Serine phosphorylation of GH-activated signal transducer and activator of transcription 5a (STAT5a) and STAT5b: impact on STAT5 transcriptional activity

Signal transducer and activator of transcription 5b (STAT5b), the major liver-expressed STAT5 form, is phosphorylated on both tyrosine and serine in GH-stimulated cells. Although tyrosine phosphorylation is known to be critical for the dimerization, nuclear translocation, and activation of STAT5b DNA-binding and transcriptional activities, the effect of STAT5b serine phosphorylation is uncertain. Presently, we identify Ser730 as the site of STAT5b serine phosphorylation in GH-stimulated liver cells. We additionally show that the serine kinase inhibitor H7 partially blocks the GH-stimulated formation of (Ser,Tyr)-diphosphorylated STAT5b without inhibiting STAT5b nuclear translocation. Evaluation of the functional consequences of STAT5b serine phosphorylation by mutational analysis revealed an approximately 50% decrease in GH-stimulated luciferase reporter gene activity regulated by an isolated STAT5-binding site when STAT5b Ser730 was mutated to alanine and under conditions where STAT5 DNA-binding activity was not diminished. No decrease in GH-stimulated reporter activity was seen with the corresponding STAT5a-Ser725Ala mutant; however, a decrease in reporter activity occurred when the second established STAT5a serine phosphorylation site, serine 779, was additionally mutated to alanine. Unexpectedly, STAT5a-Ser725,779Ala and STAT5b-Ser730Ala displayed approximately 2-fold higher GH- or PRL-stimulated transcriptional activity compared with wild-type STAT5b when assayed using an intact beta-casein promoter-luciferase reporter. Finally, STAT5b-stimulated gene transcription was abolished in cells treated with H7, but in a manner unrelated to the inhibitory effects of H7 on STAT5b Ser730 phosphorylation. These findings suggest that the effects of STAT5b and STAT5a serine phosphorylation on STAT-stimulated gene transcription can be modulated by promoter context. Moreover, in the case of STAT5a, phosphorylation of serine 779, but not serine 725, may serve to regulate target gene transcriptional activity.

Role of serine phosphorylation of Stat5a in prolactin-stimulated beta-casein gene expression

Milk production remains suppressed in mammals during late pregnancy despite high levels of lactogenic polypeptide hormones. At parturition, associated with a precipitous fall in circulating progesterone, rising glucocorticoid levels synergize with prolactin to initiate copious milk production. This synergy is mediated at least in part through the coordinated activation of glucocorticoid receptors and transcription factor Stat5, particularly Stat5a. Here we show that two proline-juxtaposed serine residues within the transactivation domain of Stat5a are phosphorylated in the mammary gland during late gestation and lactation, and that these phosphorylation sites inhibit the transcriptional activity of Stat5a in the absence of glucocorticoid receptor costimulation. Specifically, transfection assays revealed that phosphorylation of residues S725 and S779 of Stat5a cooperatively suppressed prolactin-stimulated transcription from the beta-casein promoter in both COS-7 kidney and MCF-7 mammary cells. This suppression was associated with shortened duration and reduced amplitude of nuclear DNA binding activity of wild type Stat5a relative to that of the serine phosphorylation-defective Stat5 mutant. However, costimulation of glucocorticoid receptors completely reversed the suppressive effect of Stat5a serine phosphorylation on beta-casein gene transcription. We propose that serine phosphorylation within the transactivation domain may limit the activity of Stat5a in the absence of proper coactivation by glucocorticoid receptors.

Flavonoids from Glycyrrhiza pallidiflora hairy root cultures

Glycyrrhiza pallidiflora hairy roots were induced from axenic young plants by direct infection with Agrobacterium rhizogenes. The chemical constituents were then investigated after mass culture. The isoflavone, licoagroisoflavone and the coumestan, licoagroside C, were isolated along with seven known flavonoids. Their structures were determined on the basis of spectroscopic evidence.

Functional uncoupling of the Janus kinase 3-Stat5 pathway in malignant growth of human T cell leukemia virus type 1-transformed human T cells

