Lack of prostate cancer radiosensitization by androgen deprivation

The majority of clinical trials have shown that high-grade prostate cancer patients treated with androgen deprivation (AD) plus radiation (RT) have a survival advantage over those treated with RT alone. One possible mechanism for such a favorable interaction is that AD sensitizes cells to radiation. Animal model studies have provided suggestive evidence that AD sensitizes cells to radiation, but this mechanism is difficult to confirm conclusively in vivo. This question was investigated in LNCaP cells grown in vitro.

LNCaP cells were cultured in vitro in Dulbecco's modified Eagle's medium (DMEM)-F12 medium, containing 10% fetal bovine serum (complete medium [CM]). AD was achieved by culture in charcoal-stripped serum (SS)-containing medium. Replacement of androgen was done by adding the synthetic androgen R1881 at 1 x 10(-10) M to SS. Apoptosis was measured with a terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) assay. Clonogenic survival was used to determine overall cell death, and the results were corrected for differences in plating efficiency from the various growth conditions.

LNCaP cells were grown in CM, SS, or SS + R1881 medium, and cell counts obtained at 3, 4, and 5 days. Cell number increased exponentially in CM, whereas no increase in cell number was observed in SS medium. Cell counts from growth in SS + R1881 were intermediate between these extremes. Apoptosis was measured to determine if the combination of AD + RT in vitro resulted in supra-additive cell death, as has been previously described in an in vivo model system. The cells were cultured for 3 days before RT and apoptosis quantified 24 h after RT. There was a consistent supra-additive increase in apoptosis in cells exposed to AD + RT (2 or 8 Gy), as compared to either treatment given individually. In contrast, significant radiosensitization by AD was not observed by clonogenic survival even when the conditions of AD were varied. No radiosensitization was observed upon incubation in SS medium for 3, 4, or 5 days before RT, or extending AD after RT for 6 h before plating or 24 h after plating.

The results show that in LNCaP prostate tumor cells supra-additive apoptosis does not translate into radiosensitization by clonogenic survival. Because clonogenic survival is a measure of overall cell death, either the level of apoptosis is too small a component of overall cell death or the increases in apoptosis occurred in a subpopulation that would have been killed by other mechanisms. Although the findings indicate that AD does not act by sensitizing prostate cancer cells to RT, the additive cell death and growth inhibitory effects of AD + RT are clinically meaningful.