Suppression of proliferation and phosphorylation of Jak3 and STAT5 in malignant T-cell lymphoma cells by derivatives of octylamino-undecyl-dimethylxanthine

IL-2R signal transduction involves tyrosine phosphorylation of several proteins including Jak3 and STAT5. In the present study we examined the effect of two octylamino-undecyl-dimethylxanthine (OUDMX) derivatives, designated CT2576 and CT5589, on proliferation and protein tyrosine phosphorylation in human malignant T-cell lymphoma lines. These T-cell lines (PB-1, 2A, and 2B), obtained from a progressive T-cell lymphoma involving skin, are IL-2 independent but have constitutively activated IL-2R-associated signal transduction pathway common to IL-2 and several other cytokines: IL-4, IL-7, IL-9, and IL-15. CT2576, characterized previously on the functional level as an inhibitor of IL-2 signaling and, on the biochemical level, as an inhibitor of phosphatidic acid biosynthesis, suppressed completely growth of the malignant T cell lymphoma lines. CT5589 which is a novel analog of the CT2576, displayed a similar, although weaker, effect. Furthermore, both CT compounds inhibited constitutive tyrosine phosphorylation of two proteins: Jak3 and STAT5 which are key downstream elements in the signal transduction pathway activated by IL-2 and the other cytokines. The CT compounds inhibited also Jak3 phosphorylation induced by IL-2 in the IL-2 dependent SZ-4 cells. Inhibition of phosphorylation by CT2576 and CT5589 was only partially selective since phosphorylation of several other proteins was also affected. Phosphorylation of many others was, however, unaffected. These findings demonstrate that the OUDMX derivatives suppress proliferation of malignant T lymphocytes. Furthermore, they suggest that this suppression may be mediated by inhibition of the IL-2R-associated Jak/STAT signaling pathway. A potential role for OUDMX derivatives in therapy of human T-cell lymphoma should be further explored.     

Requirement for distinct Janus kinases and STAT proteins in T cell proliferation versus IFN-gamma production following IL-12 stimulation

While IL-12 is known to activate JAK2 and TYK2 and induce the phosphorylation of STAT4 and STAT3, little is known regarding how the activation of these signaling molecules is related to the biologic effects of IL-12. Using an IL-12-responsive T cell clone (2D6), we investigated their requirements for proliferation and IFN-gamma production of 2D6 cells. 2D6 cells could be maintained with either IL-12 or IL-2. 2D6 lines maintained with IL-12 (2D6(IL-12)) or IL-2 (2D6(IL-2)) exhibited comparable levels of proliferation, but produced large or only small amounts of IFN-gamma, respectively, when restimulated with IL-12 after starvation of either cytokine. 2D6(IL-12) induced TYK2 and STAT4 phosphorylation. In contrast, their phosphorylation was marginally induced in 2D6(IL-2). The reduced STAT4 phosphorylation was due to a progressive decrease in the amount of STAT4 protein along with the passages in IL-2-containing medium. 2D6(IL-12) and 2D6(IL-2) similarly proliferating in response to IL-12 induced comparable levels of JAK2 activation and STAT5 phosphorylation. JAK2 was associated with STAT5, and IL-12-induced STAT5 phosphorylation was elicited in the absence of JAK3 activation. These results indicate that IL-12 has the capacity to induce/maintain STAT4 and STAT5 proteins, and that TYK2 and JAK2 activation correlate with STAT4 phosphorylation/IFN-gamma induction and STAT5 phosphorylation/cellular proliferation, respectively.     

Growth hormone stimulates tyrosine phosphorylation of JAK2 and STAT5, but not insulin receptor substrate-1 or SHC proteins in liver and skeletal muscle of normal rats in vivo

GH has been shown to stimulate tyrosine phosphorylation of JAK2, several STAT proteins, insulin receptor substrate-1 (IRS-1), and SHC proteins in cultured cells. The goal of this study was to determine GH effects on protein tyrosine phosphorylation in liver and skeletal muscle of normal rats in vivo. Nonfasted male Sprague-Dawley rats (225-250 g) were injected with GH iv, and tissues were obtained after 5, 15, 30, or 60 min. At a maximally effective GH dose (1.5 mg/kg body weight), phosphotyrosine antibody immunoblots demonstrated marked stimulation of the tyrosine phosphorylation of JAK2 (maximal at 5 min) and a 95,000 Mr protein (maximal at 15 min) in both liver and skeletal muscle. The 95,000 Mr protein was recognized and immunodepleted by STAT5 antibody, but not by other STAT protein antibodies. Although basal tyrosine phosphorylation of IRS-1 and SHC was evident, GH did not stimulate tyrosine phosphorylation of either of these proteins in liver or skeletal muscle. In conclusion, GH stimulates the tyrosine phosphorylation of JAK2 and STAT5, but not IRS-1, SHC, or other STAT proteins in liver and skeletal muscle of normal rats. These results differ from findings in cultured cells and support the concept that selectivity for tyrosine kinase substrates is an important determinant of postreceptor signaling specificity in vivo.     