Human T cell leukemia virus type 1 (HTLV-1) transforms cytokine-dependent T lymphocytes and causes adult T cell leukemia. Janus tyrosine kinase (Jak)3 and transcription factors Stat5a and Stat5b are essential for the proliferation of normal T cells and are constitutively hyperactivated in both HTLV-1-transformed human T cell lines and lymphocytes isolated from HTLV-1-infected patients; therefore, a critical role for the Jak3-Stat5 pathway in the progression of this disease has been postulated. We recently reported that tyrphostin AG-490 selectively blocked IL-2 activation of Jak3/Stat5 and growth of murine T cell lines. Here we demonstrate that disruption of Jak3/Stat5a/b signaling with AG-490 (50 microM) blocked the proliferation of primary human T lymphocytes, but paradoxically failed to inhibit the proliferation of HTLV-1-transformed human T cell lines, HuT-102 and MT-2. Structural homologues of AG-490 also inhibited the proliferation of primary human T cells, but not HTLV-1-infected cells. Disruption of constitutive Jak3/Stat5 activation by AG-490 was demonstrated by inhibition of 1) tyrosine phosphorylation of Jak3, Stat5a (Tyr(694)), and Stat5b (Tyr(699)); 2) serine phosphorylation of Stat5a (Ser(726)) as determined by a novel phosphospecific Ab; and 3) Stat5a/b DNA binding to the Stat5-responsive beta-casein promoter. In contrast, AG-490 had no effect on DNA binding by p50/p65 components of NF-kappaB, a transcription factor activated by the HTLV-1-encoded phosphoprotein, Tax. Collectively, these data suggest that the Jak3-Stat5 pathway in HTLV-1-transformed T cells has become functionally redundant for proliferation. Reversal of this functional uncoupling may be required before Jak3/Stat5 inhibitors will be useful in the treatment of this malignancy.

Epithelial defect in prostates of Stat5a-null mice

The transcription factor Stat5a critically mediates prolactin (PRL)-induced mammary gland development and lactogenesis. PRL also stimulates growth and differentiation of prostate tissue. Specifically, hyperprolactinemia gives rise to prostate hyperplasia, and prostate size is reduced in PRL-deficient mice. We therefore investigated the importance of Stat5a for prostate development and function by examining Stat5a-null mice. The absence of Stat5a in mice was associated with a distinct prostate morphology characterized by an increased prevalence of local disorganization within acinar epithelium of ventral prostates. Affected acini were typically filled with desquamated, granular epithelial cells that had become embedded in dense, coagulated secretory material. These features were reminiscent of acinar cyst formation and degeneration frequently observed in human benign prostate hyperplasia, however, cystic changes in prostate acini of Stat5a-deficient mice were not associated with increased prostate size or morphologic hallmarks of epithelial hyperplasia. Instead, immunohistochemistry of the prostate-specific secretory marker, probasin, suggested that hypersecretory function of the epithelium could underlie local congestion and cyst formation in prostates of Stat5a-null mice. Serum testosterone and PRL levels were normal in Stat5a knockout mice, but prostate PRL receptor expression was reduced as determined by immunohistochemistry. Expression levels or activation states of other PRL signal transduction proteins, including Stat5b, Stat3, Stat1, ERK1, and ERK2 were not altered. The present study offers the first evidence for a direct role of Stat5a in the maintenance of normal tissue architecture and function of the mouse prostate.

Prolactin is a survival factor for androgen-deprived rat dorsal and lateral prostate epithelium in organ culture

PRL is one of several polypeptide factors that regulate growth and differentiation of prostate epithelium besides steroid hormones. This hormone may also participate in the development of pathologic changes of the prostate, as evidenced by marked prostate hyperplasia in hyperprolactinemic mice. We have previously demonstrated expression of PRL receptors and androgen-dependent local production of PRL in rat and human prostate epithelium, suggesting the existence of an autocrine loop. We now show that PRL acts as a survival factor for epithelial cells of rat dorsal and lateral prostate but not ventral prostate, using long-term organ cultures as an in vitro model. Culture of prostate explants in androgen-free medium was associated with a transient surge of apoptosis during the first 2-4 days of culture in rat ventral, dorsal, and lateral prostate tissues, as quantified by either nuclear morphology or in situ DNA fragmentation analysis. PRL significantly inhibited apoptosis in androgen-deprived dorsal and lateral prostate cultures, by 40-60%, as determined by the two methods. The present study has established conditions and methodology for analysis of apoptosis in organ cultures of rat prostate and suggests a physiological role for PRL as a survival factor for prostate epithelium.

Stat5a and Stat5b: fraternal twins of signal transduction and transcriptional activation

Stat5a and Stat5b are discretely encoded transcription factors that mediate signals for a broad spectrum of cytokines. Their activation is often an integral component of redundant cytokine signal cascades involving complex cross-talk and pleiotropic gene regulation by Stat5 has been implicated in cellular functions of proliferation, differentiation and apoptosis with relevance to processes of hematopoiesis and immunoregulation, reproduction, and lipid metabolism. Although Stat5a and Stat5b show peptide sequence similarities of > 90%, targeted gene disruptions in mice yield distinctive phenotypes. Prolactin-directed mammary gland maturation fails without functional Stat5a, while disruption of Stat5b in males mitigates growth hormone effects on hepatic function and body mass. The molecular basis for this biologic dichotomy is probably multifaceted. Limited structural dissimilarities between the Stat5a and Stat5b transactivation domains, or subtle differences in the DNA-binding affinities of Stat5 dimer pairs undoubtedly influence gene regulation, but cell-dependent asymmetries in availability of phosphorylated Stat5 can be an underlying factor. Differences in serine phosphorylation(s) of Stat5a and Stat5b, or Stat5 associations with adaptor proteins or co-transcription factors are other potential sources of functional disparity and the signal amplitude, frequency or duration also can be significant. In addition to Stat5 signal attenuation by phosphatase actions or classical feedback inhibition, truncated forms of Stat5 lacking in transactivation capacity may compete upstream for activation and diminish access of full length molecules to DNA binding sites.

