Tyrosine kinase and phosphatidylinositol 3-kinase activation are required for cyclic adenosine 3',5'-monophosphate-dependent potentiation of deoxyribonucleic acid synthesis induced by insulin-like growth factor-I in FRTL-5 cells

IGF1 receptor signaling pathways

Insulin-like growth factors (IGFs) bind specifically to the IGF1 receptor on the cell surface of targeted tissues. Ligand binding to the α subunit of the receptor leads to a conformational change in the β subunit, resulting in the activation of receptor tyrosine kinase activity. Activated receptor phosphorylates several substrates, including insulin receptor substrates (IRSs) and Src homology collagen (SHC). Phosphotyrosine residues in these substrates are recognized by certain Src homology 2 (SH2) domain-containing signaling molecules. These include, for example, an 85 kDa regulatory subunit (p85) of phosphatidylinositol 3-kinase (PI 3-kinase), growth factor receptor-bound 2 (GRB2) and SH2-containing protein tyrosine phosphatase 2 (SHP2/Syp). These bindings lead to the activation of downstream signaling pathways, PI 3-kinase pathway and Ras-mitogen-activated protein kinase (MAP kinase) pathway. Activation of these signaling pathways is known to be required for the induction of various bioactivities of IGFs, including cell proliferation, cell differentiation and cell survival. In this review, the well-established IGF1 receptor signaling pathways required for the induction of various bioactivities of IGFs are introduced. In addition, we will discuss how IGF signals are modulated by the other extracellular stimuli or by themselves based on our studies.

Phosphatidylinositol 3-Kinase-Associated Protein (PI3KAP)/XB130 Crosslinks Actin Filaments through Its Actin Binding and Multimerization Properties In Vitro and Enhances Endocytosis in HEK293 Cells

Actin-crosslinking proteins control actin filament networks and bundles and contribute to various cellular functions including regulation of cell migration, cell morphology, and endocytosis. Phosphatidylinositol 3-kinase-associated protein (PI3KAP)/XB130 has been reported to be localized to actin filaments (F-actin) and required for cell migration in thyroid carcinoma cells. Here, we show a role for PI3KAP/XB130 as an actin-crosslinking protein. First, we found that the carboxyl terminal region of PI3KAP/XB130 containing amino acid residues 830-840 was required and sufficient for localization to F-actin in NIH3T3 cells, and this region is directly bound to F-actin in vitro. Moreover, actin-crosslinking assay revealed that recombinant PI3KAP/XB130 crosslinked F-actin. In general, actin-crosslinking proteins often multimerize to assemble multiple actin-binding sites. We then investigated whether PI3KAP/XB130 could form a multimer. Blue native-PAGE analysis showed that recombinant PI3KAP/XB130 was detected at 250-1200 kDa although the molecular mass was approximately 125 kDa, suggesting that PI3KAP/XB130 formed multimers. Furthermore, we found that the amino terminal 40 amino acids were required for this multimerization by co-immunoprecipitation assay in HEK293T cells. Deletion mutants of PI3KAP/XB130 lacking the actin-binding region or the multimerizing region did not crosslink actin filaments, indicating that actin binding and multimerization of PI3KAP/XB130 were necessary to crosslink F-actin. Finally, we examined roles of PI3KAP/XB130 on endocytosis, an actin-related biological process. Overexpression of PI3KAP/XB130 enhanced dextran uptake in HEK 293 cells. However, most of the cells transfected with the deletion mutant lacking the actin-binding region incorporated dextran to a similar extent as control cells. Taken together, these results demonstrate that PI3KAP/XB130 crosslinks F-actin through both its actin-binding region and multimerizing region and plays an important role in endocytosis.

The association between thyroid autoantibodies in serum and abnormal function and structure of the thyroid

Objectives

To evaluate the association between thyroid autoantibodies and abnormalities in thyroid function and structure, and to investigate any risk factors.

Methods

A cross-sectional survey was undertaken in Chengdu residents ≥18 years with no previous thyroid disease. The study participants provided demographic and clinical data. Thyroid function and serum concentrations of the thyroid autoantibodies antithyroperoxidase antibody (TPOAb) and antithyroglobulin antibody (TgAb) were measured.

