Role of tyrosine phosphorylation in potassium channel activation. Functional association with prolactin receptor and JAK2 tyrosine kinase

The subfornical organ: A novel site for prolactin action

Prolactin (PRL) is a peptide hormone that performs over 300 biological functions, including those that require binding to prolactin receptor (PRL-R) in neurones within the central nervous system (CNS). To enter the CNS, circulating PRL must overcome the blood-brain barrier. Accordingly, areas of the brain that do not possess a blood-brain barrier, such as the subfornical organ (SFO), are optimally positioned to interact with systemic PRL. The SFO has been classically implicated in energy and fluid homeostasis but has the potential to influence oestrous cyclicity and gonadotrophin release, which are also functions of PRL. We aimed to confirm and characterise the expression of PRL-R in the SFO, as well as identify the effects of PRL application on membrane excitability of dissociated SFO neurones. Using a quantitative real-time polymerase chain reaction, we found that PRL-R mRNA in the SFO of male and female Sprague Dawley rats did not significantly differ between juvenile and sexually mature rats (P = .34), male and female rats (P = .97) or across the oestrous cycle (P = .54). Patch-clamp recordings were obtained in juvenile male rats to further investigate the actions of PRL at the SFO. Dissociated SFO neurones perfused with 1 μmol L^-1 PRL resulted in 2 responsive subpopulations of neurones; 40% depolarised (n = 15/43, 11.3 ± 1.7 mV) and 14% hyperpolarised (n = 6/43, -6.7 ± 1.4 mV) to PRL application. Within the range of 10 pmol L^-1 to 1 μmol L^-1 , the concentrations of PRL were not significantly different in either the magnitude (P = .53) or proportion (P = .19) of response. Furthermore, PRL application significantly reduced the transient K⁺ current in 67% of SFO neurones in voltage-clamp configuration (n = 6/9, P = .02). The stability in response to PRL and expression of PRL-R in the SFO suggests that PRL function is conserved across physiological states and circulating PRL concentrations, prompting further investigations aiming to clarify the nature of PRL function in the SFO.

Prolactin receptor in regulation of neuronal excitability and channels

Prolactin (PRL) activates PRL receptor isoforms to exert regulation of specific neuronal circuitries, and to control numerous physiological and clinically-relevant functions including; maternal behavior, energy balance and food intake, stress and trauma responses, anxiety, neurogenesis, migraine and pain. PRL controls these critical functions by regulating receptor potential thresholds, neuronal excitability and/or neurotransmission efficiency. PRL also influences neuronal functions via activation of certain neurons, resulting in Ca(2+) influx and/or electrical firing with subsequent release of neurotransmitters. Although PRL was identified almost a century ago, very little specific information is known about how PRL regulates neuronal functions. Nevertheless, important initial steps have recently been made including the identification of PRL-induced transient signaling pathways in neurons and the modulation of neuronal transient receptor potential (TRP) and Ca(2+) -dependent K(+) channels by PRL. In this review, we summarize current knowledge and recent progress in understanding the regulation of neuronal excitability and channels by PRL.

EGFR tyrosine kinase regulates human small-conductance Ca2+-activated K+ (hSKCa1) channels expressed in HEK-293 cells

SKCa (small-conductance Ca(2+)-activated K(+)) channels are widely distributed in different tissues, including the brain, pancreatic islets and myocardium and play an important role in controlling electrical activity and cellular functions. However, intracellular signal modulation of SKCa channels is not fully understood. The present study was designed to investigate the potential regulation of hSKCa1 (human SKCa1) channels by PTKs (protein tyrosine kinases) in HEK (human embryonic kidney)-293 cells expressing the hSKCa1 (KCNN1) gene using approaches of whole-cell patch voltage-clamp, immunoprecipitation, Western blotting and mutagenesis. We found that the hSKCa1 current was inhibited by the broad-spectrum PTK inhibitor genistein, the selective EGFR (epidermal growth factor receptor) kinase inhibitors T25 (tyrphostin 25) and AG556 (tyrphostin AG 556), but not by the Src-family kinases inhibitor PP2. The inhibitory effect of these PTK inhibitors was significantly antagonized by the PTP (protein tyrosine phosphatase) inhibitor orthovanadate. The tyrosine phosphorylation level of hSKCa1 channels was reduced by genistein, T25 or AG556. The reduced tyrosine phosphorylation was countered by orthovanadate. Interestingly, the Y109F mutant hSKCa1 channel lost the inhibitory response to T25 or AG556, and showed a dramatic reduction in tyrosine phosphorylation levels and a reduced current density. These results demonstrate the novel information that hSKCa1 channels are inhibited by genistein, T25 and AG556 via EGFR tyrosine kinase inhibition, which is related to the phosphorylation of Tyr(109) in the N-terminus. This effect may affect electrical activity and cellular functions in brain, pancreatic islets and myocardium.

Prolactin induces a hyperpolarising current in rat paraventricular oxytocinergic neurones

Prolactin and oxytocin are important reproductive hormones implicated in several common adaptive functions during pregnancy, pseudopregnancy and lactation. Recently, extracellular recordings of supraoptic neurones have shown that prolactin may modulate the electrical activity of oxytocinergic neurones. However, no study has been conducted aiming to establish whether prolactin directly influences this activity in oxytocinergic paraventricular neurones. In the present study, we addressed this question by studying the effects of prolactin on the electrical activity and voltage-current relationship of identified paraventricular neurones in rat brain slices. Whole-cell recordings were obtained and neurones were classified on the basis of their morphological and electrophysiological fingerprint (i.e. magnocellular or parvicellular) and neuropeptide phenotype (i.e. oxytocinergic or non-oxytocinergic). We report that prolactin elicited a hyperpolarising current in 37% of the neurones in this nucleus, of which the majority (67%) were identified as putative magnocellular oxytocin neurones and the reminder (33%) were regarded as oxytocin-negative, parvicellular neuroendocrine neurones. Our results suggest that, in addition to the well-established negative feedback loop between prolactin-secreting lactotrophs and dopaminergic neurones in the arcuate nucleus, an inhibitory feedback loop also exists between lactotrophs and oxytocinergic paraventricular neurones.

Protein tyrosine kinase involvement in learning-produced changes in Hermissenda type B photoreceptors

Learning-correlated changes in the excitability and photoresponses of Hermissenda's ocular type B photoreceptors are mediated by reductions in two distinct K(+) currents, I(A) and I(K-Ca). The suppression of these K(+) currents has been linked to conditioning-produced activation of protein kinase C (PKC). The question of whether PKC accounts completely for the changes in excitability and K(+) currents or whether other kinase(s) are involved has received little attention. In the present experiments, we asked whether protein tyrosine kinases (PTKs) might also contribute to conditioning-produced alterations in B cells. We found that the PTK inhibitors genistein and lavendustin A greatly reduced cumulative depolarization of type B cells, a short-term correlate of associative learning. This disruption occurred even when PKC activation had been either occluded by preexposure of type B cells to a phorbol ester or otherwise prevented by the pseudosubstrate inhibitor peptide PKC[19-31]. PTK inhibitors also increased the amplitude of the transient (I(A)) and delayed (I(Delayed)) components of voltage-dependent K(+) current that have previously been shown to be selectively reduced by conditioning and to contribute to cumulative depolarization. Genistein partially prevented the reduction of I(A) and I(Delayed) due to in vitro conditioning and blocked the changes in their voltage dependencies. Ionophoresis of pervanadate ion, a potent inhibitor of protein tyrosine phosphatases, depolarized type B photoreceptors and occluded conditioning-produced cumulative depolarization. Pervanadate also suppressed I(A) and I(Delayed), reduced their voltage dependence, and altered inactivation kinetics for I(A), mimicking conditioning. Western blot analysis using a phosphotyrosine antibody indicated that conditioning increased the phosphotyrosine content of many proteins within the Hermissenda CNS. Collectively, our results suggest that in addition to PKC, one or more PTKs play an important role in conditioning-produced changes in type B cell excitability. PTKs and PKCs converge to effect reductions in B cell K(+) currents during conditioning, apparently through distinct biophysical mechanisms.

Differences in adrenocortical secretory and gene expression responses to stimulation in vitro by ACTH or prolactin between high- and low-avoidance Hatano rats

Rats of the Hatano high-avoidance (HAA) and low-avoidance (LAA) strains have been genetically selected on the basis of their two-way active avoidance behavior, and have different endocrine responses to stress. The present study focused on the adrenal steroid hormone responses of the Hatano strains and identifies differences in regulation of the adrenal cortex in vitro of HAA and LAA rats. Although incubation with prolactin (PRL) and/or adrenocorticotrophic hormone (ACTH) resulted in a dose-dependent increase of corticosterone and progesterone release by adrenal cells from both HAA and LAA male rats, the responses were markedly increased for adrenal cells from LAA rats as compared with HAA rats. This finding suggested that adrenal glands of HAA rats are less sensitive to PRL and/or ACTH than adrenals from LAA rats. Several possible intra-adrenal regulators were investigated. The basal level of expression of steroidogenic acute regulatory protein (StAR) and the long form of the PRL receptor (PRLR-L) mRNAs was higher in adrenals of LAA rats. ACTH treatment of adrenal cells from HAA rats resulted in statistically significant increases in melanocortin receptor 2 (MC2R) mRNA expression, while neither ACTH nor PRL altered MC2R mRNA expression in adrenal cells of LAA rats. Conversely, the increase in PRLR-L mRNA expression induced by PRL was observed only in adrenal cells from LAA rats. Treatment of adrenal cells with PRL and/or ACTH increased the expression of StAR and CYP11A1 mRNAs for both Hatano strains. However, the induction of StAR mRNA expression was higher in LAA rats, but the CYP11A1 response was lower. These findings indicate that adrenal cells of the LAA strain have higher sensitivity to secretagogues than those of the HAA strain. These results suggest that PRL may also be important in stimulating secretion of adrenal steroid hormones.

