Fibroblast growth factors (FGFs) constitute a large family of at least nine distinct polypeptide growth factors (7,31,34). FGFs play an important role in the regulation of cell growth, differentiation, embryogenesis, and angiogenesis (34). Like other growth factors, FGFs exert their action by binding to and activating a distinct family of growth factor receptors that has been previously classified as subclass IV (20, 74). The FGF receptor family consists of at least four distinct gene products, each composed of an extracellular ligand-binding domain that contains three immunoglobulin-like domains, a single transmembrane domain, and a cytoplasmic domain that contains protein tyrosine kinase activity (interrupted by an insertion of 14 amino acids in the kinase domain). One of the characteristic features of the FGF receptor family is the occurrence of numerous receptor isoforms that are produced from alternatively spliced transcripts in both the intracellular and extracellular domains (9,14,27,32,33). As with other growth factors, binding of FGF to FGF receptors leads to receptor dimerization (3, 70, 73) and subsequent tyrosine autophosphorylation and phosphorylation of target substrates (6, 13, 23). Autophosphorylation on tyrosine is considered to have at least two functions. One such function is the stimulation of the intrinsic protein tyrosine kinase activity by an allosteric mechanism, as seen with the insulin receptor (22,63,81,84,86). Also, many autophosphorylation sites serve as binding sites for signaling proteins that contain Src homology 2 (SH2) domains or the recently identified phosphotyrosine interaction (also called phosphotyrosine-binding [PTB]) domains (4,5,8,35,36,39,48,56,57). Binding of SH2 or PTB domain-containing proteins to activated growth factor receptors has been shown to be important for the activation of downstream signaling molecules. For example, binding of Shc and phospholipase C␥ (PLC␥) through PTB and SH2 domains, respectively, to the activated nerve growth factor receptor (Trk) has been shown to be required for the nerve growth factor-induced activation of Ras signaling pathways and neuronal differentiation of PC12 cells (16,55,72). Several studies have demonstrated that mutation of autophosphorylation sites in platelet-derived growth factor (PDGF) receptor and in colony-stimulating factor 1 receptor can impair mitogenic signaling in some cell lines (19,76,77).Very little is known about the cellular substrates and target proteins involved in signaling processes that lead to FGFmediated mitogenesis. So far only PLC␥ has been shown to associate with the activated FGF receptor 1 (FGFR1) (flg); the identities of other targets remain unclear. We have previously identified tyrosine 766 of FGFR1 as the major autophosphorylation site and have shown that this tyrosine and its flanking sequences represent a high affinity binding site for one of the SH2 domains of PLC␥ (52). Mutation of this tyrosine to phenylalanine results in a receptor that is no longer able to stimulate phosphatidylinosito...

Mitogen-activated protein (MAP) kinases are critical mediators of innate immune responses. In response to lipopolysaccharide (LPS), MAP kinases are rapidly activated and play an important role in the production of proinflammatory cytokines. Although a number of MAP kinase phosphatases (MKPs) have been identified, their roles in the control of cytokine production have not been well defined. In the present report, we investigated the role of MKP-1 in alveolar macrophages stimulated with LPS. We found that LPS triggered transient activation of three MAP kinase subfamilies, ERK, JNK, and p38, in both immortalized and primary murine alveolar macrophages. MKP-1 was rapidly induced by LPS, and its induction correlated with the dephosphorylation of these MAP kinases. Blocking MKP-1 with triptolide prolonged the activities of both JNK and p38 in immortalized alveolar macrophages. Stimulation of primary alveolar macrophages isolated from MKP-1-deficient mice with LPS resulted in a prolonged p38 phosphorylation compared with wild type alveolar macrophages. Accordingly, these MKP-1-deficient alveolar macrophages also mounted a more robust and rapid tumor necrosis factor ␣ production than their wild type counterparts. Adenovirus-mediated MKP-1 overexpression significantly attenuated tumor necrosis factor ␣ production in immortalized alveolar macrophages. Finally, MKP-1 was induced by a group of corticosteroids frequently prescribed for the treatment of inflammatory lung diseases, and the anti-inflammatory potencies of these drugs closely correlated with their abilities to induce MKP-1. Our studies indicated that MKP-1 plays an important role in dampening the inflammatory responses of alveolar macrophages. We speculate that MKP-1 may represent a novel target for therapeutic intervention of inflammatory lung diseases.

