECM synthesis and remodeling can lead to various diseases, while overproduction of ECM components can cause severe conditions like organ fibrosis and influence cancer progression and therapy resistance. Collagens are the most abundant core ECM proteins in physiological and pathological conditions and are predominantly synthesized by fibroblasts. Previous efforts to target aberrant collagen synthesis in fibroblasts by inhibiting pro-fibrotic signaling cascades have been ineffective. More recently, metabolic rewiring downstream of pro-fibrotic signaling has emerged as a critical regulator of collagen synthesis in fibroblasts. Here, we propose that targeting the metabolic pathways involved in ECM biomass generation provides a novel avenue for treating conditions characterized by excessive collagen accumulation. This review summarizes the unique metabolic challenges collagen synthesis imposes on fibroblasts and discusses how underlying metabolic networks could be exploited to create therapeutic opportunities in cancer and fibrotic disease. Finally, we provide a perspective on open questions in the field and how conceptual and technical advances will help address them to unlock novel metabolic vulnerabilities of collagen synthesis in fibroblasts and beyond.

Keywords: Cancer; Collagen; ECM; Fibroblasts; Fibrosis; Glycine; Metabolism; Nutrients; Proline.

Figure 1:. Biochemical composition and amino acid profiles of cellular and extracellular matrix (ECM) biomass.

(A) Left: composition of cell dry mass, adapted from [19]. Right: composition of ECM dry mass, based on [20]. (B) Left: relative whole-body collagen content in mice, based on [21]. Right: human pancreatic ductal adenocarcinoma (PDAC) ECM protein composition, based on [22]. (C) Comparison of amino acid abundance in all human proteins (left) versus all collagens (right). Based on data from the NCBI protein database with the assumption that each protein has the same abundance. Letters represent amino acid letter codes.

Figure 2:. Metabolic rewiring that promotes collagen synthesis in fibroblasts.

In a pro-fibrotic environment, fibroblasts increase glucose and glutamine uptake to sustain the epigenetic, bioenergetic, biosynthetic, and post-translational requirements of collagen synthesis. Thick arrows indicate the pathway fluxes that TGFβ stimulation or the CAF state induces. Metabolites, enzymes, and arrows in grey denote proposed pathways that have not yet been experimentally verified. Question marks denote unknowns. 3PG: 3-Phosphoglyceric acid; 3PHP: 3-phosphohydroxypyruvate; 3PS: 3-phospho-L-serine; αKG: alpha-ketoglutarate; Ac: acetyl; ACLY: ATP citrate lyase; AT: aminotransferase; ADP: Adenosine diphosphate; ATP: Adenosine triphosphate; eIF5A: eukaryotic translation initiation factor 5A; EP300: Histone acetyltransferase p300; ETC: Electron transport chain; G: glycine; Glu: glutamate; GLUT1: Glucose transporter 1; GLS: Glutaminase; GSA: Glutamate-γ-semialdehyde; Hy: Hypusine; Hyp: Hydroxyproline; HK2: Hexokinase 2; HMT: Histone methyltransferases; KDM: Histone lysine demethylase; LDH: Lactate dehydrogenase; Me: methyl; mTHF: Methyltetrahydrofolate; NAD: Nicotinamide adenine dinucleotide; NADK2: Nicotinamide adenine dinucleotide kinase 2; NNMT: nicotinamide N-methyltransferase; OAA: Oxaloacetate; OAT: Ornithine aminotransferase; ODC: Ornithine decarboxylase P: proline; P5CS: Pyrroline-5-carboxylate synthase; PDH: Pyruvate Dehydrogenase; PDK: Pyruvate Dehydrogenase kinase; PHGDH: phosphoglycerate dehydrogenase; PSAT1: phosphoserine aminotransferase 1; PYCR1: Pyrroline-5-carboxylate reductase 1; SAM: S-adenosyl-methionine; SAH: S-adenosylhomocysteine; SHMT2: serine hydroxymethyltransferase 2; TCA: tricarboxylic acid; THF: Tetrahydrofolate.

Figure 3:. Metabolic adaptations in the tumor microenvironment (TME).

(A) Metabolic rewiring of fibroblasts: In the TME, glutamine and glucose are scarce, whereas lactate is abundant. Metabolic fluxes for collagen synthesis are altered compared to standard culture conditions (Figure 2). Lactate-derived pyruvate is used via PDH and PC to maintain the TCA cycle and replenish glutamate pools. This maintains tRNA charging through de novo glutamine synthesis and supports proline biosynthesis. Lactate utilization also allows the remaining glutamine and glucose to be used efficiently for proline and glycine biosynthesis, respectively. Colored lines represent the flux of nutrients inside the cell. In blue: lactate, in red: glucose, in orange: glutamine. αKG: alpha-ketoglutarate; ADP: Adenosine diphosphate; ATP: Adenosine triphosphate; GLS: Glutaminase; GS: Glutamine synthetase; LDH: Lactate dehydrogenase; MCT1: Monocarboxylate transporter 1; NAD: Nicotinamide adenine dinucleotide; NADK2: Nicotinamide adenine dinucleotide kinase 2; OAA: Oxaloacetate; P5CS: Pyrroline-5-carboxylate synthase; PC: Pyruvate carboxylase; PYCR1: Pyrroline-5-carboxylate reductase 1; TCA: tricarboxylic acid. (B) The nutrient-dictated extracellular matrix (ECM): during tumorigenesis, nutrients are used by fibroblasts to produce ECM. The ECM, when in excess, alters the biomechanical and biophysical properties of the TME, causing reduced perfusion. This alters nutrient availability for cancer-associated fibroblasts (CAFs). We propose that CAFs produce an ECM that is different in quantity and quality in such altered nutrient conditions: the nutrient-dictated extracellular matrix.