21.3 Cancer stem cells and metastasis

Cancer Stem Cells

Cancer stem cells and tumor dynamics

Not all cancer cells are equal. Within a tumor, a small subpopulation called cancer stem cells (CSCs) acts as the engine behind tumor growth, spread, and recurrence. What makes them so dangerous is that they share two defining properties with normal stem cells:

• Self-renewal: CSCs can divide to produce more CSCs, maintaining a persistent pool of these cells within the tumor.
• Differentiation: CSCs can give rise to the various non-stem cancer cell types found in a tumor, which is a major source of tumor heterogeneity (the mix of different cell types within a single tumor).

Because of these properties, CSCs are considered the driving force behind tumor initiation. A single CSC can, in principle, seed an entirely new tumor. They also contribute to tumor progression by promoting angiogenesis (the formation of new blood vessels to supply the tumor) and invasion into surrounding tissues.

Perhaps most clinically significant, CSCs are often resistant to conventional chemotherapy and radiation. These treatments may kill the bulk of tumor cells but leave CSCs intact. The surviving CSCs can then repopulate the tumor, leading to relapse and recurrence.

Molecular mechanisms of stem-like properties

Several signaling pathways and molecular regulators work together to maintain the stem-like state of CSCs.

Key signaling pathways:

• Notch signaling regulates whether a CSC self-renews or differentiates. Abnormal Notch activation tips the balance toward maintaining the CSC pool.
• Wnt signaling promotes CSC proliferation and survival, supporting the expansion of the CSC population.
• Hedgehog signaling helps maintain CSCs and drives tumor growth. Like Notch and Wnt, it's a normal developmental pathway that becomes hijacked in cancer.

These three pathways are frequent targets in experimental CSC therapies precisely because they're so central to CSC maintenance.

Transcription factors:

The transcription factors Oct4, Sox2, and Nanog are the same core regulators that maintain pluripotency in normal embryonic stem cells. In CSCs, they keep cells in an undifferentiated, self-renewing state. Their expression is a hallmark of stemness.

Epigenetic modifications:

Changes in DNA methylation and histone modifications can activate stemness-related genes while silencing genes that would push the cell toward differentiation. These epigenetic shifts don't alter the DNA sequence itself but reshape which genes are accessible for transcription.

The CSC niche:

CSCs don't survive in isolation. They depend on a specialized microenvironment (niche) made up of stromal cells, extracellular matrix components, and secreted signaling molecules. The niche provides survival signals that help CSCs maintain their stem-like properties, much like how normal stem cell niches support tissue-resident stem cells.

Metastasis

Process of metastasis and cell invasion

Metastasis is the spread of cancer cells from the primary tumor to distant organs. It's the leading cause of cancer-related death, and it unfolds through a series of distinct steps:

1. Local invasion: Cancer cells break through the basement membrane and invade surrounding tissue.
2. Intravasation: Cancer cells enter the bloodstream or lymphatic system.
3. Circulation: Cancer cells travel through the circulatory system. Most circulating tumor cells die during this stage, but a small fraction survives.
4. Extravasation: Surviving cancer cells exit the bloodstream and invade the tissue of a distant organ.
5. Colonization: Cancer cells establish a secondary tumor (metastasis) at the new site. This is the least efficient step; many cells that reach a distant organ fail to colonize it.

Several molecular mechanisms enable cancer cells to complete these steps:

• Epithelial-mesenchymal transition (EMT): Cancer cells lose their epithelial characteristics (tight cell-cell adhesion, polarity) and gain mesenchymal traits (increased motility, invasiveness). EMT is a key switch that transforms stationary tumor cells into migratory ones.
• Matrix metalloproteinases (MMPs): These enzymes degrade the extracellular matrix, clearing a physical path for invading cancer cells.
• Cell adhesion molecules: Proteins like integrins and cadherins regulate how cancer cells attach to and detach from their surroundings. Downregulation of E-cadherin, for example, is a hallmark of EMT and loosens cell-cell connections that would otherwise keep cells anchored in place.

Challenges in targeting cancer stem cells

Eliminating CSCs is one of the biggest unsolved problems in cancer therapy. Several factors make this difficult:

• Treatment resistance: CSCs often express drug efflux pumps, enhanced DNA repair mechanisms, and anti-apoptotic proteins that help them survive chemotherapy and radiation.
• Dormancy: CSCs can enter a quiescent state, essentially hiding from therapies that target rapidly dividing cells. They may reactivate months or years later, causing recurrence.
• Identification: CSCs are rare within a tumor and share many markers with normal stem cells, making it hard to distinguish and isolate them without harming healthy tissue.

Current research strategies focus on several approaches:

• Developing therapies that specifically target CSC signaling pathways (Notch, Wnt, Hedgehog) or CSC surface markers
• Combining CSC-targeted therapies with conventional treatments to attack both CSCs and bulk tumor cells simultaneously
• Investigating how the tumor microenvironment supports CSCs, since disrupting niche signals could strip CSCs of the support they need to survive