Src-mediated tyrosine phosphorylation of dynamin is required for beta2-adrenergic receptor internalization and mitogen-activated protein kinase signaling

Dynamin and its role in membrane fission

Dynamin, a 100-kDa GTPase, is an essential component of vesicle formation in receptor-mediated endocytosis, synaptic vesicle recycling, caveolae internalization, and possibly vesicle trafficking in and out of the Golgi. In addition to the GTPase domain, dynamin also contains a pleckstrin homology domain (PH) implicated in membrane binding, a GTPase effector domain (GED) shown to be essential for self-assembly and stimulated GTPase activity, and a C-terminal proline-rich domain (PRD), which contains several SH3-binding sites. Dynamin partners bind to the PRD and may either stimulate dynamin's GTPase activity or target dynamin to the plasma membrane. Purified dynamin readily self-assembles into rings or spirals. This striking structural property supports the hypothesis that dynamin wraps around the necks of budding vesicles where it plays a key role in membrane fission. The focus of this review is on the relationship between the GTPase and self-assembly properties of dynamin and its cellular function.

Involvement of mitogen-activated protein kinase in agonist-induced phosphorylation of the mu-opioid receptor in HEK 293 cells

Agonist exposure of many G protein-coupled receptors stimulates an activation of extracellular signal-regulated protein kinases (ERKs) 1 and 2, members of the mitogen-activated protein kinase (MAPK) family. Here, we show that treatment of human embryonic kidney (HEK) 293 cells stably transfected to express the rat micro-opioid receptor (MOR1) with [D-Ala2,MePhe4,Gly5-ol]enkephalin (DAMGO) stimulated a rapid and transient (3-5-min) activation and nuclear translocation of MAPK. Exposure of these cells to the MAPK kinase 1 inhibitor PD98059 not only prevented MAPK activation but also inhibited homologous desensitization of the mu-opioid receptor. We have therefore determined the effect of PD98059 on agonist-induced mu-receptor phosphorylation. DAMGO stimulated a threefold increase in MOR1 phosphorylation within 20 min that could be reversed by the antagonist naloxone. PD98059 produced a dose-dependent inhibition of agonist-promoted mu-receptor phosphorylation with an IC50 of 20 microM. DAMGO also induced MOR1 internalization that peaked at 30 min. Confocal microscopy revealed that DAMGO-induced MOR1 internalization was also largely inhibited in the presence of PD98059. U0126, another chemically unrelated inhibitor of the MAPK cascade, mimicked the effect of PD98059 on mu-receptor phosphorylation and desensitization. MOR1 itself, however, appears to be a poor substrate for MAPK because mu-receptors immunoprecipitated from stably transfected HEK 293 cells were not phosphorylated by exogenous ERK 2 in vitro. The fact that morphine also triggered MAPK activation but did not induce MOR1 internalization indicates that receptor internalization was not required for MOR1-mediated mitogenic signaling. We conclude that MOR1 stimulates a rapid and intemalization-independent MAPK activation. Activation of the MAPK cascade in turn may not only relay mitogenic signals to the nucleus but also trigger initial events leading to phosphorylation and desensitization of the mu-opioid receptor.

The MEK pathway is required for stimulation of p21(WAF1/CIP1) by transforming growth factor-beta

Transforming growth factor-beta (TGF-beta)can induce the cyclin-dependent kinase inhibitors p21 and p15 in a variety of cell types. We have shown previously that Smad3 is required for the growth inhibitory activity of TGF-beta, whereas overexpression of Smads is not sufficient to activate the expression of p21 in HaCaT cells. These data suggest that an additional signaling pathway may be involved in stimulating p21 in HaCaT cells. Given the recent finding that the mitogen-activated protein kinase (MAPK) pathway can cause p21 induction and arrest cells, we examined the involvement of this pathway for p21 and p15 induction by TGF-beta. We found that TGF-beta can regulate the MAPK pathway, leading to the increased transactivation ability of transcription factor Elk. Constitutively active components in the MAPK pathway activate p21 expression, and inhibitors or dominant negative constructs for the MAPK pathway significantly decrease p21 induction by TGF-beta. Both constitutively active MEK and inhibitors for MEK have no effect on Smad activity, including DNA binding, localization, and interaction with coactivator p300/CBP. These findings suggest that the MAPK pathway may be an independent pathway that is involved in p21 and p15 induction by TGF-beta.

Dynamin is required for the activation of mitogen-activated protein (MAP) kinase by MAP kinase kinase

Internalization of activated receptors from the plasma membrane has been implicated in the activation of mitogen-activated protein (MAP) kinase. However, the mechanism whereby membrane trafficking may regulate mitogenic signaling remains unclear. Here we report that dominant-negative dynamin (K44A), an inhibitor of endocytic vesicle formation, abrogates MAP kinase activation in response to epidermal growth factor, lysophosphatidic acid, and protein kinase C-activating phorbol ester. In contrast, dynamin-K44A does not affect the activation of Ras, Raf, and MAP kinase kinase (MEK) by either agonist. Through immunofluorescence and subcellular fractionation studies, we find that activated MEK is present both at the plasma membrane and in intracellular vesicles but not in the cytosol. Our findings suggest that dynamin-regulated endocytosis of activated MEK, rather than activated receptors, is a critical event in the MAP kinase activation cascade.