Prolactin regulation of gonadotropin-releasing hormone neurons to suppress luteinizing hormone secretion in mice

Hyperprolactinemia causes infertility, but the mechanisms involved are not known. The present study aimed to determine whether and how prolactin may influence LH secretion in the adult female mouse. Using ovariectomized, estrogen-treated (OVX+E) mice, we found that 7 d of intracerebroventricular prolactin potently suppressed serum LH levels (P < 0.05). To examine whether this central action of prolactin may involve the GnRH neurons, the effects of acute and chronic prolactin on cAMP response element-binding protein phosphorylation (pCREB) in GnRH neurons were examined using dual-label immunocytochemistry. In diestrous and OVX+E mice, a single sc injection of ovine prolactin resulted in a significant (P < 0.05) doubling of the number of GnRH neurons expressing pCREB. OVX+E mice treated with five injections of ovine prolactin over 48 h showed a 4-fold increase in the number of GnRH neurons with pCREB. To determine whether GnRH neurons might be regulated directly by prolactin, we examined prolactin receptor (PRL-R) mRNA expression in green fluorescent protein-tagged GnRH neurons by single-cell RT-PCR. As a positive control, PRL-R mRNA was measured in arcuate dopaminergic neurons obtained from green fluorescent protein-tagged tyrosine hydroxylase neurons. Three of 23 GnRH neurons (13%) were identified to express PRL-R transcripts, whereas nine of 11 arcuate dopaminergic neurons (82%) were found to coexpress PRL-R mRNA. These data demonstrate that prolactin suppresses LH levels in the mouse, as it does in other species, and indicate that it acts centrally to regulate intracellular signaling within GnRH neurons. This is likely to occur, at least in part, through the direct regulation of a subpopulation of GnRH neurons.

Involvement of JAK2 and Src kinase tyrosine phosphorylation in human growth hormone-stimulated increases in cytosolic free Ca2+and insulin secretion

We previously reported that human growth hormone (hGH) increases cytoplasmic Ca2+concentration ([Ca2+]i) and proliferation in pancreatic β-cells (Sjöholm Å, Zhang Q, Welsh N, Hansson A, Larsson O, Tally M, and Berggren PO. J Biol Chem 275: 21033–21040, 2000) and that the hGH-induced rise in [Ca2+]iinvolves Ca2+-induced Ca2+release facilitated by tyrosine phosphorylation of ryanodine receptors (Zhang Q, Kohler M, Yang SN, Zhang F, Larsson O, and Berggren PO. Mol Endocrinol 18: 1658–1669, 2004). Here we investigated the tyrosine kinases that convey the hGH-induced rise in [Ca2+]iand insulin release in BRIN-BD11 β-cells. hGH caused tyrosine phosphorylation of Janus kinase (JAK)2 and c-Src, events inhibited by the JAK2 inhibitor AG490 or the Src kinase inhibitor PP2. Although hGH-stimulated rises in [Ca2+]iand insulin secretion were completely abolished by AG490 and JAK2 inhibitor II, the inhibitors had no effect on insulin secretion stimulated by a high K+concentration. Similarly, Src kinase inhibitor-1 and PP2, but not its inactive analog PP3, suppressed [Ca2+]ielevation and completely abolished insulin secretion stimulated by hGH but did not affect responses to K+. Ovine prolactin increased [Ca2+]iand insulin secretion to a similar extent as hGH, effects prevented by the JAK2 and Src kinase inhibitors. In contrast, bovine GH evoked a rise in [Ca2+]ibut did not stimulate insulin secretion. Neither JAK2 nor Src kinase inhibitors influenced the effect of bovine GH on [Ca2+]i. Our study indicates that hGH stimulates rise in [Ca2+]iand insulin secretion mainly through activation of the prolactin receptor and JAK2 and Src kinases in rat insulin-secreting cells.

Expression of the long form of the prolactin receptor in magnocellular oxytocin neurons is associated with specific prolactin regulation of oxytocin neurons

Magnocellular neurons of the supraoptic (SON) and paraventricular nuclei (PVN) show considerable plasticity during pregnancy and lactation. Prolactin receptors (PRL-R) have been identified in both these nuclei. The aim of this study was to investigate the cell type(s) expressing mRNA for the long form of prolactin receptor (PRL-RL) and to determine whether patterns of expression change during pregnancy and lactation. In addition, we examined effects of prolactin on excitability of oxytocin and vasopressin neurons. Sections from brains of nonpregnant, pregnant, and lactating rats were hybridized with an 35S-labeled probe to label PRL-RL mRNA together with digoxigenin-labeled probes to detect either oxytocin or vasopressin mRNA. In the SON, PRL-RL mRNA was predominantly colocalized with oxytocin mRNA, with over 80% of oxytocin neurons positive for PRL-RL mRNA. Very few (<10%) vasopressin neurons expressed PRL-RL mRNA. In the PVN, PRL-RL mRNA was also predominantly found in oxytocin neurons, and the proportion of PRL-RL-positive oxytocin neurons increased significantly during pregnancy and lactation. As in the SON, relatively few vasopressin cells contained PRL-RL mRNA. For in vivo electrophysiology, nonpregnant rats were anesthetized, and then extracellular single neuron activity was recorded in identified oxytocin and vasopressin neurons. After a period of baseline recording, the effect of prolactin (1 μg icv) on firing rate was examined. Prolactin treatment of nonpregnant rats induced a significant decrease in firing rates of oxytocin neurons. There was no effect of prolactin on the activity of vasopressin neurons. Together, these data provide strong evidence that prolactin directly and specifically regulates activity of oxytocin neurons.

Protein kinase A inhibits intermediate conductance Ca2+-activated K+ channels expressed in Xenopus oocytes

Intermediate-conductance (IK) Ca(2+)-activated K(+) channels are expressed in many different cell types where they perform a variety of functions including cell volume regulation, transepithelial secretion, lymphocyte activation and cell cycle progression. IK channels are thought to be regulated by phosphorylation; however, whether kinases act directly on the channel is unclear. Using IK channels heterologously expressed in Xenopus oocytes, we demonstrate that IK channels are potently inhibited (60%) by the catalytic subunit of protein kinase A (PKA). Inhibition of IK channel current by PKA is abolished by mutation of four phosphorylation residues (S312, T327, S332, and T348) in the putative calmodulin-binding region of the channel. Evidence for direct modulation of the IK channel by PKA was further demonstrated using GST fusion proteins. The major site of phosphorylation was found to be serine 332; however, other residues were also phosphorylated. We conclude that IK channels can be directly regulated by the cAMP second-messenger system. The mechanism appears to involve direct phosphorylation by PKA of a modulatory locus in the cytoplasmic region of the channel, the site at which calmodulin is thought to interact. Modulation of IK channels by protein kinases may be an important mechanism regulating cell function.

Effects of Prolactin on Ionic Membrane Conductances in the Human Malignant Astrocytoma Cell Line U87-MG

Prolactin (PRL) is involved in numerous biological processes in peripheral tissues and the brain. Although numerous studies have been conducted to elucidate the signal transduction pathways associated with the PRL receptor, very few have examined the role of ion conductances in PRL actions. We used the patch-clamp technique in “whole cell” configuration and microspectrofluorimetry to investigate the effects of PRL on membrane ion conductances in the U87-MG human malignant astrocytoma cell line, which naturally expresses the PRL receptor. We found that a physiological concentration (4 nM) of PRL exerted a biphasic action on membrane conductances. First, PRL activated a Ca2+-dependent K+current that was sensitive to CTX and TEA. This current depended on PRL-induced Ca2+mobilization, through a JAK2-dependent pathway from a thapsigargin- and 2-APB-sensitive Ca2+pool. Second, PRL also activated an inwardly directed current, mainly due to the stimulation of calcium influx via nickel- and 2-APB-sensitive calcium channels. Both phases resulted in membrane hyperpolarizations, mainly through the activation of Ca2+-dependent K+channels. As shown by combined experiments (electrophysiology and microspectrofluorimetry), the PRL-induced Ca2+influx increased with cell membrane hyperpolarization and conversely decreased with cell membrane depolarization. Thus PRL-induced membrane hyperpolarizations facilitated Ca2+influx through voltage-independent Ca2+channels. Finally, PRL also activated a DIDS-sensitive Cl-current, which may participate in the PRL-induced hyperpolarization. These PRL-induced conductance activations are probably related to the PRL proliferative effect we have already described in U87-MG cells.

