Changes in clonogen number and radiation sensitivity in mouse jejunal crypts after treatment with dimethylsulfoxide and retinoic acid.

Retinoic acid (RA) and the drug carrier dimethylsulfoxide (DMSO) have been shown to reduce cellular radiation sensitivity in vitro because of their hydroxyl radical scavenging properties. Both agents have also been shown to induce differentiation in vitro and in vivo. As intestinal crypts are multicellular systems, crypt survival after irradiation depends not only on the cellular sensitivity of the clonogenic cells, but also on the number of clonogenic cells in each crypt, which may be changed by treatments with agents which induce differentiation. In the present experiments we examined the effects of DMSO and RA on the radiosensitivity of mouse jejunal crypts in vivo using the microcolony assay. Mice were treated with five daily intraperitoneal doses of 0-500 microgram RA in 0.1 ml DMSO per mouse, the last dose applied 4 h before the start of irradiation. The results showed a clear protection by 100 and 500 micrograms/day RA in 0.1 ml DMSO for crypt survival over the dose range of 9-16 Gy. The D0 was increased from 1.30 Gy for untreated controls to 1.59 Gy after treatment with DMSO alone, and to 1.85 Gy after treatment with 100 micrograms/day RA in DMSO. Split-dose experiments showed a reduction in the number of clonogens by a factor of about 2 from DMSO treatment alone, with no additional effect of RA on the number of clonogens. Despite this reduction, the number of BrdUrd-labeled cells per crypt remained roughly the same, as did the count of cells per longitudinal villus section. We conclude that, in addition to the protective effects of RA in DMSO, there is induced differentiation of crypt clonogens which is counteracted by increased proliferative activity of transit cells with the result that villus cellularity is maintained.