Targeting cancer stem cells in therapy Alexandrina Burlacu, PhD “Nicolae Simionescu” Institute of Cellular Biology and Pathology, Bucharest, Romania Dept. of Regenerative Medicine, Laboratory of Stem Cell Biology

Workshop: Trends in nanomaterials field for cancer detection / therapy Bucharest, September 11th, 2015

Summary

What are stem cells? What is cancer and how does it appear? What is a cancer stem cell?

Brief introduction into stem cells Unspecialized cells that retain the ability to divide throughout life (self-renew) and give rise to cells that differentiate further into one or more specialized types

Symmetrical division

Asymmetrical division

Origin and classification of stem cells Plasticity = Potential to generate specialized cell types

√ Totipotents (216 specialized cell types; egg cell) √ Pluripotents (embryonic stem cells) √ Multipotents (adult stem cells, hematopoietic stem cells, etc) √ Unipotents/progenitor (produce a single differentiated cell type)

Adult stem cells ASC retain the ability to self-renew and give rise to highly specialized cells to replace the lost cells (contribute to homeostasis and tissue repair).

Localized in all tissues in the body e.g., bone marrow, gut epithelium, liver, pancreatic duct, CNS, skeletal muscle.

Bone marrow as an important source of adult stem cells Heterogeneous population of blood cells (red cells, megakaryocytes, myeloid cells –monocytes/macrofages and neutrophils - and lymphocytes), precursors of blood cells, HSC and stromal cells enclosed into an extracellular matrix enriched in fibronectin, collagen and proteoglycans. HSC = Multipotent cells that give rise to all blood cells

The main source of HSC (0.001% - 0.003% of total BM cells)

A subpopulation of cells (mesenchymal stem cells, MSC) with potential to differentiate towards cells of mesenchymal origin (adipocytes, chondrocytes, osteoblasts) MSC represent 10x less than HSC in bone marrow

Characteristics of adult stem cells  Cell surface markers HSC: Lin- (cells are negative for all blood lineages); CD34 (sialomucin); CD133; Sca-1 (stem cell antigen 1, Ly-6A/E); c-kit (Stem Cell Factor Receptor, CD117) MSC: Negative for hematopoietic markers (CD45, CD34, CD14, CD11), CD31, CD56, etc; Positive for CD105, CD73, CD44, CD90, Stro-1, VCAM-1, ICAM-1, CD166, CD29.

 Biological properties HSC: capacity to reconstitute the entire blood system of a recipient MSC: support for hematopoiesis immunomodulatory properties (reduce the incidence of GvHD) differentiation into specialized cells

 Quiescence divide little or not at all until required to differentiate, yet capable of self-renewal  Dye efflux properties through ATP-dependent transporters

Summary

What are stem cells? What is cancer and how does it appear? What is a cancer stem cell?

Introduction to cancer

Cancer = collection of diseases in which some of the body’s cells begin to divide without stopping and spread into surrounding tissues. Cancer can start almost anywhere in the human body.

Cancerous tumors are malignant, which means they can spread into, or invade, nearby tissues. As tumor grows, some cancer cells can break off and travel to distant places in the body through the blood or the lymph system and form new tumors far from the original tumor. Many cancers form solid tumors, which are abnormal masses of tissue (cancers of the blood, such as leukemias, generally do not form solid tumors).

Tumor heterogeneity:  

Inter-tumor heterogeneity Intra-tumor heterogeneity

Evidence for cancer stem cells

Small numbers of disseminated cancer cells have often been detected at sites distant from primary tumors in patients that never manifested metastatic diseases. • •

immune surveillance was highly effective at killing disseminated cancer cells before they could form a detectable tumour most cancer cells lacked the ability to form a new tumour such that only the dissemination of rare (“special”) cancer cells could lead to metastatic disease For leukemia and multiple myeloma: only a small subset of cancer cells is capable of extensive proliferation •

Bruce et al., 1963: only 1%–4% of lymphoma cells can form colonies in vitro or initiate carcinoma in mouse spleen).

Solid cancers: cells are phenotypically heterogeneous; only a small proportion of cells are clonogenic in culture and in vivo •

only a few cancer cells are tumorigenic

Cancer stem cells

Al-Hajj, 2003: demonstrated the presence of CSCs in breast cancer (first report of CSCs in solid cancer)

To date, CSCs have been discovered in a broad spectrum of solid tumors: lung cancer, colon cancer, prostate cancer, ovarian cancer, brain cancer, and melanoma, among others.

Summary

What are stem cells? What is cancer and how does it appear? What is a cancer stem cell?

Cancer stem cells  cells within a tumor that possess the capacity to selfrenew and to cause the heterogeneous lineages of cancer cells that comprise the tumor;  identified as rare cell populations in many cancers, including leukemia and solid tumors;  thought to be responsible for:  Cancer initiation  Cancer progression  Cancer metastasis  Cancer recurrence  Cancer drug resistance

 can generate tumor cells with different phenotypes, which results in the growth of the primary tumor and emergence of new tumors;  Most tumors are thought to arise from a single cell that develops into a heterogeneous population;

The origin of CSC Hypothesis #1: Cancers originate in a stem cell Hypothesis #2: Cancer cells originate from a progenitor cells Hypothesis #3: Cancers may arise from differentiated cells (the cancer-inducing genetic alteration confers the capabilities of a stem cell to a differentiated cell)

Targeting CSC in cancer therapy

If tumor growth and metastasis are driven by a small population of cancer stem cells, this might explain the failure to develop therapies able to eradicate solid tumors. Currently available drugs can only shrink metastatic tumors, but these effects are usually transient and often do not appreciably extend the life of patients.

CSC hypothesis: tumors have a core population of drug-resistant pluripotent cells that can respond to therapy by generating a new tumor population The existence of CSCs implies the presence of a small pool of slow cycling cells that carry the oncogenic mutation and are apparently insensitive to antiproliferative treatments, although their cellular descendants are sensitive.

Targeting CSC in cancer therapy

Reasons for treatment failure  acquisition of drug resistance by the cancer cells as they evolve  Inefficiency in killing cancer stem cells

 By selectively targeting surface markers of CSCs  By intervening aberrant pathways, (to suppress specific characteristics of CSCs)  By molecular drugs inhibiting ABC cassette  By controlling tumor microenvironment (cutting off the growth of blood vessels or exploiting the environmental pH, etc).

Final words  Although CSCs have been identified in numerous human cancers, whether or not CSCdirected therapies will ultimately lead to cures remains an open question.  CSC-directed therapies show great promise for improving clinical outcomes, but it will be important for researchers to verify the properties of CSCs in each tumor type since they likely will not be uniform with respect to their biologic properties, consistent with their heterogeneous molecular origins.