The Epigenetic Repression of Mir-375 Is the Dominant Mechanism for the Constitutive Activation of PDPK1/RSK2 Signaling Axis in Multiple Myeloma

[Introduction] Multiple myeloma (MM) is a cytogenetically/molecularly heterogeneous hematologic malignancy that remains mostly incurable, and the identification of a universal and relevant therapeutic target molecule is essential for the further development of therapeutic strategy. We have recently identified that 3-phosphoinositide-dependent protein kinase 1 (PDPK1), a serine threonine kinase, and its major downstream substrate RSK2, a member of the 90 kDa ribosomal S6 kinase family of serine threonine kinases, were universally active in eleven human MM-derived cell lines (HMCLs) examined regardless of the type of cytogenetic abnormality, the mutation state of RAS, RAF and FGFR3 genes and myeloma cells of approximately 90% of symptomatic patients at diagnosis. Our study also disclosed that PDPK1/RSK2 signaling axis played pivotal roles in myeloma pathophysiology by regulating series of downstream molecules, such as c-MYC, IRF4, D-type cyclins, or PLK1, while the inactivation of either PDPK1 or N-terminal domain of RSK2 resulted in the induction of apoptosis in myeloma cells which was accompanied by the activation of BH3-only proteins BIM and BAD (Shimura Y, Mol Caner Ther 2012; Chinen Y, Cancer Res 2014). Here we assessed the underlying mechanism for PDPK1 overexpression in MM.

[Methods] The miR-375 expression level was analyzed by the quantitative RT-PCR in 11 HMCLs and 92 patient-derived myeloma cells isolated by CD138-positive cell sorting (normal plasma cells (N=10), MGUS (N=21), newly diagnosed MM (NDMM) (N=27), relapsed/refractory (RRMM) (N=34). The pre-miR-375 precursor molecule (miR-375 mimics), the siRNA targeted against PDPK1, or a negative control RNA-oligonucleotides was transfected into 8 cell lines by utilizing Hemagglutinating Virus of Japan (HVJ)-envelope vector. The copy number abnormality of PDPK1 gene was assessed by double-color FISH for PDPK1 gene and the centromere of chromosome 16. The methylation status of miR-375 promoter site was analyzed by methylation-specific PCR (MSP). This study was conducted in accordance with the Declaration of Helsinki and with the approval of the Institutional Review Boards. Patient-derived samples were obtained with informed consent, and normal bone marrow plasma cells were obtained from volunteers who were not affected by hematologic disease.

[Result] The level of miR-375 expression was calculated with 2^-ΔCt methods. Human U6 snRNA was examined as the reference. The median log₁₀2^-ΔCt ± SD of normal plasma cells, MGUS, NDMM, RRMM and HMCLs were -2.46 ± 0.67, -3.64 ± 0.68, -4.23 ± 0.95, -3.92 ± 1.24 and -3.69 ± 0.29 respectively. When compared to normal plasma cells, the miR-375 expression was significantly decreased in NDMM and RRMM (p<0.01, respectively), and tended to be decreased in MGUS (p=0.083) and HMCLs (p=0.097). As the causative of miR-375 repression, our study disclosed that the promoter sites of miR-375 gene were hypermethylated in 8/8 of HMCLs when examined by MSP. The interphase FISH for PDPK1 with centromere chromosome 16 indicated the copy number of PDPK1 gene was increased in 11/11 HMCLss, however, this was never the case with patient-derived myeloma cells (0/7). Importantly, the miR-375 gene transfection resulted in the reduction of PDPK1 expression in 7 of 8 HMCLs, and it simultaneously caused the reduction of the expression levels of IGF1 receptor and JAK2, the known targets of miR-375. Furthermore, when treated with 5-Azacitidine and/or Trichostatin A, miR-375 was markedly upregulated, suggesting that the overlapping epigenetic deregulations, such as DNA hypermethylation or histone deacetylation, are involved in the silencing of miR-375.

[Conclusion and Discussion] PDPK1 is activated by autophosphorylation and, therefore, its expression level is the crucial determinant for its activity, Because our study revealed the miR-375 expression as the major regulator of PDPK1 expression, it is suggested that the abnormally repressed miR-375 is the major causative for the constitutive hyperactivation of the PDPK1/RSK2 signaling axis in MM. Moreover, since the decreased miR-375 expression was observed in plasma cells of MGUS and was more pronounced in MM, miR-375 repression by epigenetic deregulation may be involved in both disease development and progression of MM.