Deletion of CD47 from Schwann cells and macrophages hastens myelin disruption/dismantling and scavenging in Schwann cells and augments myelin debris phagocytosis in macrophages

BACKGROUND

Myelin that surrounds axons breaks in trauma and disease; e.g., peripheral nerve and spinal cord injuries (PNI and SCI) and multiple sclerosis (MS). Resulting myelin debris hinders repair if not effectively scavenged by Schwann cells and macrophages in PNI and by microglia in SCI and MS. We showed previously that myelin debris evades phagocytosis as CD47 on myelin ligates SIRPα (signal regulatory protein-α) on macrophages and microglia, triggering SIRPα to inhibit phagocytosis in phagocytes. Using PNI as a model, we tested the in vivo significance of SIRPα-dependent phagocytosis inhibition in SIRPα null mice, showing that SIRPα deletion leads to accelerated myelin debris clearance, axon regeneration and recovery of function from PNI. Herein, we tested how deletion of CD47, a SIRPα ligand and a cell surface receptor on Schwann cells and phagocytes, affects recovery from PNI.

METHODS

Using CD47 null (CD47-/-) and wild type mice, we studied myelin disruption/dismantling and debris clearance, axon regeneration and recovery of function from PNI.

RESULTS

As expected from CD47 on myelin acting as a SIRPα ligand that normally triggers SIRPα-dependent phagocytosis inhibition in phagocytes, myelin debris clearance, axon regeneration and function recovery were all faster in CD47-/- mice than in wild type mice. Unexpectedly compared with wild type mice, myelin debris clearance started sooner and CD47-deleted Schwann cells displayed enhanced disruption/dismantling and scavenging of myelin in CD47-/- mice. Furthermore, CD47-deleted macrophages from CD47-/- mice phagocytosed more myelin debris than CD47-expressing phagocytes from wild type mice.

CONCLUSIONS

This study reveals two novel normally occurring CD47-dependent mechanisms that impede myelin debris clearance. First, CD47 expressed on Schwann cells inhibits myelin disruption/dismantling and debris scavenging in Schwann cells. Second, CD47 expressed on macrophages inhibits myelin debris phagocytosis in phagocytes. The two add to a third mechanism that we previously documented whereby CD47 on myelin ligates SIRPα on macrophages and microglia, triggering SIRPα-dependent phagocytosis inhibition in phagocytes. Thus, CD47 plays multiple inhibitory roles that combined impede myelin debris clearance, leading to delayed recovery from PNI. Similar inhibitory roles in microglia may hinder recovery from other pathologies in which repair depends on efficient phagocytosis (e.g., SCI and MS).

Natural loss of function of ephrin-B3 shapes spinal flight circuitry in birds

Flight in birds evolved through patterning of the wings from forelimbs and transition from alternating gait to synchronous flapping. In mammals, the spinal midline guidance molecule ephrin-B3 instructs the wiring that enables limb alternation, and its deletion leads to synchronous hopping gait. Here, we show that the ephrin-B3 protein in birds lacks several motifs present in other vertebrates, diminishing its affinity for the EphA4 receptor. The avian ephrin-B3 gene lacks an enhancer that drives midline expression and is missing in galliforms. The morphology and wiring at brachial levels of the chicken embryonic spinal cord resemble those of ephrin-B3 null mice. Dorsal midline decussation, evident in the mutant mouse, is apparent at the chick brachial level and is prevented by expression of exogenous ephrin-B3 at the roof plate. Our findings support a role for loss of ephrin-B3 function in shaping the avian brachial spinal cord circuitry and facilitating synchronous wing flapping.

Deletion of SIRPα (signal regulatory protein-α) promotes phagocytic clearance of myelin debris in Wallerian degeneration, axon regeneration, and recovery from nerve injury

BACKGROUND

Recovery of function from traumatic nerve injury depends on the ability of severed axons to grow/regenerate back to their target tissues. This is achieved by successfully crossing the lesion site where physical impact severed axons, determined by the type of trauma, followed by successfully growing throughout the Wallerian degenerating nerve segment located distal to and beyond the lesion site, determined by the nature of Wallerian degeneration. The protracted removal of myelin debris in Wallerian degeneration, which leads residual myelin debris to slow down axon growth, impedes recovery of function. We focused in this study on mechanism(s) that delay the removal of myelin debris in Wallerian degeneration and so impede recovery. Previously, we showed that myelin debris inhibited its own phagocytosis in primary cultured macrophages and microglia as CD47 on myelin ligated SIRPα (signal regulatory protein-α) on phagocytes, and sequentially, SIRPα generated "don't eat me" signaling. We also demonstrated that serum inhibited phagocytosis in a SIRPα-dependent manner. Herein, we aimed to determine whether SIRPα-dependent inhibition of phagocytosis in macrophages impedes the in vivo removal of myelin debris in Wallerian degeneration, further leading to impaired healing.

