Apoptosis and Response to Radiation: Implications for Radiation Therapy

Tumor growth is the result of two opposing processes--cell divisionand cell loss. As long as division outpaces loss, tumors will continueto grow. The form of "active" cell death called apoptosis isnow known to be controlled by specific genes, and it is hoped that manipulatingthe expression of these genes could shift the balance in favor of cellloss.

The review by Dr. Meyn covers promising early results using gene therapytechniques to administer p53 (a tumor-suppressor protein involved in apoptosisand growth regulation) or bcl-2X_s (an inhibitor of bcl-2that, itself, inhibits apoptosis). These results support the idea thatpromoting apoptosis may be a viable way of controlling tumor growth.

Dr. Meyn was actively involved in apoptosis research as it relates toradiation damage before it became fashionable. He points out that almost40 years ago, radiobiologists labeled this form of cell death as "interphasecell death" to distinguish it from the mitotic cell death that occurredas radiation damaged cells attempted to divide. What was not appreciateduntil relatively recently is that rapid interphase death may be preventedby a simple gene mutation or by overexpression of certain other gene products.Such genetic changes are often present in solid tumors.

An important, valid point made in Dr. Meyn's review is that "time"is the critical element needed to define the significance of apoptosisin radiotherapy. He makes the distinction between apoptosis as a primarymode of cell death occurring within a few hours of irradiation and "secondary"apoptosis observed at much longer times (24 to 96 hours) after irradiation.Clearly, damaged cells that fail to undergo apoptosis within the firstfew hours after irradiation can still undergo reproductive cell death.However, Dr. Meyn argues (and is probably right) that the manner in whichthose cells die--whether by secondary apoptosis, necrosis, or simple senescence--islargely immaterial for practical purposes.

Radiosensitivity of Cells Capable of Primary Apoptosis

Dr. Meyn's second point is that cells capable of undergoing primaryapoptosis are more radiosensitive. Again, results from a number of studiesusing cells in culture support this position. He could probably go furtherand add that while it is possible to reduce radiation cell killing by preventingprimary apoptosis, it may be impossible to prevent secondary apoptosis.Secondary apoptosis seems to be a response to lethal (irreparable) damage.Since most cells capable of primary apoptosis are of hematologic or lymphoidorigin, the relevance of apoptosis to the response of most solid tumorshas been questioned.

Dr. Meyn approaches this question by summarizing a great deal of workcarried out in over a dozen solid murine tumors. Squamous cell carcinomas,hepatocarcinomas, and fibrosarcomas did not undergo apoptosis in responseto radiation. However, the apoptotic response of adenocarcinomas and lymphomaspeaked at about 4 hours after irradiation, suggesting that this was primaryapoptosis by his definition. In these studies, Dr. Meyn found that lessradiation was required to control tumors with significant amounts of radiation-inducedapoptosis--the expected result if cells capable of under going primaryapoptosis are more radiosensitive.

Perhaps more interesting, the spontaneous apoptotic index correlatedwith tumor curability in the same series of murine tumours. Dr. Meyn arguesthat if tumors contain more apoptotic cells, there must be fewer clonogensto kill, and the tumor should be controlled with lower radiation doses.This observation has important implications, since it suggests that theability to undergo apoptosis could have a greater influence on outcomethan other properties of solid tumors known to affect response to radiotherapy,such as hypoxia and tumor growth kinetics.

Clinical Results Less Clear-Cut

However, results from the clinic are not as clear-cut. Dr. Meyn reportsthat some studies show a correlation between high pretreatment apoptoticindex and survival/response, while others do not. Gordon Steel's "bucketand tap" analogy is useful in thinking about this situation.[1] Steelillustrates the relationships among cell production by mitosis (the tapdripping tumor cells into a bucket), cell loss by apoptosis or necrosis(holes in the bucket that release the cells), and tumor growth (the accumulationof cells in the bucket). Influencing the speed of the dripping tap, orthe size of the holes in the bucket, will alter tumor volume. However,if there are more holes in the bucket (ie, a higher apoptotic fraction),mitosis will have to increase to maintain tumor growth. This could resultin a higher growth fraction and, perhaps, a lower likelihood of curingsome tumors with radiation.

Alternatively, tumor cell types with a high incidence of spontaneousapoptosis could be dying because they do not tolerate a nutrient-poor (hypoxic?)tumor microenvironment. Such tumors should be more radiocurable.

Unfortunately, resolving these issues is not easy because measurementsof apoptotic index in vivo are difficult to perform and interpret. Apoptoticindex varies with time, radiation dose, and fractionation schedule. Aspointed out in an article by Potten[2] in reference to apoptosis in thegut and by Dewey et al[3] in relation to solid tumors, the apoptotic yieldmeasured in vivo rarely relates to the yield of reproductively sterilizedcells. This is perhaps not surprising considering that the duration ofthe apoptotic process (ie, how rapidly apoptotic cells develop, how longthey survive in vivo) is largely unknown, as is the contribution of othercell types within the tumor.

The inability to directly correlate reproductive cell death with apoptoticfraction is clearly a limitation in defining the kinetics and significanceof the process in vivo. However, this should not be a detriment to designingeffective strategies to promote apoptosis in tumors that have lost thiscapacity.

References:

1. Steel GG: Growth Kinetics of Tumours, p 176. Oxford, Clarendon Press,1977.

2. Potten CS: What is an apoptotic index measuring? A commentary. BrJ Cancer 74:1743-1748, 1996.

3. Dewey WC, Ling CC, Meyn RE: Radiation-induced apoptosis: Relevanceto radiotherapy. Int J Radiat Oncol Biol Phys 33:781-796, 1995.