Download Othon Iliopoulos, MD Associate Professor of Medicine, Harvard

Othon Iliopoulos, MD
Associate Professor of Medicine, Harvard Medical School
Director, MGH GU Genetics Program
Attending Oncologist, MGH Cancer Center
A. Introduction to the Iliopoulos Laboratory
Cancer cells transform their metabolism to adapt to the needs of fast growth and to compete with the
surrounding normal cells for nutrients and oxygen. In addition to a reprogrammed metabolism, cancer
cells stimulate the growth of new blood vessels that bring blood to them, a phenomenon known for
many years as “cancer angiogenesis”. The overall goal of my laboratory is to understand the main
mechanisms underlying the reprogramming of cancer cell metabolism and cancer angiogenesis and to
develop mechanism-based strategies for selectively killing cancer cells.
We use Renal Cell Carcinoma (RCC) as a disease model to study cancer metabolism and angiogenesis.
The overwhelming majority of the RCC tumors (more than 90%) lack a tumor suppressor protein called
VHL. The main function of this protein in the cells is to keep Hypoxia Inducible Factor 2 (HIF2a) under
physiologic control; it allows the expression of HIF2a only when the cells lack nutrients or the oxygen
level drops within a cell (hypoxia). HIF2a is a protein that binds DNA and activates the expression of
many genes that interfere with cancer angiogenesis and metabolism. In other words, HIF2a is a “master
regulator” of cancer metabolism and angiogenesis. RCC that lack VHL express continuously HIF2a,
independently of the oxygen or nutrient levels within the cell. This inappropriate activation turns HIF2a
into an oncogenic “driver” of RCC tumors. Additional mutations in metabolic enzymes such Succinate
Dehydrogenase (SDH) and Fumarate Hydratase (FH) are also linked to the development of RCC.
Comprehensive molecular analysis of RCC by the TCGA Investigators Network identified mutations and
gene expression changes in many enzymes regulating intermediary metabolism and lipid synthesis. The
investigators concluded that “….remodeling cellular metabolism thus constitutes a recurrent pattern in
ccRCC that correlates with tumor stage and severity and offers new views on the opportunities for
disease treatment”.
We have already contributed seminal observations in understanding how RCC-associated mutations and
lack of oxygen and nutrients reprogram RCC metabolism and angiogenesis. We are using RCC disease as
a model disease that helps us to dissect the fundamental mechanisms of cancer angiogenesis and to
develop strategies that will exploit these mechanisms for the development of novel anti-cancer
therapies.
B. Primary Areas of Focus
1) Development of HIF2a inhibitors for treatment of RCC and other HIF2a-dependent tumors
Upregulation of HIF2a, as well as the genes that are “switched on” by HIF2a, are detected in all solid and
hematologic malignancies. HIF2a expression levels bear prognostic significance. This is because rapidly
proliferating tumors outgrow their blood vessels and therefore they “starve” for oxygen and nutrients.
The importance of HIF2a as a therapeutic target for prostate cancer, glioblastoma and many other
tumors beyond RCC, was validated experimentally in many animal models.
It is currently well accepted that HIF2a inhibitors could be powerful agents for the treatment of RCC and
many other human cancers. Based on this concept we screen libraries of chemical compounds and
discovered chemical molecules that significantly and specifically decrease the expression of HIF2a. We
spend significant effort in trying to understand how the HIF2a inhibitors work and we discovered that
they suppress the expression of HIF2a by activating a cellular protein that senses iron levels. The
consequences of these observations were important at several levels. First, we showed that the iron
sensing mechanisms of the cells talk to the oxygen sensing mechanisms, this way linking the biochemical
pathways that sense cellular iron levels to the ones that sense cellular oxygen levels. Both nutrients are
delivered by blood and therefore our observations provided insights into how cancer cells “safeguard”
their supply of nutrients by multiple “checkpoints”. Next we studied the therapeutic potential of these
HIF2a inhibitors in two animal models (zebrafish and mice) and showed that the HIF2a inhibitors we
discovered are “active” and they reverse the consequences of VHL protein loss. They are therefore very
promising agents for targeting cancer angiogenesis.
2) Targeting the metabolic reprogramming of RCC and HIF2a expressing tumors for therapy
My laboratory provided significant insights into how HIF2a expressing cells reprogram their cancer
metabolism and use the resources of their environment in a “smart” way to compete with normal cells.
Normal cells use glucose to produce energy and to generate the building blocks (amino acids, lipids and
nucleotides) required for proliferation (anabolism). They actively uptake glucose from their environment
and transfer it to a central cellular “generator” where they use glucose’s carbons to generate the
building blocks. This generator is called TCA cycle, short for Tricarboxylic Acid cycle. It has been known
for long time that cancer cells do not use glucose efficiently, and instead of fueling it into the TCA cycle,
they use it to produce lactic acid that acidifies their environment. This observation was made by Otto
Warburg in 1922. In addition, Otto Warburg observed that cells that grow in low oxygen concentration
do the same: they consume glucose to produce lactic acid. These observations raised the question of
which are the alternative resource(s) for biomass production in cancer cells growing in hypoxia, in which
major amount of glucose is diverted to lactic acid synthesis.
