Hypotonicity activates transcription through ERK-dependent and -independent pathways in renal cells

Role of nitric oxide on pressor mechanisms within the dorsomedial and rostral ventrolateral medulla in anaesthetized cats

1. The role of nitric oxide (NO) in central cardiovascular regulation and the correlation between NO and glutamate-induced mechanisms is not clear. Microinjection of glutamate (3 nmol/30 nL) into dorsomedial medulla (DM) and rostral ventrolateral medulla (RVLM) increased arterial blood pressure (BP) and sympathetic vertebral nerve activity (VNA). Thus, in the present study, we examined the modulation by NO of glutamate-induced pressor responses in the DM and RVLM of cats. 2. Histochemical methods using nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) as a marker to stain neurons containing NO synthase (NOS), showed positive findings of NOS in both the DM and RVLM. 3. Microinjection of N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, into the DM or RVLM did not alter resting BP and VNA, but it did cause a dose-dependent attenuation of glutamate-induced pressor responses. Interestingly, the increase in NO levels that resulted from pretreatment with L-arginine (L-Arg) or sodium nitroprusside (SNP) did not alter resting BP and VNA, but still inhibited glutamate-induced pressor responses in the DM and RVLM in a dose-dependent manner. 4. We also examined whether NO modulated the pressor responses induced by activation of different excitatory amino acid receptors. N-Methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) were used. Consistent with the results from the initial glutamate studies, we observed that not only L-NAME, but also L-Arg and SNP attenuated pressor responses induced by NMDA and AMPA. No difference was found between the effects of NO on NMDA- and AMPA-induced pressor responses. 5. To investigate the possibility of a loss of agonist selectivity, the effects of D-2-amino-5-phosphonovalerate (D-AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on AMPA and NMDA responses in the DM were examined. The results showed that CNQX did not alter NMDA-induced pressor responses, while D-AP5 failed to alter AMPA-induced responses. 6. Our results suggest that activation of the glutamate-induced pressor mechanism is regulated by changes in NO levels in the DM and RVLM. This implies that NO may play a permissive role to allow operation of the glutamate-activation mechanism.

Hypoxia activates jun-N-terminal kinase, extracellular signal-regulated protein kinase, and p38 kinase in pulmonary arteries

Chronic alveolar hypoxia is the major cause of pulmonary hypertension. The cellular mechanisms involved in hypoxia- induced pulmonary arterial remodeling are still poorly understood. Mitogen-activated protein kinase (MAPK) is a key enzyme in the signaling pathway leading to cellular growth and proliferation. The purpose of this investigation was to determine the roles that MAPKs, specifically Jun-N-terminal kinase (JNK), extracellular signal-regulated protein kinase (ERK), and p38 kinase, play in the hypoxia-induced pulmonary arterial remodeling. Rats were exposed to normobaric hypoxia (10% O(2)) for 1, 3, 7, or 14 d. Hypoxia caused significant remodeling in the pulmonary artery characterized by thickening of pulmonary arterial wall and increases in tissue mass and total RNA. JNK, ERK, and p38 kinase tyrosine phosphorylations and their activities were significantly increased by hypoxia. JNK activation peaked at Day 1 and ERK/p38 kinase activation peaked after 7 d of hypoxia. The results from immunohistochemistry show that hypoxia increased phospho-MAPK staining in both large and small intrapulmonary arteries. Hypoxia also upregulated vascular endothelial growth factor messenger RNA (mRNA) and platelet-derived growth factor receptor mRNA levels in pulmonary artery with a time course correlated to the activation of ERK and p38 kinase. The gene expressions of c-jun, c-fos, and egr-1, known as downstream effectors of MAPK, were also investigated. Hypoxia upregulated egr-1 mRNA but downregulated c-jun and c-fos mRNAs. These data suggest that hypoxia-induced activation of JNK is an early response to hypoxic stress and that activation of ERK and p38 kinase appears to be associated with hypoxia-induced pulmonary arterial remodeling.