METHODS

Using SIRPα null (SIRPα-/-) and littermate wild-type (SIRPα+/+) mice, we studied the recovery of sensory and motor functions from nerve injury and, further, axon regeneration, SIRPα expression, myelin debris removal, and the phagocytic capacity and presence of macrophages in Wallerian degeneration.

RESULTS

Myelin debris removal, axon regeneration, and the recovery of functions were all faster in SIRPα-/- mice than in wild-type mice. Between the two cell types that mostly scavenge myelin debris, macrophages but not Schwann cells expressed SIRPα in wild-type mice, and furthermore, SIRPα-/- macrophages phagocytosed significantly more than wild-type macrophages.

CONCLUSIONS

Our findings suggest an intrinsic normally occurring SIRPα-dependent mechanism that impedes the in vivo removal of myelin debris in Wallerian degeneration by inhibiting the phagocytosis of myelin debris in macrophages, hence preventing fast growing axons from fully implementing their regenerative potential. Thus, accelerating the removal of myelin debris by eliminating SIRPα-dependent inhibition of phagocytosis will most likely advance recovery of functions from nerve injury.

Prostaglandin E2 stimulates cystogenesis through EP4 receptor in IMCD-3 cells

Previously, we demonstrated that prostaglandin E(2) (PGE(2)) induced cAMP and cyst formation through PGE(2) receptor-2 (EP2) activity in human autosomal-dominant polycystic kidney disease (ADPKD) epithelial cells. In this study, we determined the role of EP2 and EP4 receptors in mediating PGE(2) stimulation of cAMP signaling and cystogenesis in mouse renal epithelial cells using the inner medullary collecting duct-3 (IMCD-3) cell line. In contrast to human ADPKD cells, using novel EP2 and EP4 antagonists, we found that IMCD-3 cells expressed functional EP4 but not EP2, which stimulated cAMP formation and led to cyst formation in 3D culture system. The involvement of EP4 receptors in IMCD-3 cells was further supported by the specific effect of EP4 siRNA that inhibited PGE(2)-induced cystogenesis. We also observed different cellular localization of EP2 or EP4 receptors in IMCD-3 transfected cells. Collectively, our results suggest an important role of different expression of EP2 or EP4 receptors in the regulation of cystogenesis.

MKL1 mediates TGF-β1-induced α-smooth muscle actin expression in human renal epithelial cells

Transforming growth factor-β1 (TGF-β1) is known to induce epithelial-mesenchymal transition in the kidney, a process involved in tubulointerstitial fibrosis. We hypothesized that a coactivator of the serum response factor (SRF), megakaryoblastic leukemia factor-1 (MKL1), stimulates α-smooth muscle actin (α-SMA) transcription in primary cultures of renal tubular epithelial cells (RTC), which convert into myofibroblasts on treatment with TGF-β1. Herein, we study the effect of MKL1 expression on α-SMA in these cells. We demonstrate that TGF-β1 stimulation of α-SMA transcription is mediated through CC(A/T)6-rich GG elements known to bind to SRF. These elements also mediate the MKL1 effect that dramatically activates α-SMA transcription in serum-free media. MKL1 fused to green fluorescent protein localizes to the nucleus and induces α-SMA expression regardless of treatment with TGF-β1. Using proteasome inhibitors, we also demonstrate that the proteolytic ubiquitin pathway regulates MKL1 expression. These data indicate that MKL1 overexpression is sufficient to induce α-SMA expression. Inhibition of endogenous expression of MKL1 by small interfering RNA abolishes TGF-β1 stimulation of α-SMA expression. Therefore, MKL1 is also absolutely required for TGF-β1 stimulation of α-SMA expression. Western blot and immunofluorescence analysis show that overexpressed and endogenous MKL1 are located in the nucleus in non-stimulated RTC. Chromatin immunoprecipitation assay demonstrates that TGF-β1 induces binding of endogenous SRF and MKL1 to the α-SMA promoter in chromatin. Since MKL1 constitutes a potent factor regulating α-SMA expression, modulation of endogenous MKL1 expression or activity may have a profound effect on myofibroblast formation and function in the kidney.

