CONCEPTS, LABELING PROCEDURES, AND DESIGN OF CELL-PROLIFERATION STUDIES RELATING TO CARCINOGENESIS

Chemicals may induce cell proliferation directly as mitogens or indire ctly via cell death with subsequent proliferation to replace lost cell s. Chemically induced proliferation has been demonstrated to play a ro le in the carcinogenic process. A wide range of procedures and techniq ues are currently being used to define the quantitative relationship b etween the extent and duration of chemically induced cell proliferatio n and carcinogenic potential in different species and target organs. H owever, a limited database and nonstandard protocols and procedures fo r measuring cell proliferation have made it difficult to compare resul ts between laboratories. Comparison of frequencies of S phase between control and treated animals is the most commonly used end point in cel l proliferation studies and may be regarded as an indirect indication of a proliferative response. This response can be ascertained as label ing indexes (LI; percentage of cells in S phase) after the administrat ion of the DNA precursor labels (tritiated thymidine; H-3-TdR; bromode oxyuridine, BrdU) or through immunostaining of the endogenous cell rep lication marker, proliferating cell nuclear antigen (PCNA). Both appro aches are applicable to tissue sections. An important issue in the des ign of experimental studies for measuring LI is determining how and wh en to investigate proliferative responses in relation to the chemical treatment regimen. Variables to consider when designing cell prolifera tion studies include the animal's age, chemical dose and method of tre atment, choice and dose of label, time and length that the label is ad ministered, and methods of quantitation. Study design considerations d epend on the experimental objective. A common approach to characterize the complex relationship of cell proliferation and carcinogenic activ ity has been to focus on relatively early (less than 90 days) prolifer ative responses in the target tissue. However, a larger database on th e duration and nature of the chemically induced proliferative response under bioassay conditions in the target cell population is required t o more clearly establish the role of this end point in the cancer proc ess. in addition, studies must also investigate mitogenic versus cytot oxic induction of cell proliferation in normal and preneoplastic cells and differential toxicity that may provide a preferential growth adva ntage to spontaneous or chemically induced intermediate or malignant c ells.