Results

A total of 1334 subjects were included in this study. The prevalence of TPOAb and TgAb positivity was significantly higher in female than in male subjects. The prevalence of thyroid autoantibodies in those with subclinical hypothyroidism and clinical hyper- and hypothyroidism was significantly greater than in euthyroid subjects. The concentration of TPOAb and TgAb in subjects with both TPOAb and TgAb was significantly higher than in those who exhibited only one type of thyroid autoantibody. Using multivariate logistic regression analysis, female sex, thyroid volume, thyroid hypo- and heteroechogenicity were found to be risk factors for the presence of autoantibodies.

Conclusions

Thyroid autoantibodies were common in the general population. Women with thyroid enlargement, hypoechogenicity and heteroechogenicity might benefit from routine screening for thyroid autoantibodies and thyroid function.

Tumor necrosis factor (TNF)-α-induced repression of GKAP42 protein levels through cGMP-dependent kinase (cGK)-Iα causes insulin resistance in 3T3-L1 adipocytes

Insulin receptor substrates (IRSs) have been shown to be major mediators of insulin signaling. Recently, we found that IRSs form high-molecular weight complexes, and here, we identify by yeast two-hybrid screening a novel IRS-1-associated protein: a 42-kDa cGMP-dependent protein kinase-anchoring protein (GKAP42). GKAP42 knockdown in 3T3-L1 adipocytes suppressed insulin-dependent IRS-1 tyrosine phosphorylation and downstream signaling, resulting in suppression of GLUT4 translocation to plasma membrane induced by insulin. In addition, GLUT4 translocation was also suppressed in cells overexpressing GKAP42-N (the IRS-1 binding region of GKAP42), which competed with GKAP42 for IRS-1, indicating that GKAP42 binding to IRS-1 is required for insulin-induced GLUT4 translocation. Long term treatment of 3T3-L1 adipocytes with TNF-α, which induced insulin resistance, significantly decreased the GKAP42 protein level. We then investigated the roles of cGMP-dependent kinase (cGK)-Iα, which bound to GKAP42, in these changes. cGK-Iα knockdown partially rescued TNF-α-induced decrease in GKAP42 and impairment of insulin signals. These data indicated that TNF-α-induced repression of GKAP42 via cGK-Iα caused reduction of insulin-induced IRS-1 tyrosine phosphorylation at least in part. The present study describes analysis of the novel TNF-α-induced pathway, cGK-Iα-GKAP42, which regulates insulin-dependent signals and GLUT4 translocation.

ERK phosphorylation is not increased in papillary thyroid carcinomas with BRAF(V600E) mutation compared to that of corresponding normal thyroid tissues

BACKGROUND

An association between a BRAF(V600E) mutation and upregulation of mitogen-activated protein kinase (MAPK) pathways in human papillary thyroid carcinoma (PTC) tissues has not been demonstrated well outside of in vitro studies. The aims of this study were to evaluate the activation status of extracellular signal-regulated kinase 1/2 (ERK1/2) in human PTCs with BRAF(V600E) mutations compared to that of corresponding normal thyroid tissue and to determine the expressions of Raf kinase inhibitor protein (RKIP) and MAPK phosphatase 3 (MKP-3), possible regulators of ERK1/2 activation.

METHODS

We analyzed the presence of BRAF(V600E) mutation and the expressions of BRAF, total ERK, p-ERK, RKIP, and MKP-3 in 33 PTCs and corresponding normal thyroid gland tissues using western blot analysis.

RESULTS

BRAF(V600E) mutation was found in 28 (84.8%) of 33 PTCs, 96.4% (27/28) of which showed decreased p-ERK activity, while 75% (21/28) showed increased MKP-3 expression. There were significant differences in p-ERK and MKP-3 expressions between BRAF(V600E) (+) PTCs and normal thyroid glands (p < 0.001). There were no differences in expressions of BRAF, total ERK, and RKIP between PTCs and normal thyroid tissue, irrespective of the presence of BRAF(V600E) mutation.

CONCLUSIONS

In human BRAF(V600E) (+) PTCs, ERK phosphorylation is decreased compared to normal thyroid glands and the observed decrease in ERK1/2 MAPK phosphorylation in BRAF(V600E) (+) PTCs may be associated with increased MKP-3 activity.

