Elevated expression of SAG/ROC2/Rbx2/Hrt2 in human colon carcinomas: SAG does not induce neoplastic transformation, but antisense SAG transfection inhibits tumor cell growth

Sensitive-to-apoptosis gene (SAG)/regulator of cullins (ROC)2/Rbx2/Hrt2 is a newly identified component of SCF E3 ubiquitin ligase that controls cell- cycle progression by promoting ubiquitination and degradation of cell-cycle inhibitors. We recently found that SAG protects cells from apoptosis induc ed by redox agents, promotes S-phase entry and cell growth under serum star vation, and is required for yeast growth. In the present study, we report t hat the SAG protein level was elevated in six of 10 human colon carcinoma t issues (60%) as compared with adjacent normal tissues from the same patient . SAG overexpression in preneoplastic cells in a JB6 tumor promotion-and-pr ogression model did not induce neoplastic transformation, and SAG overexpre ssion in NIH/3T3 cells did not induce transforming foci formation, suggesti ng that SAG is not a dominant oncogene. However, when DLD-1 human colon car cinoma cells were transfected with antisense SAG, monolayer growth was sign ificantly inhibited, as shown by a decreased number of stable colonies in t he plate after normalization with transfection efficiency. Stable clones th at expressed antisense SAG showed a 50% decrease in their ability to form c olonies when grown in soft agar versus clones that did not express antisens e SAG. We found an inverse correlation in four of 10 tumors between the lev els of SAG and p27, a cyclin-dependent kinase inhibitor. We concluded that SAG is not causally related to cellular transformation, but its overexpress ion may be important for the maintenance of tumor cell phenotype. Therefore , targeting SAG expression may have therapeutic value in cancer treatment. (C) 2001 Wiley-Liss, Inc.