A lymphoma growth inhibitor blocks some but not all prolactin-stimulated signaling pathways

Cytokines and hormones activate a network of intracellular signaling pathways to regulate cell division, survival and differentiation. In parallel, a series of growth inhibitory mechanisms critically restrict cell population sizes. For example, mitogens can be opposed in crowded cell cultures through contact-inhibition or by autocrine release of antiproliferative substances. Here, we characterize a small, heat-stable growth inhibitor secreted by a rat T lymphoma line when cultured at high cell density. Short term incubation (<60 min) of prolactin-responsive Nb2 lymphoma cells at high density selectively blocked prolactin stimulation of p42/p44 mitogen-activated protein kinases and transcription factors Stat1 and Stat3 but not prolactin activation of Stat5 or the tyrosine kinase Jak2. The selective effects of cell density on prolactin signaling were reversible. Furthermore, exposure of cells at low density to conditioned media from cells incubated at high density had the same inhibitory effects on prolactin signaling. This selective inhibition of discrete prolactin signals was mimicked by short term preincubation of cells at low density with staurosporine or genistein but not with bis-indoleyl maleimide, cyclic nucleotide analogs, calcium ionophore A23187, or phorbol 12-myristate 13-acetate. A heat-stable, proteinase K-resistant, low molecular weight factor with these characteristics was recovered from high density culture medium. The partially purified inhibitor suppressed Nb2 cell growth with a sigmoidal concentration response consistent with a saturable, receptor-mediated process.

The effects of S-3-1 on lipid peroxidation and scavenging free radicals in vitro

S-3-1 is a simplified synthetic analogue of the active principle of Salvia miltiorrhioza. Electron spin resonance spectrometry using 5,5'-dimethyl-1-pyrroline-N-oxide as a scavenger of free radicals indicated that 200 microg/ml of S-3-1 scavenged 1,1-diphenyl-2-picrylhydrazyl radicals completely. 25 microg/ml of this compound quenched 100% of superoxide anion and a concentration of 250 microg/ml of S-3-1 quenched 63% of hydroxyl radicals. It was also shown that 3.56 mg/ml of S-3-1 could inhibit lipid peroxidation in microsome fraction from rat liver induced by FeSO4 and cysteine. These results show that S-3-1 is an effective antioxidant by scavenging free radicals.

Differential control of the phosphorylation state of proline-juxtaposed serine residues Ser725 of Stat5a and Ser730 of Stat5b in prolactin-sensitive cells

Transcription factors of the Stat family are controlled by protein kinases. Phosphorylation of a positionally conserved tyrosine residue is obligatory for Stat dimerization, nuclear translocation, and specific DNA binding. Studies of Stat1 and Stat3 have suggested that serine phosphorylation may also regulate function. We now identify serine residues located in a conserved PSP motif of Stat5a (Ser725) and Stat5b (Ser730) as major phosphorylation sites, using mutagenesis, phosphoamino acid analysis, and site-specific anti-Stat5-phosphoserine antibodies. Unexpectedly, phosphorylation control of this PSP motif differed between the highly homologous Stat5a and Stat5b proteins. Whereas Ser725 of Stat5a was constitutively phosphorylated both in COS-7 cells and Nb2 lymphocytes, phosphorylation of Ser730 of Stat5b was markedly stimulated by prolactin. The data also suggested the existence of a second major serine phosphorylation site in Stat5a. Interestingly, constitutive phosphorylation of the PSP motif was suppressed by PD98059 but not by staurosporine under conditions in which both agents inhibited mitogen-activated protein kinases. Furthermore, pretreatment of cells with staurosporine, PD98059, H7, or wortmannin did not prevent either Stat5a or Stat5b from becoming maximally serine-phosphorylated after prolactin exposure. We propose that two pathways regulate Stat5 serine phosphorylation, one that is prolactin-activated and PD98059-resistant and one that is constitutively active and PD98059-sensitive and preferentially targets Stat5a. Finally, phosphorylation of the PSP motif of Stat5a or Stat5b was not essential for DNA binding or transcriptional activation of a beta-casein reporter gene in COS-7 cells, suggesting that serine kinase control of Stat5 activity differs from that of Stat1 and Stat3.

