Measurements of cell cycle phase fractions, particularly S-phase, are useful for studies of cell biology and carcinogenesis. Up-regulation of histone gene expression is tightly coupled to the G1-S-phase transition of the cell cycle, and mRNA levels rise 30–100-fold during S-phase. Labeling of histone H3 mRNA using in situ hybridization (ISH) was assessed as a measure of S-phase cells and compared with that found using in vivo 5-bromodeoxyuridine (BrdUrd) labeling in formalin-fixed rat colonic crypts under baseline, modified 72-h starvation, and 24-h refeeding conditions. The labeling index scored in single-labeled sections by histone H3 ISH tightly correlated with that found by in vivo BrdUrd labeling (r = 0.99, P < 0.0001) and clearly discriminated between the control, starved, and refed states (P < 0.001). In 180 crypt sections double labeled using histone H3 ISH and BrdUrd, 92% of 1572 labeled cells exhibited both nuclear BrdUrd and cytoplasmic histone H3 label. It is concluded that histone H3 ISH is an accurate measure of the S-phase fraction and provides an alternative to in vivo BrdUrd labeling in rat colon. This finding warrants validation in human studies.

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This work was supported in part by NIH Grant CA06294.

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