The Suppressor of Cytokine Signaling-3 Gene Is Overexpressed in Human De Novo Follicular Lymphomas and Promotes IL-3-Independent Proliferation of the BaF3 Pro-B Cell Line.

The recent generation of mice lacking functional SOCS3 in hepatocytes, macrophages, and neutrophils reveals SOCS3 to be an essential regulator of IL-6 signaling via mediation of gp130-related cellular complexes, as well as a negative regulator of G-CSF signaling in myeloid cells. Although SOCS3 would appear to be a critical physiologic regulator of inflammatory responses, its possible role in hematologic malignancies and the underlying mechanisms which regulate its expression in B cells remain to be clearly defined. We previously showed that CD19+ B cells isolated from E_μ-Bcl-2 transgenic mice express high levels of SOCS3 in addition to overexpression of Bcl-2. Moreover, hematopoietic cell lines transduced to stably overexpress Bcl-2 exhibited marked induction of SOCS3 compared to controls, suggesting Bcl-2-associated pathways may play a role in the induction of SOCS3. In the current study, we describe SOCS3 overexpression limited to neoplastic follicular lymphoma (FL) cells in Bcl-2-associated human de novo FL and show that overexpression of SOCS3 is capable of stimulating cytokine-independent cellular proliferation of the BaF3 pro-B cell line. We measured SOCS3 protein levels by immunohistochemistry in paraffin-embedded biopsies from twelve patients diagnosed with de novo, untreated histologic grade I or II FL which harbored t(14;18) and Bcl-2 overexpression. In 9/12 de novo FL cases examined, immunostaining with two distinct antibodies to SOCS3 revealed marked overexpression of SOCS3 protein that, within the follicular center cell region, was limited to neoplastic FL cells and co-localized with Bcl-2 primarily in the nucleus of positive cells. In contrast, SOCS3 protein was not detected by immunostaining in germinal center follicular B cells from benign hyperplastic tonsil tissue. To further evaluate the role of SOCS3 in B cell biology, the IL-3-dependent BaF3 pro-B cell line was stably transduced with either a retroviral expression construct containing a 675bp human SOCS3 cDNA (BaF3^SOCS3) or with vector only control (BaF3^Δ). Whereas no SOCS3 protein was detected in control cells, high level expression of SOCS3 in transduced BaF3^SOCS3 cells was confirmed by Western analysis using SOCS3 anti-sera. Furthermore, Bcl-2 protein was not detected in either BaF3^SOCS3 or control cell lines. 2 x 10⁵ BaF3^SOCS3, BaF3^Δ, and non-transduced BaF3 cell lines were initially grown in the presence 10% fetal bovine serum (FBS) and 5% WEHI 3B cell-conditioned medium as a source of IL-3. IL-3 was then removed by washing with DMEM/10% FBS. Cell viability was then measured by recording absorbance at 490nm using incorporation of the MTS tetrazolium compound. Interestingly, BaF3^SOCS3 cells overexpressing SOCS3 did not undergo apoptosis but were able to proliferate in the absence of IL-3, with percent viable cells approaching 400% at > 96 hours, which represented the final time-point measured. In contrast, BaF3^Δ and non-transduced BaF3 cells underwent apoptotic cell death between 8 and 36 hours in response to IL-3 withdrawal. Thus, SOCS3 overexpression confers IL-3-independent cell proliferation to the BaF3 cell line. These data indicate that unlike its negative regulatory effect on G-CSF signaling in myeloid cells, overexpression of SOCS3 in B cells may promote B cell proliferation rather than growth suppression and may play an important role in the pathogenesis of de novo FL in humans.