Transforming growth factor-beta3 increases gap-junctional communication among folliculostellate cells to release basic fibroblast growth factor

Loss of Transforming Growth Factor Beta Signaling in Aortic Smooth Muscle Cells Causes Endothelial Dysfunction and Aortic Hypercontractility

[Figure: see text].

Gap junction-mediated cell-to-cell communication in oral development and oral diseases: a concise review of research progress

Homoeostasis depends on the close connection and intimate molecular exchange between extracellular, intracellular and intercellular networks. Intercellular communication is largely mediated by gap junctions (GJs), a type of specialized membrane contact composed of variable number of channels that enable direct communication between cells by allowing small molecules to pass directly into the cytoplasm of neighbouring cells. Although considerable evidence indicates that gap junctions contribute to the functions of many organs, such as the bone, intestine, kidney, heart, brain and nerve, less is known about their role in oral development and disease. In this review, the current progress in understanding the background of connexins and the functions of gap junctions in oral development and diseases is discussed. The homoeostasis of tooth and periodontal tissues, normal tooth and maxillofacial development, saliva secretion and the integrity of the oral mucosa depend on the proper function of gap junctions. Knowledge of this pattern of cell-cell communication is required for a better understanding of oral diseases. With the ever-increasing understanding of connexins in oral diseases, therapeutic strategies could be developed to target these membrane channels in various oral diseases and maxillofacial dysplasia.

The anterior pituitary gap junctions: potential targets for toxicants

The anterior pituitary regulates endocrine organs and physiological activities in the body. Environmental pollutants and drugs deleterious to the endocrine system may affect anterior pituitary activity through direct action on anterior pituitary cells. Within the gland, endocrine and folliculostellate cells are organized into and function as individual tridimensional networks, each network regulating its activity by coordinating the connected cells' responses to physiological or pathological cues. The gap junctions connecting endocrine cells and/or folliculostellate cells allow transmission of information among cells that is necessary for adequate network function. Toxicants may affect gap junctions as well as the physiology of the anterior pituitary. However, whether toxicants effects on anterior pituitary hormone secretion involve gap junctions is unknown. The folliculostellate cell gap junctions are sensitive to hormones, cytokines and growth factors. These cells may be an interesting experimental model for evaluating whether toxicants target anterior pituitary gap junctions.

Establishment of a confluent monolayer model with human primary trophoblast cells: novel insights into placental glucose transport

STUDY HYPOTHESIS

Using optimized conditions, primary trophoblast cells isolated from human term placenta can develop a confluent monolayer in vitro, which morphologically and functionally resembles the microvilli structure found in vivo.

STUDY FINDING

We report the successful establishment of a confluent human primary trophoblast monolayer using pre-coated polycarbonate inserts, where the integrity and functionality was validated by cell morphology, biophysical features, cellular marker expression and secretion, and asymmetric glucose transport.

WHAT IS KNOWN ALREADY

Human trophoblast cells form the initial barrier between maternal and fetal blood to regulate materno-fetal exchange processes. Although the method for isolating pure human cytotrophoblast cells was developed almost 30 years ago, a functional in vitro model with primary trophoblasts forming a confluent monolayer is still lacking.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Human term cytotrophoblasts were isolated by enzymatic digestion and density gradient separation. The purity of the primary cells was evaluated by flow cytometry using the trophoblast-specific marker cytokeratin 7, and vimentin as an indicator for potentially contaminating cells. We screened different coating matrices for high cell viability to optimize the growth conditions for primary trophoblasts on polycarbonate inserts. During culture, cell confluency and polarity were monitored daily by determining transepithelial electrical resistance (TEER) and permeability properties of florescent dyes. The time course of syncytia-related gene expression and hCG secretion during syncytialization were assessed by quantitative RT-PCR and enzyme-linked immunosorbent assay, respectively. The morphology of cultured trophoblasts after 5 days was determined by light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Membrane makers were visualized using confocal microscopy. Additionally, glucose transport studies were performed on the polarized trophoblasts in the same system.

