Cell and tissue kinetics Flashcards
Which of the following CDK or cyclin is paired with the correct phase transition?
A. CDK1 (CDC2) – G2 into M
B. CDK4 – S into G2
C. cyclin A – G2 into M
D. cyclin B – S into G2
E. cyclin D – M into G1
A
CDK1 (and cyclin A/B) is associated with the G2 to M cell cycle phase transition.
The other CDKs and Cyclins are appropriately paired as follows:
- G1 phase -> S phase: CDK4 and Cyclin D1; CDK2 and Cyclin E
- S phase -> G2 phase: CDK2 and Cyclin A
- G2 phase -> M phase: CDK1 and Cyclin B/A
Irradiation of an exponentially-growing population of cells in culture with a dose that kills 90% of cells tends to select surviving cells that are initially in which phase of the cell cycle?
A. G0
B. G1
C. S
D. G2
E. M
C
A dose that kills 90% of the cells in the population would leave a surviving cell population heavily enriched in the radioresistant cells in late S phase. Radiosensitivity across the cell cycle is ranked as follows from least to most sensitive: Late S, Early S, G1, G2≈M.
The typical cell cycle time (TC) for proliferating cells in human tumors is in the range of:
A. <1 day
B. 1-5 days
C. 6-25 days
D. 26-100 days
E. >100 days
B
The typical TC for tumor cells in vivo is generally in the range of 1-5 days.
Which of the following statements concerning the cell cycle kinetics of tumors is TRUE?
A. Often, the cell loss factor (Φ) decreases several weeks after the start of radiotherapy
B. The growth fraction (GF) is the ratio of the number of viable cells to the sum of viable and non-viable cells
C. If the volume doubling time (TD) is 60 days and the potential doubling time (Tpot) is 3 days, then the cell loss factor is 5%
D. Tpot has proven useful in predicting tumor response to accelerated radiotherapy
E. Typically, the cell loss factor is not of major importance in determining a tumor’s volume doubling time
A
The cell loss factor (Φ) is the percent of newly produced cells that die or fail to continue dividing. Most human tumors have a ϕ of around 77 % and explains why tumors grow in-vivo far more slowly than would be expected based upon cell doubling time.
CLF=1‐Tpot/Td
Where:
- Tpot= potential doubling time = the time necessary to double the number of proliferating tumor cells in the absence of spontaneous cell loss.
- Td= volume doubling time = the actual doubling time observed.
Cell loss factor (CLF, ϕ) often appears to decrease several weeks after the start of radiotherapy, which has the net effect of slowing tumor regression. The growth fraction is the ratio of the number of proliferating cells to the sum of proliferating and quiescent cells (Answer Choice B).
If the observed tumor volume doubling time (TD) is 60 days and the potential doubling time (Tpot) calculated from the cell cycle time and the growth fraction is 3 days, then the cell loss factor is 95% (Answer Choice C).
Although Tpot (as measured from a tumor biopsy derived from patients previously given bromodeoxyurdine) has not proven to be a robust predictor of long-term outcome after accelerated radiotherapy, it might still be useful for the pre-selection of patients most likely to benefit from accelerated treatment (Answer Choice D).
For carcinomas, the cell loss factor is usually the major determinant of the discrepancy between a tumor’s potential doubling time and its overall volume doubling time (Answer Choice E).
Exponentially growing cells are pulse-labeled with tritiated thymidine and sampled as a function of time thereafter. The time required for the percent of labeled mitoses to reach 50% of its maximum value corresponds approximately to:
A. TS
B. TC
C. TG2
D. TG1 + TS/2
E. TG2 + TM/2
E
The question refers to the ‘percent-labeled mitoses technique’ using tritiated thymidine. The radiolabeled thymidine is added to the growth medium of cells for about 20 minutes (flash labeling). Only those cells in S-phase take up the label. The length of time required for the first radioactively-labeled S-phase cells to first enter mitosis, as measured using the percent-labeled mitosis technique, would correspond to the duration of the G2 phase (TG2). The additional time required for the cells to completely fill the mitotic compartment (i.e., 100% labeled mitoses) would be equal to the length of M (TM). The time to reach 50% of the maximum point, therefore, corresponds to TG2 plus TM/2.
