TACI expression is associated with a mature bone marrow plasma cell signature and C-MAF overexpression in human myeloma cell lines.

BACKGROUND AND OBJECTIVES: BAFF and APRIL stimulate the growth of multiple myeloma (MM) cells. BAFF and APRIL share two receptors--TACI and BCMA--and BAFF binds to a third receptor, BAFF-R. We previously reported that TACI gene expression is bimodal in 18 human MM cell lines (HMCL), being either present or absent, unlike BCMA that is expressed on all HMCL. BAFF-R is lacking. TACI expression is a good indicator of a BAFF-binding receptor in HMCL. In primary MM cells, the level of TACI expression correlates with a characteristic phenotypic pattern: TACIhighMM cells resemble bone marrow plasma cells and TACIlow resemble plasmablasts. The aim of this study was to further characterize the role of TACI expression in MM DESIGN AND METHODS: Using gene expression profiling, we investigated whether these patterns are kept in TACI+ or TACI- HMCL.
RESULTS: Eighty genes/EST interrogated by Affymetrix microarrays were differentially expressed between TACI+ and TACI- HMCL, particularly c-maf, cyclin D2, and integrin beta7. Triggered by the finding that TACI and c-maf expressions correlate in TACI+ HMCL, we demonstrated that TACI activation influences c-maf expression: (i) activation of TACI by BAFF or APRIL increases c-maf, cyclin D2, and integrin beta7 gene expressions in TACI+ HMCL, (ii) blocking of autocrine BAFF/APRIL stimulation in some TACI+ HMCL by the TACI-Fc fusion protein reduces c-maf, cyclin D2, and integrin beta7 gene expression, (iii) nucleofection of siRNA to c-maf decreases c-maf mRNA levels and reduces the expression of cyclin D2 and integrin beta7 gene expressions, without affecting TACI expression INTERPRETATION AND
CONCLUSIONS: We conclude that TACI activation can upregulate c-maf expression which, in turn, controls cyclin D2, and integrin beta7 gene expression.