Prolactin stimulates cell proliferation through a long form of prolactin receptor and K+ channel activation

PRL (prolactin) has been implicated in the proliferation and differentiation of numerous tissues, including the prostate gland. However, the PRL-R (PRL receptor) signal transduction pathway, leading to the stimulation of cell proliferation, remains unclear and has yet to be mapped. The present study was undertaken to develop a clear understanding of the mechanisms involved in this pathway and, in particular, to determine the role of K(+) channels. We used androgen-sensitive prostate cancer (LNCaP) cells whose proliferation is known to be stimulated by PRL. Reverse transcriptase PCR analysis showed that LNCaP cells express a long form of PRL-R, but do not produce its intermediate isoform. Patch-clamp techniques showed that the application of 5 nM PRL increased both the macroscopic K(+) current amplitude and the single K(+)-channel open probability. This single-channel activity increase was reduced by the tyrosine kinase inhibitors genistein, herbimycin A and lavandustine A, thereby indicating that tyrosine kinase phosphorylation is required in PRL-induced K(+) channel stimulation. PRL enhances p59( fyn ) phosphorylation by a factor of 2 after a 10 min application in culture. In addition, where an antip59( fyn ) antibody is present in the patch pipette, PRL no longer increases K(+) current amplitude. Furthermore, the PRL-stimulated proliferation is inhibited by the K(+) channel inhibitors alpha-dendrotoxin and tetraethylammonium. Thus, as K(+) channels are known to be involved in LNCaP cell proliferation, we suggest that K(+) channel modulation by PRL, via p59( fyn ) pathway, is the primary ionic event in PRL signal transduction, triggering cell proliferation.

Regulation of Ca2+-dependent K+ current by alphavbeta3 integrin engagement in vascular endothelium

Interactions between endothelial cells and extracellular matrix proteins are important determinants of endothelial cell signaling. Endothelial adhesion to fibronectin through alpha(v)beta(3) integrins or the engagement and aggregation of luminal alpha(v)beta(3) receptors by vitronectin triggers Ca2+ influx. However, the underlying signaling mechanisms are unknown. The electrophysiological basis of alpha(v)beta(3) integrin-mediated changes in endothelial cell Ca2+ signaling was studied using whole cell patch clamp and microfluorimetry. The resting membrane potential of bovine pulmonary artery endothelial cells averaged -60 +/- 3 mV. In the absence of intracellular Ca2+ buffering, the application of soluble vitronectin (200 microg/ml) resulted in activation of an outwardly rectifying K+ current at holding potentials from -50 to +50 mV. Neither a significant shift in reversal potential (in voltage clamp mode) nor a change in membrane potential (in current clamp mode) occurred in response to vitronectin. Vitronectin-activated current was significantly inhibited by pretreatment with the alpha(v)beta(3) integrin antibody LM609 by exchanging extracellular K+ with Cs+ or by the application of iberiotoxin, a selective inhibitor of large-conductance, Ca2+-activated K+ channels. With intracellular Ca2+ buffered by EGTA in the recording pipette, vitronectin-activated K+ current was abolished. Fura-2 microfluorimetry revealed that vitronectin induced a significant and sustained increase in intracellular Ca2+ concentration, although vitronectin-induced Ca2+ current could not be detected. This is the first report to show that an endothelial cell ion channel is regulated by integrin activation, and this K+ current likely plays a crucial role in maintaining membrane potential and a Ca2+ driving force during engagement and activation of endothelial cell alpha(v)beta(3) integrin.

Coupling of c-Src to large conductance voltage- and Ca2+-activated K+ channels as a new mechanism of agonist-induced vasoconstriction

The voltage-dependent and Ca(2+)-activated K(+) channel (MaxiK, BK) and the cellular proto-oncogene pp60(c-Src) (c-Src) are abundant proteins in vascular smooth muscle. The role of MaxiK channels as a vasorelaxing force is well established, but their role in vasoconstriction is unclear. Because Src participates in regulating vasoconstriction, we investigated whether c-Src inhibits MaxiK as a mechanism for agonist-induced vasoconstriction. Functional experiments in human and rat show that inhibitors of Src (Lavendustin A, PP2) but not inactive compounds (Lavendustin B, PP3) induce a pronounced relaxation of coronary or aortic smooth muscle precontracted with 5-hydroxytriptamine, phenylephrine, or Angiotensin II. Iberiotoxin, a MaxiK blocker, antagonizes the relaxation induced by Lavendustin A or PP2, indicating that c-Src inhibits the Iberiotoxin-sensitive component, likely MaxiK channels. In agreement, coronary muscle MaxiK currents were enhanced by Lavendustin A. To investigate the molecular mechanism of c-Src action on MaxiK channels, we transiently expressed its alpha subunit, hSlo, with or without c-Src in HEK293T cells. The voltage sensitivity of hSlo was right-shifted by approximately 16 mV. hSlo inhibition by c-Src is due to channel direct phosphorylation because: (i) excised patches exposed to protein tyrosine phosphatase (CD45) resulted in a partial reversal of the inhibitory effect by approximately 10 mV, and (ii) immunoprecipitated hSlo channels were recognized by an anti-phosphotyrosine Ab. Furthermore, coexpression of hSlo and c-Src demonstrate a striking colocalization in HEK293T cells. We propose that MaxiK channels via direct c-Src-dependent phosphorylation play a significant role supporting vasoconstriction after activation of G protein-coupled receptors by vasoactive substances and neurotransmitters.

K depletion increases protein tyrosine kinase-mediated phosphorylation of ROMK

We purified His-tagged ROMK1 and carried out in vitro phosphorylation assays with (32)P-radiolabeled ATP to determine whether ROMK1 protein is a substrate for PTK. Addition of active c-Src and [(32)P]ATP to the purified ROMK1 protein resulted in the phosphorylation of the ROMK1 protein. However, c-Src did not phosphorylate R1Y337A in which tyrosine residue 337 was mutated to alanine. Furthermore, phosphopeptide mapping identified two phosphopeptides from the trypsin-digested ROMK1 protein. In contrast, no phosphorylated peptide has been found in the trypsin-digested R1Y337A protein. This suggested that two phosphorylated peptides might contain the same tyrosine residue. Also, addition of c-Src and [(32)P]ATP phosphorylated the synthesized peptide corresponding to amino acid sequence 333-362 of the COOH terminus of ROMK1. We then examined the effect of dietary K intake on the tyrosine-phosphorylated ROMK level. Although the ROMK channels pulled down by immunoprecipitation with ROMK antibody were the same from rats on a K-deficient diet or on a high-K diet, more ROMK channels were phosphorylated by PTK in rats on a K-deficient diet than those on a high-K diet. We conclude that ROMK1 can be phosphorylated by PTK and that tyrosine residue 337 is the key site for the phosphorylation. Also, the tyrosine phosphorylation of ROMK is modulated by dietary K intake. This strongly suggests that PTK is an important member of the aldosterone-independent signal transduction pathway for regulating renal K secretion.

CNTF inhibits high voltage activated Ca2+ currents in fetal mouse cortical neurones

Neurotrophic factors yield neuroprotection by mechanisms that may be related to their effects as inhibitors of apoptosis as well as their effects on ion channels. The effect of ciliary neurotrophic factor (CNTF) on high-threshold voltage-activated Ca channels in cultured fetal mouse brain cortical neurones was investigated. Addition of CNTF into serum-free growth medium resulted in delayed reduction of the Ca2+ currents. The currents decreased to 50% after 4 h and stabilized at this level during incubation with CNTF for 48 h. Following removal of CNTF the inhibition was completely reversed after 18 h. CNTF reduced the current of all pharmacological subtypes of Ca channels as shown by use of selective blockers of L, N, and P/Q type Ca channels (nifedipine, omega-conotoxin MVIIA, omega-agatoxin IVA). The Ca channel depression was mediated via the CNTF receptor, because enzymatic cleavage of the alpha-subunit glycerophosphatidylinositol anchor of the receptor eliminated the response. The CNTF effect was not elicited through pertussis toxin-sensitive G proteins. Other neurotrophic factors like neurotrophin-3 and insulin-like growth factor-I had no effect on the Ca2+ currents. These results may have important implications for the possible functions of CNTF in the nervous system, such as altered synaptic activity, neuronal excitability and susceptibility to brain ischaemia.

