Cellular signaling following irradiation modulates the progression of the cell cycle.

The division of mammalian cells (mitosis) is preceded by other cell cycle stages, during which cell type specific proteins are expressed (G₁-phase), DNA synthesis occurs (S-phase) and mitosis is prepared by the expression of structural and regulatory proteins (G₂-phase). The progression of the cell cycle is regulated by fluctuating amounts of proteins, especially cyclins, and their associated kinases (cdk, cyclin-dependent kinase). Superimposed checkpoint controls are in charge of monitoring the course, maintenance of the order and timing of the cell cycle transitions. In the case of DNA damage, the cell cycle is inhibited by a complex network of signalling and transducing pathways. This transient cell cycle delay is thought to provide cells with additional time to repair the DNA lesions before the cell cycle progresses. The understanding of the pattern and the mechanisms driving the cell cycle delay is very important, because it is determinant for the Relative Biological Efficiency (RBE), which is the basis of the treatment planning in the heavy ion radiotherapy.

Fig. 1: Differentiation stages of normal human fibroblasts. The cells differentiate from mitotically active (MF I, MF II, MF III) to postmitotic cells (PMF IV, PMF V, PMF VI)
[Fournier C, Kraft-Weyrather W, Kraft G.
Survival, differentiation and collagen secretion of human fibroblasts after irradiation with carbon ions and X-rays
Phys Med. 1998 Jul;14 Suppl 1:44-7.]

The DNA damage resulting from exposure to densely ionising irradiation (heavy ions, neutrons) is more complex and more difficult to repair than the damage caused by sparsely ionising irradiation (X- and γ-rays). Comparing the effect of the same doses, the progression of the cell cycle is delayed in more cells and the delay is also more extended after densely than after sparsely ionising irradiation. Consistent with the hypothesis, that this could be related to the amount of DNA damage not being successfully repaired, heavily damaged cells often do not re-enter the cell cycle at all and remain permanently arrested. This permanent arrest is mainly observed in human fibroblasts (cells originating from the connective tissue) for a time period spanning over several days after irradiation. At later times (weeks after exposure), premature differentiation of fibroblast cells within the exposed populations is observed, distinguishing the mitotically active from the terminally differentiated, postmitotic cells by morphological features (Fournier et al., 2001, see figure 1). In addition to the morphological changes, the differentiated cells share with senescent fibroblasts other characteristics. In fibroblasts the premature differentiation is an alternative way to survive radiation insult, whereas in other cell types, for example lymphocytes, programmed cell death (apoptosis) or necrosis is observed. The regulation of the cell cycle can be described as the starting point for the factors determining the fate of cells after radiation insult.

Currently, we investigate in our studies the particular pattern of heavy ion induced cell cycle delay and the corresponding regulation (Fournier et al., 2004). Inhibitors of cyclin dependent kinases, e.g. CDKN1A (formerly known as p21) and CDKN2a (p16) are of special interest, because they are also involved in the regulation of cellular senescence and premature differentiation. It is not clear up to now, to what extent these two pathways overlap. Furthermore, the so called "bystander" effects in non irradiated cells in the neighbourhood of high LET irradiated cells are investigated, especially with respect to the expression of cell cycle regulatory proteins. For these experiments, a direct targeting of single cells is realized by using the GSI microbeam. We focused here also on human fibroblasts, because they are known to be important mediators of side effects occurring during conventional radiotherapy. A positive bystander effect in this cell type would be indicative for a potential impact on radiation risk assessment and the estimation of side effects related to charged particle radiotherapy.

Relevant publications:

Fournier, C., Winter, M., Zahnreich, S., Nasonova, E., Melnikova, L. and Ritter, S.
Interrelation amongst differentiation, senescence and genetic instability in long-term cultures of fibroblasts exposed to different radiation qualities
Radiother. Oncol. 83(3):277-82 (2007) PMID: 17499869

Fournier, C., Wiese, C. and Taucher-Scholz, G.
Accumulation of the cell cycle regulators TP53 and CDKN1A (p21) in human fibroblasts after exposure to low- and high-LET radiation
Radiation Research 161. 675-684 (2004).

Fournier, C. and Taucher-Scholz, G.
Radiation induced cell cycle arrest: an overview of specific effects following high LET exposure
Radiotherapy Oncology 73 (Suppl. 2). S119-122 (2004).

Fournier C., Scholz M., Weyrather W.K., Rodemann H.P., Kraft G.
Changes of fibrosis-related parameters after high- and low-LET irradiation of fibroblasts
Int J Radiat Biol. 77(6):713-22 (2001) PMID: 11403711

contact: c.fourniergsi.de