New Therapeutic Strategies to Treat Metastatic Cancer

Keynote: Hypoxia-Inducible Factors Promote Evasion of Anti-Tumor Immunity

Physician scientist Gregg Semenza is a leader in cancer biology, where he has dedicated his career to uncovering how cells sense and adapt to oxygen availability. He was recognized for his groundbreaking work in 2019 when he was jointly awarded the Nobel Prize in Physiology or Medicine with William Kaelin and Peter Ratcliffe. Semenza discovered that a group of DNA-binding transcription factors called hypoxia-inducible factors (HIFs) maintain oxygen homeostasis. HIFs bind over 4,000 target genes to activate transcription, but cancer cells hijack this system to leverage HIFs to adapt to the hypoxic environment. “In most cancers, the HIFs are highly expressed…the higher the expression the poorer the outcome in many different cancers,” said Semenza.

Reduced oxygen availability is a hallmark of breast cancers.

To better understand this process, Semenza’s lab examined HIF binding targets in melanoma and breast cancer cells. They found that one particular target, anti-apoptotic protein BIRC2, becomes highly expressed under hypoxic conditions. A knockdown of BIRC2 decreased tumor growth and increased intratumoral activated T cells, suggesting that BIRC2 expression impairs anti-cancer immunity. This finding suggests that inhibition of BIRC2 expression combined with immune checkpoint inhibitors may improve treatment outcomes for breast cancer and melanoma.

Single Cell Resolution of the Pre-Metastatic Niche

Rosandra Kaplan is a physician scientist studying the development of metastatic cancer. Specifically, she aims to understand the “pre-metastatic window” in which, prior to metastasis, the tumor microenvironment begins to support the dissemination of tumor cells throughout the body. “Patients with localized disease are at risk for metastasis pretty much indefinitely while they remain alive with their cancer,” Kaplan explained. “This is a window of opportunity to really understand how we can therapeutically modulate [this risk].” Kaplan’s lab investigates the pre-metastatic window in mice by implanting tumor cells into their leg muscles, which metastasizes to the lungs. Whole-lung bioluminescence and flow cytometry are used to monitor disease progression. Single-cell sequencing of the pre-metastatic lung cell population revealed a significant upregulation of immune suppression genes and a downregulation of T cell immunity. They found that hematopoietic stem and progenitor cells from bone marrow differentiate into immunosuppressive myeloid cells prior to metastasis. Additional work in Kaplan’s lab showed that delivery of anti-tumor factors restored T cell populations in the lung and significantly prolonged mouse survival. Next, Kaplan aims to translate this approach to patients in the clinic.

Persistence of Tumor Associated Microbiota in Metastases

Susan Bullman examines the connection between microbes and cancer. “If a bacterial agent can contribute to cancer initiation or progression, then these organisms are viable targets for the prevention and treatments of cancers,” said Bullman. One potential target is Fusobacterium nucleatum, an anaerobic bacterium that is enriched in colorectal cancer. Bullman wondered whether Fusobacterium plays a role in metastasis as well. She obtained primary tumor and associated liver metastasis samples from over 100 patients and found that Fusobacterium indeed persisted in metastatic tissue. Whole-genome sequencing of the cultured bacteria further revealed that the same strain was present in both the primary tumor and the metastases. Bullman’s lab then worked with collaborators to implant patient-derived xenografts of CRC into mice. Not only did they discover that Fusobacterium remains viable across multiple generations, but they found that antibiotic treatment of mice led to significant reduction in tumor growth. These exciting findings suggest that modulation of the microbiome may possess therapeutic potential for this disease.

Neural Regulation of Hematopoietic Stem Cells and Cancer Cells

Paul Frenette studies hematopoietic stem cells (HPSCs) and their connection to cancer. HPSCs are influenced by the sympathetic nervous system, which mobilizes their transition from bone marrow to blood (and vice versa) under homeostasis. In prostate cancer, tumor sites are invaded by nerves, so the nervous systems’ interaction with HSPCs may influence the tumors.

Nerves at the tumor site invade prostate cancer.

To understand the factors that drive this process, Frenette’s lab uses mouse models of prostate cancer. Their work revealed that activation of the muscarinic acetylcholine receptors of the cholinergic nervous system promotes tumor progression and metastases to distant sites. Frenette’s lab surgically removed nerve connections to the prostate to confirm the sympathetic nervous system’s role in this process. Interestingly, these mice did not develop tumors, indicating that sympathetic signals are critical for tumor initiation. Next steps in the lab aim to uncover further detail into the mechanisms in this process that allow cancer cells to spread.

