Causes and Consequences of the
Hostile Tumor Microenvironment

By RJ Gillies, PhD

In 1976, Sidney Weinhouse stated that, since "our perspectives have broadened over the years, the burning issues of glycolysis and respiration in cancer now flicker only dimly". While this may have been accurate in 1976, it is certainly not true today. As Yogi Berra said, "It's like deja vu, all over again". Interest in glycolysis and cancer has been rekindled due to advances in molecular phenotyping, the discovery of hypoxia-inducible factor, advances in evolutionary models of carcinogenesis and, most important, through the almost universal observation of elevated glucose uptake in tumors by positron emission tomography (PET).

Virtually every student of cancer biology is aware of the "Vogelgram" developed by Fearon and Vogelstein to describe multistage carcinogenesis in molecular genetic terms. Although this model accurately explains the genetic program that occurs during colorectal carcinogenesis, it inadequately accounts for the progression of tumors as an evolutionary system. This deficit has been addressed by Robert Weinberg in a recent Nature editorial, wherein he describes the progression of tumors in evolutionary terms. We believe that hypoxia and the acidic tumor microenvironment apply the selective pressures that drive this evolving system. This hostile environment is intimately coupled to glycolysis and hence, the "aerobic glycolysis phenotype", first described by Otto Warburg in 1924, may be central to the process of carcinogenesis itself. Although Warburg observed high glycolysis in a few tumors, the universality of this phenomenon was not appreciated until recently, with the wide application of fluorodeoxyglucose (FdG) PET scans. A recent review by Sam Gambhir and Mike Phelps of over 14,000 patient scans has shown that FdG uptake is significantly elevated in over 90% of all metastatic cancers. Hence, although it is almost axiomatic that "tumors are glycolytic", the mechanisms are not known.

We believe that the elevated aerobic glycolysis in tumors can be caused by high aerobic levels of hypoxia-inducible factor (HIF-1a), which is selected for during the transient hypoxia that occurs during tumorigenesis. HIF-1a is normally targeted for degradation in the presence of oxygen through proline hydroxylation, which is recognized by ubiquitin ligase. If it is not degraded, it forms a heterodimer with HIF-1ß to make an active transcription factor that upregulates the expression of dozens of genes, including most glycolytic enzymes. The resulting upregulated glycolysis leads to lower tumor pH, and this induces a number of tumor phenotypes, including increased invasion and metastasis.

We are testing the above hypothesis primarily using molecular imaging tools. We have shown that the microenvironments of dozens of tumor types are acidic, as measured with NMR. Further investigations have used both lowly metastatic (MCF-7) and highly metastatic (MDA-mb-231 and -435) cells. MCF-7 cells do not glycolyze rapidly under aerobic conditions, whereas the metastatic cells do. When grown as tumors, the pH of the metastatic tumors is much lower than that of the MCF-7. Transfecting the -435 cells to express nm-23 (an adenylate kinase) suppresses metastasis and leads to elevated pHe. Artificially raising the pHe of 231 tumors by treating the host animals with bicarbonate reduces the number of lung metastases by a factor of two. The elevated glycolysis in the -435 and -231 cells may be due to upregulated HIF-1a, which was shown to be high in Western blots. In these cells, the HIF-1a induced transcripts are also constitutively high, even in the presence of oxygen. By comparison, the HIF-1a and transcript levels in MCF-7 cells are low under normoxia and are strongly induced by hypoxia.

Mechanistically, hypoxia and acidity have their roots in poor perfusion and elevated metabolism. Although perfusion, hypoxia and acidosis have been studied for many years, our understanding of the complex interrelationships between these microenvironmental parameters has improved in the past decade with the advent of non-invasive imaging techniques. We have made a number of contributions to the relationship between vasculature and perfusion, including a recent review entitled "MR imaging of the tumor microenvironment", which can be downloaded from the departmental website:

http://www.biochem.arizona.edu/gillies/publications/JMRI2002.pdf.

In 1907, Professor E. Goldmann stated 'Above all things, I hope to be able to report on a first attempt to penetrate into the darkness of physiological conditions existing in malignant growths'. Results of the last decade are bringing this ambition to reality. We are now able to dynamically image tumor perfusion, tumor oxygenation and tumor pH with increasing spatio-temporal resolution, and these are providing novel windows into the process of carcinogenesis itself.

Literature Cited

Bernards R and Weinberg R. A progression puzzle. Nature 418:823, 2002.

Fearon ER and Vogelstein B. A genetic model for colorectal tumorigenesis. Cell 61:759-767, 1990.

Gambhir SS et al. A tabulated summary of the FDG PET literature. J. Nucl. Med.42(Suppl):1S-93S, 2001.

Goldmann E. The growth of malignant disease in man and the lower animals, with special reference to the vascular system. Proc Royal Acad Med 1:1-13, 1907.

Warburg O. The Metabolism of Tumours. London: Arnold Constable, 1930.

Weinhouse SZ. Krebsforsh. 87:115-126, 1976.

Representative Publications from Gillies' group and collaborators on this subject:

Book. "The Tumor Microenvironment: Causes and Consequences of hypoxia and acidity" Novartis FoundationSymposium 240. RJ Gillies (chair)

Bhujwalla ZM, Artemov D, Ballesteros P, Cerdan S, Gillies RJ and Solaiyappan M. (2002) Combined vascular and extracellular pH imaging of solid tumors. NMR in Biomedicine 15:114-119.

Bhujwalla ZM, Aboagye EO, Gillies RJ, Chacko VP, Mendola CE and Backer JM. (1999) Nm23-transfected MDA-MB-435 human breast carcinoma cells form tumors with altered phospholipid metabolism and pH:  ³¹P nuclear magnetic resonance study in vivo and in vitro. Magn. Reson. Med. 41:897-903.

Gillies RJ, Raghunand N, Karczmar G and Bhujwalla ZM (2002) MR Imaging of the tumor microenvironment. J Magn. Reson. Imaging 16:430-450.

Gillies RJ, Schornack PA, Secomb TW and Raghunand N. (1999) Causes and Effects of Heterogeneous Perfusion in Tumors. Neoplasia 1:197-207.

Gillies RJ. (1999) Angiostatin's partners (letter). Science 284:434-435.

Raghunand N, He X, van Sluis R, Mahoney B, Baggett B, Taylor CW, Paine-Murrietta G, Roe D, Bhujwalla ZM and Gillies RJ. (1999) Enhancement Of Chemotherapy By Manipulation of Tumor pH. Br. J. Cancer 80, 1005-1011.

Raghunand N, Mahoney B, Van Sluis R, Baggett B, Gillies RJ. (2001) Acute metabolic alkalosis enhances response of C3H mouse mammary tumors to the weak-base mitoxantrone. Neoplasia 3:227-235.

Feature 1 | Feature 2 | Talk From Tom | Outreach Update
BMCB Retreat | Profiles | Faces | Catalyst Home

Biological Sciences West
P.O. Box 210088 ·Tucson, AZ 85721-0088
Tel: (520) 621-9185 FAX (520) 621-9288
Department of Biochemistry and Molecular Biophysics
The University of Arizona
Updated June 1, 2004

http://www.biochem.arizona.edu/
All contents copyright ©2002-2004. All rights reserved.
cherylr@u.arizona.edu