Activation of Signal Transducers and Activators of Transcription 1 and 3 by Leukemia Inhibitory Factor, Oncostatin-M, and Interferon-γ in Adipocytes

Abstract

We have recently demonstrated that signal transducers and activators of transcription (STATs) 1, 3, 5A, 5B, and 6 are expressed in both cultured and native adipocytes. Our current studies have focused on the activation of STATs 1 and 3 by leukemia inhibitory factor (LIF), oncostatin-M (OSM), and interferon-␥ (IFN␥) in 3T3-L1 adipocytes. IFN␥ is shown to be a potent activator of STAT 1 as indicated by both tyrosine phosphorylation and nuclear translocation. However, LIF and OSM, which are potent inducers of STAT 3, are less potent activators of STAT 1 as measured by both tyrosine phosphorylation and nuclear translocation. Both STATs 1 and 3 were translocated to the nucleus in a time-dependent fashion following LIF treatment. In addition, IFN␥ resulted in a time-and dose-dependent effect on STATs 1 and 3 nuclear translocation. Growth hormone, a potent activator of STATs 5A and 5B, had a minimal effect on STAT 1 and STAT 3 tyrosine phosphorylation. Preincubation with either insulin or growth hormone had no detectable effects on the tyrosine phosphorylation or nuclear translocation of STATs 1 and 3 induced by LIF, OSM, or IFN␥. The effects of LIF and IFN␥ on STAT 1 and 3 tyrosine phosphorylation and nuclear translocation were confirmed in native rat adipocytes. In 3T3-L1 adipocytes, a low level of serine phosphorylation of STAT 3 on residue 727 was observed and was markedly enhanced by insulin, LIF, or OSM. This increase in STAT 3 Ser 727 phosphorylation was dependent upon the activation of MAPK, since the MAPK kinase inhibitor (PD98059) reduced STAT 3 Ser 727 phosphorylation to basal levels. The inhibition of MAPK had no effect on the ability of STATs 1 and 3 to be tyrosinephosphorylated or translocate to the nucleus. These studies demonstrate the highly specific and quantitative activation of STATs 1 and 3 by LIF, OSM, and IFN␥ in adipocytes and indicate that STAT 3 is a substrate for MAPK in adipocytes.The signal transducer and activator of transcription (STAT) 1 family of transcription factors is composed of seven family members (STATs 1, 2, 3, 4, 5A, 5B, and 6) that, in response to stimulation of various receptors, mainly those for cytokines, are phosphorylated on tyrosine residues, which causes their translocation to the nucleus. Each STAT family member shows a distinct pattern of activation by cytokines and upon nuclear translocation can regulate the transcription of particular genes (1). The order of events for STAT activation can be briefly described as follows: 1) ligand binding of cell surface receptor; 2) receptor association with a Janus kinase family member; 3) Janus kinase tyrosine phosphorylation of STAT proteins; 4) dimerization of the STATs; 5) translocation to the nucleus; and 6) DNA binding. STATs have been shown to bind at least three different DNA consensus sequences, and this binding regulates the transcription of specific genes (1, 2). It has also been demonstrated that STATs can be activated independently of Janus kinases (1, 3) and that serine phosphorylation may also contribute to the abil...

Interferon-γ Induces X-linked Inhibitor of Apoptosis-associated Factor-1 and Noxa Expression and Potentiates Human Vascular Smooth Muscle Cell Apoptosis by STAT3 Activation

“…In contrast, less STAT3 activation by IFN-␥ was found in macrophages from STAT1-deficient versus wild type mice (8). Convincing evidence of STAT3 phosphorylation and binding to promoter elements in the presence of IFN-␥-induced STAT1 activation has been observed in certain human cell types, including VSMCs (15), hematopoietic progenitors (9, 10), neutrophils (10,11), myelocytic leukemia cells (9, 11), adipocytes (35), hepatocytes (36), hepatoma cells (37), neuroblastoma cells (38), and synovial cells (39) but not endothelial cells (23), lymphocytes (11), or monocytes and eosinophils (10). However, IFN-␥ has also been described to dephosphorylate constitutively activated STAT3 in tuberous sclerosis complex-deficient mouse embryonic fibroblasts and human prostate cancer cells (40,41).…”

Serine phosphorylation of STATs

“…For example, STAT1 S727 phosphorylation in macrophages is sensitive to SB203580 in stress of LPS responses, but insensitive to this inhibitor in the IFN-g response . Similarly, STAT3 S727 phosphorylation in response to EGF, PDGF, insulin, IL-2, BCR or TCR stimulation is sensitive to PD98059 and, where tested, insensitive to H7, whereas the opposite is true for IL-6 or IFN-a (Chung et al, 1997;Ng and Cantrell, 1997;Stephens et al, 1998;Su et al, 1999). (2) S727 phosphorylation of dierent STATs by signals from the same cell surface receptor can be routed through distinct signaling paths.…”

Adipogenesis

“…Numerous findings in the last decade have highlighted the differential effects of gp130 cytokines on important adipocyte functions. These studies have shown that adipocytes in vitro and adipose tissue in vivo are responsive to gp130 cytokines (Balhoff and Stephens 1998;Stephens et al 1998;Lagathu et al 2003;Rotter et al 2003;Zvonic et al 2003Zvonic et al , 2004Tenney et al 2005;White et al 2008) and exert differential effects on adipogenesis. To date, no gp130 cytokines have been shown to promote adipogenesis, but as shown in Table 1, several cytokines in this family have been shown to inhibit adipocyte development (Keller et al 1993;Meng et al 2001;Zvonic et al 2003Zvonic et al , 2004Sopasakis et al 2004;Hogan and Stephens 2005;Miyaoka et al 2006;Song et al 2007;White et al 2008).…”