In 1926, Otto Warburg observed that normal and tumor cells differ in their ability to metabolize glucose.
He hypothesized that unlike most normal tissues, cancer cells rely upon glycolysis for energy production, despite the presence of sufficient oxygen (aerobic glycolysis or “the Warburg Effect”).
In addition, Warburg hypothesized that tumor cells develop mitochondrial dys-function that leads to impaired aerobic respiration and a switch toward glycolytic metabolism, with increased L-lactate production.
However, subsequent studies showed that most tumor cells still maintain at least some mitochondrial function suggesting that there might be a different explanation for the increased aerobic glycolysis of cancer cells.
In the last years, it has been increasingly recognized that the tumor microenvironment plays an important role in promoting tumor growth and metastasis. Especially Cancer-associated fibroblast (CAF) play a mayor role in this processes.
Lisanti and colleagues have recently proposed a new model that can explain cancer metabolism in which cancer cells induce oxidative stress and autophagy in cancer-associated fibroblasts, leading to the onset of inflammation, autophagy, mitophagy and aerobic glycolysis in the tumor microenvironment Cancer cells metabolically fertilize the tumor microenvironment with hydrogen peroxide, driving the Warburg effect: implications for PET imaging of human tumors. 2011. (Fig1)
Fig 1. Schematic rapresentation of the Reverse warburg Effect.
The oxidative stress triggers the activation of two transcription factor Hif-1α and NFkB.
This activation was also seen by Sonveaux and collegues within cancer cell mass in the hypoxic compartment.
There is a metabolic reprogramming orchestrated by Hif-1α through the activation of different metabolic enzyme like: lactate dehydrogenase (LDHA), pyruvate dehydrogenase kinase 1 (PDK1) which inactivate the enzyme responsible of conversion of pyruvate to acetil-CoA, and BNIP3 which triggers selective mitochondrial autophagy (Fig.2).
In this case the authors demonstrate the existence of a “metabolic symbiosis” between hypoxic and aerobic cancer cells, in which lactate produced by hypoxic cells is taken up by aerobic cells, which use it as their principal substrate for oxidative phosphorylation.
As a result, the limited glucose available to the tumor is used most efficiently: hypoxic cells downregulate oxidative phosphorylation in order to maintain redox homeostasis and must consume large amount of glucose to maintain energy homeostasis.
Fig 2. Intratumoral hypoxia and metabolic symbiosis. Tumors are characterized by gradients of O2 levels, based on the distance of tumor cells from a functional blood vessel.
In this context monocarboxylate transporter 1 (MCT1) play an important role. Its expression is upregulated in aerobic cells which can use lactate and, in concert with the O2-dependent expression of LDHB, to utilize it as an energy substrate, thereby freeing these cells from the need to take up large quantities of glucose. (Tumor metabolism: cancer cells give and take lactate. 2008)
Lisanti et al. Demonstrate that there is a similar mechanism in CAF.
This type of stromal metabolism produces high-energy nutrients (lactate, ketones and glutamine), as well as recycled chemical building blocks (nucleotides, amino acids, fatty acids), to literally “feed” cancer cells. This phenomenon was called the “Reverse Warburg effect” in which cancer cells induce aerobic glycolysis in CAF with the increased production and secretion of lactate.
Secreted L-lactate is transferred to adjacent cancer cells, and converted to pyruvate, thus functioning as a substrate for mitochondrial oxidative phosphorylation and togheter with ketones are able to induce cancer cell migration functioning as chemoattractants.
