Tyrosine phosphorylation of a c-Src-like protein is increased in membranes of CD4- CD8- T lymphocytes from lpr/lpr mice

SRC family kinases in hamster spermatozoa: evidence for the presence of LCK

Sperm capacitation is a prerequisite for successful fertilization. Increase in tyrosine phosphorylation is considered the hallmark of capacitation and attempts to understand its regulation are ongoing. In this regard, we attempted to study the role of SRC family kinases (SFKs) in the hamster sperm functions. Interestingly, we found the presence of the lymphocyte-specific protein tyrosine kinase, LCK, in mammalian spermatozoa and further characterized it in terms of its localization and function. LCK was found in spermatozoa of several species, and its transcript was identified in the hamster testis. Autophosphorylation of LCK at the Y394 residue increased as capacitation progressed, indicating an upregulation of LCK activity during capacitation. Inhibition of LCK (and perhaps the other SFKs) with the use of a specific inhibitor showed a significant decrease in protein tyrosine phosphorylation of several proteins, implying LCK/SFKs as key tyrosine kinase(s) regulating tyrosine phosphorylation during hamster sperm capacitation. Dihydrolipoamide dehydrogenase was identified as a substrate for LCK/SFK. LCK/SFKs inhibition significantly reduced the percentage fertilization (in vitro) but had no effect on sperm motility, hyperactivation and acrosome reaction. In summary, this is the first report on the presence of LCK, an SFK of hematopoietic lineage in spermatozoa besides being the first study on the role of SFKs in the spermatozoa of Syrian hamsters.

Hyperactive mTOR pathway promotes lymphoproliferation and abnormal differentiation in autoimmune lymphoproliferative syndrome

Autoimmune lymphoproliferative syndrome (ALPS) is a human disorder characterized by defective Fas signaling, resulting in chronic benign lymphoproliferation and accumulation of TCRαβ(+) CD4(-) CD8(-) double-negative T (DNT) cells. Although their phenotype resembles that of terminally differentiated or exhausted T cells, lack of KLRG1, high eomesodermin, and marginal T-bet expression point instead to a long-lived memory state with potent proliferative capacity. Here we show that despite their terminally differentiated phenotype, human ALPS DNT cells exhibit substantial mitotic activity in vivo. Notably, hyperproliferation of ALPS DNT cells is associated with increased basal and activation-induced phosphorylation of serine-threonine kinases Akt and mechanistic target of rapamycin (mTOR). The mTOR inhibitor rapamycin abrogated survival and proliferation of ALPS DNT cells, but not of CD4(+) or CD8(+) T cells in vitro. In vivo, mTOR inhibition reduced proliferation and abnormal differentiation by DNT cells. Importantly, increased mitotic activity and hyperactive mTOR signaling was also observed in recently defined CD4(+) or CD8(+) precursor DNT cells, and mTOR inhibition specifically reduced these cells in vivo, indicating abnormal programming of Fas-deficient T cells before the DNT stage. Thus, our results identify the mTOR pathway as a major regulator of lymphoproliferation and aberrant differentiation in ALPS.

Src family tyrosine kinase regulates acrosome reaction but not motility in porcine spermatozoa

During the capacitation process, spermatozoa acquire the ability to fertilize an oocyte, and upregulation of cAMP-dependent protein tyrosine phosphorylation occurs. Recently, Src family tyrosine kinase (SFK) has been involved in spermatozoa capacitation as a key PKA-dependent tyrosine kinase in several species. This work investigates the expression and role of SFK in porcine spermatozoa. SFK members Lyn and Yes are identified in porcine spermatozoa by western blotting as well as two proteins named SFK1 and SFK2 were also detected by their tyrosine 416 phosphorylation, a key residue for SFK activation. Spermatozoa with SFK1 and SFK2 increase their Y416 phosphorylation time-dependently under capacitating conditions compared with noncapacitating conditions. The specific SFK inhibitor SU6656 unaffected porcine spermatozoa motility or viability. Moreover, SFK inhibition in spermatozoa under capacitating conditions leads to a twofold increase in both nonstimulated and calcium-induced acrosome reaction. Our data show that capacitating conditions lead to a time-dependent increase in actin polymerization in boar spermatozoa and that long-term incubation with SFK inhibitor causes a reduction in the F-actin content. In summary, this work shows that the SFK members Lyn and Yes are expressed in porcine spermatozoa and that SFK1 and SFK2 are phosphorylated (activated) during capacitation. Our results point out the important role exerted by SFK in the acrosome reaction, likely mediated in part by its involvement in the actin polymerization process that accompanies capacitation, and rule out its involvement in porcine spermatozoa motility.