Chemoprevention of cancer of uterine cervix: a study on chemoprevention of retinamide II from cervical precancerous lesions

Dysplasia of the uterine cervix is a recognized precancerous condition. Because of the observed ability of retinoids to suppress various cell lines in vitro, a number of clinical studies have examined the effect these agents have on cervical dysplasia, with the object of developing a means of chemoprevention of cervical malignancies in women at risk. Three cervical cancer chemoprevention trials with Retinamide II (RII) have been conducted at the Cancer Institute, Chinese Academy of Medical Sciences, Beijing, China. A pilot study used RII to intervene in cases of precancerous cervical dysplasia. Twenty-seven women with mild, moderate, or severe cervical dysplasia, pathologically confirmed, were treated by RII suppositories, 10 mg QD, given intravaginally for 6 months (each course lasting 3 months). The results indicated that after the second course, the overall response rate was 96.29% and the complete response rate was 88.89%. In general, side effects were mild. A little cervical and vaginal irritation was well tolerated. In the second double-blind study, patients with precancerous cervical lesions were randomized into two groups, one treated with RII suppository intravaginally and the other with a placebo, once daily for 50 days in two courses. Precancerous lesions in 68.76% of patients in the treatment arm disappeared, with an overall effective rate of 74.29% after two courses of treatment with RII. Its curative effect was approximately that of laser beam radiation and electrocautery (P > 0.05), and differed significantly (P < 0.01) from that of traditional antiinflammatories. RII can be a major measure in prevention and treatment of cervical cancer in high-incidence areas in China. In the third trial, we are conducting a randomized double-blind study placebo controlled, in a high-incidence area of cervical cancer (Xiang-Yuan county, Shang Xi Province, China). At present, the patients are being followed up and the study will be completed after 2 years.

Research and development of cancer chemopreventive agents in China

Since the late 1970s, a comprehensive search for cancer chemopreventive agents has been established in our Institute. A series of new retinoids have been synthesized and screened on the basis of established methodologies of experimental chemoprevention in vitro as well as in vivo. Pharmacological studies demonstrated that N-4-(carboxyphenyl)retinamide (RII) induces cell differentiation of HL-60 cells and inhibits dimethylnitrosamine-induced carcinogenesis of the forestomach in mice, 7,12-dimethylbenz[a]anthracene (DMBA)-induced papilloma in mouse skin, and DMBA-induced carcinogenesis of the buccal pouch in Syrian golden hamsters. It significantly promoted lymphoblastic transformation and activated macrophages. In further studies, RII significantly inhibited ornithine decarboxylase activity. After 6 months of chronic toxicological studies in rats and dogs, RII was recommended for clinical trial. Phase II studies found that RII is effective in treating oral and vulvar leukoplakia. It is also effective in treating myelodysplastic syndrome and dysplasia of uterine cervix. The chalcone retinoidal compounds were discovered when the search for new retinoids with less toxicity and higher potency led to third-generation retinoids, which were synthesized and screened. Structure-activity relationship studies found that 3,5-di-tert-butyl-4-methoxy-4-carboxyl chalcone (R9158) is the most active inhibitor of a variety of cancer cells. It has no effect on the Colony Forming Unit-Granulocyte/Macrophage (CFU-GM) of bone marrow in mice. In in vivo studies, R9158 showed a remarkable inhibition of chondrosarcoma in rats. It had no cross-resistance to vincristine, but was cross-resistant to all-trans retinoic acid. Red ginseng, a processed Panax ginseng, is considered a typical tonic in traditional Chinese medicine. Our studies demonstrated that red ginseng extract inhibited DMBA-induced skin papilloma significantly. Experiments showed that glycyrrhetinic acid inhibited croton oil-induced ear edema in mice. It also inhibited epidermal ornithine decarboxylase as well as the rapid DNA damage induced by the carcinogen benzo[a]pyrene (B[a]P). Our pharmacological studies demonstrated that Chinese gallotannin inhibited the malignant transformation of B[a]P-induced V79 cells in vitro and B[a]P-induced pulmonary adenoma in A/J mice in vivo significantly.

Prolactin activates Stat1 but does not antagonize Stat1 activation and growth inhibition by type I interferons in human breast cancer cells