EP2 receptor mediates PGE2-induced cystogenesis of human renal epithelial cells

Autosomal-dominant polycystic kidney disease (ADPKD) is characterized by formation of cysts from tubular epithelial cells. Previous studies indicate that secretion of prostaglandin E2 (PGE2) into cyst fluid and production of cAMP underlie cyst expansion. However, the mechanism by which PGE2 directly stimulates cAMP formation and modulates cystogenesis is still unclear, because the particular E-prostanoid (EP) receptor mediating the PGE2 effect has not been characterized. Our goal is to define the PGE2 receptor subtype involved in ADPKD. We used a three-dimensional cell-culture system of human epithelial cells from normal and ADPKD kidneys in primary cultures to demonstrate that PGE2 induces cyst formation. Biochemical evidence gathered by using real-time RT-PCR mRNA analysis and immunodetection indicate the presence of EP2 receptor in cystic epithelial cells in ADPKD kidney. Pharmacological evidence obtained by using PGE2-selective analogs further demonstrates that EP2 mediates cAMP formation and cystogenesis. Functional evidence for a role of EP2 receptor in mediating cAMP signaling was also provided by inhibiting EP2 receptor expression with transfection of small interfering RNA in cystic epithelial cells. Our results indicate that PGE2 produced in cyst fluid binds to adjacent EP2 receptors located on the apical side of cysts and stimulates EP2 receptor expression. PGE2 binding to EP2 receptor leads to cAMP signaling and cystogenesis by a mechanism that involves protection of cystic epithelial cells from apoptosis. The role of EP2 receptor in mediating the PGE2 effect on stimulating cyst formation may have direct pharmacological implications for the treatment of polycystic kidney disease.

Limitations of Commonly Used Internal Controls for Real-Time RT-PCR Analysis of Renal Epithelial-Mesenchymal Cell Transition

BACKGROUND/AIMS

Progressive renal fibrotic disease is accompanied by the massive accumulation of myofibroblasts as defined by alpha smooth muscle actin (alphaSMA) expression. We quantitated gene expression using real-time RT-PCR analysis during conversion of primary cultured human renal tubular cells (RTC) to myofibroblasts after treatment with transforming growth factor-beta1 (TGF-beta1). We report herein the limitations of commonly used reference genes for mRNA quantitation.

METHODS

We determined the expression of alphaSMA and megakaryoblastic leukemia-1 (MKL1), a transcriptional regulator of alphaSMA, by quantitative real-time PCR using three common internal controls, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), cyclophilin A and 18S rRNA.

RESULTS

Expression of GAPDH mRNA and cyclophilin A mRNA, and to a lesser extent, 18S rRNA levels varied over time in culture and with exposure to TGF-beta1. Thus, depending on which reference gene was used, TGF-beta1 appeared to have different effects on expression of MKL1 and alphaSMA.

CONCLUSIONS

RTC converting to myofibroblasts in primary culture is a valuable system to study renal fibrosis in humans. However, variability in expression of reference genes with TGF-beta1 treatment illustrates the need to validate mRNA quantitation with multiple reference genes to provide accurate interpretation of fibrosis studies in the absence of a universal internal standard for mRNA expression.

Homologous and heterologous regulation of somatostatin receptor 2

We previously demonstrated that phosphorylation of somatostatin receptor 2A (sst2A) is rapidly increased in transfected cells both by agonist and by the protein kinase C (PKC) activator phorbol myristate acetate (PMA). Here, we investigate whether PKC-mediated receptor phosphorylation is involved in the homologous or heterologous regulation of endogenous sst2 receptors in AR42J pancreatic acinar cells upon stimulation by agonist or by cholecystokinin (CCK) or bombesin (BBS). Somatostatin, PMA, CCK, and BBS all increased sst2A receptor phosphorylation 5- to 10-fold within minutes. Somatostatin binding also caused rapid internalization of the ligand-receptor complex, and PMA, CCK, and BBS all stimulated this internalization further. Additionally, sst2 receptor-mediated inhibition of adenylyl cyclase was desensitized by all treatments. Somatostatin, as well as peptidic (SMS201-995) and nonpeptidic (L-779,976) sst2 receptor agonists increased the EC(50) for somatostatin inhibition 20-fold. In contrast, pretreatment with BBS, CCK, or PMA caused a modest 2-fold increase in the EC(50) for cyclase inhibition. Whereas the PKC inhibitor GF109203X abolished sst2A receptor phosphorylation by CCK, BBS, and PMA, it did not alter the effect of somatostatin, demonstrating that these reactions were catalyzed by different kinases. Consistent with a functional role for PKC-mediated receptor phosphorylation, GF109203X prevented PMA stimulation of sst2 receptor internalization. Surprisingly, however, GF109203X did not inhibit BBS and CCK stimulation of sst2A receptor endocytosis. These results demonstrate that homologous and heterologous hormones induce sst2A receptor phosphorylation by PKC-independent and -dependent mechanisms, respectively, and produce distinct effects on receptor signaling and internalization. In addition, the heterologous hormones also modulate sst2 receptor internalization by a novel mechanism that is independent of receptor phosphorylation.