Identification of SRC as a key PKA-stimulated tyrosine kinase involved in the capacitation-associated hyperactivation of murine spermatozoa

Fertilization of the mammalian oocyte depends on the ability of spermatozoa to undergo a process known as capacitation as they ascend the female reproductive tract. A fundamental feature of this process is a marked increase in tyrosine phosphorylation by an unusual protein kinase A (PKA)-mediated pathway. To date, the identity of the intermediate PKA-activated tyrosine kinase driving capacitation is still unresolved. In this study, we have identified SRC as a candidate intermediate kinase centrally involved in the control of sperm capacitation. Consistent with this conclusion, the SRC kinase inhibitor SU6656 was shown to suppress both tyrosine phosphorylation and hyperactivation in murine spermatozoa. Moreover, SRC co-immunoprecipitated with PKA and this interaction was found to lead to an activating phosphorylation of SRC at position Y416. We have also used difference-in-2D-gel-electrophoresis (DIGE) in combination with mass spectrometry to identify a number of SRC substrates that become phosphorylated during capacitation including enolase, HSP90 and tubulin. Our data further suggest that the activation of SRC during capacitation is negatively controlled by C-terminal SRC kinase. The latter was localized to the acrosome and flagellum of murine spermatozoa by immunocytochemistry, whereas capacitation was associated with an inactivating serine phosphosphorylation of this inhibitory kinase.

Defective activity of ERK-1 and ERK-2 mitogen-activated protein kinases in peripheral blood T lymphocytes from patients with systemic lupus erythematosus: potential role of altered coupling of Ras guanine nucleotide exchange factor hSos to adapter protein Grb2 in lupus T cells

The integrity of the Ras/Raf/mitogen-activated protein kinase (MAPK) cascade is critical for maintenance of T cell tolerance, a process that fails in patients with systemic lupus erythematosus (SLE). In this study we have examined the activity of mitogen-activated protein kinases ERK-1 and ERK-2 in resting and TCR-activated peripheral blood T lymphocytes from patients with SLE. We also examined the binding of Ras guanine nucleotide exchange factor, human Son of Sevenless (hSos), to cytosolic adapter protein growth factor receptor-bound protein 2. T cells from lupus patients showed diminished catalytic activity and TCR-driven dual phosphorylation of ERK-1 and ERK-2 upon stimulation through the TCR/CD3 receptor, a defect that may be related to altered translocation of hSos to the Ras/Raf membrane complex and diminished nuclear translocation of trans-acting factor AP-1. Defective MAPK activity triggered by TCR/ CD3 activation may alter the coordination of signals needed for normal interleukin-2 production and maintenance of tolerance in lupus T cells.

Fyn and Lck tyrosine kinases regulate tyrosine phosphorylation of p105CasL, a member of the p130Cas docking protein family, in T-cell receptor-mediated signalling

We have previously shown that engagement of the T-cell receptor (TCR)/CD3 complex with anti-CD3 antibody induces tyrosine phosphorylation of p105CasL (CasL), a member of the p130Cas docking protein family. In the present work, we attempted to determine which protein tyrosine kinases (PTKs) regulate TCR-mediated phosphorylation of CasL. We show here that an association between CasL and two types of Src family PTKs, Fyn and Lck, is induced by anti-CD3 cross-linking of human H9 T cells. In contrast, ZAP-70, another PTK that also plays a critical role in the TCR signalling, failed to bind CasL, even after anti-CD3 stimulation. In vitro kinase assays revealed that Fyn and Lck, but not ZAP-70, were capable of phosphorylating CasL. Moreover, we found that CasL was constitutively hyperphosphorylated in vivo in splenocytes of MRL-MP-lpr/lpr mice, in which overproduction and excessive activation of Fyn and Lck have previously been shown to occur. Constitutive in vivo binding of CasL to both kinases was also demonstrated in lpr splenocytes. These results strongly suggest that CasL is a substrate for Fyn and Lck PTKs in TCR signal transduction.