Phosphatidylinositol 3-kinase (PI3K) activity bound to insulin-like growth factor-I (IGF-I) receptor, which is continuously sustained by IGF-I stimulation, is required for IGF-I-induced cell proliferation

Continuous stimulation of cells with insulin-like growth factors (IGFs) in G(1) phase is a well established requirement for IGF-induced cell proliferation; however, the molecular components of this prolonged signaling pathway that is essential for cell cycle progression from G(1) to S phase are unclear. IGF-I activates IGF-I receptor (IGF-IR) tyrosine kinase, followed by phosphorylation of substrates such as insulin receptor substrates (IRS) leading to binding of signaling molecules containing SH2 domains, including phosphatidylinositol 3-kinase (PI3K) to IRS and activation of the downstream signaling pathways. In this study, we found prolonged (>9 h) association of PI3K with IGF-IR induced by IGF-I stimulation. PI3K activity was present in this complex in thyrocytes and fibroblasts, although tyrosine phosphorylation of IRS was not yet evident after 9 h of IGF-I stimulation. IGF-I withdrawal in mid-G(1) phase impaired the association of PI3K with IGF-IR and suppressed DNA synthesis the same as when PI3K inhibitor was added. Furthermore, we demonstrated that Tyr(1316)-X-X-Met of IGF-IR functioned as a PI3K binding sequence when this tyrosine is phosphorylated. We then analyzed IGF signaling and proliferation of IGF-IR(-/-) fibroblasts expressing exogenous mutant IGF-IR in which Tyr(1316) was substituted with Phe (Y1316F). In these cells, IGF-I stimulation induced tyrosine phosphorylation of IGF-IR and IRS-1/2, but mutated IGF-IR failed to bind PI3K and to induce maximal phosphorylation of GSK3β and cell proliferation in response to IGF-I. Based on these results, we concluded that PI3K activity bound to IGF-IR, which is continuously sustained by IGF-I stimulation, is required for IGF-I-induced cell proliferation.

Phosphatidylinositol 3-kinase-binding protein, PI3KAP/XB130, is required for cAMP-induced amplification of IGF mitogenic activity in FRTL-5 thyroid cells

We previously demonstrated that long-term pretreatment of rat FRTL-5 thyroid cells with TSH or cAMP-generating reagents potentiated IGF-I-dependent DNA synthesis. Under these conditions, cAMP treatment increased tyrosine phosphorylation of a 125-kDa protein (p125) and its association with a p85 regulatory subunit of phosphatidylinositol 3-kinase (p85 PI3K), which were suggested to mediate potentiation of DNA synthesis. This study was undertaken to identify p125 and to elucidate its roles in potentiation of DNA synthesis induced by IGF-I. Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) analysis revealed p125 to be a rat ortholog of human XB130, which we named PI3K-associated protein (PI3KAP). cAMP treatment elevated PI3KAP/XB130 mRNA and protein levels as well as tyrosine phosphorylation and interaction with p85 PI3K leading to increased PI3K activities associated with PI3KAP/XB130, supporting the role of PI3KAP/XB130 in DNA synthesis potentiation. Importantly, PI3KAP/XB130 knockdown attenuated cAMP-dependent potentiation of IGF-I-induced DNA synthesis. Furthermore, c-Src was associated with PI3KAP/XB130 and was activated in response to cAMP. Addition of Src family kinase inhibitors, PP1 or PP2, during cAMP treatment abolished tyrosine phosphorylation of PI3KAP/XB130 and its interaction with p85 PI3K. Finally, introduction of PI3KAP/XB130 into NIH3T3 fibroblasts lacking endogenous PI3KAP/XB130 enhanced IGF-I-induced DNA synthesis; however, a mutant Y72F incapable of binding to p85 PI3K did not show this response. Together, these data indicate that cAMP-dependent induction of PI3KAP/XB130, which is associated with PI3K, is required for enhancement of IGF mitogenic activities.

Thyroglobulin (Tg) activates MAPK pathway to induce thyroid cell growth in the absence of TSH, insulin and serum

The growth of thyroid cells is tightly regulated by thyroid stimulating hormone (TSH) through the cyclic adenosine 3', 5'-monophosphate (cAMP) signaling pathway by potentiating the mitogenic activity of insulin and insulin-like growth factors (IGFs). However, we recently reported that thyroglobulin (Tg), a major product of the thyroid, also induces the growth of thyroid cells cultured in 0.2% serum in the absence of TSH and insulin. In this report, we demonstrate that Tg induced phosphorylation of molecules of the c-Raf/MEK/ERK pathway of the mitogen-activated protein kinase (MAPK). The MEK-1/2 inhibitor PD98059 suppressed Tg-induced phosphorylation of ERK1/2 and reduced bromodeoxyuridine (BrdU) incorporation. Tg also induced expression of the essential transcriptional factors c-Myc, c-Fos and c-Jun and phosphorylation of the retinoblastoma (Rb) protein. The present results, together with the previous report, suggest that Tg utilizes multiple signaling cascades to induce thyroid cell growth independent of TSH/cAMP stimulation.