Effects of prolactin on intracellular calcium concentration and cell proliferation in human glioma cells

Prolactin (PRL) has several physiological effects on peripheral tissues and the brain. This hormone acts via its membrane receptor (PRL-R) to induce cell differentiation or proliferation. Using reverse transcription-polymerase chain reaction (RT-PCR) combined with Southern blot analysis, we detected PRL-R transcripts in a human glioma cell line (U87-MG) and in primary cultured human glioblastoma cells. These transcripts were deleted or not in their extracellular domains. We examined the effects of PRL on intracellular free Ca2+ concentration ([Ca2+](i)) in these cells in order to improve our understanding of the PRL transduction mechanism, which is still poorly documented. [Ca2+](i) was measured by microspectrofluorimetry using indo-1 as the Ca2+ fluorescent probe. Spatiotemporal aspects of PRL-induced Ca2+ signals were investigated using high-speed fluo-3 confocal imaging. We found that physiological concentrations (0.4-4 nM) of PRL-stimulated Ca2+ entry and intracellular Ca2+ mobilization via a tyrosine kinase-dependent mechanism. The two types of Ca2+ responses observed were distinguishable by their kinetics: one showing a slow (type I) and the other a fast (type II) increase in [Ca2+](i). The amplitude of PRL-induced Ca2+ increases may be sufficient to provoke several physiological responses, such as stimulating proliferation. Furthermore, PRL induced a dose-dependent increase in [3H]thymidine incorporation levels and in cellular growth and survival, detected by the MTT method. These data indicate that PRL induced mitogenesis of human glioma cells.

Calcium-activated potassium channel SK1- and IK1-like immunoreactivity in injured human sensory neurones and its regulation by neurotrophic factors

Calcium-activated potassium ion channels SK and IK (small and intermediate conductance, respectively) may be important in the pathophysiology of pain following nerve injury, as SK channels are known to impose a period of reduced excitability after each action potential by afterhyperpolarization. We studied the presence and changes of human SK1 (hSK1)- and hIK1-like immunoreactivity in control and injured human dorsal root ganglia (DRG) and peripheral nerves and their regulation by key neurotrophic factors in cultured rat sensory neurones. Using specific antibodies, hSK-1 and hIK-1-like immunoreactivity was detected in a majority of large and small/medium-sized cell bodies of human DRG. hSK1 immunoreactivity was decreased significantly in cell bodies of avulsed human DRG (n = 8, surgery delay 8 h to 12 months). There was a decrease in hIK1-like immunoreactivity predominantly in large cells acutely (<3 weeks after injury), but also in small/medium cells of chronic cases. Twenty-three injured peripheral nerves were studied (surgery delay 8 h to 12 months); in five of these, hIK1-like immunoreactivity was detected proximally but not distally to injury, whereas neurofilament staining confirmed the presence of nerve fibres in both regions. These five nerves, unlike the others, had all undergone Wallerian degeneration previously and the loss of hIK1-like immunoreactivity may therefore reflect reduced axonal transport of this ion channel across the injury site in regenerated fibres, as well as decreased expression in the cell body. In vitro studies of neonatal rat DRG neurones showed that nerve growth factor (NGF) significantly increased the percentage of hSK1-positive cells, whereas neurotrophin 3 (NT-3) and glial cell line-derived neurotrophic factor (GDNF) failed to show a significant effect. NT-3 stimulated hIK1 expression, while NGF and GDNF were ineffective. As expected, NGF increased expression of the voltage-gated sodium channel SNS1/PN3 in this system. Decreased retrograde transport of these neurotrophic factors in injured sensory neurones may thus reduce expression of these ion channels and increase excitability. Blockade of IK1-like and other potassium channels by aminopyridines (4-AP and 3,4-DAP) may also explain the paraesthesiae induced by these medications. Selective potassium channel openers are likely to represent novel therapies for pain following nerve injury.

Potassium channels and vascular proliferation

Potassium channels are currently the focus of much attention because of their recently discovered role in the regulation of vascular smooth muscle growth. Dramatic alterations in the expression and activity of K+ channels causing marked changes in the cell's electrical properties accompany enhanced growth of smooth muscle cells (SMCs). These findings indicate that alterations in K+ channel function are important for SMC proliferation. However, the mechanisms by which changes in K+ channel activity influence cellular growth pathways are poorly understood. The emergent electrical properties caused by modulation of K+ channels are associated with marked differences in the spatial and temporal organization of Ca2+ signaling. Thus, changes in K+ channel function may represent a universal mechanism by which Ca2+ signals are targeted towards activation of gene expression and cell growth. As enhanced growth of smooth muscle underlies many cardiovascular diseases and clinical pathologies, the identification of an important role for K+ channels in SMC proliferation indicates a new source of therapeutic targets to regulate proliferative vascular disorders.

hTid-1, a human DnaJ protein, modulates the interferon signaling pathway

The Jak family of protein-tyrosine kinases are crucial for the signaling of a large number of different polypeptide ligands, including the interferons, many cytokines, erythropoietin, and growth factors. Through their interaction with receptors, the Jaks initiate a signaling cascade resulting in the activation of gene transcription and ultimately a cellular response to various ligands. In addition to their role in cellular signaling, alteration of Jak activity has been implicated in several disease states. In identifying Jak2-interacting proteins with the yeast two-hybrid system, we cloned the human homologue of the Drosophila melanogaster tumor suppressor gene lethal () tumorous imaginal discs, which encodes the protein Tid56. Drosophila Tid56 and its human homologue hTid-1 represent members of the DnaJ family of molecular chaperones. The TID1 gene encodes two splice variants hTid-1(S) and hTid-1(L). We confirmed the interaction between Jak2 and hTid-1(S) or hTid-1(L) by immunoprecipitation from COS-1 cells expressing these proteins. The interaction between endogenous hTid-1 and Jak2 was shown in HEp2 cells. We further showed that hTid-1 interacts with the human interferon-gamma (Hu-IFN-gamma) receptor subunit IFN-gamma R2. In addition, using a chimeric construct where the extracellular domain of IFN-gamma R2 was fused to the kinase domain of Jak2, we showed that hTid-1 binds more efficiently to the chimera with an active kinase domain than to a similar construct with an inactive kinase domain. Additionally, the data demonstrate that hTid-1 isoforms as well as Jak2 interact with Hsp70/Hsc70 in vivo, and the interaction between Hsp70/Hsc70 and hTid-1 is reduced after IFN-gamma treatment. Furthermore, both hTid-1(S) and hTid-1(L) can modulate IFN-gamma-mediated transcriptional activity.

Regulation of ion channels by protein tyrosine phosphorylation

Ion channels are regulated by protein phosphorylation and dephosphorylation of serine, threonine, and tyrosine residues. Evidence for the latter process, tyrosine phosphorylation, has increased substantially since this topic was last reviewed. In this review, we present a comprehensive summary and synthesis of the literature regarding the mechanism and function of ion channel regulation by protein tyrosine kinases and phosphatases. Coverage includes the majority of voltage-gated, ligand-gated, and second messenger-gated channels as well as several types of channels that have not yet been cloned, including store-operated Ca2+ channels, nonselective cation channels, and epithelial Na+ and Cl- channels. Additionally, we discuss the critical roles that channel-associated scaffolding proteins may play in localizing protein tyrosine kinases and phosphatases to the vicinity of ion channels.

Effects of protein tyrosine kinase and protein tyrosine phosphatase on apical K(+) channels in the TAL

We have previously demonstrated that the protein level of c-Src, a nonreceptor type of protein tyrosine kinase (PTK), was higher in the renal medulla from rats on a K-deficient (KD) diet than that in rats on a high-K (HK) diet (Wang WH, Lerea KM, Chan M, and Giebisch G. Am J Physiol Renal Physiol 278: F165-F171, 2000). We have now used the patch-clamp technique to investigate the role of PTK in regulating the apical K channels in the medullary thick ascending limb (mTAL) of the rat kidney. Inhibition of PTK with herbimycin A increased NP(o), a product of channel number (N) and open probability (P(o)), of the 70-pS K channel from 0.12 to 0.42 in the mTAL only from rats on a KD diet but had no significant effect in tubules from animals on a HK diet. In contrast, herbimycin A did not affect the activity of the 30-pS K channel in the mTAL from rats on a KD diet. Moreover, addition of N-methylsulfonyl-12,12-dibromododec-11-enamide, an agent that inhibits the cytochrome P-450-dependent production of 20-hydroxyeicosatetraenoic acid, further increased NP(o) of the 70-pS K channel in the presence of herbimycin A. Furthermore, Western blot detected the presence of PTP-1D, a membrane-associated protein tyrosine phosphatase (PTP), in the renal outer medulla. Inhibition of PTP with phenylarsine oxide (PAO) decreased NP(o) of the 70-pS K channel in the mTAL from rats on a HK diet. However, PAO did not inhibit the activity of the 30-pS K channel in the mTAL. The effect of PAO on the 70-pS K channel was due to indirectly stimulating PTK because pretreatment of the mTAL with herbimycin A abolished the inhibitory effect of PAO. Finally, addition of exogenous c-Src reversibly blocked the activity of the 70-pS K channel in inside-out patches. We conclude that PTK and PTP have no effect on the low-conductance K channels in the mTAL and that PTK-induced tyrosine phosphorylation inhibits, whereas PTP-induced tyrosine dephosphorylation stimulates, the apical 70-pS K channel in the mTAL.

