PTP1B, Distinctively Expressed in GCB-Like and ABC-Like DLBCL, Is a New Regulator of IL-4-STAT6 Intracellular Signaling.

Abstract

Gene expression profiling studies sub-classified diffuse large B-cell lymphomas (DLBCL) into two clinically distinct types: Germinal Center B cell (GCB)-like and Activated B-cell (ABC)-like tumors, characterized by long and short survival, respectively. We have recently reported distinct responsiveness of GCB-like and ABC-like DLBCL cell lines to IL-4 stimulation. In ABC-like lymphoma cells, STAT6 signaling is ameliorated due to enhanced STAT6 dephosphorylation resulting in non-inducibility of IL-4 target genes in these tumors. (Lu et al: Blood 2005; 105:2924). Furthermore, we have demonstrated that IL-4 may increase doxorubicin and complement-mediated rituxmab cellular cytotoxicity in GCB-like cell-lines, but protects the ABC-like DLBCL. At least some of the observed differences were attributed to the differential expression of TC-PTP - a new STAT6 nuclear protein tyrosine phosphatase (PTP) (Lu et al MCB 2007; 27:2166). PTP1B, another PTP highly expressed in ABC-like DLBCL, exhibits a 71% amino acid identity in the catalytic domain with TC-PTP. In contrast to TC-PTP that is mainly located in nuclei, PTP1B localizes to the endoplasmic reticulum. Herein, we demonstrate that PTP1B may serve as a physiological cytoplasmic PTP of phosphorylated STAT6. Over-expression of PTP1B leads to STAT6 dephosphorylation while PTP1B knock down augment STAT6 phosphorylation. We demonstrate interactions between endogenous PTP1B and STAT6 and delineate the STAT6 and PTP1B protein domains responsible for the interaction: the catalytic domain of PTP1B and the C terminal transactivation domain of STAT6. Furthermore, we demonstrate that PTP1B may also dephosphorylate JAK1 and JAK3. However, by using the PTK inhibitor, staurosporine, we show that the PTP1B effect on STAT6 is direct. We confirm a potential regulatory role of PTP1B on IL-4-STAT6 signaling in a cellular context by demonstrating that over-expression of PTP1B ameliorates IL-4-induced STAT6 reporter luciferase activity and IL-4 induced expression of its target genes. Furthermore, we delineate a new negative regulatory loop of IL-4-JAK-STAT6 signaling. We demonstrate that IL-4 induces PTP1B mRNA and enhances PTP1B protein stability. The effects of IL-4 on PTP1B mRNA are STAT6 independent and are mediated by PIK3 intracellular pathway. These observations further highlight the complexity of the intracellular regulation of cytokine-STAT signaling and may have important implications for both neoplastic and inflammatory processes. In summary, our findings identify STAT6 as a new target of PTP1B and suggest that distinct PTP expression profiles may contribute to different biological characteristics of GCB-like and ABC-like DLBCL.