Exploring the role of cyclin D1 in the pathogenesis of multiple myeloma beyond cell cycle regulation.

Cyclins D could be a unifying event in multiple myeloma (MM), even though MM is not typically considered a proliferative disease. In this study, we hypothesized that cyclins D might have additional roles in the pathogenesis of MM beyond cell cycle control. We showed that overexpression of CCND1 and CCND2 in MM cell lines lacking these proteins revealed a mutually exclusive expression pattern, with both cyclins D localized in the cytoplasm and no impact on proliferation. To investigate non-canonical roles of cyclin D1, we performed transcriptome analysis and multidimensional flow cytometry. Cyclin D1 overexpression led to upregulation of several key cell adhesion pathway proteins, including STAT1 and ZO-1, along with alterations in the actin cytoskeleton and decreased adhesion to certain matrices. Immunophenotypic analysis showed a significant reduction in CD56 expression following cyclin D1 overexpression, validated in a cohort of 85 MM patients, in which 73% with high cyclin D1 were CD56-negative. High cyclin D1 was also associated with increased circulating tumor cells (CTCs) (P < 0.001). Overall, we revealed novel functions of cyclin D1 in MM pathogenesis, particularly in cell adhesion and dissemination.