If the mitotic index of a cell line is 5%, the growth fraction is 100%, the cell cycle time is 14 hours, and the correction factor, lambda, is 0.7, what is the approximate length of mitosis (TM)?
A. 0.2 hours
B. 1 hour
C. 2 hours
D. 4 hours
E. 8 hours
B
Using the equation MI = (lambda)TM/TC, where MI is the mitotic index, TM is the length of mitosis and TC is the total cell cycle time, then TM = (MI)(TC/lambda) = (0.05)(14 hours)/0.7 = 1 hour. Of note, the TM for most mammalian cells is typically ~1 hour.
Which of the following is the main reason why the volume doubling time of a tumor rarely equals its potential doubling time?
A. High cell loss factor
B. High metastatic propensity
C. Long cell cycle time
D. Low hypoxic fraction
E. Low growth fraction
A
A tumor’s volume doubling time rarely equals its potential doubling time because most tumors have a high cell loss factor. Formation of metastases represents only one of many reasons for cell loss, and usually is only a minor contributor (Answer Choice B).
Human tumor cells typically have cell cycle times of a few days whereas tumor volume doubling times are generally on the order of months (Answer Choice C).
The presence of a high hypoxic fraction would probably contribute to a low growth fraction, which would affect both Tpot and volume doubling time. If hypoxia were a significant cause of cell death, it would affect the cell loss factor and therefore affect the volume doubling time. The presence of non-proliferating cells affects both the tumor volume doubling time and the potential doubling time and does not cause a difference between them. In addition, non-viable cells (whether hypoxic or aerobic) have similar effects on the tumor volume doubling time and the potential doubling time (Answer Choice E).
Which of the following statements concerning tumor kinetics is TRUE?
A. Cell-cycle times (Tc) are longer than potential doubling times (Tpot) because of the presence of non-proliferating cells
B. The Tpot is usually shorter than the volume doubling time because the growth fraction (GF) is usually less than 100%
C. Tpot can be determined if the mitotic index (MI) and the duration of S phase (Ts) are known
D. Tumors with long values for Tpot are good candidates for accelerated radiotherapy
E. In the absence of cell loss, Tpot would equal the volume doubling time (TD) of the tumor
E
The cell loss factor (Φ) is equal to 1-(Tpot/TD). Therefore, if the cell loss factor were zero, then the Tpot would equal the TD.
The mean TC is shorter than the Tpot becauseTpot also considers the presence of quiescent cells, and the growth fraction in tumors is generally less than 100% (Answer Choice A).
For solid tumors the Tpot is generally much shorter than the TD because the cell loss factor is typically quite high. The GF is taken into account in the determination of Tpot, so it does not affect the relationship between the Tpot and the TD (Answer Choice B).
Tpot can be calculated from the labeling index (LI) and the duration of S phase (TS) using the equation Tpot = TS/LI. (where λ is a constant ranging from about 0.6 to 1.0; Answer Choice C).
It has been suggested that tumors with short pretreatment values for Tpot, (suggesting the presence of rapidly proliferating cells and a high growth faction), would be most likely to benefit from accelerated radiotherapy, but this has not been confirmed in clinical trials performed to date (Answer Choice D).
Which of the following substrates and target sites of the ATM kinase are implicated in the control of the G2-checkpoint in irradiated cells?
A. CHK2 (CHEK2) and MDM2
B. NBS1 (NBN) and CHK2
C. CHK2 and CDC25C
D. CHK2 and p53 (TP53)
E. PUMA and p53 (TP53)
C
Regulation of the G2 checkpoint by ATM is thought to occur via the activation of CHK2, which phosphorylates CDC25C phosphatase thereby preventing it from dephosphorylating CDK1 (CDC2), a step necessary for the progression from G2 into M phase. The remaining proteins listed are all targets for phosphorylation by the ATM kinase, and, consequently, are implicated in various cell cycle control pathways although not the G2 checkpoint. CHK2 and MDM2 are involved in control of the G1–S phase transition (Answer Choice A). ATM also phosphorylates MDM2, which reduces the ability of MDM2 to negatively regulate p53. NBS1 and CHK2 are implicated in S phase progression (Answer Choice B). Upon phosphorylation by CHK2, p53 is stabilized and causes cell cycle arrest in G1 (Answer Choice D). PUMA (“p53-upregulated modulator of apoptosis”) is a pro-apoptotic gene that can induce cell death via a p53- dependent pathway.
