CANCER: CARCINOGENS and CARCINOGENESIS

BIOM 255 Drugs and Disease: The pathophysiological and molecular bases of disease and drug therapy March 3, 2009 Hyam Leffert, MD

OVERVIEW

Agents (molecules, radiation or viruses) that cause cancer are called carcinogens. This causal process is called

carcinogenesis

.

In humans, it is generally accepted that most epithelial cancers are caused by environmental exposure to certain kinds of chemicals.

However, carcinogenesis is clearly influenced by large numbers of genes and non-carcinogenic environmental factors (notably diet, and underlying inflammation).

This discussion will review basic principles of cancer and chemical carcinogenesis, focusing upon hepatocarcinogenesis as a model system.

We will also consider two outstanding questions:

1. What molecular events are thought to be critical for carcinogenesis?

2. What carcinogen-targeted cells give rise to cancer?

References 1. Hanahan D, Weinberg RA. The hallmarks of cancer. Cell. 2000 Jan 7;100(1):57-70. [Assigned reading] 2. Balmain A, Harris CC. Carcinogenesis in mouse and human cells: parallels and paradoxes. Carcinogenesis. 2000 Mar;21(3):371-7. 3. Vogelstein B, Kinzler KW. Cancer genes and the pathways they control. Nature Med. 2004; 10(8):789-799. 4. Luch A. Nature and nurture - lessons from chemical carcinogenesis. Nat Rev Cancer. 2005 Feb;5(2):113 25. [Assigned reading] 5. Li HC, Stoicov C, Rogers AB, Houghton J. Stem cells and cancer: Evidence for bone marrow stem cells in epithelial cancers. World J Gastroenterol. 2006 Jan 21;12(3):363-71. 6. Weinberg RA. The Biology of Cancer. 2007. Garland Science, New York, NY (This recently published textbook and accompanying CD-R is in the SOM LIBRARY RESERVE)

BASIC PRINCIPLES

WHAT DO CANCERS LOOK LIKE?

Cancerous tissues and metastatic cancer cells (that migrate away from primary sites) display disorganized arrays of normal macroscopic and microscopic tissue structure.

Hepatocellular Carcinoma (http://www.emanet.org/) Ovarian Carcinoma (from BIOM 255 lecture of S. Howell, MD) Acute Myelogenous Leukemia (Bone Marrow) (http://en.wikipedia.org/) Breast Carcinoma (Inflammatory) (http://www.emedicine.com/)

PROPERTIES OF CANCER CELLS

(diagram is from Reference #1)

THOUSANDS OF KNOWN CARCINOGENIC AGENTS ARE IN THE ENVIRONMENT

• • • • • •

Organic molecules (PAHs, aromatic amines) Inorganic molecules (vinyl chloride) Heavy metals (lead, arsenic, chromium[VI]) Viruses (HBV, HCV, HPV, HIV) Radiation (gamma, X-ray, high energy beta) Inert substances (asbetos) (See Reference #2, and Reference #4 for Summary Table)

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CARCINOGENESIS INVOLVES 2 STEPS

1.

INITIATION (“initiators”) Rapid Usually requires metabolic activation (procarcinogen



carcinogen) Sufficient to induce carcinogenesis if exposure dose is high enough (controversial) Precedes promotion May cause mutations or epigenetic changes

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2. PROMOTION (“promoters”) Follows initiation Long period of time Augments effects of low dose initiator Associated with increased survival and/or clonal expansion of initiated clones

METABOLIC ACTIVATION REQUIRES SPECIALIZED ENZYMES: e.g. CYP450s (receptors, co-factors and many other enzymes are also involved) (diagram is from Reference #4)

PROPOSED CARCINOGENIC EVENTS Genotoxic and non-genotoxic effects of carcinogens (diagram is from Reference #4)

PROPOSED CARCINOGENIC PATHWAYS (diagram is from Reference #1; see also Reference #3)

LIVER CANCER

Diagram of the HEPATIC LOBULE

HEPATOCARCINOGENESIS

• Hepatocellular carcinomas (‘HCC’, parenchymal cell-like) are the most frequent type of liver cancer.

• Millions of people world-wide are diagnosed yearly with HCC, mostly in Southeast Asia and Africa.

• High mortality rates with widespread metastasis (preference for lung).

• Gender preference due to endocrine status (male >> female).

• HCC is strongly associated with viral (HBV, HCV), chemical (aflatoxin-B 1 ) and dietary exposures (alcohol). • Inflammation may act as a promoter of HCC.

• Non-HCC hepatic angiosarcomas are caused by exposure to vinyl chloride.

Diethylnitrosamine (DEN) metabolized by CYP2E1 induces hepatocarcinogenesis in genetically targeted mutant mice: Hepatocyte Ikk ß-deletion, oxidative stress, cell death, liver inflammation and growth advantages of Ikk ß-deleted hepatocytes augment carcinogenesis in this animal model; tumor suppressor(s) may also be involved.

(from Maeda S. et al. CELL 121:977-990, 2005; Koch KS et al., BBRC 380:349-354,2009)

(from Reference #5)

CARCINOGEN-TARGETED CELLS…which one(s) ?

- The cellular target(s) of chemical carcinogens are unknown.

- The key questions are whether parenchymal cells (e.g. hepatocytes) or non parenchymal cells (e.g. stem cells) are the targets? Or both…? - The answers are critical since mechanism and drug therapy studies may be directed towards the wrong cell type(s).

- Some investigations suggest that cellular targets may also include non resident cells that enter tissues via the blood or lymphatic system. - Carcinogens may also alter the functions of cells that do not generate tumors but are required for tumor cell clonal expansion and survival (i.e. tumor progression).

HEPATOCARCINOGENESIS (HCC) LINEAGE MODELS (Diagram is from Koch KS, Leffert HL. Normal liver progenitor cells in culture, in Stem Cells Handbook, ed. S. Sell, Humana Press, Inc., Totowa, NJ, pp. 367-384, 2004).