Type I interferons (IFN alpha and IFN beta) are presently used in the adjuvant treatment of several human cancers. However, these cytokines have demonstrated only modest success in breast cancer therapy, and research efforts have focused on improving their efficacy. Recent progress in understanding the molecular mechanisms underlying the antiproliferative effects of IFNs has identified the cytoplasmic transcription factor Stat1 as a critical mediator. It is, therefore, possible that IFN-induced growth inhibition of mammary epithelial cells is counteracted by other cytokines that also use Stat1. One such candidate IFN-antagonist with particular relevance to breast cancer is the mammotropic hormone prolactin (PRL). The main goal of this study was to examine whether PRL would interfere with type I IFN (IFN alpha/beta) signal transduction by competing for limited cytoplasmic Stat factors. A second aim was to test whether pretreatment of mammary tumor cell lines with IFN gamma could enhance the effect of IFN alpha/beta. By analyzing the effect of PRL on IFN alpha/beta-induced tyrosine phosphorylation of Stat proteins and their binding to IFN-regulated genes, we now report that costimulation of PRL receptors did not interfere with IFN alpha/beta signals in several human breast cancer cell lines, including T47D, MCF-7, and BT-20. Specifically, PRL did not affect IFN alpha/beta-induced tyrosine phosphorylation or heterodimerization of Stat1 and Stat2 in any cell line. Instead, IFN alpha/beta- and PRL-induced tyrosine phosphorylation of Stat1 was additive and occurred without evidence of competition for limited concentrations of cytoplasmic Stat1. A similar additive relationship was observed on IFN alpha/beta- and PRL-induced Stat3 tyrosine phosphorylation. Furthermore, electrophoretic mobility shift assays showed that type I IFNs induced predominantly Stat1-Stat2 or Stat1-Stat3 heteromeric complexes with various IFN-response elements of IFN-stimulated genes, whereas PRL induced Stat1 homodimers. Despite significant mutual use of Stats by IFNs and PRL, these results indicated a high degree of signaling specificity in the two receptor systems, and that cytoplasmic levels of Stat proteins were not limiting. Similarly, PRL did not interfere with the growth-inhibitory effect of IFN beta. On the other hand, the study indicated that pretreatment of human breast cancer cell lines with IFN gamma enhanced the growth-inhibitory action of type I IFNs, suggesting a possible avenue for improving the effect of type I IFNs in the treatment of breast cancer patients.

Prolonged STAT1 activation related to the growth arrest of malignant lymphoma cells by interferon-alpha

Multiple biologic effects of interferon-alpha (IFN-alpha), including cell growth inhibition and antiviral protection, are initiated by tyrosine phosphorylation of STAT proteins. Although this signal pathway has been intensively investigated, the relevance of STAT signal persistence has received scant attention. Using paired isogenic lymphoma cells (Daudi), which either are sensitive or resistant to growth inhibition by IFN-alpha, we found comparable initial tyrosine phosphorylation of multiple STAT proteins; however, the phosphorylation durations and associated DNA-binding activities diverged. Phosphorylation and DNA-binding capacity of STAT1 decreased after 4 to 8 hours in resistant cells, as compared with 24 to 32 hours in sensitive cells, whereas phosphorylation of STAT3 and STAT5b was briefer in both lines. Functional significance of the prolonged STAT1 signal, therefore, was explored by experimental interruption of tyrosine phosphorylation, either by premature withdrawal of the IFN-alpha or deferred addition of pharmacologically diverse antagonists: staurosporine (protein kinase inhibitor), phorbol 12-myristate 13-acetate (growth promoter), or aurintricarboxylic acid (ligand competitor). Results indicated that an approximately 18-hour period of continued STAT1 phosphorylation was associated with growth arrest, but that antiviral protection developed earlier. These differences provide novel evidence of a temporal dimension to IFN-alpha signal specificity and show that duration of STAT1 activation may be a critical variable in malignant cell responsiveness to antiproliferative therapy.

Cancer chemopreventive and therapeutic activities of red ginseng

Red ginseng extract A and B are the active components of Panax ginseng. Red ginseng is a classical traditional Chinese medicine. Among Chinese herbs, red ginseng has been considered as one of the tonics. Many studies indicated that red ginseng could enhance immune function of the human body. The effects of red ginseng extracts on transplantable tumors, proliferation of lymphocyte, two-stage model and rat liver lipid peroxidation were studied. In a two-stage model, red ginseng extracts had a significant cancer chemoprevention. At 50-400 mg/kg, they could inhibit DMBA/Croton oil-induced skin papilloma in mice, decrease the incidence of papilloma, prolong the latent period of tumor occurrence and reduce tumor number per mouse in a dose-dependent manner. Red ginseng extract B could effectively inhibit the Fe2+/cysteine-induced lipid peroxidation of rat liver microsome, suggesting that red ginseng extract B has a stronger antioxidative effect than that of extract A. The results indicated that red ginseng extracts (50 approximately 400 mg/kg) could significantly inhibit the growth of transplantable mouse sarcoma S180 and melanoma B16. Red ginseng extracts A (0.5 mg/ml) and B (0.1 and 0.25 mg/ml) might effectively promote the transformation of T lymphocyte, but there was no influence on lymphocyte proliferation stimulated by concanavalin A. This suggests that red ginseng extracts have potent tumor therapeutic activity and improve the cell immune system.