Identification of microRNAs that mediate thyroid cell growth induced by TSH

TSH is a major regulator of thyroid cell growth and endocrine function. It is known that cAMP and phosphatidylinositol 3-kinase (PI3K) are responsible for mediating the action of TSH. Activation of these signals results in the induction of a series of transcription factors and cell cycle regulating proteins, which induce cell proliferation. In addition to such canonical transcriptional regulation, it was recently shown that microRNA (miRNA or miR) constitutes another key mechanism for the regulation of gene expression. However, whether TSH action is mediated by miRNA in the thyroid is unknown. In this study, we have performed miRNA microarray analysis and demonstrated that TSH significantly decreases expression of 47 miRNA in thyroid cells. Among these, we have shown, using their specific agonists, that overexpression of miR-16 and miR-195 suppressed cell cycle progression and DNA synthesis that was induced by TSH. In silico analysis predicted that Mapk8, Ccne1, and Cdc6, the expression of which was up-regulated by TSH, are potential target genes for these miRNA, and overexpression of miR-16 and miR-195 suppressed expression of these target genes. The decrease of miR-16 and miR-195 expression by TSH was reproduced by forskolin and N(6),2'-O-dibutyryladenosine cAMP and reversed by the protein kinase A inhibitor H89 and the PI3K inhibitor LY294002. These results suggest that TSH activates cAMP/protein kinase A and PI3K cascades to decrease miR-16 and miR-195, which induce Mapk8, Ccne1, and Cdc6 to activate cell proliferation.

Paraquat-induced oxidative stress represses phosphatidylinositol 3-kinase activities leading to impaired glucose uptake in 3T3-L1 adipocytes

Accumulated evidence indicates that oxidative stress causes and/or promotes insulin resistance; however, the mechanism by which this occurs is not fully understood. This study was undertaken to elucidate the molecular mechanism by which oxidative stress induced by paraquat impairs insulin-dependent glucose uptake in 3T3-L1 adipocytes. We confirmed that paraquat-induced oxidative stress decreased glucose transporter 4 (GLUT4) translocation to the cell surface, resulting in repression of insulin-dependent 2-deoxyglucose uptake. Under these conditions, oxidative stress did not affect insulin-dependent tyrosine phosphorylation of insulin receptor, insulin receptor substrate (IRS)-1 and -2, or binding of the phosphatidylinositol 3'-OH kinase (PI 3-kinase) p85 regulatory subunit or p110alpha catalytic subunit to each IRS. In contrast, we found that oxidative stress induced by paraquat inhibited activities of PI 3-kinase bound to IRSs and also inhibited phosphorylation of Akt, the downstream serine/threonine kinase that has been shown to play an essential role in insulin-dependent translocation of GLUT4 to the plasma membrane. Overexpression of active form Akt (myr-Akt) restored inhibition of insulin-dependent glucose uptake by paraquat, indicating that paraquat-induced oxidative stress inhibits insulin signals upstream of Akt. Paraquat treatment with and without insulin treatment decreased the activity of class Ia PI 3-kinases p110alpha and p110beta that are mainly expressed in 3T3-L1 adipocytes. However, paraquat treatment did not repress the activity of the PI 3-kinase p110alpha mutated at Cys(90) in the p85 binding region. These results indicate that the PI 3-kinase p110 is a possible primary target of paraquat-induced oxidative stress to reduce the PI 3-kinase activity and impaired glucose uptake in 3T3-L1 adipocytes.

Thyroglobulin (Tg) induces thyroid cell growth in a concentration-specific manner by a mechanism other than thyrotropin/cAMP stimulation

