Dominant suppression of lymphocyte apoptosis by hepatoma cells

Suppression of apoptosis appears to contribute to the development of various diseases, including autoimmune disorders and cancer. Numerous genes that encode activators and suppressors of apoptosis have been identified; however, such genes have not been shown to be expressed in all cell types. Furthermore, the sensitivity of different cell types to induction of apoptosis varies widely. We have employed a genetic approach using somatic cell hybridization to determine if apoptosis is a dominant or a recessive process in cells. These studies have utilized cell fusion partners with differing sensitivity to induction of apoptosis. The apoptosis-sensitive cells chosen were BW5147 murine thymoma cells. These cells readily undergo apoptosis in response to glucocorticoids and calcium ionophore. The resistant fusion partners were HTC rat hepatoma cells, which possess an intact glucocorticoid signal transduction pathway but are resistant to induction of apoptosis by either agent. Neither cell type expresses detectable Bcl-2 protein. Heterokaryons were identified by their retention of fluorescent cytosolic dyes and by nuclear morphology and cell size. The three types of heterokaryons observed were intratypic HTC/HTC and BW5147/BW5147 heterokaryons and intertypic BW5147/HTC heterokaryons. Glucocorticoid receptor was shown by immunohistochemistry to undergo hormone-dependent translocation to all nuclei in intertypic heterokaryons. BW5147/BW5147 heterokaryons die after treatment with glucocorticoid and calcium ionophore, whereas both HTC/HTC and BW5147/HTC hybrids survive. The presence of multiple BW5147 cells fused to a single HTC cell did not affect this outcome. This demonstrates that HTC cells are able to dominantly suppress apoptosis in all BW5147/HTC heterokaryons. Thus, HTC cells contain activities that can suppress apoptosis in lymphocytes. (C) 1997 Academic Press