Active and inactive metabolic pathways in tumor spheroids: determination by GC-MS

Current Advances in 3D Bioprinting for Cancer Modeling and Personalized Medicine

Tumor cells evolve in a complex and heterogeneous environment composed of different cell types and an extracellular matrix. Current 2D culture methods are very limited in their ability to mimic the cancer cell environment. In recent years, various 3D models of cancer cells have been developed, notably in the form of spheroids/organoids, using scaffold or cancer-on-chip devices. However, these models have the disadvantage of not being able to precisely control the organization of multiple cell types in complex architecture and are sometimes not very reproducible in their production, and this is especially true for spheroids. Three-dimensional bioprinting can produce complex, multi-cellular, and reproducible constructs in which the matrix composition and rigidity can be adapted locally or globally to the tumor model studied. For these reasons, 3D bioprinting seems to be the technique of choice to mimic the tumor microenvironment in vivo as closely as possible. In this review, we discuss different 3D-bioprinting technologies, including bioinks and crosslinkers that can be used for in vitro cancer models and the techniques used to study cells grown in hydrogels; finally, we provide some applications of bioprinted cancer models.

Reductive carboxylation supports redox homeostasis during anchorage-independent growth

Cells receive growth and survival stimuli through their attachment to an extracellular matrix (ECM). Overcoming the addiction to ECM-induced signals is required for anchorage-independent growth, a property of most malignant cells. Detachment from ECM is associated with enhanced production of reactive oxygen species (ROS) owing to altered glucose metabolism. Here we identify an unconventional pathway that supports redox homeostasis and growth during adaptation to anchorage independence. We observed that detachment from monolayer culture and growth as anchorage-independent tumour spheroids was accompanied by changes in both glucose and glutamine metabolism. Specifically, oxidation of both nutrients was suppressed in spheroids, whereas reductive formation of citrate from glutamine was enhanced. Reductive glutamine metabolism was highly dependent on cytosolic isocitrate dehydrogenase-1 (IDH1), because the activity was suppressed in cells homozygous null for IDH1 or treated with an IDH1 inhibitor. This activity occurred in absence of hypoxia, a well-known inducer of reductive metabolism. Rather, IDH1 mitigated mitochondrial ROS in spheroids, and suppressing IDH1 reduced spheroid growth through a mechanism requiring mitochondrial ROS. Isotope tracing revealed that in spheroids, isocitrate/citrate produced reductively in the cytosol could enter the mitochondria and participate in oxidative metabolism, including oxidation by IDH2. This generates NADPH in the mitochondria, enabling cells to mitigate mitochondrial ROS and maximize growth. Neither IDH1 nor IDH2 was necessary for monolayer growth, but deleting either one enhanced mitochondrial ROS and reduced spheroid size, as did deletion of the mitochondrial citrate transporter protein. Together, the data indicate that adaptation to anchorage independence requires a fundamental change in citrate metabolism, initiated by IDH1-dependent reductive carboxylation and culminating in suppression of mitochondrial ROS.

Mitochondria in Cancer Energy Metabolism: Culprits or Bystanders?

Cancer is a disease characterized by uncontrolled growth. Metabolic demands to sustain rapid proliferation must be compelling since aerobic glycolysis is the first as well as the most commonly shared characteristic of cancer. During the last decade, the significance of metabolic reprogramming of cancer has been at the center of attention. Nonetheless, despite all the knowledge gained on cancer biology, the field is not able to reach agreement on the issue of mitochondria: Are damaged mitochondria the cause for aerobic glycolysis in cancer? Warburg proposed the damaged mitochondria theory over 80 years ago; the field has been testing the theory equally long. In this review, we will discuss alterations in metabolic fluxes of cancer cells, and provide an opinion on the damaged mitochondria theory.

Rapid uptake of glucose and lactate, and not hypoxia, induces apoptosis in three-dimensional tumor tissue culture

The spatial arrangement of cellular metabolism in tumor tissue critically affects the treatment of cancer. However, little is known about how diffusion and cellular uptake relate to intracellular metabolism and cell death in three dimensions. To quantify these mechanisms, fluorescent microscopy and multicellular tumor cylindroids were used to measure pH and oxygen profiles, and quantify the distribution of viable, apoptotic and necrotic cells. Spheroid dissociation, enzymatic analysis, and mass spectrometry were used to measure concentration profiles of glucose, lactate and glutamine. A mathematical model was used to integrate these measurements and calculate metabolic rate parameters. It was found that large cylindroids, >500 μm in diameter, contained apoptotic and necrotic cells, whereas small cylindroids contained apoptotic but not necrotic cells. The center of cylindroids was found to be acidic but not hypoxic. From the edge to the center, concentrations of glucose, lactate and glutamine decreased rapidly. Throughout the cell masses lactate was consumed and not produced. These measurements indicate that apoptosis was the primary mechanism of cell death; acidity was not caused by lactic acid; and cell death was caused by depletion of carbon sources and not hypoxia. The mathematical model showed that the transporter enzymes for glucose and lactate were not saturated; oxygen uptake was limited by intracellular metabolism; and oxygen uptake was not limited by membrane-transport or diffusion. Unsaturated transmembrane uptake may be the cause of both proliferative and apoptotic regimes in cancer. These results suggest that transporter enzymes are excellent targets for treating well oxygenated tumors.

The conditional role of inflammation in pregnancy and cancer

Cancer growth is characterized by proliferation of tumor cells in conjunction with invasion of all different immune cells that also invade healing wounds. This inflammatory response is necessary for cell proliferation but a second purpose of the inflammatory process is so that a low Th1/Th2 ratio is present with overexpression of IL-10, TGF-β and IFN-γ. Down regulation of NO activity also shifts the balance between M1 and M2 macrophages. Both aspects allow the antigenous nature of the tumor to escape anti-tumor effects of the host. Support for this view comes from observations in pregnancy in which the placenta exhibits identical immune responses and downregulation of NO production to allow trophoblast cells to invade the uterine tissues without being rejected. Cell proliferation requires a metabolic set-up in which the organism produces adequate substrate for growth. This also bears the characteristics of a systemic inflammatory response delivering a similar substrate mix required for cancer and fetal growth. This arrangement is clearly beneficial in pregnancy and therefore supports the view that cancer growth is facilitated by the organism: the cancerous tumor elicits an immunological response opposing anti-tumor effects and induces the host to produce building blocks for growth.