Thyroglobulin (Tg), a major product of the thyroid gland, serves as a macromolecular precursor of thyroid hormone biosynthesis. In addition, Tg stored in the thyroid follicles is a potent regulator of thyroid-specific gene expression. In conjunction with thyroid stimulating hormone (TSH) and iodide, Tg regulates thyroid follicle function, which is the minimal functional unit of the thyroid gland. In the present study, we show that Tg stimulates growth of FRTL-5 thyroid cells in the absence of TSH, insulin and serum. Unlike TSH, Tg did not increase cellular cyclic AMP (cAMP) levels; rather, the TSH signal counteracted Tg-induced cell growth. A specific inhibitor of A-kinase, H-89, did not modulate the effect of Tg. Tg increased kinase activity of Akt to the same level as TSH, insulin and 5% serum, while LY294002 abolished Tg-induced growth. Interestingly, low Tg concentrations maximized growth-promotion activity and induction of the apical iodide transporter (PDS; SLC26A4), whereas high Tg concentrations suppressed both cell growth and the expression of thyroid-specific genes. These results suggest that a low levels of Tg in the follicular lumen might stimulates cell growth and iodide transport to accelerate the iodide organification process; however, elevated Tg levels in the follicle might then shut down all of these functions.

Growth hormone inhibition of glucose uptake in adipocytes occurs without affecting GLUT4 translocation through an insulin receptor substrate-2-phosphatidylinositol 3-kinase-dependent pathway

Growth hormone (GH) pretreatment of 3T3-L1 adipocytes resulted in a concentration- and time-dependent inhibition of insulin-stimulated glucose uptake. Surprisingly, this occurred without significant effect on insulin-stimulated glucose transporter (GLUT) 4 translocation or fusion with the plasma membrane. In parallel, the inhibitory actions of chronic GH pretreatment also impaired insulin-dependent activation of phosphatidylinositol (PI) 3-kinase bound to insulin receptor substrate (IRS)-2 but not to IRS-1. In addition, insulin-stimulated Akt phosphorylation was inhibited by GH pretreatment. In contrast, overexpression of IRS-2 or expression of a constitutively active Akt mutant prevented the GH-induced insulin resistance of glucose uptake. Moreover, small interfering RNA-mediated IRS-2 knockdown also inhibited insulin-stimulated Akt activation and glucose uptake without affecting GLUT4 translocation and plasma membrane fusion. Together, these data support a model in which chronic GH stimulation inhibits insulin-dependent activation of phosphatidylinositol 3-kinase through a specific interaction of phosphatidylinositol 3-kinase bound to IRS-2. This inhibition leads to suppression of Akt activation coupled to glucose transport activity but not translocation or plasma membrane fusion of GLUT4.

Distinct modes of activation of phosphatidylinositol 3-kinase in response to cyclic adenosine 3', 5'-monophosphate or insulin-like growth factor I play different roles in regulation of cyclin D1 and p27Kip1 in FRTL-5 cells

Bioactivities of IGFs in various cells are often potentiated in the presence of other hormones. In previous studies we showed that pretreatment of rat FRTL-5 thyroid cells with TSH or other cAMP-generating agents markedly potentiated DNA synthesis induced by IGF-I. Under these conditions we found that phosphatidylinositol (PI) 3-kinase was activated in response to either cAMP or IGF stimulus, and both activation modes were indispensable for the potentiation of DNA synthesis. The present studies were undertaken to elucidate how cAMP and/or IGF-I stimulus regulated the G1 cyclin-cyclin dependent kinase (CDK)-inhibitor system, and to determine the roles of PI 3-kinase activation by cAMP or IGF-I stimulus in this system. We found that cAMP pretreatment enhanced IGF-I-dependent increases in cyclin D1, due to synergistic increases in mRNA and elevation of translation rates. Furthermore, cAMP pretreatment enhanced IGF-I-induced protein degradation of the CDK inhibitor, p27(Kip1). These changes well explained an increase in cyclin E, leading to marked activation of G1 CDKs, followed by retinoblastoma protein phosphorylation. Our results using a PI 3-kinase inhibitor showed that cAMP-dependent PI 3-kinase activation plays an important role in the increase in cyclin D1 translation. In contrast, IGF-I-dependent PI 3-kinase activation was required for the increase in cyclin D1 mRNA levels and degradation of p27(Kip1). Together, the present study elucidates the role of cAMP and IGF-I in differentially activating PI 3-kinase as a mediator of multiple molecular events. These events converge in the regulation of cyclin D1 and p27(Kip1), leading to cAMP-dependent potentiation of IGF-I-dependent CDK activation and DNA synthesis.