Regulation of radiation-induced apoptosis in oncogene-transfected fibroblasts: influence of H-ras on the G2 delay

The high-affinity receptor (R) for IL-5 consists of a unique alpha chain (IL-5R alpha) and a beta chain (beta c) that is shared with the receptors for IL-3 and granulocyte macrophage colony stimulating factor (GM-CSF). We defined two regions of IL-5R alpha for the IL-5-induced proliferative response, the expression of nuclear proto-oncogenes, and the tyrosine phosphorylation of cellular proteins including beta c, SH2/SH3-containing proteins and JAK2 kinase. In the studies described here, we demonstrate that IL-5, IL-3 or GM-CSF stimulation induces the tyrosine phosphorylation of JAK2, and to a lesser extent JAK1, and of STAT5. Mutational analysis revealed that one of the proline residues, particularly Pro352 and Pro355, in the membrane-proximal proline-rich sequence (Pro352-Pro353-X-Pro355) of the cytoplasmic domain of IL-5R alpha is required for cell proliferation, and for both JAK1 and JAK2 activation. In addition, transfectants expressing chimeric receptors which consist of the extracellular domain of IL-5R alpha and the cytoplasmic domain of beta c responded to IL-5 for proliferation and tyrosine phosphorylation of JAK1. Intriguingly, electrophoretic mobility shift assay analysis revealed that STAT5 was activated in cells showing either JAK1 or JAK2 tyrosine phosphorylation. These results indicate that activation of JAK1, JAK2 and STAT5 is critical to coupling IL-5-induced tyrosine phosphorylation and ultimately mitogenesis, and that Pro352 and Pro355 in the proline-rich sequence appear to play more essential roles in cell growth and in both JAK1/STAT5 and JAK2/STAT5 activation than Pro353 does.     

IL-2 induces STAT4 activation in primary NK cells and NK cell lines, but not in T cells

IL-2 exerts potent but distinct functional effects on two critical cell populations of the immune system, T cells and NK cells. Whereas IL-2 leads to proliferation in both cell types, it enhances cytotoxicity primarily in NK cells. In both T cells and NK cells, IL-2 induces the activation of STAT1, STAT3, and STAT5. Given this similarity in intracellular signaling, the mechanism underlying the distinct response to IL-2 in T cells and NK cells is not clear. In this study, we show that in primary NK cells and NK cell lines, in addition to the activation of STAT1 and STAT5, IL-2 induces tyrosine phosphorylation of STAT4, a STAT previously reported to be activated only in response to IL-12 and IFN-alpha. This activation of STAT4 in response to IL-2 is not due to the autocrine production of IL-12 or IFN-alpha. STAT4 activated in response to IL-2 is able to bind to a STAT-binding DNA sequence, suggesting that in NK cells IL-2 is capable of activating target genes through phosphorylation of STAT4. IL-2 induces the activation of Jak2 uniquely in NK cells, which may underlie the ability of IL-2 to activate STAT4 only in these cells. Although the activation of STAT4 in response to IL-2 occurs in primary resting and activated NK cells, it does not occur in primary resting T cells or mitogen-activated T cells. The unique activation of the STAT4-signaling pathway in NK cells may underlie the distinct functional effect of IL-2 on this cell population.     

Granulocyte-macrophage colony-stimulating factor induces a unique set of STAT factors in murine dendritic cells

Cytokine-mediated signaling pathways were studied in mouse dendritic cells (DC) by analysis of the activation pattern of STAT factors. Electrophoretic mobility shift assays were performed to detect STAT isoform-specific complexes. Granulocyte-macrophage colony-stimulating factor (GM-CSF) simultaneously induced complexes containing STAT1, STAT3, STAT5A, STAT5B and STAT6. In non-DC, a similar broad activation pattern of STAT factors by GM-CSF or other cytokines has not been observed so far. By comparison, in peritoneal macrophages, GM-CSF induced a complex with the properties of a truncated form of STAT5. Other cytokines tested on DC either failed to induce STAT factors [interleukin (IL)-1 beta, IL-2, IL-15], or activated the same STAT factors as observed in peritoneal macrophages (IL-4, IFN-gamma). Our results implicate a specific effect of GM-CSF on STAT signaling in DC which might account for the cell type-specific effect of this cytokine on development and function.     

Identification of interleukin-2 receptor-associated tyrosine kinase p116 as novel leukocyte-specific Janus kinase

Janus tyrosine kinase (JAK) has recently been linked to signal transduction by cytokine receptors of the hematopoietin family. We have recently described a 116-kDa tyrosine kinase (p116) present in interleukin-2 (IL-2) receptor complexes in human YT cells that showed functional characteristics of a JAK kinase. These included receptor association, rapid and transient tyrosine phosphorylation kinetics in response to ligand, and in vitro autophosphorylating tyrosine kinase activity (Kirken, R. A., Rui, H., Evans, G. A., and Farrar, W. L. (1993) J. Biol. Chem. 268, 22765-22770). Here we extend these observations by demonstrating structural homologies between IL-2-modulated p116 and prolactin-modulated JAK2 in the rat T cell line Nb2. These include similar net charge as determined by nonequilibrium pH gradient electrofocusing and related primary structure based upon phosphopeptide mapping of V8 protease-digested hyperphosphorylated proteins. This putative JAK kinase underwent marked tyrosine phosphorylation in response to IL-2, IL-4, and IL-7, lymphoid growth factors that use the common IL-2 receptor gamma-chain, but not in response to prolactin. Furthermore, polyclonal antisera to JAK1, JAK2, or tyrosine kinase 2 did not recognize either rat or human p116. However, we identified the IL-2-modulated p116 as the recently cloned novel leukocyte Janus kinase, L-JAK, using an antiserum to a peptide corresponding to the COOH terminus of human L-JAK.