MAIN RESULTS AND THE ROLE OF CHANCE

During 5-day culture, the highly pure trophoblasts were cultured on inserts coated with reconstituted basement membrane matrix . They exhibited a confluent polarized monolayer, with a modest TEER and a size-dependent apparent permeability coefficient (Papp) to fluorescently labeled compounds (MW ∼400-70 000 Da). The syncytialization progress was characterized by gradually increasing mRNA levels of fusogen genes and elevating hCG secretion. SEM analyses confirmed a confluent trophoblast layer with numerous microvilli, and TEM revealed a monolayer with tight junctions. Immunocytochemistry on the confluent trophoblasts showed positivity for the cell-cell adhesion molecule E-cadherin, the tight junction protein 1 (ZO-1) and the membrane proteins ATP-binding cassette transporter A1 (ABCA1) and glucose transporter 1 (GLUT1). Applying this model to study the bidirectional transport of a non-metabolizable glucose derivative indicated a carrier-mediated placental glucose transport mechanism with asymmetric kinetics.

LIMITATIONS, REASONS FOR CAUTION

The current study is only focused on primary trophoblast cells isolated from healthy placentas delivered at term. It remains to be evaluated whether this system can be extended to pathological trophoblasts isolated from diverse gestational diseases.

WIDER IMPLICATIONS OF THE FINDINGS

These findings confirmed the physiological properties of the newly developed human trophoblast barrier, which can be applied to study the exchange of endobiotics and xenobiotics between the maternal and fetal compartment, as well as intracellular metabolism, paracellular contributions and regulatory mechanisms influencing the vectorial transport of molecules.

LARGE-SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

This study was supported by the Swiss National Center of Competence in Research, NCCR TransCure, University of Bern, Switzerland, and the Swiss National Science Foundation (grant no. 310030_149958, C.A.). All authors declare that their participation in the study did not involve factual or potential conflicts of interests.

Biphasic Effect of Basic Fibroblast Growth Factor on Anterior Pituitary Folliculostellate TtT/GF Cell Coupling, and Connexin 43 Expression and Phosphorylation

Basic fibroblast growth factor (bFGF) is a mitogenic and differentiating cytokine. In the anterior pituitary, folliculostellate (FS) cells constitute the major source of bFGF. bFGF affects endocrine cell proliferation and secretion in the anterior pituitary. In addition, bFGF increases its own expression by acting directly on FS cells. FS cell Cx43-mediated gap junction intercellular communication allows the establishment of an intrapituitary network for the transmission of information. In the present study, we assessed how bFGF regulates FS cell coupling. Time course studies were carried out on the FS cell line TtT/GF. Short-term bFGF treatment induced a transient cell uncoupling and the phosphorylation in Ser368 of membrane-bound Cx43 without modifying Cx43 levels. We demonstrated the involvement of the protein kinase C (PKC) isoform α in the phosphorylation of Cx43 in S368. Moreover, we showed that bFGF induced PKCα activation by stimulating its expression, phosphorylation and association with the plasma membrane. The long-term incubation with bFGF increased TtT/GF cell coupling, total Cx43 levels and Cx43 accumulation at the cell membrane of cytoplasmic projections. The Cx43 level increase was a result of the stimulation of Cx43 gene transcription as mediated by the extracellular-regulated kinase 1/2 signalling pathway. Taken together, the data show that bFGF modulates TtT/GF cell coupling by activating different pathways that lead to opposite effects on Cx43 phosphorylation and expression depending on the duration of the exposure of the cells to bFGF. A short-term bFGF exposure reduces cell-to-cell communication as a mean of desynchronising FS cells. By contrast, long-term exposure to bFGF enhances cell-to-cell communication and facilitates coordination among FS cells.

Cellular and molecular specificity of pituitary gland physiology

The anterior pituitary gland has the ability to respond to complex signals derived from central and peripheral systems. Perception of these signals and their integration are mediated by cell interactions and cross-talk of multiple signaling transduction pathways and transcriptional regulatory networks that cooperate for hormone secretion, cell plasticity, and ultimately specific pituitary responses that are essential for an appropriate physiological response. We discuss the physiopathological and molecular mechanisms related to this integrative regulatory system of the anterior pituitary gland and how it contributes to modulate the gland functions and impacts on body homeostasis.