53BP2S, interacting with insulin receptor substrates, modulates insulin signaling

It is well known that insulin receptor substrates (IRS) act as a mediator for signal transduction of insulin, insulin-like growth factors, and several cytokines. To identify proteins that interact with IRS and modulate IRS-mediated signals, we performed yeast two-hybrid screening with IRS-1 as bait. Out of 109 cDNA-positive clones identified from a human placental cDNA library, two clones encoded 53BP2, p53-binding protein 2 (53BP2S), a short form splicing variant of the apoptosis-stimulating protein of p53 that possesses Src homology region 3 domain, and ankyrin repeats domain, and had been reported to interact with p53, Bcl-2, and NF-kappaB. Interaction of 53BP2S with IRS-1 was confirmed by glutathione S-transferase pull-down and co-immunoprecipitation assays in COS-7 cells and 3T3-L1 adipocytes. The Src homology region 3 domain and ankyrin repeats domain of 53BP2S were responsible for its interaction with IRS-1, whereas the phosphotyrosine binding domain and a central domain (amino acid residues 750-861) of IRS-1 were required for its interaction with 53BP2S. In CHO-C400 cells, expression of 53BP2S reduced insulin-stimulated IRS-1 tyrosine phosphorylation with a concomitant enhancement of IRS-2 tyrosine phosphorylation. In addition, the amount of the phosphatidylinositol 3-kinase regulatory p85 subunit associated with tyrosine-phosphorylated proteins, and activation of Akt was enhanced by 53BP2S expression. Although 53BP2S also enhanced Akt activation in 3T3-L1 adipocytes, insulin-induced glucose transporter 4 translocation was markedly inhibited in accordance with reduction of insulin-induced AS160 phosphorylation. Together these data demonstrate that 53BP2S interacts and modulates the insulin signals mediated by IRSs.

p66Shc expression in proliferating thyroid cells is regulated by thyrotropin receptor signaling

It is almost unanimously accepted that thyrocyte proliferation is synergistically activated by TSH and insulin/IGF-I. Moreover, it was recently suggested that p66Shc, which is an adaptor molecule of the IGF-I receptor, might play a critical role in this synergistic effect. In this study, we undertook to confirm the role and the mechanism underlying the regulation of p66Shc expression via TSH receptor in thyrocytes. We have found that p66Shc expression is elevated in proliferating human thyroid tissues, including adenomatous goiter, adenoma, Graves' disease, and thyroid cancer, but not in normal thyroid. Among growth factors, TSH increased p66Shc expression both in vivo and in vitro; however, IGF-I, epidermal growth factor, or insulin did not. TSH and Graves' Ig increased the p66Shc expression via the TSH receptor-G(s)-cAMP pathway. However, interestingly, IGF-I or epidermal growth factor increased the tyrosine phosphorylations of p66Shc, and this was enhanced by TSH pretreatment. A similar synergism was observed during the DNA synthesis. When we measured the p66Shc levels induced by individual Igs from 130 patients with Graves' disease, TSH receptor stimulating activity and goiter size showed a weak correlation. We conclude that the expression of p66Shc is regulated by signaling through the TSH receptor in proliferating thyroid cells and that p66Shc appears to be an important mediator of the synergistic effect between TSH and IGF-I with respect to thyrocyte proliferation. Moreover, we suggest that TSH potentiates the regulatory effect of IGF-I on thyrocyte growth, at least in part, by increasing the expression of p66Shc.

Regulation of the phosphatidylinositol 3-kinase, Akt/protein kinase B, FRAP/mammalian target of rapamycin, and ribosomal S6 kinase 1 signaling pathways by thyroid-stimulating hormone (TSH) and stimulating type TSH receptor antibodies in the thyroid gland

Thyroid-stimulating hormone (TSH) regulates the growth and differentiation of thyrocytes by activating the TSH receptor (TSHR). This study investigated the roles of the phosphatidylinositol 3-kinase (PI3K), PDK1, FRAP/mammalian target of rapamycin, and ribosomal S6 kinase 1 (S6K1) signaling mechanism by which TSH and the stimulating type TSHR antibodies regulate thyrocyte proliferation and the follicle activities in vitro and in vivo. The TSHR immunoprecipitates exhibited PI3K activity, which was higher in the cells treated with either TSH or 8-bromo-cAMP. TSH and cAMP increased the tyrosine phosphorylation of TSHR and the association between TSHR and the p85alpha regulatory subunit of PI3K. TSH induced a redistribution of PDK1 from the cytoplasm to the plasma membrane in the cells in a PI3K- and protein kinase A-dependent manner. TSH induced the PDK1-dependent phosphorylation of S6K1 but did not induce Akt/protein kinase B phosphorylation. The TSH-induced S6K1 phosphorylation was inhibited by a dominant negative p85alpha regulatory subunit or by the PI3K inhibitors wortmannin and LY294002. Rapamycin inhibited the phosphorylation of S6K1 in the cells treated with either TSH or 8-bromo-cAMP. The stimulating type TSHR antibodies from patients with Graves disease also induced S6K1 activation, whereas the blocking type TSHR antibodies from patients with primary myxedema inhibited TSH- but not the insulin-induced phosphorylation of S6K1. In addition, rapamycin treatment in vivo inhibited the TSH-stimulated thyroid follicle hyperplasia and follicle activity. These findings suggest an interaction between TSHR and PI3K, which is stimulated by TSH and cAMP and might involve the downstream S6K1 but not Akt/protein kinase B. This pathway may play a role in the TSH/stimulating type TSH receptor antibody-mediated thyrocyte proliferation in vitro and in the response to TSH in vivo.