c-Src-mediated phosphorylation of the epidermal growth factor receptor on Tyr845 and Tyr1101 is associated with modulation of receptor function

Accumulating evidence indicates that interactions between the epidermal growth factor receptor (EGFR) and the nonreceptor tyrosine kinase c-Src may contribute to an aggressive phenotype in multiple human tumors. Previous work from our laboratory demonstrated that murine fibroblasts which overexpress both these tyrosine kinases display synergistic increases in DNA synthesis, soft agar growth, and tumor formation in nude mice, and increased phosphorylation of the receptor substrates Shc and phospholipase gamma as compared with single overexpressors. These parameters correlated with the ability of c-Src and EGFR to form an EGF-dependent heterocomplex in vivo. Here we provide evidence that association between c-Src and EGFR can occur directly, as shown by receptor overlay experiments, and that it results in the appearance of two novel tyrosine phosphorylations on the receptor that are seen both in vitro and in vivo following EGF stimulation. Edman degradation analyses and co-migration of synthetic peptides with EGFR-derived tryptic phosphopeptides identify these sites as Tyr845 and Tyr1101. Tyr1101 lies within the carboxyl-terminal region of the EGFR among sites of receptor autophosphorylation, while Tyr845 resides in the catalytic domain, in a position analogous to Tyr416 of c-Src. Phosphorylation of Tyr416 and homologous residues in other tyrosine kinase receptors has been shown to be required for or to increase catalytic activity, suggesting that c-Src can influence EGFR activity by mediating phosphorylation of Tyr845. Indeed, EGF-induced phosphorylation of Tyr845 was increased in MDA468 human breast cancer cells engineered to overexpress c-Src as compared with parental MDA 468 cells. Furthermore, transient expression of a Y845F variant EGFR in murine fibroblasts resulted in an ablation of EGF-induced DNA synthesis to nonstimulated levels. Together, these data support the hypothesis that c-Src-mediated phosphorylation of EGFR Tyr845 is involved in regulation of receptor function, as well as in tumor progression.

Abnormal pattern of tyrosine phosphorylation in unstimulated peripheral blood T lymphocytes from patients with systemic lupus erythematosus

Previous reports have shown abnormal responses mediated via the TCR/CD3 pathway in T lymphocytes from systemic lupus erythematosus (SLE) patients. Recently, we and others have reported augmented TCR/CD3-mediated responses in lupus T cells. It is possible that the pattern of downstream biochemical signals triggered by TCR/CD3 ligation may be altered in T lymphocytes from patients with SLE, thus leading to abnormal distal cell responses. In this paper we have examined the phosphorylation of proteins on tyrosine residues in peripheral blood T lymphocytes from a group of SLE patients and controls. We found a lower frequency of constitutively tyrosine-phosphorylated 119- and 113-kDa substrates and an enhanced frequency of tyrosine-phosphorylated 66- and 25-kDa proteins in unstimulated cultures of SLE T lymphocytes, suggesting an altered pattern of tyrosine phosphorylation in T cells from patients in vivo. Additionally, the protein tyrosine phosphatase (PTP) activity of CD45 immunoprecipitates was lower in unstimulated lupus T cells and was enhanced after stimulation via the CD3 pathway in lupus but not control T lymphocytes. The present results seem to suggest abnormal regulation of in-vivo tyrosine phosphorylation in T cells from patients with SLE.

Characterization of human epidermal growth factor receptor and c-Src interactions in human breast tumor cells