They also shows that in human breast cancer samples lacking stromal Cav-1, the transporter for the cellular extrusion of lactate (MCT4) is specifically upregulated in stromal cells, whereas the transporter for the uptake of lactate (MCT1) is specifically upregulated in epithelial cancer cells. (Fig.3)
Fig3. Pyruvate kinase expression (PKM1 and PKM2) in cancer associated fibroblasts drives tumor growth. Cancer cells secrete hydrogen peroxide (H2O2) which initiates oxidative stress in adjacent stromal fibroblasts. This, in turn, drives the activation of two main transcription factors, namely HIF1 and NFkB. NFkB is the master regulator of the innate immune response, resulting in the secretion of inflammatory cytokines. HIF1-a stabilization induces autophagy, mitophagy, and aerobic glycolysis. As a consequence, PKM1 and PKM2 expression results in the production of excess L-lactate and ketone bodies (3-hydroxy-butyrate) that are extruded via a mono-carboxylate transporter, known as MCT4. These high-energy nutrients are then taken up and recycled by cancer cells using another mono-carboxylate transporter, namely MCT1. Thus, L-lactate and ketones produced by stromal fibroblasts are transferred to cancer cells and used as fuel for mitochondrial oxidative metabolism (OXPHOS), producing large amounts of ATP.
In these samples they also show that there is an overexpression of 3 classes of protein:
- fibroblast marker (vimentin, calponin, collagen)
- glycolytic enzyme (PKM2 and LDHA)
- antioxidants (catalase and peroxiredoxin)
Pyruvate kinase
Pyruvate kinase (PK), a rate-limiting enzyme during glycolysis, catalyzes the production of pyruvate and adenosine 5′-triphosphate (ATP) from phosphoenolpyruvate (PEP) and adenosine 5′-diphosphate (ADP). (Fig.4)
Fig 4.Chemical reaction of Pyruvate Kinase.
Four mammalian PK isoenzymes (M1, M2, L, and R) exist, which are present in different cell types.
L and R isoform are respectively expressed in liver and red blood cells. PKM1 is a constitutively active form of PK that is found in normal adult cells. In contrast, PKM2 is found predominantly in the fetus and also in tumor cells, where the abundance of other isoforms of PK is low. PKM2 can exist in either active tetramers or inactive dimers, but in tumor cells, it predominantly occurs in dimers with low activity.
Christofk et al demonstrate that in tumor tissue and cell line there is a switch from the adult isoform PDKM1 to the fetal one (PDKM2). [3].
Chiavarina et al show that both PDK1 and PDK2 isoform are highly expressed in tumor stroma of breast cancer patient, localized intracellularly or secreted. [4]
They also demonstate that PDK1, upregulated in fibroblast, increase the lactate production, whereas PDK2 is involved in the induction of autophagy mediated by NFkB pathway and the activation of ketone body metabolism. (Fig.5)
Fig. 5 Upper panel: lacatate production; autophagy protein expression; NFkB and Hif-1α expression in CAF overexpressing PKM1 and PKM2. Lower panel: Ketones production an protein enzyme involved in ketogenesis and ketones utilization in CAF overexpressing PKM1 and PKM2
PDK1 and PDK2 overexpressing fibroblasts also incresed the mitochondrial activity of cancer cells and promote tumor growth. (Fig6)
Fig.6 In vivo tumor growth and mitochondrial activity visualization using Mito Tracker.
References
- Martinez-Outschoorn UE, Lin Z, Trimmer C, Flomenberg N, Wang C, Pavlides S, Pestell RG, Howell A, Sotgia F, Lisanti MP. Cancer cells metabolically "fertilize" the tumor microenvironment with hydrogen peroxide, driving the Warburg effect: implications for PET imaging of human tumors. Cell Cycle. 2011 Aug 1;10(15):2504-20. [1].
- Christofk HR, Vander Heiden MG, Harris MH, Ramanathan A, Gerszten RE, Wei R, Fleming MD, Schreiber SL, Cantley LC. The M2 splice isoform of pyruvate kinase is important for cancer metabolism and tumour growth. Nature. 2008 Ma 13;452(7184):230-3.
[3].
- Chiavarina B, Whitaker-Menezes D, Martinez-Outschoorn UE, Witkiewicz AK, Birbe RC, Howell A, Pestell RG, Smith J, Daniel R, Sotgia F, Lisanti MP. Pyruvate kinase expression (PKM1 and PKM2) in cancer-associated fibroblasts drives stromal nutrient production and tumor growth. Cancer Biol Ther. 2011 Dec 15;12(12). [4]
