PRL3 Is a Mediator of IL6/STAT3 Signaling and Defines a Population of Multiple Myeloma Patients Distinct from Those That Activate NFkB.

Previous work in our laboratory has identified the importance of the NFB signaling pathway for plasma cell (PC) survival and accumulation in Multiple Myeloma (MM). Although up to 43% of patient PCs appear to have activated NFkB by gene expression profiling (GEP), the remainder must rely on an alternate signaling pathway for their survival. We examined our MM GEP datasets and supervised the clustering of patients according to an index of NFkB activity. The majority of patients not activating NFkB were associated with a high IL-6/Stat3 signature, which included the protein phosphatase of regenerating liver-3 (PRL-3). Interestingly, analysis of the PRL-3 promoter revealed competing binding sites for both NFkB and STAT3 within the first intron, suggesting that these play a role in the transcriptional regulation of this gene. PRL-3 has been previously demonstrated to be absent in most normal tissues while its expression in MM cells is quite elevated. As normal PCs have low levels of PRL-3 expression, we determined the upper limit of normal to be 3SD above the mean of these cells. Using this cutoff we report that elevated PRL-3 expression is observed in plasma cells from 1/15 normal individuals, 4/22 MGUS patients and 47/126 patients with either smoldering or overt MM (6.7%, 18.2% and 45.6% respectively). Less than 10% of patients were seen to have both an elevated NFkB index and high PRL-3 expression. Patients with higher expression of PRL-3 were found to have statistically significant higher mean values of creatinine, C-reactive protein, lower albumin levels and a lower overall survival in our Mayo Clinic MM dataset. To examine the role PRL-3 in MM biology we first stimulated MM cell lines with IL-6 and observed PRL-3 up-regulation in two cell lines (INA6, OPM1) but not in JK6L (an NFkB activated cell line). Using a luciferase reporter we have determined that STAT3 induction of PRL3 expression requires the first intron of the PRL3 gene which contains the putative STAT3 binding site. Finally we show that PRL-3 inhibition by shRNA leads to decreased cell growth and loss of IL-6 responsiveness in vitro in at least one cell line. Based on these results, we propose that the majority of MM patients depend on either the NFkB or STAT3 signaling pathways for their survival and that these are to a large degree mutually exclusive. Our experimental results suggest that PRL-3 plays a role in IL-6/STAT3 signaling and that it is a promising target for drug development.