We established a myeloma cell line (RPMI8226) with cyclin D1 overexpression in which the transfected cyclin D1 gene was stably expressed. D1 transfectants showed down-regulation of cyclin D2. Cell proliferation analysis did not show any differences among RPMI8226, mock control, and D1 transfectants. The number of S-phase cells increased while the number of G0/G1- and G2/M-phase cells decreased in D1 transfectants, which indicates a prolonged S-phase caused by cyclin D1 transfection. A decreased number of G2/M-phase cells was also detected in myeloma cells of patients with translocation t(11;14)(q13;q32). Western blot analysis revealed an increase in the hyperphosphorylated form of retinoblastoma (Rb) protein in D1 transfectants; however, the expression of p53, p16, Bax, Bad, Bcl-2, and Mcl-1 did not significantly change. Treatment with anti-myeloma drugs (melphalan, dexamethasone, bortezomib and immunomodulatory compounds) induced apoptosis earlier in D1 transfectants compared with RPMI8226 and mock control via the activation of both caspase-8 and -9. However, we could not detect a relationship between cyclin D1 expression and the response to treatment with VAD and bortezomib. Therefore, we assume that high sensitivity to anti-myeloma drugs depends on the duration of the S-phase, but a clinical response might depend on the number of myeloma cells with cyclin D1 overexpression.

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