Agonist-dependent phosphorylation of the G protein-coupled receptor kinase 2 (GRK2) by Src tyrosine kinase

GRK2 is a member of the G protein-coupled receptor kinase (GRK) family, which phosphorylates the activated form of a variety of G protein-coupled receptors (GPCR) and plays an important role in GPCR modulation. It has been recently reported that stimulation of the mitogen-activated protein kinase cascade by GPCRs involves tyrosine phosphorylation of docking proteins mediated by members of the Src tyrosine kinase family. In this report, we have investigated the possible role of c-Src in modulating GRK2 function. We demonstrate that c-Src can directly phosphorylate GRK2 on tyrosine residues, as shown by in vitro experiments with purified proteins. The phosphorylation reaction exhibits an apparent K(m) for GRK2 of 12 nM, thus suggesting a physiological relevance in living cells. Consistently, overexpression of the constitutively active c-Src Y527F mutant in COS-7 cells leads to tyrosine phosphorylation of co-expressed GRK2. In addition, GRK2 can be detected in phosphotyrosine immunoprecipitates from HEK-293 cells transiently transfected with this Src mutant. Interestingly, phosphotyrosine immunoblots reveal a rapid and transient increase in GRK2 phosphorylation upon agonist stimulation of beta(2)-adrenergic receptors co-transfected with GRK2 and wild type c-Src in COS-7 cells. This tyrosine phosphorylation is maximal within 5 min of isoproterenol stimulation and reaches values of approximately 5-fold over basal conditions. Furthermore, GRK2 phosphorylation on tyrosine residues promotes an increased kinase activity toward its substrates. Our results suggest that GRK2 phosphorylation by c-Src is inherent to GPCR activation and put forward a new mechanism for the regulation of GPCR signaling.

G protein-coupled receptors as targets for prolactin actions

Prolactin (PRL) is known to be involved in a wide range of biological functions including osmoregulation, lactation, reproduction, and immunomodulation. The first step in PRL action involves its interaction with a specific membrane receptor that belongs to the cytokine receptor superfamily. In spite of the lack of a kinase domain, receptors of the cytokine superfamily induce tyrosine phosphorylation of cellular substrates including the receptors. The role of PRL in female reproductive functions is well known and a direct effect on ovarian and testicular steroidogenesis has been established. In the ovary, PRL binds to a specific membrane receptor and exerts an inhibitory effect on follicular steroidogenesis. This effect is the result of an impairment involving FSH stimulation of G protein-coupled receptors (GPCR) and cyclic AMP-mediated activation of aromatase cytochrome P450 gene expression. This observation may indicate a direct connection between tyrosine phosphorylation and follicle-stimulating hormone (FSH) receptor (FSHR) transduction pathways, as is the case for growth factor receptors with intrinsic tyrosine kinase activity, which share several downstream signaling elements with GPCRs. Some studies leading to our understanding of these pathways are reviewed.

C, Rodríguez-Frade JM. Characterization of RANTES- and Aminooxypentane-RANTES- Triggered Desensitization Signals Reveals Differences in Recruitment of the G Protein-Coupled Receptor Complex

The trafficking of lymphocyte populations is a complex process controlled by a vast array of molecules. In this process, cells must be able to sense small changes in chemoattractant gradients. Migration through a chemotactic gradient probably employs an on-off mechanism in which chemokine receptor desensitization, internalization, and recycling may be important steps. This multistep process requires the coordinated action of many factors, including G protein-coupled receptor kinases, arrestins, clathrin, and GTP-hydrolyzing proteins such as dynamin. In this report, we show that RANTES and its derivative, aminooxypentane (AOP)-RANTES, a potent RANTES antagonist as well as an inhibitor of HIV-1 infection, both promote CCR5 desensitization involving G protein-coupled receptor kinases-2 and β-arrestin equally well. An important difference between the two molecules is that (AOP)-RANTES is more efficient than RANTES in promoting Ser/Thr phosphorylation of the receptor and association of G protein-coupled receptor kinases-2, β-arrestin, and clathrin to the CCR5. After stimulation with either ligand, we observe rapid, transient association of dynamin to CCR5, implicating this protein in receptor sensitization, but this association is faster and longer-lasting following (AOP)-RANTES stimulation. In summary, we show that chemokine receptor internalization takes place through the formation of clathrin vesicles and involves dynamin activity. We provide compelling evidence that the differences between RANTES and (AOP)-RANTES in Gαi activation condition subsequent signaling events, including internalization and receptor recycling.

Membrane microdomains and caveolae

Glycosphingolipid- and cholesterol-enriched microdomains, or rafts, within the plasma membrane of eukaryotic cells have been implicated in many important cellular processes, such as polarized sorting of apical membrane proteins in epithelial cells and signal transduction. Until recently, however, the existence of such domains remained controversial. The past year has brought compelling evidence that microdomains indeed exist in living cells. In addition, several recent papers have suggested that caveolae, which are considered to be a specific form of raft, and caveolins, the major membrane proteins of caveolae, are involved in the dynamic cholesterol-dependent regulation of specific signal transduction pathways.