Mechanism by which cAMP activates PI3-kinase and increases bile acid secretion in WIF-B9 cells

Previous studies in rat bile canalicular membrane vesicles and WIF-B9 cells revealed that cAMP-induced trafficking of ATP-binding cassette (ABC) transporters to the canalicular membrane and their activation require phosphoinositide 3-kinase (PI3-K) products. In the present studies, canalicular secretion of fluorescein isothiocyanate-glycocholate in WIF-B9 cells was increased by cAMP and a decapeptide that enhances PI3-K activity; these effects were inhibited by wortmannin. To determine the mechanism(s) whereby cAMP activates PI3-K, we examined signal transduction pathways in WIF-B9 and COS-7 cells. cAMP activated PI3-K in both cell lines in a phosphotyrosine-independent manner. PI3-K activity increased in association with p110 beta in both cell lines. The effect of cAMP was KT-5720 sensitive, suggesting involvement of protein kinase A. Expression of a dominant-negative beta-adrenergic receptor kinase COOH terminus (beta-ARKct), which blocks G beta gamma signaling, decreased PI3-K activation in both cell lines. cAMP increased GTP-bound Ras in COS-7 but not WIF-B9 cells. Expression of dominant-negative Ras abolished cAMP-mediated PI3-K, which suggests that the effect is downstream of Ras and G beta gamma. These data indicate that cAMP activates PI3-K in a cell type-specific manner and provide insight regarding mechanisms of PI3-K activation required for bile acid secretion.

TSH signaling and cell survival in 3T3-L1 preadipocytes

Thyroid-stimulating hormone (TSH) action in adipose tissue remains largely unknown. Our previous work indicates that human preadipocytes express functional TSH receptor (TSHR) protein, demonstrated by TSH activation of p70 S6 kinase (p70 S6K). We have now studied murine 3T3-L1 preadipocytes to further characterize TSH signaling and cellular action. Western blot analysis of 3T3-L1 preadipocyte lysate revealed the 100-kDa mature processed form of TSHR. TSH activated p70 S6K and protein kinase B (PKB/Akt), as measured by immunoblot analysis. Preincubation with wortmannin or LY-294002 completely blocked TSH activation of p70 S6K and PKB/Akt, implicating phosphoinositide 3-kinase (PI3K) in their regulation. TSH increased phosphotyrosine protein(s) in the 125-kDa region and augmented the associated PI3K activity fourfold. TSH had no effect on cAMP levels in 3T3-L1 preadipocytes, suggesting that adenylyl cyclase is not involved in TSH activation of the PI3K-PKB/Akt-p70 S6K pathway. 3T3-L1 preadipocyte cell death was reduced by 29-76% in serum-deprived (6 h) preadipocytes treated with 1-20 microM TSH. In the presence of 20 microM TSH, an 88% reduction in terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL)-positive cells was observed in serum-starved (3 h) 3T3-L1 preadipocytes as well as a 93% reduction in the level of cleaved activated caspase 3. In summary, TSH acts as a survival factor in 3T3-L1 preadipocytes. TSH does not stimulate cAMP accumulation in these cells but instead activates a PI3K-PKB/Akt-p70 S6K pathway.

