Initiation of Normal Thyroid Cells in Primary Culture Is Associated with Enhanced c-myc Messenger Ribonucleic Acid Levels*

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Abstract
We studied expression of the c-myc and c-ras protooncogenes during the initiation of pig thyroid cells in primary cell culture. This period is associated with major changes in differentiated thyroid cell function. After 1, 2, and 3 days of culture, polyadenylated mRNA was prepared from 30-50 replicate dishes of cells. mRNA was also prepared from a portion of the intact tissue used to prepare the dispersed follicles. Expression of c-myc and c-ras mRNA was determined by Northern blot analysis using v-myc and v-ras probes labeled by nick translation. As a nononcogene control, actin mRNA was determined using a .beta.-actin probe. In intact thyroid tissue before follicle-dispersion, c-myc (2.6 kilobases) and c-ras (1.1 kilobases) mRNA were detectable, though at relatively low levels, c-myc mRNA expression increased in cultured cells, to 430%, 670%, and 330% of tissue levels on the first, second, and third days of culture respectively. In contrast, to c-myc, c-ras oncogene mRNA expression was not significantly altered during the 3-day culture period. .beta.-Actin mRNA expression, like c-myc, increased to 560%, 810%, and 460% of tissue levels on the first, second, and third days of culture, respectively. Southern blot analysis of thyroid tissue and cultured cell DNA indicated that gene amplification did not account for the increase in c-myc mRNA levels in cultured cells. In conclusion, c-myc, but not c-ras, oncogene expression is enhanced during the transition of thyroid follicular cells into monolayer culture. .beta.-Actin mRNA expression is also enhanced during the initiation of primary thyroid cell culture. These data are consistent with a role for c-myc expression in the regulation of normal thyroid cell differentiation function.