Regulation of ROMK1 channels by protein-tyrosine kinase and -tyrosine phosphatase

We have used the two-electrode voltage clamp technique and the patch clamp technique to investigate the regulation of ROMK1 channels by protein-tyrosine phosphatase (PTP) and protein-tyrosine kinase (PTK) in oocytes coexpressing ROMK1 and cSrc. Western blot analysis detected the presence of the endogenous PTP-1D isoform in the oocytes. Addition of phenylarsine oxide (PAO), an inhibitor of PTP, reversibly reduced K(+) current by 55% in oocytes coinjected with ROMK1 and cSrc. In contrast, PAO had no significant effect on K(+) current in oocytes injected with ROMK1 alone. Moreover, application of herbimycin A, an inhibitor of PTK, increased K(+) current by 120% and completely abolished the effect of PAO in oocytes coexpressing ROMK1 and cSrc. The effects of herbimycin A and PAO were absent in oocytes expressing the ROMK1 mutant R1Y337A in which the tyrosine residue at position 337 was mutated to alanine. However, addition of exogenous cSrc had no significant effect on the activity of ROMK1 channels in inside-out patches. Moreover, the effect of PAO was completely abolished by treatment of oocytes with 20% sucrose and 250 microg/ml concanavalin A, agents that inhibit the endocytosis of ROMK1 channels. Furthermore, the effect of herbimycin A is absent in the oocytes pretreated with either colchicine, an inhibitor of microtubules, or taxol, an agent that freezes microtubules. We conclude that PTP and PTK play an important role in regulating ROMK1 channels. Inhibiting PTP increases the internalization of ROMK1 channels, whereas blocking PTK stimulates the insertion of ROMK1 channels.

Enhancement in activities of large conductance calcium-activated potassium channels in CA1 pyramidal neurons of rat hippocampus after transient forebrain ischemia

It has been reported previously that the neuronal excitability persistently suppresses and the amplitude of fast afterhyperpolarization (fAHP) increases in CA1 pyramidal cells of rat hippocampus following transient forebrain ischemia. To understand the conductance mechanisms underlying these post-ischemic electrophysiological alterations, we compared differences in activities of large conductance Ca(2+)-activated potassium (BK(Ca)) channels in CA1 pyramidal cells acutely dissociated from hippocampus before and after ischemia by using inside-out configuration of patch clamp techniques. (1) The unitary conductance of BK(Ca) channels in post-ischemic neurons (295 pS) was higher than that in control neurons (245 pS) in symmetrical 140/140 mM K(+) in inside-out patch; (2) the membrane depolarization for an e-fold increase in open probability (P(o)) showed no significant differences between two groups while the membrane potential required to produce one-half of the maximum P(o) was more negative after ischemia, indicating no obvious changes in channel voltage dependence; (3) the [Ca(2+)](i) required to half activate BK(Ca) channels was only 1 microM in post-ischemic whereas 2 microM in control neurons, indicating an increase in [Ca(2+)](i) sensitivity after ischemia; and (4) BK(Ca) channels had a longer open time and a shorter closed time after ischemia without significant differences in open frequency as compared to control. The present results indicate that enhanced activity of BK(Ca) channels in CA1 pyramidal neurons after ischemia may partially contribute to the post-ischemic decrease in neuronal excitability and increase in fAHP.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Volume-regulated chloride conductance in the LNCaP human prostate cancer cell line

Patch-clamp recordings were used to study ion currents induced by cell swelling caused by hypotonicity in human prostate cancer epithelial cells, LNCaP. The reversal potential of the swelling-evoked current suggested that Cl(-) was the primary charge carrier (termed I(Cl,swell)). The selectivity sequence of the underlying volume-regulated anion channels (VRACs) for different anions was Br(-) approximately I(-) > Cl(-) > F(-) > methanesulfonate >> glutamate, with relative permeability numbers of 1.26, 1.20, 1.0, 0.77, 0.49, and 0.036, respectively. The current-voltage patterns of the whole cell currents as well as single-channel currents showed moderate outward rectification. Unitary VRAC conductance was determined at 9.6 +/- 1.8 pS. Conventional Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 microM) and DIDS (100 microM) inhibited whole cell I(Cl,swell) in a voltage-dependent manner, with the block decreasing from 39.6 +/- 9.7% and 71.0 +/- 11. 0% at +50 mV to 26.2 +/- 7.2% and 14.5 +/- 6.6% at -100 mV, respectively. Verapamil (50 microM), a standard Ca(2+) antagonist and P-glycoprotein function inhibitor, depressed the current by a maximum of 15%. Protein tyrosine kinase inhibitors downregulated I(Cl,swell) (genistein with an IC(50) of 2.6 microM and lavendustin A by 60 +/- 14% at 1 microM). The protein tyrosine phosphatase inhibitor sodium orthovanadate (500 microM) stimulated I(Cl,swell) by 54 +/- 11%. We conclude that VRACs in human prostate cancer epithelial cells are modulated via protein tyrosine phosphorylation.

Inhibition of Ca2+-activated K+ channels by tyrosine phosphatase inhibitors in rat mesenteric artery

To investigate the possible regulation of large-conductance Ca2+-activated K+ channels (BKCa) by tyrosine phosphatases (Tyr-PPs), single-channel currents of myocytes from rat mesenteric artery were recorded in open cell-attached patches. Two structurally different Tyr-PP inhibitors, sodium orthovanadate (Na3VO4) and dephostatin, were used. The channels (236 pS) evoked at +40 mV and pCa 6, were significantly inhibited by 1 mM Na3VO4 (-81 ± 3%, n = 10; P < 0.005). Similarly, 100 µM dephostatin strongly inhibited the BKCa channels (-80 ± 7%, n = 7 ; P < 0.05). Therefore, BKCa channels in vascular smooth muscle cells may be regulated by tyrosine phosphatase-dependent signal transduction pathways, whose inhibition could attenuate the channel activity.Key words: Ca2+-activated K+ channel, vascular smooth muscle, tyrosine phosphatase, vanadate, dephostatin.

Role of protein kinases in the prolactin-induced intracellular calcium rise in Chinese hamster ovary cells expressing the prolactin receptor

There is still only limited understanding of the early steps of prolactin signal transduction in target cells. It has been shown that prolactin actions are associated with cell protein phosphorylation, Ca2+ increases, and so on. However, the link between the activation of kinases and calcium influx or intracellular Ca2+ mobilization has not yet been clearly established. Chinese hamster ovary (CHO) cells, stably transfected with the long form of rabbit mammary gland prolactin receptor (PRL-R) cDNA were used for PRL-R signal transduction studies. Spectrofluorimetric techniques were used to measure intracellular calcium ([Ca2+]i) in cell populations with Indo1 as a calcium fluorescent probe. We demonstrate that, although protein kinase C activation (PMA or DiC8) caused a calcium influx in CHO cells, prolactin-induced PKC activation was not responsible for the early effect of prolactin on [Ca2+]i. Activation of protein kinase A (PKA) or protein kinase G did not modify [Ca2+]i and inhibition of PKA pathway did not affect the prolactin response. In the same way, phosphatidylinositol-3 kinaseinhibition had no effect on the prolactin-induced Ca2+ increase. On the other hand, tyrosine kinase inhibitors (herbimycin A, lavendustin A, and genistein) completely blocked the effect of prolactin on [Ca2+]i (influx and release). W7, a calmodulin-antagonist, and a specific inhibitor of calmodulin kinases (KN-62), only blocked prolactin-induced Ca2+ influx but had no significant effect on Ca2+ release. Using pharmacological agents, we present new data concerning the involvement of protein phosphorylations in the early effects of prolactin on ionic channels in CHO cells expressing the long form of PRL-R. Our results suggest that, at least in the very early steps of prolactin signal transduction, serine-threonine phosphorylation does not participate in the prolactin-induced calcium increase. On the other hand, tyrosine phosphorylation is a crucial, very early step, since it controls K+ channel activation, calcium influx, and intracellular calcium mobilization. Calmodulin acts later, since its inhibition only blocks the prolactin-induced Ca2+ influx.

Store depletion and store-operated Ca2+ current in human prostate cancer LNCaP cells: involvement in apoptosis

1. In the present study, we investigated the mechanisms involved in the induction of apoptosis by the Ca2+-ATPase inhibitor thapsigargin (TG), in androgen-sensitive human prostate cancer LNCaP cells. 2. Exposure of fura-2-loaded LNCaP cells to TG in the presence of extracellular calcium produced an increase in intracellular Ca2+, the first phase of which was associated with depletion of intracellular stores and the second one with consecutive extracellular Ca2+ entry through plasma membrane, store-operated Ca2+ channels (SOCs). 3. For the first time we have identified and characterized the SOC-mediated membrane current (Istore) in prostate cells using whole-cell, cell-attached, and perforated patch-clamp techniques, combined with fura-2 microspectrofluorimetric and Ca2+-imaging measurements. 4. Istore in LNCaP cells lacked voltage-dependent gating and displayed an inwardly rectifying current-voltage relationship. The unitary conductance of SOCs with 80 mM Ca2+ as a charge carrier was estimated at 3.2 +/- 0.4 pS. The channel has a high selectivity for Ca2+ over monovalent cations and is inhibited by Ni2+ (0.5-3 mM) and La3+ (1 microM). 5. Treatment of LNCaP cells with TG (0.1 microM) induced apoptosis as judged from morphological changes. Decreasing extracellular free Ca2+ to 200 nM or adding 0.5 mM Ni2+ enhanced TG-induced apoptosis. 6. The ability of TG to induce apoptosis was not reduced by loading the cells with intracellular Ca2+ chelator (BAPTA-AM). 7. These results indicate that in androgen-sensitive prostate cancer cells the depletion of intracellular Ca2+ stores may trigger apoptosis but that there is no requirement for the activation of store-activated Ca2+ current and sustained Ca2+ entry in induction and development of programmed cell death.