MAPK signaling and the kidney

Following an overview of the biochemistry of mitogen-activated protein kinase (MAPK) pathways, the relevance of these signaling events to specific models of renal cell function and pathophysiology, both in vitro and in vivo, will be emphasized. In in vitro model systems, events activating the principal MAPK families [extracellular signal-regulated and c-Jun NH(2)-terminal kinase and p38] have been best characterized in mesangial and tubular epithelial cell culture systems and include peptide mitogens, cytokines, lipid mediators, and physical stressors. Several in vivo models of proliferative or toxic renal injury are also associated with aberrant MAPK regulation. It is anticipated that elucidation of downstream effector signaling mechanisms and a clearer understanding of the immediate and remote upstream activating pathways, when applied to these highly clinically relevant model systems, will ultimately provide much greater insight into the basis for specificity now seemingly absent from these signaling events.

Egr-1 gene is induced by the systemic administration of the vascular endothelial growth factor and the epidermal growth factor

Egr-1 is a transcription factor that couples short-term changes in the extracellular milieu to long-term changes in gene expression. In cultured endothelial cells, the Egr-1 gene has been shown to respond to a variety of extracellular signals. However, the physiological relevance of these findings remains unclear. To address this question, the growth factor-mediated response of the Egr-1 gene under in vivo conditions was analyzed. To that end, either vascular endothelial growth factor (VEGF) or epidermal growth factor (EGF) was injected into the intraperitoneal cavity of mice. Growth factors were delivered to all tissues examined, as evidenced by the widespread distribution of I125-labeled growth factors and the phosphorylation of their respective receptors. In Western blot analyses of whole-tissue extracts, Egr-1 protein levels were shown to be induced in the heart, brain, liver, and spleen of VEGF-treated mice, and in the heart, lung, brain, liver and skeletal muscle of EGF-treated animals. Changes in Egr-1 levels did not correlate with changes in receptor phosphorylation or ERK1/2 phosphorylation. In Northern blot analyses, VEGF induced Egr-1 mRNA levels in all tissues examined except lung and kidney, whereas EGF led to increased transcripts in all tissues except kidney. In immunofluorescence studies, VEGF induced Egr-1 in microvascular endothelial cells of the heart and liver, and EGF induced Egr-1 in the microvascular bed of skeletal muscle. Taken together, these results suggest that the Egr-1 gene is differentially regulated in response to systemically administered VEGF and EGF.

Egr-1 gene is induced by the systemic administration of the vascular endothelial growth factor and the epidermal growth factor

Egr-1 is a transcription factor that couples short-term changes in the extracellular milieu to long-term changes in gene expression. In cultured endothelial cells, the Egr-1 gene has been shown to respond to a variety of extracellular signals. However, the physiological relevance of these findings remains unclear. To address this question, the growth factor-mediated response of the Egr-1 gene under in vivo conditions was analyzed. To that end, either vascular endothelial growth factor (VEGF) or epidermal growth factor (EGF) was injected into the intraperitoneal cavity of mice. Growth factors were delivered to all tissues examined, as evidenced by the widespread distribution of I125-labeled growth factors and the phosphorylation of their respective receptors. In Western blot analyses of whole-tissue extracts, Egr-1 protein levels were shown to be induced in the heart, brain, liver, and spleen of VEGF-treated mice, and in the heart, lung, brain, liver and skeletal muscle of EGF-treated animals. Changes in Egr-1 levels did not correlate with changes in receptor phosphorylation or ERK1/2 phosphorylation. In Northern blot analyses, VEGF induced Egr-1 mRNA levels in all tissues examined except lung and kidney, whereas EGF led to increased transcripts in all tissues except kidney. In immunofluorescence studies, VEGF induced Egr-1 in microvascular endothelial cells of the heart and liver, and EGF induced Egr-1 in the microvascular bed of skeletal muscle. Taken together, these results suggest that the Egr-1 gene is differentially regulated in response to systemically administered VEGF and EGF.