Cyclic AMP regulation of neutrophil apoptosis occurs via a novel protein kinase A-independent signaling pathway

The second messenger molecule cyclic AMP dramatically modulates the apoptotic program in a wide variety of cells, accelerating apoptosis in some and delaying the rate of apoptosis in others. Human neutrophil apoptosis, a process that regulates the fate and numbers of these potentially histotoxic cells in inflammatory sites, is profoundly delayed by the cell-permeable analog of cyclic AMP, dibutyryl-cAMP. We have investigated the mechanisms underlying cyclic AMP-mediated delay of neutrophil apoptosis, and we show that cyclic AMP inhibits loss of mitochondrial potential occurring during constitutive neutrophil apoptosis. Furthermore, we demonstrate that cyclic AMP also suppresses caspase activation in these inflammatory cells. Despite increasing protein kinase A activity, this kinase is unlikely to mediate the effect of cyclic AMP on apoptosis because blockade of protein kinase A activation did not influence the survival effects of cyclic AMP. Further investigation of the signaling mechanism demonstrated that the delay of apoptosis is independent of phosphoinositide 3-kinase and MAPK activation. Our results suggest cyclic AMP delays neutrophil apoptosis via a novel, reversible, and transcriptionally independent mechanism. We show that proteasome activity in the neutrophil is vitally involved in this process, and we suggest that a balance of pro-apoptotic and anti-apoptotic proteins plays a key role in the powerful ability of cyclic AMP to delay neutrophil death.

Regulation of thyroid cell proliferation by TSH and other factors: a critical evaluation of in vitro models

TSH via cAMP, and various growth factors, in cooperation with insulin or IGF-I stimulate cell cycle progression and proliferation in various thyrocyte culture systems, including rat thyroid cell lines (FRTL-5, WRT, PC Cl3) and primary cultures of rat, dog, sheep and human thyroid. The available data on cell signaling cascades, cell cycle kinetics, and cell cycle-regulatory proteins are thoroughly and critically reviewed in these experimental systems. In most FRTL-5 cells, TSH (cAMP) merely acts as a priming/competence factor amplifying PI3K and MAPK pathway activation and DNA synthesis elicited by insulin/IGF-I. In WRT cells, TSH and insulin/IGF-I can independently activate Ras and PI3K pathways and DNA synthesis. In dog thyroid primary cultures, TSH (cAMP) does not activate Ras and PI3K, and cAMP must be continuously elevated by TSH to directly control the progression through G(1) phase. This effect is exerted, at least in part, via the cAMP-dependent activation of the required cyclin D3, itself synthesized in response to insulin/IGF-I. This and other discrepancies show that the mechanistic logics of cell cycle stimulation by cAMP profoundly diverge in these different in vitro models of the same cell. Therefore, although these different thyrocyte systems constitute interesting models of the wide diversity of possible mechanisms of cAMP-dependent proliferation in various cell types, extrapolation of in vitro mechanistic data to TSH-dependent goitrogenesis in man can only be accepted in the cases where independent validation is provided.

Effects of tyrosine kinase inhibitors on cell death induced by sodium fluoride and pertussis toxin in the pancreatic beta-cell line, RINm5F

1. Sodium fluoride causes apoptosis of pancreatic beta-cells and this response is enhanced by pre-treatment with pertussis toxin. In the present study, tyrosine kinase inhibitors were used to investigate the mechanisms of action of NaF and pertussis toxin in the beta-cell line, RINm5F. 2. Exposure of RINm5F cells to low concentrations of genistein or tyrphostin A25 resulted in significant inhibition of cell death induced by 5 mM NaF. Higher concentrations (>25 microM) were cytotoxic in the absence of NaF but, paradoxically, the combination of genistein and NaF induced less cell death than when each agent was used alone. 3. The increase in cell death induced by 100 microM genistein was markedly inhibited by ciprofloxacin, a drug which binds to topoisomerase II. Etoposide (which inhibits topoisomerase II but has no effect on tyrosine kinase activity) also caused an increase in RINm5F cell death. Neither etoposide nor ciprofloxacin altered the response to 5 mM NaF. 4. Pertussis toxin markedly enhanced the extent of RINm5F cell death induced by NaF and this effect was completely prevented by 25 microM genistein. The inhibition caused by genistein was not affected by ciprofloxacin but was reproduced by a structurally dissimilar tyrosine kinase inhibitor, herbimycin A. 5. The results demonstrate that RINm5F beta-cells express a pertussis toxin sensitive pathway that is anti-apoptotic. The activity of this pathway is most evident in cells exposed to pro-apoptotic stimuli where the effects of pertussis toxin can be blocked by inhibitors of tyrosine kinase enzymes. A genistein-sensitive tyrosine kinase does not appear to be involved in RINm5F cell survival under basal conditions.