Hypomyelination and increased activity of voltage-gated K(+) channels in mice lacking protein tyrosine phosphatase epsilon

Protein tyrosine phosphatase epsilon (PTP epsilon) is strongly expressed in the nervous system; however, little is known about its physiological role. We report that mice lacking PTP epsilon exhibit hypomyelination of sciatic nerve axons at an early post-natal age. This occurs together with increased activity of delayed- rectifier, voltage-gated potassium (Kv) channels and with hyperphosphorylation of Kv1.5 and Kv2.1 Kv channel alpha-subunits in sciatic nerve tissue and in primary Schwann cells. PTP epsilon markedly reduces Kv1.5 or Kv2.1 current amplitudes in Xenopus oocytes. Kv2.1 associates with a substrate-trapping mutant of PTP epsilon, and PTP epsilon profoundly reduces Src- or Fyn-stimulated Kv2.1 currents and tyrosine phosphorylation in transfected HEK 293 cells. In all, PTP epsilon antagonizes activation of Kv channels by tyrosine kinases in vivo, and affects Schwann cell function during a critical period of Schwann cell growth and myelination.

Protein-tyrosine phosphatase reduces the number of apical small conductance K+ channels in the rat cortical collecting duct

Previous studies have demonstrated that an increase in the activity of protein-tyrosine kinase (PTK) is involved in the down-regulation of the activity of apical small conductance K(+) (SK) channels in the cortical collecting duct (CCD) from rats on a K(+)-deficient diet (). We used the patch clamp technique to investigate the role of protein-tyrosine phosphatase (PTP) in the regulation of the activity of SK channels in the CCD from rats on a high K(+) diet. Western blot analysis indicated that PTP-1D is expressed in the renal cortex. Application of 1 microm phenylarsine oxide (PAO) or 1 mm benzylphosphonic acid, agents that inhibit PTP, reversibly reduced channel activity by 95%. Pretreatment of CCDs with PAO for 30 min decreased the mean NP(o) reversibly from control value 3.20 to 0.40. Addition of 1 microm herbimycin A, an inhibitor of PTK, had no significant effect on channel activity in the CCDs from rats on a high K(+) diet. However, herbimycin A abolished the inhibitory effect of PAO, indicating that the effect of PAO is the result of interaction between PTK and PTP. Addition of brefeldin A, an agent that blocks protein trafficking from Golgi complex to the membrane, had no effect on channel activity. Moreover, application of colchicine, a microtubule inhibitor, or paclitaxel, a microtubule stabilizer, had no effect on channel activity. In contrast, PAO still reduced channel activity in the presence of brefeldin A, colchicine, or paclitaxel. Furthermore, the effect of PAO on channel activity was absent when the tubules were bathed in 16% sucrose-containing bath solution or treated with concanavalin A. We conclude that PTP is involved in the regulation of the activity of SK channels and that inhibition of PTP may facilitate the internalization of the SK channels.

The adipoinsular axis: effects of leptin on pancreatic beta-cells

The prevalence of obesity and related diabetes mellitus is increasing worldwide. Here we review evidence for the existence of an adipoinsular axis, a dual hormonal feedback loop involving the hormones insulin and leptin produced by pancreatic beta-cells and adipose tissue, respectively. Insulin is adipogenic, increases body fat mass, and stimulates the production and secretion of leptin, the satiety hormone that acts centrally to reduce food intake and increase energy expenditure. Leptin in turn suppresses insulin secretion by both central actions and direct actions on beta-cells. Because plasma levels of leptin are directly proportional to body fat mass, an increase of adiposity increases plasma leptin, thereby curtailing insulin production and further increasing fat mass. We propose that the adipoinsular axis is designed to maintain nutrient balance and that dysregulation of this axis may contribute to obesity and the development of hyperinsulinemia associated with diabetes.

Protein tyrosine kinase regulates the number of renal secretory K channels

The apical small conductance (SK) channel plays a key role in K secretion in the cortical collecting duct (CCD). A high-K intake stimulates renal K secretion and involves a significant increase in the number of SK channels in the apical membrane of the CCD. We used the patch-clamp technique to examine the role of protein tyrosine kinase (PTK) in regulating the activity of SK channels in the CCD. The application of 100 microM genistein stimulated SK channels in 11 of 12 patches in CCDs from rats on a K-deficient diet, and the mean increase in NP(o), a product of channel number (N) and open probability (P(o)), was 2.5. In contrast, inhibition of PTK had no effect in tubules from animals on a high-K diet in all 10 experiments. Western blot analysis further shows that the level of cSrc, a nonreceptor type of PTK, is 261% higher in the kidneys from rats on a K-deficient diet than those on a high-K diet. However, the effect of cSrc was not the result of direct inhibition of channel itself, because addition of exogenous cSrc had no effect on SK channels in inside-out patches. In cell-attached patches, application of herbimycin A increased channel activity in 14 of 16 patches, and the mean increase in NP(o) was 2.4 in tubules from rats on a K-deficient diet. In contrast, herbimycin A had no effect on channel activity in any of 15 tubules from rats on a high-K diet. Furthermore, herbimycin A pretreatment increased NP(o) per patch from the control value (0.4) to 2.25 in CCDs from rats on a K-deficient diet, whereas herbimycin failed to increase channel activity (NP(o): control, 3.10; herbimycin A, 3.25) in the CCDs from animals on a high-K diet. We conclude that PTK is involved in regulating the number of apical SK channels in the kidney.

oxLDL specifically impairs endothelium-dependent, NO-mediated dilation of coronary arterioles

Our previous studies implicated that oxidized low-density lipoprotein (oxLDL), a putative atherogenic agent, impairs endothelium-dependent, nitric oxide (NO)-mediated dilation of isolated coronary arterioles to pharmacological agonists. However, it is not known whether oxLDL specifically affects NO-mediated dilation or generally impairs endothelium-dependent function, including the release of hyperpolarizing factors. In this regard, we investigated the dilation of isolated porcine coronary arterioles (50- to 100-microm luminal diameter) in response to the activation of various endothelium-dependent pathways before and after intraluminal incubation of the vessels with oxLDL (0.5 mg protein/ml for 60 min). In the absence of oxLDL, all vessels developed basal tone and dilated in response to the activation of NO synthase (by flow and adenosine), cyclooxygenase (by arachidonic acid), cytochrome P-450 monooxygenase (by bradykinin), and endothelial membrane hyperpolarization (by sucrose-induced hyperosmolarity). Incubation of the vessels with oxLDL for 60 min did not alter basal tone but did inhibit the vasodilatory responses to increased flow and adenosine in a manner similar to that of the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester. Vasodilations in response to flow and adenosine were not affected by intraluminal incubation of the vessels with either a vehicle solution or the native LDL (0.5 mg protein/ml, 60 min). In contrast with the NO-mediated response, hyperosmotic vasodilation mediated by endothelial hyperpolarization was not affected by oxLDL. Endothelium-dependent dilations to the cyclooxygenase activator arachidonic acid and to the cytochrome P-450 monooxygenase activator bradykinin and endothelium-independent vasodilation to sodium nitroprusside were also not altered by oxLDL. Collectively, these results indicate that oxLDL has a selective effect on endothelium-dependent dilation with specific impairment of the NO-mediated response, whereas cyclooxygenase and cytochrome P-450 monooxygenase-mediated dilations are spared from this inhibitory effect. In addition, oxLDL does not appear to affect vasodilation mediated by hyperpolarization of the endothelium.