Nociception in cyclooxygenase isozyme-deficient mice

Prostaglandins formed by cyclooxygenase-1 (COX-1) or COX-2 produce hyperalgesia in sensory nerve endings. To assess the relative roles of the two enzymes in pain processing, we compared responses of COX-1- or COX-2-deficient homozygous and heterozygous mice with wild-type controls in the hot plate and stretching tests for analgesia. Preliminary observational studies determined that there were no differences in gross parameters of behavior between the different groups. Surprisingly, on the hot plate (55 degrees C), the COX-1-deficient heterozygous groups showed less nociception, because mean reaction time was longer than that for controls. All other groups showed similar reaction times. In the stretching test, there was less nociception in COX-1-null and COX-1-deficient heterozygotes and also, unexpectedly, in female COX-2-deficient heterozygotes, as shown by a decreased number of writhes. Measurements of mRNA levels by reverse transcription-PCR demonstrated a compensatory increase of COX-1 mRNA in spinal cords of COX-2-null mice but no increase in COX-2 mRNA in spinal cords of COX-1-null animals. Thus, compensation for the absence of COX-1 may not involve increased expression of COX-2, whereas up-regulation of COX-1 in the spinal cord may compensate for the absence of COX-2. The longer reaction times on the hot plate of COX-1-deficient heterozygotes are difficult to explain, because nonsteroid anti-inflammatory drugs have no analgesic action in this test. Reduction in the number of writhes of the COX-1-null and COX-1-deficient heterozygotes may be due to low levels of COX-1 at the site of stimulation with acetic acid. Thus, prostaglandins made by COX-1 mainly are involved in pain transmission in the stretching test in both male and female mice, whereas those made by COX-2 also may play a role in the stretching response in female mice.

Hypertonic induction of aquaporin-5 expression through an ERK-dependent pathway

Aquaporin-5 (AQP5) is a water channel protein expressed in lung, salivary gland, and lacrimal gland epithelia. Each of these sites may experience fluctuations in surface liquid osmolarity; however, osmotic regulation of AQP5 expression has not been reported. This study demonstrates that AQP5 is induced by hypertonic stress and that induction requires activation of extracellular signal-regulated kinase (ERK). Incubation of mouse lung epithelial cells (MLE-15) in hypertonic medium produced a dose-dependent increase in AQP5 expression; AQP5 protein peaked by 24 h and returned to baseline levels within hours of returning cells to isotonic medium. AQP5 induction was observed only with relatively impermeable solutes, suggesting an osmotic pressure gradient is required for induction. ERK was selectively activated in MLE-15 cells by hypertonic stress, and inhibition of ERK activation with two distinct mitogen-activated extracellular regulated kinase kinase (MEK) inhibitors, U0126 and PD98059, blocked AQP5 induction. AQP5 induction was also observed in the lung, salivary, and lacrimal glands of hyperosmolar rats, suggesting potential physiologic relevance for osmotic regulation of AQP5 expression. This report provides the first example of hypertonic induction of an extrarenal aquaporin, as well as the first association between mitogen-activated protein kinase signaling and aquaporin expression.

RAFTK/Pyk2-mediated cellular signalling

Intracellular signal transduction following extracellular ligation by a wide variety of surface molecules involves the activation and tyrosine phosphorylation of protein tyrosine kinases (PTKs). Tyrosine phosphorylation, controlled by the coordinated actions of protein tyrosine phosphatases (PTPs) and tyrosine kinases, is a critical regulatory mechanism for various physiological processes, including cell growth, differentiation, metabolism, cell cycle regulation and cytoskeleton function. The focal adhesion PTK family consists of the focal adhesion kinase (FAK) and the RAFTK/Pyk2 kinase (also known as CAK-beta and CADTK). RAFTK/Pyk2 can be activated by a variety of extracellular signals that elevate intracellular calcium concentration, and by stress signals. RAFTK/Pyk2 is expressed mainly in the central nervous system and in cells derived from hematopoietic lineages, while FAK is widely expressed in various tissues and links transmembrane integrin receptors to intracellular pathways. This review describes the role of RAFTK/Pyk2 in various signalling cascades and details the differential signalling by FAK and RAFTK/Pyk2.

Distribution of IkappaB proteins in gastric mucosa and other organs of mouse and gerbil

The NF-kappaB/IkappaB complex is a major transcription regulator of inflammatory and immune responses. Helicobacter pylori infection causes chronic inflammation in gastric mucosa by inducing dissociation of the inhibitory IkappaB protein from the complex with a resulting increased expression of interleukin (IL)-8. To clarify which of several known IkappaB proteins could be involved in this inflammatory response, we undertook immunohistochemical examination of normal mouse stomach as well as other murine tissues for comparison, using polyclonal antibodies specific for alpha-, beta-, gamma-, and in-isoforms of IkappaB. The results showed strong immunoreactivity for the alpha-isoform in parietal cells and for the beta-isoform in pit cells of the stomach, along with the presence of these proteins in various other sites. Comparative staining revealed a similar but not identical distribution of IkappaB proteins in the Mongolian gerbil, a rodent model for H. pylori infection. The findings suggest that the alpha- and beta-isoforms are dominant IkappaB proteins in gastric parietal and foveolar cells, respectively, and point to a role for these transcription regulators in modulating pathological responses in stomach and other organs. (J Histochem Cytochem 48:191-199, 2000)

