Tumor mechanics and metabolic dysfunction

Abstract

Desmosplasia is a characteristic of most solid tumors and leads to fibrosis through abnormal extracellular matrix (ECM) deposition, remodeling, and posttranslational modifications. The resulting stiff tumor stroma not only compromises vascular integrity to induce hypoxia and impede drug delivery, but also promotes aggressiveness by potentiating the activity of key growth, invasion, and survival pathways. Intriguingly, many of the protumorigenic signaling pathways that are mechanically activated by ECM stiffness also promote glucose uptake and aerobic glycolysis, and an altered metabolism is a recognized hallmark of cancer. Indeed, emerging evidence suggests that metabolic alterations and an abnormal ECM may cooperatively drive cancer cell aggression and treatment resistance. Accordingly, improved methods to monitor tissue mechanics and metabolism promise to improve diagnostics and treatments to ameliorate ECM stiffening and elevated mechanosignaling may improve patient outcome. Here we discuss the interplay between ECM mechanics and metabolism in tumor biology and suggest that monitoring these processes and targeting their regulatory pathways may improve diagnostics, therapy, and the prevention of malignant transformation.

Keywords: Cancer; ECM stiffness; Free radicals; Mechanosignaling; Tumor metabolism; Tumor microenvironment.

Figure 1

Mechanosignaling and cellular tension drive cancer aggressiveness. A) Both normal and malignant cells sense and respond to mechanical cues from the ECM via adhesion receptors (e.g., integrins), intracellular focal adhesions, cytoskeletal networks, and molecular motors. Cell adhesion and focal adhesion in turn collaborate with G protein coupled receptors and transmembrane receptor tyrosine kinases to enhance cell growth and survival and invasion by altering phophoinositide 3-kinase (PI3K) signaling and downstream p-AKT. B) In compliant 3D gels with material properties similar to that measured in the normal mammary gland, non-malignant human mammary epithelial cells form growth-arrested, polarized acini analogous to the terminal ductual lobular units observed at the end buds of the differentiated breast. Incremental stiffening of the basement membrane gel progressively compromises tissue morphogenesis and alters EGF-dependent growth of these cells. Thus, colony size progressively increases, lumen formation is compromised, cell-cell junctions are disrupted, and tissue polarity is inhibited. Arrows indicate loss of endogenous basement membrane and disruption of basal polarity. C) Non-malignant human epithelial cells grown on stiff substrates show greater cell contractility, measured via traction force microscopy, compared to the same cells grown of softer substrates. Reprinted from Cancer Cell, 8/3, Paszek et al, Tensional homeostasis and the malignant phenotype, 241-54, copyright 2004, with permission from Elsevier.

Figure 2

Regulation of metabolism in tumor cells. Normal cells use the TCA cycle (OXPHOS) for ATP production, whereas oncogenic signaling drives a glycolytic phenotype in tumor cells. PI3K activates AKT, which stimulates glycolysis by directly regulating glycolytic enzymes and by activating mTOR. mTOR has pleotropic effects on metabolism, but facilitates the glycolytic phenotype by enhancing HIF1 activity, which engages a hypoxia-adaptive transcriptional program. HIF1 induces the expression of glucose transporters (GLUT), glycolytic enzymes and PDK1, which blocks the entry of pyruvate into the TCA cycle. The LKB1 tumor suppressor activates AMPK which opposes the glycolytic phenotype by inhibiting mTOR. MYC cooperates with HIF to induce several genes that encode glycolytic proteins, but also increases mitochondrial metabolism. The tumor suppressor p53 opposes the glycolytic phenotype by suppressing glycolysis through TIGAR, increasing mitochondrial metabolism via SCO2 and supporting expression of PTEN. OCT1 acts in an opposing manner to activate the transcription of genes that drive glycolysis and suppress oxidative phosphorylation. The switch to the PKM2 isoform affects glycolysis by slowing the pyruvate kinase reaction and diverting substrates into alternative biosynthetic and reduced NADPH-generating pathways. TCA, tricarboxylic acid; HIF1, hypoxia-inducible factor 1; LKB1, liver kinase B1; PDK1, pyruvate dehydrogenase kinase, isozyme 1; AMPK, AMP-activated protein kinase; PKM2, pyruvate kinase M2; TIGAR, TP53-induced glycolysis and apoptosis regulator; MCT, monocarboxylate transporter; PDH, pyruvate dehydrogenase. The dashed lines indicate the shift from TCA to glycolysis under the loss of p53 function. Reprinted by permission from Macmillan Publishers Ltd: Nature Reviews Cancer, 11/2, Cairns et al, Regulation of cancer cell metabolism, copyright 2011.

Figure 3

Fluorescent lifetime imaging of a high risk breast biopsy. A) Standard H&E stained section with a region of atypia. B) FLIM of the unstained adjacent section in which the weighted average lifetime of NADH (tm) and C) the fraction of free NADH (a1) is shown as a color map. D) Quantitative histograms of the pixel-by-pixel data from the regions-of-interest outlined in (B). An increase in a₁, which corresponds to a decrease in tm, in epithelium displaying atypia is an indication these cells have either increased glycolysis or decreased oxidative phosphorylation.

Figure 4

Breast malignancy associates with miR-18a and reduced PTEN expression. A) HOXA9 (top, red), PTEN (middle, red), pAKT substrate (bottom, red) and DAPI (blue) for human nonmalignant breast samples and breast tumor samples. The epithelium of normal breast tissue coexpressed appreciable quantities of nuclear HOXA9 and cytoplasmic PTEN proteins, as did the more differentiated luminal A breast tumors. In contrast, the less differentiated luminal B, basal-like, and HER2+ tumors showed reduced expression of HOXA9 and PTEN and higher levels of activated AKT. Scale bar, 100 μm. B) As measured by atomic force microscopy, miR-18a expression is correlated with breast ECM elastic modulus in human patient samples from both normal and transformed breast tissue. Reprinted by permission from Macmillan Publishers Ltd: Nature Medicine, 20/4, Mouw et al, Tissue mechanics modulate microRNA-dependent PTEN expression to regulated malignant progression, copyright 2014.

Figure 5

Metabolically active triple-negative breast cancer from women at high-risk for breast cancer. a). High glucose uptake as measured by ¹⁸Flurodeoxyglucose Positron Emission Tomography (¹⁸FDG-PET). b) Timed 3.0 Tessla Magnetic Resonance Imaging (3.0 T MRI) demonstrates the vascularity of the cancer.

Figure 6

Proposed model by which a stiffened ECM regulates metabolism. Through mechanical activation of pathways highlighted in Figures 1 and 2, ECM stiffness (and other perturbed forces within the tumor microenvironment) drives a glycolytic phenotype. ECM, extracellular matrix.