Bmps(Bone Morphogenetic Proteins) Inhibit Growth in Multiple Myeloma Cells by p53 Activation.

Only a few naturally occurring cytokines are able to inhibit myeloma cell growth, and among them BMP-4 is a potent inhibitor of growth as well as an inductor of apoptosis in myeloma cells. To study the mechanism behind BMP-4-induced growth inhibition, we performed gene expression profiling by microarrays in two human myeloma cell lines after BMP-4 stimulation for 4 hours. We found that BMP-4 upregulated several known p53 target genes like p21/Cip1, Bax, cyclin G, Gadd45 and dual specificity phosphatases, suggesting a role for p53 in BMP-mediated growth inhibition of myeloma cells. p53 is activated by several post-translational modifications, including phosphorylation at several serine/threonine residues. We found that BMP-4 treatment lead to rapid phosphorylation of p53 at Serine 15. Furthermore, we were able to co-precipitate phosphorylated Smad-1/5/8 and p53 from nuclear extracts of cells treated with BMP by applying a p53 binding element from the Gadd45 promoter. Because some myeloma cell lines are resistant for BMP-4-induced growth inhibition, we examined whether this BMP-4 resistance was associated with p53 mutations in the BMP-4-resistant cell lines. We sequenced the p53 cDNA from eight myeloma cell lines, and resistant cells all had mutated or deleted p53. Introduction of normal p53 by adenoviral constructs in the p53 mutated cell lines restored BMP-4 responsiveness to some degree. Furthermore, in a myeloma cell line with a temperature-sensitive p53 mutation, the cells were sensitive towards BMP-induced growth inhibition at the permissive temperature but resistant when p53 was inactive. In conclusion, we show that normal p53 activity is involved in BMP-4-induced growth inhibition of multiple myeloma cells.