Activation of the Jak2-Stat5 signaling pathway in Nb2 lymphoma cells by an anti-apoptotic agent, aurintricarboxylic acid

Biological effects of many hormones and cytokines are mediated through receptor-associated Jak tyrosine kinases and cytoplasmic Stat transcription factors, including critical physiological processes such as immunity, reproduction, and cell growth and differentiation. Pharmaceuticals that control Jak-Stat pathways are therefore of considerable interest. Here we demonstrate that a single Jak-Stat pathway can be activated by aurintricarboxylic acid (ATA), a negatively charged triphenylmethane derivative (475 Da) with anti-apoptotic properties. In prolactin (PRL)-dependent Nb2 lymphocytes, ATA sustained cell growth in the absence of hormone and mimicked rapid PRL-induced tyrosine phosphorylation of Jak2 and activation of Stat5a and Stat5b with tyrosine phosphorylation, heterodimerization, DNA binding, and induction of the Stat5-regulated pim-1 protooncogene. ATA also mimicked PRL activation of serine kinases ERK1 and ERK2. However, unlike PRL, ATA did not regulate Stat1 or Stat3. ATA also did not affect Jak3, which is activated in these cells by interleukin-2 family cytokines. Although the mechanism and specificity by which ATA activates Jak2, Stat5, and ERKs in Nb2 cells are still unclear, the present study demonstrates that certain hormone or cytokine effects on Jak-Stat pathways can be discretely imitated by a low molecular weight, non-peptide pharmaceutical. The results are also consistent with Stat5 involvement in lymphocyte growth and survival.

Two discrete regions of interleukin-2 (IL2) receptor beta independently mediate IL2 activation of a PD98059/rapamycin/wortmannin-insensitive Stat5a/b serine kinase

Many cytokines, hormones, and growth factors activate Janus kinases to tyrosine phosphorylate select members of the Stat transcription factors. For full transcriptional activation, Stat1 and Stat3 also require phosphorylation of a conserved serine residue within a mitogen-activated protein kinase phosphorylation consensus site. On the other hand, two recently identified and highly homologous Stat5a and Stat5b proteins lack this putative mitogen-activated protein kinase phosphorylation site. The present study set out to establish whether Stat5a and Stat5b are under the control of an interleukin-2 (IL2)-activated Stat5 serine kinase. We now report that IL2 stimulated marked phosphorylation of serine and tyrosine residues of both Stat5a and Stat5b in human T lymphocytes and in several IL2-responsive lymphocytic cell lines. No Stat5a/b phosphothreonine was detected. Phosphoamino acid analysis also revealed that Stat5a/b phosphotyrosine levels were maximized within 1-5 min of IL2 stimulation, whereas serine phosphorylation kinetics were slower. Interestingly, IL2-induced serine phosphorylation of Stat5a differed quantitatively and temporally from that of Stat5b with Stat5a serine phosphorylation leveling off after 10 min and the more pronounced Stat5b response continuing to rise for at least 60 min of IL2 stimulation. Furthermore, we identified two discrete domains of IL2 receptor beta (IL2Rbeta) that could independently restore the ability of a truncated IL2Rbeta mutant to mediate Stat5a/b phosphorylation and DNA binding to the gamma-activated site of the beta-casein gene promoter. These observations demonstrated that there is no strict requirement for one particular IL2Rbeta region for Stat5 phosphorylation. Finally, we established that the IL2-activated Stat5a/b serine kinase is insensitive to several selective inhibitors of known IL2-stimulated kinases including MEK1/MEK2 (PD98059), mTOR (rapamycin), and phosphatidylinositol 3-kinase (wortmannin) as determined by phosphoamino acid and DNA binding analysis, thus suggesting that a yet-to-be-identified serine kinase mediates Stat5a/b activation.

Prolactin stimulates serine/tyrosine phosphorylation and formation of heterocomplexes of multiple Stat5 isoforms in Nb2 lymphocytes

Transcription factors of the Stat gene family are selectively activated by many hormones and cytokines. Stat5 originally was cloned as a prolactin-stimulated DNA-binding protein, but is also activated by non-lactogenic cytokines in many cell types. The recent identification of two distinct Stat5 genes, which encode a 94-kDa Stat5a and a 92-kDa Stat5b as well as several lower molecular weight isoforms, suggests additional complexity and combinatorial possibilities for transcriptional regulation. We now report a biochemical analysis of prolactin activation of Stat proteins in Nb2 lymphocytes, which was associated with: 1) rapid tyrosine phosphorylation of Stat5a, Stat5b, a COOH-terminally truncated 80-kDa Stat5 form, Stat1alpha, and Stat3; 2) rapid and selective formation of Stat5a/b heterodimers, without involvement of Stat1alpha or Stat3; 3) marked serine, but not threonine phosphorylation of Stat5a and Stat5b; and 4) the appearance of two qualitatively distinct Stat5 protein complexes, which discriminated between oligonucleotides corresponding to the prolactin response elements of the beta-casein and interferon regulatory factor-1 gene promoters. Collectively, our analyses showed that Stat5a and Stat5b respond similarly to prolactin receptor activation, but also suggested that the two genes have evolved unique properties that may contribute to the specificity of receptors that utilize Stat5 signaling proteins.

