Protein Kinase CSNK1A Regulates the Multiple Myeloma Plasma Cell-Microenvironment Interplay

Multiple myeloma (MM) is a plasma cell malignancy characterized by a strong dependency on the bone marrow microenvironment, which makes plasma cells resistant to pro-apoptotic stimuli and favours the myeloma-associated bone disease (MMABD). Alteration of Wnt/β-catenin, Hedgehog (Hh) and NF-κB signaling by plasma cell-intrinsic or bone marrow stroma-delivered signals sustains MM cell growth and contributes to bone homeostasis alterations. Protein kinase CK1α (CSNK1A1, casein kinase 1α) belongs to a family of monomeric Ser/Thr kinases that regulate cellular processes important in cancer biology such as membrane biogenesis, transport of molecules, signal transduction, transcription, translation and the DNA damage response. CK1α is also an essential regulator of the function of the Wnt, Hh and NF-κB signalling pathways. It is known that osteogenic differentiation is dependent on the activation of Wnt/β-catenin and that CK1α exerts a negative regulative role on this pathway. Thus, CK1α could play a role in MMABD representing a rational therapeutic target. We recently demonstrated that CK1α supports MM plasma cells growth, in part through the regulation of AKT and β-catenin. CK1α is overexpressed in MM patients compared to healthy controls, and its inhibition/silencing in plasma cells causes apoptosis and cell cycle arrest (Manni et al ., 2017, Oncotarget 8(9):14604). To investigate if this kinase could regulate osteogenesis, we analyzed the generation of osteocytes from stromal cells in conditions of CK1α inactivation in BM stromal cells (BMSC). We found that at variance from malignant plasma cells, CK1α silencing in bone marrow stromal cells did not impact on cell survival but it enhanced the differentiation potential towards bone formation, inducing an osteoblastogenic transcriptional program. CK1α silencing in BMSC, led to the stabilization of β-catenin, increased mRNA expression of the osteoblast markers RUNX2, SPP1 and BGLAP, and it increased calcium deposits of stromal cells pushed to differentiate towards osteoblasts/osteocytes. Moreover, we observed that the immunomodulatory drug lenalidomide, by inducing CK1α degradation in both MM and stromal cells, potentiated the effects of CK1α silencing in BMSC, further supporting osteoblast formation. Previous papers reported that MM cells overexpress RUNX2, leading to a higher expression of RANKL, which in turn could be responsible of the inhibition of osteoblast activity in the bone marrow microenvironment (Giuliani et al., 2005 ; Blood 106, 2472). Moreover, elevated expression of RUNX2 in MM cells enhanced tumour growth and bone disease (Trotter et al., 2015, Blood 125, 3598). To investigate the effects of CK1α on RUNX2 levels in MM cells, we created a model whereby the BM microenvironment dependent INA-6 cell lines were stably transduced with lentiviral shRNA vectors to obtain CK1α silencing by addition of IPTG and then were cultured in the presence of BM stromal cells. At variance with BMSC, CK1α silencing in MM cells determined a reduction of RUNX2 in MM cells with a consequent RANKL-induced increase of RUNX2 in stromal cells. Therefore, CK1α could have a positive regulative role on RUNX2 expression in MM cells participating in the impairment of osteoblastogenesis and yet a negative regulative role still on RUNX2 levels in BMSC. Altogether, our data provide a mechanistic framework for the function of CK1α in BMSC biology and in the MM-BMSC crosstalk and support a dual beneficial therapeutic role of CK1α inhibition in MM, with the potential of targeting not only the malignant plasma cells but also the MMABD.