Modulation of gap junction channels and hemichannels by growth factors

Gap junction hemichannels and cell-cell channels have roles in coordinating numerous cellular processes, due to their permeability to extra and intracellular signaling molecules. Another mechanism of cellular coordination is provided by a vast array of growth factors that interact with relatively selective cell membrane receptors. These receptors can affect cellular transduction pathways, including alteration of intracellular concentration of free Ca(2+) and free radicals and activation of protein kinases or phosphatases. Connexin and pannexin based channels constitute recently described targets of growth factor signal transduction pathways, but little is known regarding the effects of growth factor signaling on pannexin based channels. The effects of growth factors on these two channel types seem to depend on the cell type, cell stage and connexin and pannexin isoform expressed. The functional state of hemichannels and gap junction channels are affected in opposite directions by FGF-1 via protein kinase-dependent mechanisms. These changes are largely explained by channels insertion in or withdrawal from the cell membrane, but changes in open probability might also occur due to changes in phosphorylation and redox state of channel subunits. The functional consequence of variation in cell-cell communication via these membrane channels is implicated in disease as well as normal cellular responses.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Regulation of lens gap junctions by Transforming Growth Factor beta

Using cultured lens epithelial cells, we discovered a new type of cross-talk between the FGF and TGF-β pathways, as well as a novel role for TGF-β and p38 kinase in the regulation of gap junctional intercellular communication. Our findings provide an explanation for how pathologically increased TGF-β signaling could contribute to cataract formation.

Gap junction–mediated intercellular communication (GJIC) is essential for the proper function of many organs, including the lens. GJIC in lens epithelial cells is increased by FGF in a concentration-dependent process that has been linked to the intralenticular gradient of GJIC required for lens transparency. Unlike FGF, elevated levels of TGF-β are associated with lens dysfunction. We show that TGF–β1 or -2 up-regulates dye coupling in serum-free primary cultures of chick lens epithelial cells (dissociated cell-derived monolayer cultures [DCDMLs]) via a mechanism distinct from that utilized by other growth factors. Remarkably, the ability of TGF-β and of FGF to up-regulate GJIC is abolished if DCDMLs are simultaneously exposed to both factors despite undiminished cell–cell contact. This reduction in dye coupling is attributable to an inhibition of gap junction assembly. Connexin 45.6, 43, and 56–containing gap junctions are restored, and intercellular dye coupling is increased, if the activity of p38 kinase is blocked. Our data reveal a new type of cross-talk between the FGF and TGF-β pathways, as well as a novel role for TGF-β and p38 kinase in the regulation of GJIC. They also provide an explanation for how pathologically increased TGF-β signaling could contribute to cataract formation.

Fibroblast growth factor-2 autofeedback regulation in pituitary folliculostellate TtT/GF cells

To investigate paracrine regulation of pituitary cell growth, we tested fibroblast growth factor (FGF) regulation of TtT/GF folliculostellate (FS) cells. FGF-2, and FGF-4 markedly induced cell proliferation, evidenced by induction of pituitary tumor transforming gene-1 (Pttg1) mRNA expression and percentage of cells in S phase. Signaling for FGF-2-induced FS cell proliferation was explored by specific pharmacological inhibition. A potent inhibitory effect on FGF-2 action was observed by blocking of Src tyrosine kinase with 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d] pyrimidine (>or=0.1 microm), followed by protein kinase C (PKC) inhibition with GF109203X. Treatment with FGF-2 (30 ng/ml; 10 min) activated phosphorylation of signal transducer and activator of transcription-3, ERK, stress-activated protein kinase/c-Jun N-terminal kinase, Akt, and focal adhesion kinase. Src inhibition with 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d] pyrimidine suppressed FGF-2-induced Akt and focal adhesion kinase, indicating effects downstream of FGF-2-induced Src activation. FGF-2 also markedly induced its own mRNA expression, peaking at 2-4 h, and this effect was suppressed by Src tyrosine kinase inhibition. The PKC inhibitor GF109203X abolished FGF-2 autoinduction, indicating PKC as the primary pathway involved in FGF-2 autoregulation in these cells. In addition to pituitary FGF-2 paracrine activity on hormonally active cells, these results show an autofeedback mechanism for FGF-2 in non-hormone-secreting pituitary FS cells, inducing cell growth and its own gene expression, and mediated by Src/PKC signaling.