In C3H/10T1/2 murine fibroblasts, overexpression of both c-Src and the human epidermal growth factor (EGF) receptor 1 (HER1) is required for detection of stable complexes between the two molecules and results in hyperactivation of the receptor and synergistic increases in tumor formation in nude mice, as compared with cells that overexpress only one of the pair. Elevated levels or activities of c-Src and HER1 also occur in a subset of later-stage breast cancers, suggesting that interactions between these two molecules could contribute to a more aggressive clinical course. To determine whether stable complexes between c-Src and HER1 occur in human breast cancers under the same conditions as in murine fibroblasts and whether the appearance of such complexes correlates with enhanced signaling through the EGF receptor and increased tumor growth, human breast tumor cell lines and tumor tissues were analyzed for a number of c-Src/HER1-mediated signaling events and tumorigenicity. In a panel of 14 cell lines, 10 overexpressed c-Src, and of these, five contained elevated levels of HER1 and exhibited an EGF-dependent association between HER1 and c-Src. This association was also present in a HER1/c-Src-overexpressing tumor sample from a breast cancer patient. Further analysis of signaling events revealed that phosphorylation of the HER1 substrate, Shc, and its downstream effector, mitogen-activated protein kinase, was increased in EGF-stimulated MDA-MB-468, MDA-MB-231, and BT-549 cells (which overexpress both c-Src and HER1) as compared with MCF7 and ZR-75-1 cells (which only overexpress c-Src). Furthermore, MDA-MB-468 and MDA-MB-231 cells displayed increased tumorigenicity in nude mice. These results support the hypothesis that c-Src/HER1 interactions contribute to tumor progression in certain late-stage breast tumor cells.

Impaired plasma membrane targeting of Grb2-murine son of sevenless (mSOS) complex and differential activation of the Fyn-T cell receptor (TCR)-zeta-Cbl pathway mediate T cell hyporesponsiveness in autoimmune nonobese diabetic mice

Nonobese diabetic (NOD) mouse thymocytes are hyporesponsive to T cell antigen receptor (TCR)-mediated stimulation of proliferation, and this T cell hyporesponsiveness may be causal to the onset of autoimmune diabetes in NOD mice. We previously showed that TCR-induced NOD T cell hyporesponsiveness is associated with a block in Ras activation and defective signaling along the PKC/Ras/MAPK pathway. Here, we report that several sequential changes in TCR-proximal signaling events may mediate this block in Ras activation. We demonstrate that NOD T cell hyporesponsiveness is associated with the (a) enhanced TCR-beta-associated Fyn kinase activity and the differential activation of the Fyn-TCR-zeta-Cbl pathway, which may account for the impaired recruitment of ZAP70 to membrane-bound TCR-zeta; (b) relative inability of the murine son of sevenless (mSOS) Ras GDP releasing factor activity to translocate from the cytoplasm to the plasma membrane; and (c) exclusion of mSOS and PLC-gamma1 from the TCR-zeta-associated Grb2/pp36-38/ZAP70 signaling complex. Our data suggest that altered tyrosine phosphorylation and targeting of the Grb2/pp36-38/ZAP70 complex to the plasma membrane and cytoskeleton and the deficient association of mSOS with this Grb2-containing complex may block the downstream activation of Ras and Ras-mediated amplification of TCR/CD3-mediated signals in hyporesponsive NOD T cells. These findings implicate mSOS as an important mediator of downregulation of Ras signaling in hyporesponsive NOD T cells.

Activation of Fyn tyrosine kinase upon secretagogue stimulation of bovine chromaffin cells

Adrenal medullary chromaffin cells derive from the neural crest during embryogenesis and differentiate into dedicated secretory cells that release catecholamines in response to acetylcholine in vivo or nicotinic agonists in vitro. Previous studies have indicated that tyrosine kinases participate in early secretagogue-induced events in these cells and are required for exocytosis. Abundant levels of the cytoplasmic tyrosine kinases, c-Src and c-Yes, have been detected in chromaffin cells, thereby implicating them as kinases relevant to these events. However, c-Src has been found to undergo a decrease in activity following secretagogue-stimulation, and c-Yes appears to exist in a constituitively low activity state, suggesting that other tyrosine kinases are involved. Furthermore, other members of the Src family of tyrosine kinases have been implicated as playing roles in secretion in a variety of cell types. Therefore, we sought to determine if other Src family members were present in chromaffin cells, and if so, to examine them for subcellular localization and changes in activity following treatment with nicotinic agonists. To this end, antibodies for Fyn, Lck, Lyn, and Fgr were assembled and used in immunoprecipitation, in vitro autokinase, and Western immunoblotting assays. Of these four kinases, only Fyn was found to be expressed at detectable levels. Differential centrifugation studies revealed that Fyn resides predominantly (> 95%) in the crude plasma membrane fraction and undergoes nicotinic-and carbachol-induced activation. This activation is reduced by the nicotinic antagonist, mecamylamine, is not elicited by muscarine, and is dependent upon the presence of extracellular Ca2+. These results suggest that Fyn is involved in signalling through the nicotinic receptor and may be one of the relevant kinases responsible for at least some of the tyrosine phosphorylations detected after stimulation.