Glucocorticoid block of protein kinase C signalling in mouse pituitary corticotroph AtT20 D16:16 cells

1. The regulation of large conductance calcium- and voltage-activated potassium (BK) currents by activation of the protein kinase C (PKC) and glucocorticoid signalling pathways was investigated in AtT20 D16:16 clonal mouse anterior pituitary corticotroph cells. 2. Maximal activation of PKC using the phorbol esters, 4beta-phorbol 12-myristate, 13-acetate (PMA), phorbol 12, 13 dibutyrate (PDBu) and 12-deoxyphorbol 13-phenylacetate (dPPA) elicited a rapid, and sustained, inhibition of the outward steady-state voltage- and calcium- dependent potassium current predominantly carried through BK channels. 3. The effect of PMA was blocked by the PKC inhibitors bisindolylmaleimide I (BIS; 100 nM) and chelerythrine chloride (CHE; 25 microM) and was not mimicked by the inactive phorbol ester analogue 4alpha-PMA. 4. PMA had no significant effect on the 1 mM tetraethylammonium (TEA)-insensitive outward current or pharmacologically isolated, high voltage-activated calcium current. 5. PMA had no significant effect on steady-state outward current in cells pre-treated for 2 h with 1 microM of the glucocorticoid agonist dexamethasone. Dexamethasone had no significant effect on steady-state outward current amplitude or sensitivity to 1 mM TEA and did not block PMA-induced translocation of the phorbol ester-sensitive PKC isoforms, PKCalpha and PKCepsilon, to membrane fractions. 6. Taken together these data suggest that in AtT20 D16:16 corticotroph cells BK channels are important targets for PKC action and that glucocorticoids inhibit PKC signalling downstream of PKC activation.

HIV-1 Nef Expression Inhibits the Activity of a Ca2+-Dependent K+ Channel Involved in the Control of the Resting Potential in CEM Lymphocytes

The HIV-1 Nef protein plays an important role in the development of the pathology associated with AIDS. Despite various studies that have dealt with different aspects of Nef function, the complete mechanism by which it alters the physiology of infected cells remains to be established. Nef can associate with cell membranes, therefore supporting the hypothesis that it might interact with membrane proteins as ionic channels and modify their electrical properties. By using the patch-clamp technique, we found that Nef expression determines a 25-mV depolarization of lymphoblastoid CEM cells. Both charybdotoxin (CTX) and the membrane-permeable Ca2+ chelator BAPTA/AM depolarized the membrane of native cells without modifying that of Nef-transfected cells. These data suggested that the resting potential in native CEM cells is settled by a CTX- and Ca2+-sensitive K+ channel (KCa,CTX), whose activity is absent in Nef-expressing cells. This was confirmed by direct measurements of whole-cell KCa,CTX currents. Single-channel recordings on excised patches showed that a KCa,CTX channel of 35 pS with a half-activation near 400 nM Ca2+ was present in both native and Nef-transfected cells. The measurements of free intracellular Ca2+ were not different in the two cell lines, but Nef-transfected cells displayed an increased Ca2+ content in ionomycin-sensitive stores. Taken together, these results indicate that Nef expression alters the resting membrane potential of the T lymphocyte cell line by inhibiting a KCa,CTX channel, possibly by intervening in the regulation of intracellular Ca2+ homeostasis.

Cyclic GMP-dependent protein kinase activates cloned BKCa channels expressed in mammalian cells by direct phosphorylation at serine 1072

NO-induced activation of cGMP-dependent protein kinase (PKG) increases the open probability of large conductance Ca2+-activated K+ channels and results in smooth muscle relaxation. However, the molecular mechanism of channel regulation by the NO-PKG pathway has not been determined on cloned channels. The present study was designed to clarify PKG-mediated modulation of channels at the molecular level. The cDNA encoding the alpha-subunit of the large conductance Ca2+-activated K+ channel, cslo-alpha, was expressed in HEK293 cells. Whole cell and single channel characteristics of cslo-alpha exhibited functional features of native large conductance Ca2+-activated K+ channels in smooth muscle cells. The NO-donor sodium nitroprusside increased outward current 2.3-fold in whole cell recordings. In cell-attached patches, sodium nitroprusside increased the channel open probability (NPo) of cslo-alpha channels 3.3-fold without affecting unitary conductance. The stimulatory effect of sodium nitroprusside was inhibited by the PKG-inhibitor KT5823. Direct application of PKG-Ialpha to the cytosolic surface of inside-out patches increased NPo 3.2-fold only in the presence of ATP and cGMP without affecting unitary conductance. A point mutation of cslo-alpha in which Ser-1072 (the only optimal consensus sequence for PKG phosphorylation) was replaced by Ala abolished the PKG effect on NPo in inside-out patches and the effect of SNP in cell attached patches. These results indicate that PKG activates cslo-alpha by direct phosphorylation at serine 1072.

Tyrosine kinase involvement in apamin-sensitive inhibitory responses of rat distal colon

1. It has been suggested that pituitary adenylate cyclase activating peptide (PACAP) may be involved in the non-adrenergic, non-cholinergic (NANC) inhibitory response of longitudinal muscle of rat distal colon. In this study, we have investigated the intracellular mechanism of PACAP-induced relaxation in this muscle. 2. PACAP induced an apamin-sensitive relaxation of the longitudinal muscle. The tyrosine kinase inhibitors genistein at 10 microM and tyrphostin 25 at 30 microM, but not the cyclic AMP-dependent protein kinase inhibitor Rp-8-bromoadenosine-3',5'-cyclic monophosphorothioate at 30 microM significantly inhibited the PACAP-induced relaxation to 60% and 25% of control values, respectively. PACAP did not increase the cyclic AMP content of the muscle. 3. Tyrphostin 25 at 10 microM significantly inhibited the relaxation of longitudinal muscle induced by electrical field stimulation (EFS), to 50% of control values. Apamin at 1 microM, an antagonist of small conductance Ca2+-activated K+ channels, also inhibited the relaxation, to 42 % of control values. The inhibitory effects of tyrphostin 25 and apamin were not additive (44 % of control values). 4. PACAP induced an apamin-sensitive, slow hyperpolarization of the cell membrane of the muscle. Tyrphostin 25 at 3 microM inhibited this PACAP-induced hyperpolarization. Tyrphostin 25 at 10 microM and genistein at 10 microM inhibited the apamin-sensitive inhibitory junction potentials induced by a single pulse of EFS. 5. The PACAP-induced relaxation of longitudinal muscle occurred with a concomitant decrease in intracellular Ca2+ levels ([Ca2+]i). Tyrphostin 25 at 10 microM and apamin at 1 microM abolished these PACAP-induced responses. 6. From these findings it is suggested that the activation of tyrosine kinase is involved in PACAP-induced relaxation of longitudinal muscle from rat distal colon, 'upstream of' the activation of apamin-sensitive K+ channels.

Noncatalytic inhibition of cyclic nucleotide-gated channels by tyrosine kinase induced by genistein

Rod photoreceptor cyclic nucleotide-gated (CNG) channels are modulated by tyrosine phosphorylation. Rod CNG channels expressed in Xenopus oocytes are associated with constitutively active protein tyrosine kinases (PTKs) and protein tyrosine phosphatases that decrease and increase, respectively, the apparent affinity of the channels for cGMP. Here, we examine the effects of genistein, a competitive inhibitor of the ATP binding site, on PTKs. Like other PTK inhibitors (lavendustin A and erbstatin), cytoplasmic application of genistein prevents changes in the cGMP sensitivity that are attributable to tyrosine phosphorylation of the CNG channels. However, unlike these other inhibitors, genistein also slows the activation kinetics and reduces the maximal current through CNG channels at saturating cGMP. These effects occur in the absence of ATP, indicating that they do not involve inhibition of a phosphorylation event, but rather involve an allosteric effect of genistein on CNG channel gating. This could result from direct binding of genistein to the channel; however, the time course of inhibition is surprisingly slow (>30 s), raising the possibility that genistein exerts its effects indirectly. In support of this hypothesis, we find that ligands that selectively bind to PTKs without directly binding to the CNG channel can nonetheless decrease the effect of genistein. Thus, ATP and a nonhydrolyzable ATP derivative competitively inhibit the effect of genistein on the channel. Moreover, erbstatin, an inhibitor of PTKs, can noncompetitively inhibit the effect of genistein. Taken together, these results suggest that in addition to inhibiting tyrosine phosphorylation of the rod CNG channel catalyzed by PTKs, genistein triggers a noncatalytic interaction between the PTK and the channel that allosterically inhibits gating.

Distinct cytoplasmic regions of the prolactin receptor are required for prolactin-induced calcium entry

Two cytoplasmic regions of the prolactin (PRL) receptor are well documented for their participation in PRL signal transduction, the membrane proximal box 1 and the COOH-terminal region. In order to study the role of these regions in PRL-induced Ca2+ increase, we use Chinese hamster ovary (CHO) cells stably transfected with mutated PRL receptor cDNA. These cells express the long form of PRL receptor deleted from box 1 (CHO Delta1 cells) or the 141 amino acids of the COOH-terminal region (CHO H3 cells). The patch-clamp technique in "whole-cell" configuration and microfluorimetric techniques were used singly or in combination. Data obtained for these cells were compared with those we have recently published using CHO cells expressing the wild-type long form of the PRL receptor (CHO TSE32). In contrast to CHO TSE32 cells, exposure of CHO Delta1 or H3 cells to PRL (0.05-50 nM) did not modify [Ca2+]i. We have previously shown that the PRL-induced calcium influx via voltage-insensitive, Ca2+ channels was due to the activation of tyrosine kinase-dependent K+ channels that hyperpolarize the CHO TSE32 cell membrane (hyperpolarization-driven Ca2+ influx). Therefore, two events are involved in PRL-induced Ca2+ changes (i) JAK2-activation of K+ channels and (ii) intracellular messenger-opening of Ca2+ channels. In CHO Delta1 cells, PRL (0.05-50 nM) neither hyperpolarized the membrane potential nor stimulated the JAK2-dependent K+ current, confirming the pivotal role played by box 1/JAK2 in the PRL-induced activation of K+ channels. However, when these cells were voltage-clamped below the resting membrane potential, application of 5 nM PRL resulted in an increase in Ca2+ influx. Therefore, box 1/JAK2 was not involved in the opening of these Ca2+ channels. In CHO H3 cells, 5 nM PRL activated the K+ current and hyperpolarized the membrane potential without any effect on [Ca2+]i. Moreover, PRL was also ineffective on CHO H3 cells voltage-clamped below the resting membrane potential. Therefore, the COOH-terminal region is involved in the production of the intracellular messenger that opens voltage-independent Ca2+ channels. We conclude from these findings that box 1 and COOH-terminal regions are both needed for PRL-induced Ca2+ changes.