Interleukin-13 is a potent activator of JAK3 and STAT6 in cells expressing interleukin-2 receptor-gamma and interleukin-4 receptor-alpha

The lymphocyte growth factors interleukin-2 (IL2), IL4, IL7, IL9 and IL15 use the common IL2 receptor-gamma (IL2R gamma) and activate the IL2R gamma-associated tyrosine kinase JAK3 (Janus kinase 3). IL13 is structurally related to IL4, competes with IL4 for binding to cell surface receptors and exhibits many similar biological effects. The molecular basis for this functional overlap between IL4 and IL13 has been attributed mainly to a shared use of the 140 kDa IL4R alpha, since these cytokines appear to be uniquely different in that, according to several recent reports, IL13 does not recruit the IL2R gamma or JAK3. This notion has been supported by the identification of a novel 70 kDa IL13 receptor in certain IL13-responsive cell lines that lack IL2R gamma. The present study sheds new light on the issue of functional overlap between IL13 and IL4, by demonstrating for the first time that, in cells that express both IL2R gamma and IL4R alpha, IL13 can mimic IL4-induced heterodimerization of IL2R gamma and IL4R alpha, with consequent marked activation of JAK3 and the transcription factor STAT6 (IL4-STAT). Reconstitution experiments in BA/F3 cells showed that both cytokines require the simultaneous presence of IL4R alpha and IL2R gamma to mediate JAK3 and proliferative responses, and analysis of 12 IL4R alpha variants showed that IL4 and IL13 signals were equally affected by mutations of the cytoplasmic domain. We conclude that IL13 activates the IL2R gamma-associated JAK3 tyrosine kinase in appropriate cell types, and propose that IL13 is capable of interacting with multiple receptor subunits in a cell-dependent and combinatorial manner. Consequently, we predict that partial disruption of IL13 signal transduction also contributes to the severe combined immuno-deficiency syndromes associated with inactivation of the IL2R gamma or JAK3 genes.

Prolactin recruits STAT1, STAT3 and STAT5 independent of conserved receptor tyrosines TYR402, TYR479, TYR515 and TYR580

The present study of prolactin (PRL) receptor-mediated recruitment of signal transducers and activators of transcription (STATs) demonstrates that PRL activates STAT3, in addition to STAT1 and STAT5 as previously reported, and that STAT1, STAT3 and STAT5 are mediators of PRL effects in cells whether of lymphoid, myeloid or mammary epithelial origin. Furthermore, receptor mutants M240 and T280 that do not mediate PRL-induced JAK2 activation and cell proliferation, are also unable to mediate STAT activation, supporting the proposed model of JAK2 as the initial effector protein used by PRL receptors. On the other hand, tyrosine phosphorylation analysis and electrophoretic mobility shift assays showed that receptor mutant G328, which lacks four of the five conserved cytoplasmic tyrosine residues of PRL receptors, retained the ability to activate JAK2 and STAT1, STAT3 and STAT5. These results support the notion that phosphotyrosyl residues other than those of the receptor, i.e., JAK2, are involved in recruiting STAT proteins to the activated PRL receptor complex.

Activation of JAK3, but not JAK1, is critical for IL-2-induced proliferation and STAT5 recruitment by a COOH-terminal region of the IL-2 receptor beta-chain

A number of cytokines and growth factors use the JAK-STAT pathway to signal from the cell membrane to the nucleus. While homodimerizing cytokine receptors may transmit signal via a single form of JAK (i.e. growth hormone receptors), several multicomponent cytokine receptors have been shown to require simultaneous activation of pairs of different JAK kinases (i.e. interferon receptors). Recent evidence for a preferential coupling of JAK3 to interleukin-2 receptor-gamma (IL-2R gamma) and JAK1 to IL-2R beta supports the concept of heterotrans-activation of JAK1 and JAK3 caused by IL-2-induced heterodimerization of their receptor partners. The present study verified the ability of IL-2 to cause tyrosine phosphorylation and activation of JAK1 and JAK3, but demonstrated that IL-2 stimulated JAK3 to a significantly larger extent than JAK1 in human T lymphocytes and the YT cell line. This conclusion was based upon several independent criteria, including more vigorous tyrosine phosphorylation of JAK3, more marked enzymatic activation of JAK3 as well as higher abundance of JAK3 in activated IL-2 receptor complexes. Furthermore, when human IL-2R beta was stably expressed in murine BA/F3 cells, robust IL-2-induced proliferation and JAK3 activation occurred without detectable involvement of either JAK1, JAK2 or TYK2. We therefore propose that IL-2 receptor signal transduction does not depend on equimolar heterodimerization of JAK1 and JAK3 following IL-2-induced heterodimerization of IL-2R beta and IL-2R gamma. Nonetheless, a membrane-proximal region of human IL-2R beta (Asn240-Leu335) was critical for JAK3 activation, and the amount of JAK3 present in activated IL-2 receptor complexes increased with time, suggesting that stabilization of JAK3 binding to the receptor complex relies on both IL-2R beta and IL-2R gamma. Moreover, STAT5 was found to be the predominant STAT transcription factor used by IL-2 in human T cells, and specifically required a COOH-terminal region of IL-2R beta (Ser386-Val525), while STAT5 recruitment was not correlated to activation of IL-2R gamma or JAK3.