Tumor necrosis factor-alpha-induced anterior pituitary folliculostellate TtT/GF cell uncoupling is mediated by connexin 43 dephosphorylation

The anterior pituitary folliculostellate (FS) cells are key elements of the paracrine control of the pituitary function. These cells are the source and the target of growth factors and cytokines, and are connected to other pituitary cells via Cx43-mediated gap junctions. Here, we show that acute treatment of the FS TtT/GF cell line with TNF-alpha caused a transient cell uncoupling that was accompanied by the dephosphorylation of Cx43 in Ser368. These TNF-alpha-evoked effects were dependent on protein phosphatase 2A (PP2A) and protein kinase C (PKC) activities. TNF-alpha did not affect total cell Cx43-PP2A catalytic subunit interaction, but it did induce PP2A catalytic subunit recruitment to the Triton X-100 insoluble subcellular fraction, in which Cx43-gap junction plaques are recovered. This recruitment temporally coincided with Cx43 phosphorylated in Ser368-Cx43 dephosphorylation. Cx43 did not interact with the conventional PKC-alpha, but it did interact with the atypical PKC-zeta. Moreover, this interaction was weakened by TNF-alpha. Cx43 dephosphorylation in Ser368 was followed by the tyrosine phosphorylation of the protein. The temporary closure of gap junctions during acute TNF-alpha challenge may constitute a protective mechanism to limit or confine the spread of inflammatory signals among the FS cells.

Adenosine stimulates connexin 43 expression and gap junctional communication in pituitary folliculostellate cells

Adenosine is known to stimulate interleukin (IL)-6 and vascular endothelial growth factor (VEGF) secretion from pituitary TtT/GF folliculostellate [corrected] (FS) cells indicating that it is an important paracrine regulator of anterior pituitary function. This study demonstrates that rodent anterior pituitary cell lines produce extracellular adenosine that is able to increase intercellular gap junction communication in FS cells. Ecto-5'-nucleotidase (CD73), the enzyme that generates adenosine from AMP, was demonstrated by immunocytochemistry in approximately 20% of anterior pituitary cells, and some of these cells colocalized with prolactin and growth hormone. CD73 mRNA and protein were detected in GH3 and MMQ (somatotroph-lactotroph lineages) and TtT/GF cells, and enzyme activity was demonstrated by the conversion of exogenously added fluorescent ethenoAMP to ethenoadenosine. Adenosine production, as measured by HPLC, was detected in GH3 (1 microM/h) and MMQ (3 microM/h) but not in TtT/GF cells. Adenosine (EC50: 0.5 microM) and NECA (universal adenosine receptor agonist; EC50 0.1 microM) stimulated connexin 43 (Cx43) mRNA and protein expression within 1-2 h in TtT/GF cells. Adenosine and NECA also stimulated gap junctional intercellular communication (as assessed by transmission of Alexa Fluor 488) by 6- to 8-fold in comparison with untreated TtT/GF cells. In cocultures of MMQ and TtT/GF cells, Cx43 expression in TtT/GF cells increased in proportion to the number of MMQ cells plated out. These data suggest that adenosine, formed locally in the anterior pituitary gland can stimulate gap junction communication in FS cells.

Genesis of prolactinomas: studies using estrogen-treated animals

Prolactin-secreting adenomas (prolactinomas) are the most prevalent form of pituitary tumors in humans. Our knowledge of the formation of these tumors is limited. Experimental work in animal has uncovered that estradiol exposure leads to prolactinoma formation via orchestrated events involving dopamine D2 receptors, transforming growth factor-beta(TGF-beta) isoforms and their receptors, as well as factors secondary to TGF-beta action. Additionally, these studies determined that TGF-beta and b-FGF interact to facilitate the communication between lactotropes and folliculo-stellate cells that is necessary for the mitogenic action of estradiol. The downstream signaling that governs lactotropic cell proliferation involves activation of the MAP kinase p44/42-dependent pathway.