Evidence for trans regulation of apoptosis in intertypic somatic cell hybrids

The genetic components required for glucocorticoid induction of apoptosis were studied by using somatic cell hybridization. Intertypic whole-cell hybrids were generated by crossing the glucocorticoid-resistant rat liver cell line Fado-2 with the glucocorticoid-sensitive mouse thymoma cell line BW5147.3. Morphological and biochemical criteria were used to assess sensitivity or resistance to glucocorticoid-induced cell death. Both phenotypes were observed, and all of the hybrids retained a functional glucocorticoid receptor as judged by their abilities to induce the metallothionein gene in response to dexamethasone (Dex). Sensitivity to apoptosis did not correlate with morphological phenotype in that not all suspension cells were sensitive. The effect of glucocorticoids on the expression of apoptosis-linked genes was analyzed in a subset of Dex-sensitive and Dex-resistant hybrids. p53 and c-myc mRNAs were present in parental cells as well as sensitive and resistant hybrid cells, and their levels were not affected by glucocorticoid treatment. bcl-2 expression was restricted to the thymoma cell line and was also not affected by glucocorticoids. We did not detect any bcl-2 mRNA in the hepatoma cell line and the hybrids, suggesting that, as with most tissue-specific genes, bcl-2 is regulated in trans. Furthermore, while the majority of hybrids analyzed retained a full complement of mouse chromosomes, sensitive hybrids were missing some rat chromosomes (preferentially chromosomes 16 and 19), indicating that apoptosis is subject to trans repression. Resistant cells thus appear to repress the activity or synthesis of a nuclear factor that interacts with a glucocorticoid-dependent gene(s) to activate the cell death pathway.