UC Davis

3-Methylcholanthrene (3-MC) greatly inhibits the growth of two lines of human squamous carcinoma cells, SCC-9 and SCC-12B2. Exposure of the cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin alone was much less effective and, in the presence of 3-MC, did not alter the sensitivity (EC50 = 0.3 microM) or extent of growth inhibition by the latter. The degree of 3-MC-mediated inhibition, however, was markedly alleviated by inclusion of retinoic acid (EC50 greater than or equal to 0.7 microM) and hydrocortisone (EC50 = 40 nM) or dexamethasone (EC50 = 3 nM) in the culture medium. These physiological effectors, which are known to have opposing actions on keratinocyte character in SCC cells, did not significantly alter either aryl hydrocarbon hydroxylase activity or macromolecular adduct formation. Further analysis of the cellular responses indicated that hydrocortisone and, in some experiments, retinoids increased the growth rate in 3-MC-exposed cultures, while 3-MC increased the saturation density in retinoic acid-exposed cultures, an example of interference with a physiological response of the cells. These results indicate that alteration of the differentiated state, regardless of the direction of the change, can alter the sensitivity of the cells to toxic stimuli. Further investigation of the bases of such toxic responses and their modulation by the microenvironment may enhance our understanding of the target cell specificity of polycyclic aromatic hydrocarbons.