Cancer Cell Biology: Mutations, Metabolism, and Gene Regulation

1: Carcinoma (Flat) and Sarcoma (Round)

Cancer Cell Characteristics and Hallmarks

Carcinoma (flat) and sarcoma (round) both arise from cancer in mesenchymal cells. The Hallmark of Cancer concept examines cancer at the cellular level, recognizing that cancer results from numerous different mutations.

Tumor Suppressor Proteins

A key example is the p53 tumor suppressor protein. Tumor suppressors act as the cell's "brakes" because they control cell division. When cancerous, these proteins often show a loss of function due to mutation, resulting in fewer active target proteins.

• CO2 Removal: Carbon dioxide (CO2) is the easiest molecule to remove from the cell due to its small molecular size.
• Structural Impact: Tumor suppressors affect tertiary/quaternary protein structure more significantly than secondary structure because tertiary/quaternary structures rely on hydrophobic interactions, unlike the hydrogen bonding prevalent in secondary structure.

Nucleic Acid Differences

DNA has a hydrogen atom (H) on the 2' carbon, whereas RNA has a hydroxyl group (OH) on the 2' carbon.

Cell Signaling Mechanisms

Signaling molecules utilize different pathways:

1. Endocrine: Signaling via the bloodstream.
2. Paracrine: Signaling via the extracellular fluid.
3. Autocrine: Signaling affecting the cell surface or immediate surrounding area.

All signaling molecules destined for secretion or membrane insertion are translated at the rough ER by membrane-bound ribosomes and subsequently packed into vesicles.

Chromatin Structure and Gene Control

Chromatin States

• Euchromatin: Loosely packed chromatin, created by both acetylation of histones and hypomethylation of DNA.
• Heterochromatin: Condensed chromatin with minimal histone acetylation.

Controlling protein synthesis at its source—by regulating when a protein is created—represents the most efficient way cells control protein function.

Methylation and Cancer

Typically, tumor suppressor genes are unmethylated (active), and proto-oncogenes are methylated (inactive).

Decreased methylation contributes to cancerous conditions by activating proto-oncogenes. Removing methylation activates these growth-promoting genes, leading to uncontrolled proliferation. Conversely, tumor suppressor genes are usually already active.

Cellular Metabolism in Cancer

ATP Production Pathways

Two primary methods of ATP generation are contrasted:

1. Oxidative Phosphorylation (Mitochondrial Respiration): Highly efficient, yielding 36–38 ATP per glucose molecule.
2. Warburg Effect (Aerobic Glycolysis): Low ATP efficiency, yielding only 2 ATP per glucose molecule, but characterized by rapid glucose consumption.

Metabolic Intermediates

The oxidation of glucose produces NADH and Acetyl-CoA in a reduced state; H2O is not a reduced product.

Cancer cells often exhibit high levels of phosphorylated pyruvate dehydrogenase.

Key Reactions and Redox Balance

• Pyruvate Oxidation (converts pyruvate to acetyl-CoA) and the Krebs Cycle (oxidizes acetyl-CoA) produce NADH but do not regenerate NAD+.
• The Electron Transport Chain (ETC) oxidizes NADH, converting it back to NAD+, which is essential for glycolysis and the Krebs cycle to continue.

Preventing Cancerous Conditions

Two primary epigenetic mechanisms can help prevent or reverse cancerous conditions:

1. Demethylation of Tumor Suppressor Genes: Removing DNA methylation restores normal cell cycle control.
2. Histone Acetylation: Increased histone acetylation alters chromatin structure, which can reactivate critical, silenced genes.