My laboratory used modern methods of studying metabolism to show that hypoxic cells use glutamine
as a carbon source for anabolism. Moreover, we described for the first time in mammalian cells a novel
metabolic pathway that was detected in the past only in bacteria. Specifically we showed that low
oxygen levels or HIF2a expression use glutamine in a “reverse” TCA cycle progression to produce the
metabolites required for anabolic reactions. These findings described for the first time a novel metabolic
pathway in mammalian cells (called Reductive Carboxylation), which departs from the classic TCA cycle
paradigm and it provides insights into a mechanism by which hypoxic and HIF2a expressing cancer cells
compensate for the Warburg phenomenon (Metallo et al. Nature 2012; 481(7381): 380-4). We
delineated the mechanism driving Reductive Carboxylation and we also showed that reductive
carboxylation does not only happens in cells in culture but it can be detected in human RCC tumors
growing as xenografts in mice, when labeled with appropriate metabolic tracers. We therefore provided
for the first time in vivo evidence for the utilization of glutamine in tumors through reductive
carboxylation (Gameiro et al. Cell Metabolism 2013; 17(3): 372-385).
Very importantly from the therapeutic point of view, we discovered that VHL-deficient RCC ells are
sensitive to inhibition of the glutamine pathway and that administration of a drug that blocks the
utilization of glutamine by cells (glutaminase inhibitor CB839) is well tolerated and suppresses the
growth of human RCC tumors as xenografts in mice (Gameiro et al. Cell Metabolism 2013; 17(3): 372385). As a direct outcome of our studies on glutamine metabolism and its targeting in RCC cells, DF/HCC
initiated a Phase 1 trial with the GLS1 inhibitor CB-839 (Calithera Inc) titled “CB-839 in
Advanced/Refractory Solid Tumors” (PI: Othon Iliopoulos). We are very excited to report that the first,
heavily pre-treated, patient with metastatic RCC (one out of 3 enrolled at the currently well tolerated
dose) had 40% shrinkage of their RCC tumor.
3) Understanding the mechanisms of metabolism-based antineoplastic therapy as a basis for
combination treatment of HIF2a-expressing tumors.
Despite the mounting evidence that targeting glutamine suppresses the growth of cancer cells, such as
RCC, the precise mechanisms that are responsible for growth suppression are not known. Understanding
these mechanisms will provide transformative observations that will allow the contextual use of
metabolic inhibitors and make possible to find viable targets for combination therapy with glutaminase
inhibitors. Part of my laboratory is focusing now on understanding these mechanisms. Below we provide
an example of such mechanism that we recently discovered in the lab and how we exploited it for
therapeutic purposes.
Glutamine derived carbons are required for the generation of aspartate - the carbon source required for
nucleotide synthesis. We recently showed that treatment RCC cells with the clinically available
glutaminase inhibitor CB-839 depleted these cells from nucleotides. Nucleotide depletion induced
“stress” in the replicating DNA that led to breaks in the DNA fibers. These “breaks” occurred only in RCC
cells that lacked VHL protein and not in normal kidney cells. Repair of DNA breaks is required for cells to
maintain their viability and proliferation. When we combine glutaminase inhibitor CB-839 with an FDA
approved drug that inhibits the repair of DNA breaks (Olaparib, acting as PARP inhibitor) the result was
dramatic: normal cells grew fine while VHL-deficient RCC cells died in culture. More importantly,
established RCC tumors in mice were completely suppressed by the combination. The importance of
these findings links cancer metabolism to DNA replication stress and it extends beyond RCC to many
cancers with DNA repair defects (ovarian, breast and prostate). We will be actively targeting the
metabolism of these tumors for therapy.
C. Impact of our research
My laboratory works closely with collaborators in MGH Cancer Center, Brigham and Women’s Hospital
and the Broad Institute of Harvard and MIT. We are harnessing the latest technological advances in
combination with information from cancer genomic and epigenetic studies in human tumors and animal
models. It is not an exaggeration to say that given the experience we have in HIF2a-hypoxia signaling in
animal cells, mice and zebrafish models we are on the cusp of unprecedented discoveries of how to
target RCC and HIF2a-driven tumors.
Our observations are not restricted to RCC. Cancer angiogenesis and reprogrammed cancer metabolism
are essential cellular and organism processes, the contribution of which in tumor growth has been well
documented and validated in several animal models and in the clinic. For example, drugs inhibiting one
of the targets that HIF2a upregulates in cancer cells (Vascular Endothelial Growth Factor, VEGF) are now
approved for treatment of renal, ovarian and lung cancer, as single agents or in combination with
chemotherapy. One can imagine that the limited only success is due to the untargeted expression of
many other HIF2a-deriven genes that contribute to tumor growth. The discovery of HIF2a inhibitors or
drugs that target HIF2a-driven reprogramming of tumor metabolism (such as the glutaminase inhibitor
as a single agent or in combination with olaparib) may dramatically impact the treatment of these
diseases. Our fundamental discoveries of the cellular mechanisms that reprogram cancer metabolism
and the validation of their importance in cellular and animal models of RCC and other tumors may fulfill
the therapeutic promise of targeted therapy.