Novel Activation of STAT5b in Response to Epidermal Growth Factor

Abstract

Of the seven signal transducers and activators of transcription that have been identified, STATs 1, 3, and 5a/5b can be activated not only by a multitude of cytokines but also by some growth factors. The data presented here demonstrate that, in contrast to activation by the cytokine, growth hormone (GH), the activation of STAT5b by the growth factor, epidermal growth factor (EGF), requires overexpression of the EGF receptor (EGFR The signal transducers and activators of transcription (STATs) 1 transmit signals from cytokine receptors at the membrane of the cell to the nucleus. Seven STAT proteins have been cloned and four of these (STATs 1, 3, 5a, and 5b) can be activated by a number of different cytokines (1-3). Similar to growth factor receptors, cytokine receptors are single transmembrane receptors that are activated by ligand-induced dimerization. However, the cytokine receptors do not contain a tyrosine kinase activity; rather they recruit one or more of a family of intracellular tyrosine kinases known as JAK kinases (JAKs 1, 2, 3, and tyk2) to the cytokine receptor complex. Activation of these kinases results in the tyrosine phosphorylation of one or more of the recruited STAT proteins (2). Some growth factor receptors have also been shown to activate the tyrosine phosphorylation of STAT proteins (4). Platelet-derived growth factor treatment results in the activation of STAT1 and STAT3, and EGF can activate STATs 1, 3, and 5 (5-8). Although JAK kinases are activated after growth factor stimulation, they are not required for growth factorstimulated tyrosine phosphorylation of STAT1 or STAT3 (4). In response to cytokine treatment, STAT proteins are activated by the phosphorylation of a single tyrosine residue at the C terminus of the molecule. Tyrosine phosphorylation of the STATs results in STAT dimerization through phosphotyrosine-SH2 domain interactions. Dimerization leads to nuclear translocation and binding of STAT dimers to consensus elements upstream of regulated genes. There is evidence that STATs 1, 3, and 5 are also serine-phosphorylated in their C-terminal transactivation domain resulting in maximal transcriptional activation (9). The importance of this C-terminal domain is evidenced by the fact that its truncation results in dominant negative STAT proteins that are able to inhibit the transcriptional activity of the wild type STAT proteins (2). Therefore, these C-terminal truncated proteins provide a direct means of inhibiting the biological function of wild type STATs at the cellular level. The biological actions of STAT proteins are diverse and involve a number of cellular processes, including proliferation, differentiation, and apoptosis (10).In our previous work on growth hormone (GH) receptor signaling, a member of the cytokine receptor superfamily, we identified and cloned the human forms of STAT5a and STAT5b, encoded by two separate genes that result in two highly homologous proteins (94% homology at the amino acid level) (11). Cytokine activation results in the phosphorylation of tyrosi...

EGF receptor‐mediated, c‐Src‐dependent, activation of Stat5b is downregulated in mitogenically responsive hepatocytes

“…There was also a lower sensitivity for EGF-induced activation of STAT proteins (ED 50 ¼ 10 nM) compared to other effects of EGF, such as stimulation of ERK and of DNA synthesis (ED 50 ¼ 1 nM, see e.g., Thoresen et al, 1998). Thus, a strong tyrosine phosphorylation of Stat5b seems to require a high concentration of EGF or, as recently published, overexpression of EGF receptors (Kloth et al, 2002). It is conceivable that differential sensitivity of EGF receptor responses may represent a determinant of specificity in the EGF receptor signal transduction, depending on the cellular context.…”

“…Taken together, these results suggest that EGF, but apparently not PRL or GH, may induce phosphorylation of tyrosine residues other than Tyr-699 in Stat5b. Recently the Tyr-725, -740, and -743 of Stat5b were identified as residues that are phosphorylated in response to EGF in transfected GHR293 cells (Kloth et al, 2002). The functional role of the phosphorylation of the additional Stat5 tyrosine residue(s) is not known, but could be involved in negative regulation of transcription (Kloth et al, 2002) or reflect a function of Stat5 utilized by the EGF receptor but not related to DNA binding activity.…”

Constitutive activation of STAT3 and STAT5 is present in the majority of nasopharyngeal carcinoma and correlates with better prognosis

“…It has been shown that cellular STATs can also be activated through interactions between JAK3 and LMP-1 protein of EBV (Gires et al, 1999). LMP-1 can also upregulate expression of epidermal growth factor receptor (EGFR) (Miller et al, 1995), which may then modulate expression of specific STATs upon ligand stimulation (Petersen and Haldosen, 1998;Kloth et al, 2002;Leong et al, 2002). Interestingly, in line with our findings, despite the fact that LMP-1 of EBV is known for its transforming ability and promotes cellular proliferation in various studies (Baichwal and Sugden, 1988;Kaye et al, 1993), tumours with LMP-1 expression tend to have better prognosis in EBV-associated malignancies such as Hodgkin's disease (Montalban et al, 2000;Glavina-Durdov et al, 2001) and NPC (Hu et al, 1995).…”

Neurotensin stimulates mitogenesis of prostate cancer cells through a novel c-Src/Stat5b pathway

“…To test the involvement of Stat5b in NT-induced DNA synthesis, cells were transfected with either WT Stat5b or a tyrosine mutant of Stat5b (Y699F); expression levels of Stat5b and mutants are shown in Figure 9c. This strategy was employed because Tyr 699 is required for Stat5b-mediated transcriptional activation (Kloth et al, 2002(Kloth et al, , 2003. Cells overexpressing WT Stat5b exhibited the characteristic NT-induced increase in %BrdU incorporation; cells transfected with the empty vector also showed a significant increase (P ¼ 0.01) upon NT treatment (Figure 9d).…”

“…The anti-EGFR antibody (monoclonal antibody (Mab) 108) used for immunoprecipitation was obtained from Dr Michael Weber (University of Virginia (UVA)) (Wright et al, 1996). The anti-Stat5b antibody was provided by Dr Corinne Silva (UVA) (Kloth et al, 2002); antiSrc antibodies were generated by our laboratory . Antibodies specific for phospho-Stat5A/B (Tyr 694/699 ) and protein A agarose beads were purchased from Upstate (Lake Placid, NY, USA).…”