Abnormal signal transduction through CD4 leads to altered tyrosine phosphorylation in T cells derived from MRL-lpr/lpr mice

CD4+ T cells play a crucial role in the development of lupus in MRL-lpr/lpr mice: incomplete deletion/silencing of self-reactive CD4+ T cells leads to T cell activation, which causes both polyclonal B cell activation and T cell infiltration of multiple organs. Furthermore, anti-CD4 antibody therapy ameliorates disease and prolongs survival. Because CD4 is normally involved in both tolerance induction and T cell activation, we questioned whether signaling through CD4 was normal among T cells in this strain. For this purpose, signal transduction in CD4+ T cells derived from MRL-lpr/lpr and normal mice were compared, using an autoreactive CD4+ T cell clone and freshly isolated CD4+ T cells derived from mice of varying ages. Tyrosine phosphorylation was similar among MRL and normal CD4+ T cells after cross-linking with either anti-TCR antibody or anti-CD3 antibody, and following co-culture with Con A. In constrast, cross-linking of surface CD4 resulted in deficient tyrosine phosphorylation of cellular proteins in MRL T cells. By comparison, lck protein expression in MRL CD4+ T cells was found to be lower than normal. However, following stimulation with Con A, lck enzyme activity, as detected by autophosphorylation of lck, was comparable in MRL and normal T cells. The observed differences were present in the autoreactive T cell clone as well as in T cells isolated from both pre-diseased and diseased mice, and they could not be explained by variation in surface density of CD4. These results raise the possibility that abnormal signaling through CD4 may contribute to impaired tolerance and expansion of autoreactive T cells exhibited in MRL-lpr/lpr mice.

A role for Fyn tyrosine kinase in the suckling behaviour of neonatal mice

Non-receptor-type tyrosine kinases of the Src family, such as Src, Yes and Fyn, are strongly expressed in the brain and have been suggested to have an important function in the central nervous system. We generated Fyn-deficient mice by inserting the beta-galactosidase gene (lacZ) into the fyn gene. The homozygous Fyn-mutant neonates from homozygous Fyn-deficient parents died because of a suckling problem. Neonates were, however, able to suckle milk normally when the homozygous mother's mammary glands had been activated by suckling of a heterozygous or wild-type pup. In these homozygous pups, the modified glomerular complex of the olfactory bulb, which had been suggested to play a role in perceiving pheromones, was abnormal in shape and reduced in size, and the hippocampal cell-layer was undulated. These results suggest that Fyn may be involved in the initial step of instinctive suckling behaviour in neonates.

An abnormal signal transduction pathway in CD4-CD8- double-negative lymph node cells of MRL lpr/lpr mice

We describe here a novel 26-kDa posphoprotein (p26), associated to the T cell receptor of CD4-CD8- lymph node cells of lpr/lpr mice, but not present in significant amounts in control mouse cells including polyclonally activated T cells. lpr p26 is constitutively phosphorylated on a tyrosyl residue. It is most likely a member of the G protein family and displays high GTP-binding and GTPase activities both unsensitive to interleukin 2. Bordetella pertussis toxin has no effect on the spontaneously enhanced GTP hydrolysis. The traits associated to p26 could contribute to the distinctive features of lpr/lpr double-negative cells.

Alleviation of autoimmune disease in MRL-lpr mice by administration of Ye19.1, a monoclonal specific for the lpr T cell antigen, LTA

The MRL-lpr murine model of systemic lupus erythematosus (SLE) has provided many insights into the pathology of human lupus. The model is characterized by an age-dependent expansion of a Thy-1+ alpha beta/CD3+ CD4-, CD8- T-cell subset in the nodes and spleen. In this study, a lpr T-cell specific monoclonal antibody, Ye19.1, was found to bind to a 200 kDa cell surface molecule (termed LTA) which has a phosphotyrosine phosphatase (PTPase) enzymatic function. The significance of this marker in the development of autoimmune pathology in MRL/lpr mice was also demonstrated; treatment of MRL-lpr mice with the Ye19.1 Ab was shown to retard the development of the autoimmune syndrome and to restore the T cell-dependent immune response to ovalbumin.