Structural and functional implications of age-related abnormal modifications in collagen II from intervertebral disc

Collagen II was isolated from annulus fibrosus of human donors, and the degree of enzymatic glycosylation and the extent of nonenzymatic modification was determined as a function of age. While enzymatic glycosylation did not alter with age, the fluorescence as a measure of nonenzymatic modification increased, in particular in those fractions containing highly cross-linked collagen molecules. The age-dependent increase of fluorescence was associated with a lower thermal stability of collagen II, being lowest (Tm = 36.9 degrees C instead of 42.1 degrees C) in a fraction of collagen II molecules isolated from tissue of a 69-year-old donor. In addition, collagen II from elderly donors showed an impaired propensity to form fibrils, using an in vitro assay, as evidenced by the facts that (1) more collagen molecules remained in solution and were not assembled into fibrils, and (2) the relative turbidity caused by the aggregates progressively decreased as the age of the donors increased. As seen on electron micrographs, these fibrillar aggregates were less densely packed and intermingled with precipitates such as unbanded filaments or discontinuous fibrils with split and frayed regions. It can be hypothesized that functional impairment of the intervertebral disc with aging may be caused in part by molecular alterations as are described here for collagen II obtained from aged donors.

Prolactin activates Ras via signaling proteins SHC, growth factor receptor bound 2, and son of sevenless

Identification of the signal transduction pathways used by PRL is essential for understanding the role of PRL receptors in growth and differentiation processes. Early cellular mediators of PRL receptor activation include tyrosine kinases of the Janus kinase (JAK) and SRC families, with rapid nuclear signaling via tyrosine phosphorylated signal transducers and activators of transcription. In the present study we provide the first demonstration of PRL-induced activation of Ras, an oncogenic protein that supports an alternative signaling route from the membrane to the nucleus. PRL stimulated Ras in rat Nb2-SP lymphoma cells, as detected by a 2.0-fold increase in the GTP-bound state of the molecule (P < 0.01). This activation was associated with marked tyrosine phosphorylation and increased membrane association of the 52-kilodalton form of SHC. Moreover, PRL induced binding of SHC to growth factor receptor bound 2 and the guanine-nucleotide exchange factor son of sevenless, a common method used by growth factor receptors to activate Ras. In contrast, no apparent regulation by PRL of Ras via VAV or p120 Ras-guanosine triphosphatase-activating protein was detected, based upon an absence of PRL-inducible tyrosine phosphorylation of these proteins. Collectively, these results provide a molecular bridge between activation of PRL receptor-associated tyrosine kinases and subsequent stimulation of the serine/threonine kinase Raf-1, an established Ras target that was recently shown to be activated by PRL in Nb2 cells. We conclude that PRL is able to activate Ras via recruitment of the signaling proteins SHC, growth factor receptor bound 2, and son of sevenless in Nb2 cells. Moreover, PRL induced tyrosine phosphorylation of SHC in two of three PRL-responsive human breast cancer cell lines, suggesting that SHC-mediated Ras activation is a commonly used signaling strategy by PRL.

Cloning of the gene encoding rat JAK2, a protein tyrosine kinase

A complete cDNA clone encoding the rat JAK2 protein tyrosine kinase was isolated from an Nb2-SP (rat pre-T lymphoma cell line) cDNA library. The nucleotide (nt) and deduced amino acid (aa) sequences for this clone were determined and an open reading frame of 3399 bp, encoding a protein of a deduced mass of 130 kDa, was found. The coding regions of the rat and murine Jak2 clones share 93.4% nt identity and 97.1% aa identity. Northern analysis demonstrated that the 5-kb mRNA is highly abundant in brain and spleen, less abundant in skeletal muscle and testis, and detectable in kidney, heart, lung and liver. Translation of the rat Jak2 mRNA in rabbit reticulocytes results in a protein which is specifically immunoprecipitated by antibodies (Ab) recognizing JAK2, but not by Ab recognizing JAK1.