Keynote: Non-Cell-Autonomous Drivers of Breast Tumor Progression

Kornelia Polyak studies the molecular underpinnings of breast cancer. In particular, she is interested in identifying non-cell-autonomous drivers that produce mutant cells—which influence other cells­—regardless of their genotype, to exhibit a mutant phenotype. In cancer, these drivers lead to greater tumor heterogeneity, which is associated with greater instances of metastasis and therapeutic resistance. To target non-cell-autonomous drivers of breast cancer, Polyak’s lab identified eighteen factors that had the potential to promote tumor growth. Each factor was individually overexpressed in a breast cancer cell line to create a sub-clone.

Polyclonal tumors maintain tumor heterogeneity and exhibit faster tumor growth compared to monoclonal tumors.

Interestingly, very few sub-clones were able to drive tumor growth on their own. Only the polyclonal tumors, which were created from mixing all eighteen sub-clones, led to fast growing tumors. Of the sub-clones that could support tumor growth, one called IL11 was identified as a non-cell-autonomous driver after Polyak’s lab observed that even cancer cells that did not directly interact with IL11 supported tumor growth. Ultimately, Polyak hopes that therapeutic targeting of non-cell-autonomous drivers in the clinic will improve patient outcomes.

Understanding Mechanisms of Metastatic Relapse After Targeted Therapy

Swarnali Acharyya’s lab researches drug resistance in cancer metastasis. Lung cancer is particularly susceptible to systemic metastases, including the lymph nodes, bones, and the brain. Studies have shown that the epidermal growth factor receptor (EGFR) signaling pathway is a driver of lung cancer in 20% of Caucasian patients and more than 50% of Asian patients. Although EGFR drug treatments improve patient survival, 60%-70% of patients still relapse, and many develop brain metastases. To understand what contributes to drug resistance in these patients, Acharyya’s lab created models of lung cancer in mice. As expected, they observed that drug treatment prolonged survival, but mice eventually died of brain metastasis. Isolating cells from the metastases revealed that immune response protein S100A9 was abundantly expressed in metastatic brain lesions. Acharyya suspected that targeting the CREB pathway, which involves S100A9, might reduce metastasis. Excitingly, she observed that CREB suppression in S100A9-expressing cancer cells significantly reduced brain metastasis in mice. Acharyya hopes this treatment can eventually be applied to lung cancer patients in the clinic.

Exercise Regulation of the Host-Tumor Interaction

Lee Jones is an exercise physiologist studying the effects of exercise on cancer. Observational studies have long reported that cancer can be prevented through exercise, but this work has relied on self-reporting and inconsistent definitions of exercise. “It gives us a signal that exercise might be doing something,” said Jones, “but certainly doesn’t prove that exercise delays the initiation of cancer or the progression of cancer.”

Exercise influences the interaction between the host and tumor microenvironment.

To test the role of exercise directly, the Jones lab uses genetically engineered mouse models with breast cancer, which are prescribed a “dose” of exercise in minutes per week. They discovered that the mice that exercised more than 150 minutes per week had significantly lower tumor incidence, delayed tumor progression, and reduced metastases to the lungs. To test these findings in cancer patients, the Jones lab recently developed a digital clinical trial platform called “DigitEx” that enables remote monitoring of patients and high dimensional data collection. Future work aims to expand trial enrollment to obtain conclusive data on the role of exercise in cancer treatment.

Why is Skeletal Muscle Anti-Metastatic?

Cancer scientist Cyrus Ghajar is fascinated by skeletal muscle. He is particularly puzzled by its uniquely “anti-metastatic” nature: although metastatic cancer cells can occupy skeletal muscle, they do not proliferate, typically remaining dormant for years or even decades. Understanding what differentiates skeletal muscle may enable researchers to protect more vulnerable organs, like lungs or bone marrow, from metastatic growth. Toward this goal, Ghajar’s lab models metastatic disease by introducing breast cancer cells into skeletal muscle myofibers and lung fibroblasts. The skeletal muscle myofibers suppress tumor growth while lung fibroblasts do not. To examine differences between the two, Ghajar’s lab conducted RNA sequencing and extracellular metabolomics. RNA sequencing showed that tumor cells downregulate oxidative phosphorylation pathways to promote metastasis. Extracellular metabolomics revealed that skeletal muscle exhibits enhanced oxidative stress compared to lung. Together, these results suggest that redox metabolism may play a central role in skeletal muscle-mediated suppression of metastasis. Ghajar hopes that future work can leverage this finding to advance metastatic cancer therapies.