Hyperosmolality induces activation of cPKC and nPKC, a requirement for ERK1/2 activation in NIH/3T3 cells

Protein kinase C (PKC) has been reported to be associated with the activation of extracellular signal-regulated kinase (ERK) by hyperosmolality. However, it is unclear whether hyperosmolality induces PKC activation and which PKC isoforms are involved in ERK activation. In this study, we demonstrate that NaCl increases total PKC activity and induces PKCalpha, PKCdelta, and PKCepsilon translocation from the cytosol to the membrane in NIH/3T3 cells, suggesting that hyperosmotic stress activates conventional PKC (cPKC) and novel PKC (nPKC). Further studies show that NaCl-inducible ERK1 and ERK2 (ERK1/2) activation is a consequence of cPKC and nPKC activation, because either downregulation with 12-O-tetradecanoylphorbol 13-acetate or selective inhibition of cPKC and nPKC by GF-109203X and rottlerin largely inhibited the stimulation of ERK1/2 phosphorylation by NaCl. In addition, we show that NaCl increases diacylglycerol (DAG) levels and that a phospholipase C (PLC) inhibitor, U-73122, inhibits NaCl-induced ERK1/2 phosphorylation. These results, together, suggest that a hyperosmotic NaCl-induced signaling pathway that leads to activation of ERK1/2 may sequentially involve PLC activation, DAG release, and cPKC and nPKC activation.

PI3K signaling in the murine kidney inner medullary cell response to urea

Growth factors and other stimuli increase the activity of phosphatidylinositol-3 kinase (PI3K), an SH2 domain-containing lipid kinase. In the murine kidney inner medullary mIMCD3 cell line, urea (200 mM) increased PI3K activity in a time-dependent fashion as measured by immune complex kinase assay. The PI3K effector, Akt, was also activated by urea as measured by anti-phospho-Akt immunoblotting. In addition, the Akt (and PI3K) effector, p70 S6 kinase, was activated by urea treatment in a PI3K-dependent fashion. PI3K inhibition potentiated the proapoptotic effect of hypertonic and urea stress. Urea treatment also induced the tyrosine phosphorylation of Shc and the recruitment to Shc of Grb2. Coexistence of activated Shc and PI3K in a macromolecular complex was suggested by the increase in PI3K activity evident in anti-Shc immunoprecipitates prepared from urea-treated cells. Taken together, these data suggest that PI3K may regulate physiological events in the renal medullary cell response to urea stress and that an upstream tyrosine kinase conferring activation of both PI3K and Shc may govern urea signaling in these cells.

The PEST domain of IkappaBalpha is necessary and sufficient for in vitro degradation by mu-calpain

Polypeptide sequences enriched in proline (P), glutamate (E), serine (S), and threonine (T), dubbed PEST domains, are proposed to expedite the degradation of proteins. The proteolysis of one PEST-containing protein, IkappaBalpha, is prerequisite to the activation of the transcription factor NF-kappaB. Two mechanisms of IkappaBalpha degradation in vivo have been described, one well characterized through the ubiquitin-proteasome pathway, and another less characterized through calpain. In this report, a mutational analysis was done to identify any regions of IkappaBalpha that facilitate its recognition and proteolysis by calpain in vitro. These studies revealed that the PEST sequence of IkappaBalpha is critical for its calpain-dependent degradation. Furthermore, the IkappaBalpha-PEST domain binds to the calmodulin-like domain of the large subunit of mu-calpain (muCaMLD). Transfer of the IkappaBalpha-PEST domain to a protein incapable of either binding to or being degraded by mu-calpain allowed for the interaction of the chimeric protein with muCaMLD and resulted in its susceptibility to calpain proteolysis. Moreover, the muCaMLD of calpain acts as a competitive inhibitor of calpain-dependent IkappaBalpha degradation. Our data demonstrate that the IkappaBalpha-PEST sequence acts as a modular domain to promote the physical association with and subsequent degradation by mu-calpain and suggest a functional role for PEST sequences in other proteins as potential calpain-targeting units.