Introduction of an activated N-ras oncogene alters the growth characteristics of the interleukin 6-dependent myeloma cell line ANBL6.

Multiple myeloma (MM) is a late-stage B-cell cancer with an unknown etiology. Activating mutations of the N-ras and K-ras oncogenes occur with a high frequency in myeloma and, therefore, may play a role in the pathogenesis of the disease. To study the role of N-ras-activating mutations in the regulation of myeloma tumor growth, we introduced a constitutively active N-ras cDNA containing a glutamine to arginine (CAA-CGA) amino acid substitution at codon 61 into the interleukin 6 (IL-6)-dependent myeloma cell line ANBL6. Expression of the mutant N-ras cDNA resulted in significant IL-6-independent growth, as well as augmentation of growth at suboptimal concentrations of IL-6. The IL-6-independent growth pattern was not the result of activation of autocrine IL-6 production in the mutant N-ras-expressing population because neutralizing antibodies to the IL-6 receptor and to IL-6 had no effect on the rate of DNA synthesis in the absence of IL-6. Furthermore, mutant N-ras expression decreased the percentage of cells undergoing apoptosis in the absence of IL-6. These data suggest that activating mutations of the ras oncogenes may result in growth factor independence accompanied by a suppression of apoptosis in MM. Therefore, the use of therapies designed to block IL-6 action in MM may have less of an impact on tumors bearing activated ras mutations.