KineticsMatthew C. Ward and Richard Blake Ross

CELL KINETICS

MATTHEW C. WARD AND RICHARD BLAKE ROSS

Question 1

What are cyclin-dependent kinases (Cdks)? How are they related to cyclins?

Question 2

What are the names of the three most important checkpoints a cell encounters during the cell cycle?

Question 3

At which checkpoint are p53 and Rb most relevant?

Question 4

What is the mitotic index (MI) and how is it calculated?

Question 1 What are cyclin-dependent kinases (Cdks)? How are they related to cyclins?

Answer 1

Cdks are enzymes that regulate the cell cycle. Cdks complex with cyclins (labeled A–H), which are proteins. The activation of a cyclin protein by a Cdk will allow the cell to progress through the cell cycle. While Cdk levels are constant throughout the cell cycle, cyclin levels vary based on the cell cycle phase.

Hall EJ, Giaccia AJ. Cell, tissue and tumor kinetics. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:372–390.

Question 2 What are the names of the three most important checkpoints a cell encounters during the cell cycle?

Answer 2

The cell encounters the G1/S checkpoint, S phase checkpoint, and G2/M checkpoint.

Hall EJ, Giaccia AJ. Cell, tissue and tumor kinetics. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:372–390.

Question 3 At which checkpoint are p53 and Rb most relevant?

Answer 3

The purpose of a cell’s checkpoint is to check for errors prior to initiating the next step of the cell cycle. The proteins p53 and Rb are keys within the G1 checkpoint whose purpose is to detect DNA damage prior to initiating DNA synthesis. p53 also mediates the G2/M checkpoint.

Hall EJ, Giaccia AJ. Cell, tissue and tumor kinetics. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:372–390.

Question 4 What is the mitotic index (MI) and how is it calculated?

Answer 4

The MI is the fraction of cells undergoing mitosis. The MI can be observed by counting the number of cells undergoing mitosis or calculated using the formula:

where T_M is the time required for a cell to undergo mitosis and T_C is the time required for a cell to undergo the entire cell cycle. λ is a correction factor accounting for the uneven distribution of cells throughout the cell cycle.

Hall EJ, Giaccia AJ. Cell, tissue and tumor kinetics. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:372–390.

Question 5

What is the labeling index (LI) and how is it calculated?

Question 6

What technique, beyond the mitotic index (MI) and labeling index (LI), is required to determine the length of all phases in the cell cycle?

Question 7

Which phase of the cell cycle is the most variable and accounts for the majority of variation in cycle times between malignant and benign cell lines?

Question 8

What are the general principles of flow cytometry?

Question 5 What is the labeling index (LI) and how is it calculated?

Answer 5

The LI is used to determine the length of time required for a cell to pass through the S phase of the cell cycle. It is calculated by “flash labeling” cells with 5-bromodeoxyuridine and counting the proportion of cells that are labeled. The formula:

can be used to calculate the duration of the S phase (T_s) of the cell cycle.

Hall EJ, Giaccia AJ. Cell, tissue and tumor kinetics. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:372–390.

Question 6 What technique, beyond the mitotic index (MI) and labeling index (LI), is required to determine the length of all phases in the cell cycle?

Answer 6

Although the MI and LI can be used to quickly and simply determine the length of the M and S phase of the cell cycle, the percent-labeled mitoses technique is necessary to determine the time required for cells to pass through the additional phases (G1 and G2). This technique involves labeling a cohort of cells and serially sampling the cells, each time counting the fraction undergoing mitosis. It is a time and labor-intensive process and is difficult to perform in vivo due to the large number of samples required.

Hall EJ, Giaccia AJ. Cell, tissue and tumor kinetics. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:372–390.

Question 7 Which phase of the cell cycle is the most variable and accounts for the majority of variation in cycle times between malignant and benign cell lines?

Answer 7

The G1 phase is highly variable whereas S, G2, and M are fairly consistent across cell lines.

Hall EJ, Giaccia AJ. Cell, tissue and tumor kinetics. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:372–390.

Question 8 What are the general principles of flow cytometry?

Answer 8

Flow cytometry is a method by which single cells (usually fluorescently labeled) are passed through a light beam (usually a laser matched to the fluorescent dye) in order to count the fraction of cells labeled with the dye.

Hall EJ, Giaccia AJ. Cell, tissue and tumor kinetics. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:372–390.

Question 9

What is the growth fraction (GF)?

Question 10

Why do tumors typically grow more slowly than is predicted by the calculated cell cycle time?

Question 11

How is the cell loss factor defined?

Question 12

How does the tumor diameter doubling time relate to the cell doubling time?

Question 9 What is the growth fraction (GF)?

Answer 9

The GF is defined as the fraction of cells proliferating. Although highly variable, the typical GF for cancerous cell lines is between 30% and 50%.

Hall EJ, Giaccia AJ. Cell, tissue and tumor kinetics. In: Hall EJ, Giaccia AJ, eds. Radiobiology for the Radiologist. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:372–390.

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