Endothelin-1 (ET-1) induces cell proliferation and differentiation through multiple G-protein-linked signaling systems, including p21 ras activation. Whereas p21 ras activation and desensitization by receptor tyrosine kinases have been extensively investigated, the kinetics of p21 ras activation induced by engagement of G-protein-coupled receptors remains to be fully elucidated. In the present study we show that ET-1 induces a biphasic activation of p21 ras in rat glomerular mesangial cells. The first peak of activation of p21 ras , at 2-5 min, is mediated by immediate association of phosphorylated Shc with the guanosine exchange factor Sos1 via the adaptor protein Grb2. This initial activation of p21 ras results in activation of the extracellular signal-regulated kinase (ERK) cascade. We demonstrate that ET-1 signaling elicits a negative feedback mechanism, modulating p21 ras activity through ERKdependent Sos1 phosphorylation, findings which were confirmed using an adenovirus MEK construct. Subsequent to p21 ras and ERK deactivation, Sos1 reverts to the non-phosphorylated condition, enabling it to bind again to the Grb2/Shc complex, which is stabilized by persistent Shc phosphorylation. However, the resulting secondary activation of p21 ras at 30 min does not lead to ERK activation, correlating with intensive, ET-1-induced expression of MAP kinase phosphatase-1, but does result in increased p21 ras -associated phosphatidylinositol 3-kinase activity. Our data provide evidence that ET-1-induced biphasic p21 ras activation causes sequential stimulation of divergent downstream signaling pathways.

Cyclooxygenase-2 (Cox-2), an inducible form of the enzyme that catalyzes the first step in the synthesis of prostanoids, has been shown to be overexpressed in a wide range of tumors and possesses proangiogenic and antiapoptotic properties. To understand the molecular mechanism of Cox-2 action we used adenovirus-mediated transfer of rat Cox-2 cDNA into renal rat mesangial cells and determined the differential gene expression using cDNA microarrays. One of the several genes that were highly up-regulated by over expressed Cox-2 was MDR1. MDR1 or P-glycoprotein (P-gp), the product of the MDR1 gene, is implicated as the primary cause of multidrug resistance (MDR) in tumors where it acts as an efflux pump for chemotherapeutic agents. It is also expressed in normal tissues of the liver and kidney where it functions to actively transport lipophilic xenobiotics. Reverse transcriptase-PCR analysis confirmed the results of the microarray, showing increased mRNA levels for MDR1 in Cox-2 overexpressing cells. This increase in mRNA translated to an increase in MDR1 protein expression, which was dose-dependent on Cox-2 expression. Furthermore, using rhodamine 123 efflux assay we observed a significant increase in P-gp activity in Cox-2 overexpressing renal mesangial cells. The specific Cox-2 inhibitor NS398 was able to block the Cox-2-mediated increase in MDR1 expression and activity, suggesting that Cox-2 products may be implicated in this response. These results prove the existence of a causal link between Cox-2 and P-gp activity, which would have implications for kidney function and multidrug resistance in tumors where Cox-2 is overexpressed.

The Nijmegen Biomedical Study is a population-based cross-sectional study conducted in the eastern part of the Netherlands. As part of the overall study, we provide reference values of estimated glomerular filtration rate (GFR) for this Caucasian population without expressed risk. Age-stratified, randomly selected inhabitants received a postal questionnaire on lifestyle and medical history. In a large subset of the responders, serum creatinine was measured. The GFR was then measured using the abbreviated Modification of Diet in Renal Disease (MDRD) formula. To limit possible bias, serum creatinine was calibrated against measurements performed in the original MDRD laboratory. The study cohort included 2823 male and 3274 female Caucasian persons aged 18-90 years. A reference population of apparently healthy subjects was selected by excluding persons with known hypertension, diabetes, cardiovascular-or renal diseases. This healthy study cohort included 1660 male subjects and 2072 female subjects, of which 869 of both genders were 65 years or older. The median GFR was 85 ml/min/1.73 m 2 in 30-to 34-year-old men and 83 ml/min/1.73 m 2 in similar aged women. In these healthy persons, GFR declined approximately 0.4 ml/min/year. Our study provides age-and gender-specific reference values of GFR in a population of Caucasian persons without identifiable risk.