Constitutive activation of delayed-rectifier potassium channels by a src family tyrosine kinase in Schwann cells

In the nervous system, Src family tyrosine kinases are thought to be involved in cell growth, migration, differentiation, apoptosis, as well as in myelination and synaptic plasticity. Emerging evidence indicates that K+ channels are crucial targets of Src tyrosine kinases. However, most of the data accumulated so far refer to heterologous expression, and native K+-channel substrates of Src or Fyn in neurons and glia remain to be elucidated. The present study shows that a Src family tyrosine kinase constitutively activates delayed-rectifier K+ channels (IK) in mouse Schwann cells (SCs). IK currents are markedly downregulated upon exposure of cells to the tyrosine kinase inhibitors herbimycin A and genistein, while a potent upregulation of IK is observed when recombinant Fyn kinase is introduced through the patch pipette. The Kv1.5 and Kv2.1 K+-channel alpha subunits are constitutively tyrosine phosphorylated and physically associate with Fyn both in cultured SCs and in the sciatic nerve in vivo. Kv2.1- channel subunits are found to interact with the Fyn SH2 domain. Inhibition of Schwann cell proliferation by herbimycin A and by K+-channel blockers suggests that the functional linkage between Src tyrosine kinases and IK channels could be important for Schwann cell proliferation and the onset of myelination.

Role of tyrosine phosphorylation in leptin activation of ATP-sensitive K+ channels in the rat insulinoma cell line CRI-G1

1. Using whole-cell and cell-attached recording configurations, the role of phosphorylation in leptin activation of ATP-sensitive K+ (KATP) channels was examined in the rat CRI-G1 insulinoma cell line. 2. Whole-cell current clamp recordings demonstrated that, following dialysis with the non-hydrolysable ATP analogue 5'-adenylylimidodiphosphate (AMP-PNP; 3-5 mM), the leptin-induced hyperpolarization and increase in K+ conductance were completely inhibited. 3. Under current clamp conditions, application of the broad-spectrum protein kinase inhibitor H-7 (10 microM) had no effect on the resting membrane potential or slope conductance of CRI-G1 insulinoma cells and did not occlude the actions of leptin. 4. Application of the tyrosine kinase inhibitors genistein (10 microM), tyrphostin B42 (10 microM) and herbimycin A (500 nM) all resulted in activation of KATP channels. In cell-attached recordings, the presence of tyrphostin B42 (10 microM) in the pipette solution activated tolbutamide-sensitive KATP channels in CRI-G1 cells. In contrast, the inactive analogues of genistein and tyrphostin B42 were without effect. 5. The serine/threonine-specific protein phosphatase inhibitors okadaic acid (50 nM) and cyclosporin A (1 microM) did not prevent or reverse leptin activation of KATP channels. In contrast, whole-cell dialysis with the tyrosine phosphatase inhibitor orthovanadate (500 microM) prevented the actions of both leptin and tyrphostin B42. 6. In conclusion, leptin activation of KATP channels appears to require inhibition of tyrosine kinases and subsequent dephosphorylation. This process is likely to occur prior to activation of phosphoinositide 3-kinase (PI 3-kinase) as wortmannin prevented activation of KATP channels by tyrphostin B42.

Effect of inhibition of tyrosine phosphatases on voltage-operated calcium channel currents in rabbit isolated ear artery cells

1. The effect of increasing cellular tyrosine phosphorylation by inhibiting endogenous tyrosine phosphatases was examined on voltage-operated calcium channel currents in vascular smooth muscle cells. 2. In single ear artery smooth muscle cells of the rabbit, studied by the whole cell voltage clamp technique, intracellular application of the tyrosine phosphatase inhibitors, sodium orthovanadate (100 microM) and peroxyvanadate (100 microM orthovanadate + 1 mM H2O2) increased voltage-operated calcium channel currents by 56% and 83%, respectively. 3. Bath application of two other membrane permeant tyrosine phosphatase inhibitors, phenylarsine oxide (100 microM) and dephostatin (50 microM) also increased voltage-operated calcium channel currents by 48% and 52%, respectively. 4. The selective tyrosine kinase inhibitor, tyrphostin-23 (100 microM) reduced calcium channel currents by 41%. Pre-incubation with tyrphostin-23 abolished the effects of peroxyvanadate, phenylarsine oxide and dephostatin on calcium channels. 5. Western blot analysis of rabbit ear artery cell lysates showed increased tyrosine phosphorylation of several endogenous proteins following treatment with peroxyvanadate. 6. These results indicate that a number of structurally dissimilar inhibitors of tyrosine phosphatases increase voltage-operated calcium channel currents in arterial smooth muscle cells presumably due to increased tyrosine phosphorylation.

Long-term exposure to retinoic acid induces the expression of IRK1 channels in HERG channel-endowed neuroblastoma cells

The modulation of inward K+ conductances was studied during neuronal differentiation of human SH-SY5Y neuroblastoma cells. Under standard culture conditions, these cells express the herg gene, and the HERG current is the main inward K+ current regulating their Vrest. After 10-20 days exposure to Retinoic Acid (RA), SH-SY5Y cells showed, in addition to HERG currents, a novel current characterized by inward rectification, dependence on the extracellular K+ concentration, and blockade by Cs+ and Ba2+, the main features of the IRK1 current. The appearance of this current is accompanied by a strong hyperpolarisation of Vrest. RT-PCR experiments confirmed that a transcript of the IRK1 (Kir 2.1) gene actually appears in SH-SY5Y cells treated for 10-20 days with RA. On the whole, data here presented demonstrate that RA-induced neuronal differentiation of neuroblastoma cells is accompanied by the switch from a HERG-driven to a IRK1-driven control of Vrest, similarly to what happens in normal differentiating neurons; however, in tumor cells, this switch does not imply the abolition of HERG channel expression.

Single-channel activity of the Ca2+-dependent K+ channel is modulated by FK506 and rapamycin

Single-channel patch clamp recordings were performed in primary cultured neurons from rat dorsal hippocampi. Ca2+-dependent and TEA-sensitive K+ current was recorded from the neurons. Application of immunosuppressants FK506 and rapamycin to the channel inside the plasma membrane of the neurons significantly prolonged the mean open time of the channel. Calcineurin autoinhibitory fragment and W-7 induced no significant alteration in the mean open time of the channel. These results suggest that modulation of the activity of the Ca2+-dependent K+ channel by FK506 and rapamycin is directly through association of immunosuppressants with FKBP12.

Voltage-activated potassium channels in mammalian neurons and their block by novel pharmacological agents

1. Electrophysiological studies have shown that a number of different types of potassium (K) channel currents exist in mammalian neurons. Among them are the voltage-gated K channel-currents which have been classified as fast-inactivating A-type currents (KA) and slowly inactivating delayed-rectifier type currents (KDR). 2. Two major molecular superfamilies of K channel have been identified; the KIR superfamily and the Shaker-related superfamily with a number of different pore-forming alpha-subunits in each superfamily. 3. Within the Shaker-related superfamily are the KV family, comprising of at least 18 different alpha-subunits that almost certainly underlie classically defined KA and KDR currents. However, the relationship between each of these cloned alpha-subunits and native voltage-gated K currents remains, for the most part, to be established. 4. Classical pharmacological blockers of voltage-gated K channels such as tetraethylammonium ions (TEA), 4-aminopyridine (4-AP), and certain toxins lack selectivity between different native channel currents and between different cloned K channel currents. 5. A number of other agents block neuronal voltage-gated K channels. All of these compounds are used primarily for other actions they possess. They include organic calcium (Ca) channel blockers, divalent and trivalent metal ions and certain calcium signalling agents such as caffeine. 6. A number of clinically active tricyclic compounds such as imipramine, amitriptyline, and chlorpromazine are also potent inhibitors of neuronal voltage-gated K channels. These compounds are weak bases and it appears that their uncharged form is required for activity. These compounds may provide a useful starting point for the rational design of novel selective K channel blocking agents.