Differential regulation of the clusterin gene by Ha-ras and c-myc oncogenes and during apoptosis

Clusterin (ApoJ) is an extracellular glycoprotein expressed during processe s of tissue differentiation and regression that involve programmed cell dea th (apoptosis). Increased clusterin expression has also been found in tumor s, however, the mechanism underlying this induction is not known. Apoptotic processes in tumors could be responsible for clusterin gene activation. Al ternatively, oncogenic mutations could modulate signal transduction, thereb y inducing the gene. We examined the response of the rat clusterin gene to two oncogenes, Ha-ras and c-myc, in transfected Rat1 fibroblasts. While c-m yc overexpression did not modify clusterin gene activity, the Ha-ras oncoge ne produced a seven to tenfold repression of clusterin mRNA; this down-regu lation was also observed in the presence of c-myc. Since no induction of th e clusterin gene was observed by the two oncogenes, we tested the alternati ve mechanism involving apoptosis. Growth factor withdrawal induced apoptosi s, as shown by DNA degradation and micronuclei formation in the floating ce lls. Concomittantly we observed a three to tenfold increase in the amount o f clusterin mRNA in the adhering cells of Rat1 and the c-myc transformed ce ll lines, and a weaker induction in the Ha-ras transformed cell line. On th e basis of our results, we suggest that clusterin gene induction in the vit al cells is produced by signaling molecules that are generated by the apopt otic cells. We conclude that apoptotic processes, not oncogenic mutations, are responsible for increased clusterin expression in tumors. J. Cell. Phys iol. 177:593-605, 1998. (C) 1998 Wiley-Liss, Inc.