In the present study, we investigated the function and the mechanism of action of RGS3, a member of a family of proteins called regulators of G protein signaling (RGS). Polyclonal antibodies against RGS3 were produced and characterized. An 80-kDa protein was identified as RGS3 by immunoprecipitation and immunoblotting with anti-RGS3 antibodies in a human mesangial cell line (HMC) stably transfected with RGS3 cDNA. Coimmunoprecipitation experiments in RGS3-overexpressing cell lysates revealed that RGS3 bound to aluminum fluoride-activated Galpha11 and to a lesser extent to Galphai3 and that this binding was mediated by the RGS domain of RGS3. A role of RGS3 in postreceptor signaling was demonstrated by decreased calcium responses and mitogen-activated protein (MAP) kinase activity induced by endothelin-1 in HMC stably overexpressing RGS3. Moreover, depletion of endogenous RGS3 by transfection of antisense RGS3 cDNA in NIH 3T3 cells resulted in enhanced MAP kinase activation induced by endothelin-1. The study of intracellular distribution of RGS3 indicated its unique cytosolic localization. Activation of G proteins by AlF4-, NaF, or endothelin-1 resulted in redistribution of RGS3 from cytosol to the plasma membrane as determined by Western blotting of the cytosolic and particulate fractions with RGS3 antiserum as well as by immunofluorescence microscopy. Agonist-induced translocation of RGS3 occurred by a dual mechanism involving both C-terminal (RGS domain) and N-terminal regions of RGS3. Thus, coexpression of RGS3 with a constitutively active mutant of Galpha11 (Galpha11-QL) resulted in the binding of RGS3, but not of its N-terminal fragment, to the membrane fraction and in its interaction with Galpha11-QL in vitro without any stimuli. However, both full-length RGS3 and its N-terminal domain translocated to the plasma membrane upon stimulation of intact cells with endothelin-1 as assayed by immunofluorescence microscopy. The effect of endothelin-1 was also mimicked by calcium ionophore A23187, suggesting the importance of Ca2+ in the mechanism of redistribution of RGS3. These data indicate that RGS3 inhibits G protein-coupled receptor signaling by a complex mechanism involving its translocation to the membrane in addition to its established function as a GTPase-activating protein.

The intracellular mechanisms involved in the activation of extracellular signal-regulated kinase (ERK) are relatively well understood. However, the intracellular signaling pathways which regulate the termination of ERK activity remain to be elucidated. Mitogen-activated protein kinase phosphatase 1 (MKP-1) has been shown to dephosphorylate and inactivate ERK in vitro and in vivo. In the present study, we show in NIH3T3 fibroblasts that activation of the stress-activated protein kinase (SAPK) pathway by either specific extracellular stress stimuli or via induction of MEKK, an upstream kinase of SAPK, results in MKP-1 gene expression. In contrast, selective stimulation of the ERK pathway by 12-O-tetradecanoylphorbol-13-acetate or following expression of constitutively active MEK, the upstream dual specificity kinase of ERK did not induce the transcription of MKP-1. Hence, these findings demonstrate the existence of cross-talk between the ERK and SAPK signaling cascades since activation of SAPK induced the expression of MKP-1 that can inactivate ERK. This mechanism may modulate the cellular response to stimuli which employ the SAPK signal transduction pathway.

In recent years, BK virus (BKV) nephritis after renal transplantation has become a severe problem. The exact mechanisms of BKV cell entry and subsequent intracellular trafficking remain unknown. Since human renal proximal tubular epithelial cells (HRPTEC) represent a main natural target of BKV nephritis, analysis of BKV infection of HRPTEC is necessary to obtain additional insights into BKV biology and to develop novel strategies for the treatment of BKV nephritis. We coincubated HRPTEC with BKV and the cholesteroldepleting agents methyl beta cyclodextrin (MBCD) and nystatin (Nys), drugs inhibiting caveolar endocytosis. The percentage of infected cells (detected by immunofluorescence) and the cellular levels of BKV large T antigen expression (detected by Western blot analysis) were significantly decreased in both MBCD-and Nys-treated HPRTEC compared to the level in HRPTEC incubated with BKV alone. HRPTEC infection by BKV was also tested after small interfering RNA (siRNA)-dependent depletion of either the caveolar structural protein caveolin-1 (Cav-1) or clathrin, the major structural protein of clathrin-coated pits. BKV infection was inhibited in HRPTEC transfected with Cav-1 siRNA but not in HRPTEC transfected with clathrin siRNA. The colocalization of labeled BKV particles with either Cav-1 or clathrin was investigated by using fluorescent microscopy and image cross-correlation spectroscopy. The rate of colocalization of BKV with Cav-1 peaked at 4 h after incubation. Colocalization with clathrin was insignificant at all time points. These results suggest that BKV entered into HRPTEC via caveolae, not clathrin-coated pits, and that BKV is maximally associated with caveolae at 4 h after infection, prior to relocation to a different intracellular compartment.