Which of the following pairs of chemicals could be used with flow cytometry to determine the S phase fraction of a cell population and estimate of relative DNA content?
A. Bromodeoxyuridine (BrdU) and propidium iodide
B. Tritiated thymidine and hydroxyurea
C. Dichlorohydrofluorescein and cytochrome c
D. H2AX and ethidium bromide
E. Sphingomyelin and ceramide
A
Bromodeoxyuridine (BrdU) is incorporated into DNA in place of thymidine, so it can be used to label cells in S-phase. The incorporated bromodeoxyuridine is assayed using a fluorescently-labeled anti-BrdUrd antibody. Propidium iodide fluoresces when incorporated into DNA. The amount of fluorescence is directly proportional to the DNA content, which, in turn, is a reflection of the cell cycle phase in which the cell is located.
If a tumor is comprised of cells characterized by a high growth fraction and a short cell cycle time, which of the following would most likely describe its behavior prior to and after treatment with a curative dose of radiation?
A. Slow growth, slow regression
B. Slow growth, rapid regression
C. Rapid growth, rapid regression
D. Rapid growth, slow regression
C
Tumor types with a high growth fraction and short cell cycle time would be expected to grow more rapidly. Such a tumor would also be expected to regress rapidly after irradiation since irradiated cells generally die as they attempt to progress through mitosis.
What is the most probable range of cell cycle time (Tc) and tumor doubling time (Td) for human tumors?
A. Tc, 1 to 5 days and Td, 20 to 30 days
B. Tc, 1 to 5 days and Td, 40 to 100 days
C. Tc, 0.5 to 1 days and Td, 20 to 30 days
D. Tc, 0.5 to 1 days and Td, 40 to 100 days
E. Tc, 1 to 2 days and Td, 120 to 300 days
B
The volume doubling time (TD) of human tumors is characteristically 40 to 100 days, while the cell cycle time is relatively short, generally between 1 to 5 days. This has important implications, which often are overlooked, in the use of cell cycle-specific chemotherapeutic agents or radiosensitizing drugs for which it is the cell cycle time that is relevant.
What is the main reason for the great disparity between the cell cycle time of individual dividing cells and the overall doubling time of the tumor?
A. Intratumor oxygen partial pressure (pO2)
B. Growth fraction
C. Cell loss factor
D. Body temperature where the tumor grows
E. Extra- and intra-cellular acidity (pH)
C
The growth of a tumor depends on the fraction of cycling or proliferating cells (growth fraction), the cell cycle time, and cell loss (the number of newly produced cells that die or fail to continue dividing). Physical stresses such as low intra-tumoral oxygen concentration, low pH and high temperature both lengthen the cell cycle and slow down tumor growth. The high rate of cell loss in human tumors largely accounts for the great disparity between Tc and the volume doubling time (TD). Values for the cell-loss factor vary from 0% to more than 90% for tumors in laboratory animals.
The cell loss factor represents the ratio of the rate of cell loss to the rate of new cell production. Which of the following is not a dominant cause of cell loss in tumors?
A. Death from inadequate nutrition
B. Apoptosis (programmed cell death)
C. Death from immunologic attack
D. Metastasis
E. Cell migration
E
Cell migration within a tumor has recently been described from the study of microbeam radiation therapy (MRT). Since cell migration occurs within 200 um (the interspace between microbeams), the cells still stay in the tumor mass without affecting cell loss from the tumor.
In an untreated tumor with a potential doubling time of 3 days and a cell loss factor of 80%, the volume doubling time is:
A. 2.4 days
B. 3.5 days
C. 3.75 days
D. 15 days
E. 20 days
D
The cell loss factor (Φ) is equal to 1-(Tpot/TD). Rearranging this, TD = Tpot/(1-Φ). TD = 3 days/(1-0.8) = 15 days.