High insulin requirements and poor metabolic control do not modify the expression, regulation and PKC mediated activation of the p21ras pathway in PBMC from type II diabetic patients

AIMS

To asses whether clinically severe insulin resistance and poor metabolic control in patients with type 11 diabetes are associated with aberrant expression or function of the p21ras pathway.

METHODS

We examined the expression and function of the p21ras pathway in resting and activated PBMC from 10 insulin treated patients with type II diabetes characterized by high insulin requirements and poor metabolic control (IR group) and 10 age and sex matched well controlled patients treated by diet alone or oral hypoglycemic medications (WC group).

RESULTS

Levels of p21ras and its regulatory elements: p21rasGAP and hSOS1, were comparable in the two groups. The induced activities of p21ras and its associated down-stream regulatory enzyme MAP-kinase following TPA stimulation were also comparable in the IR and WC patients.

CONCLUSIONS

Taken together, these data indicate that clinically significant severe insulin resistance does not modify the expression, regulation and activation of p21ras pathway in PBMC of patients with type II diabetes.

Modulation of the murine peroxisome proliferator-activated receptor gamma 2 promoter activity by CCAAT/enhancer-binding proteins

Peroxisome proliferator-activated receptor gamma (PPARgamma) and CCAAT/enhancer-binding proteins (C/EBPs) are transcriptional regulators essential for adipocyte differentiation and function. Previous findings indicate that PPARgamma2 transcription is regulated by members of the C/EBP family. We demonstrate here that C/EBPalpha and C/EBPdelta, but not C/EBPbeta, induce the activity of the PPARgamma2 promoter in transiently transfected 3T3-L1 preadipocytes and bind to two juxtaposed low affinity C/EBP binding sites. Results obtained with chimeras containing interchanged C/EBPalpha-C/EBPbeta N-terminal transactivation domain and C-terminal DNA binding dimerization domain indicate that the N-terminal part of C/EBPbeta prevents it from binding to the PPARgamma2 promoter. Indeed, deletion mutants of C/EBPbeta lacking the N-terminal part of the molecule are able to bind to the PPARgamma2 promoter. We further demonstrate that deletion of a region located between amino acids 184-212, upstream of the DNA binding domain, permits C/EBPbeta binding to the PPARgamma2 promoter, implicating an inhibitory region in C/EBPbeta for modulating DNA binding specificity to the PPARgamma2 promoter. In summary, this study indicates that C/EBPbeta but not C/EBPalpha or C/EBPdelta is unable to bind to C/EBP binding sites in the mouse PPARgamma2 promoter. The lack of binding is due to a region N-terminal of the C/EBPbeta DNA binding domain. Our findings illustrate a mechanism by which C/EBP isoforms differentially modulate the transactivation of the PPARgamma2 promoter.

Vanadate activates membranous nonreceptor protein tyrosine kinase in rat adipocytes

The insulin-like effects of vanadate are independent of the insulin receptor and insulin receptor substrate 1 (IRS-1) phosphorylation. A cytosolic protein tyrosine kinase (CytPTK), sensitive to inhibition by nanomolar concentrations of staurosporine (concentration at which 50% inhibition occurs [IC50], 1-2 nmol/l), has been implicated in some (i.e., glucose oxidation, lipogenesis) but not all (i.e., hexose uptake, inhibition of lipolysis) of the insulin-like effects of vanadate. We report here the existence of another nonreceptor protein tyrosine kinase in rat adipocytes, located exclusively in the plasma membranes (MembPTK), which we suggest is associated with hexose uptake and the antilipolytic activity of vanadate. MembPTK is a nonglycoprotein with an estimated molecular weight of 55-60 kDa. In a cell-free experiment, vanadate activates MembPTK seven- to ninefold (median effective dose, 17 +/- 2 micromol/l). Vanadate-activated MembPTK is inhibited by staurosporine (IC50, 60 +/- 5 nmol/l). In intact adipocytes, staurosporine antagonized vanadate-induced hexose uptake (IC50, 6.0 +/- 0.3 micromol/l) and significantly reversed the antilipolytic effect of vanadate (IC50, 5.0 +/- 0.4 micromol/l). After vanadate treatment, a phosphorylated P55 protein is immunoprecipitated by antibodies to both phosphotyrosine and phosphatidylinositol (PI) 3-kinase. In conclusion, rat adipocytes contain an additional vanadate-activatable nonreceptor membranous protein tyrosine kinase that may participate in the effects of vanadate not carried out by CytPTK. We also suggest that after treatment with vanadate, MembPTK is activated by autophosphorylation and interacts with PI 3-kinase. This may explain how vanadate activates PI 3-kinase without involving receptor activation and IRS-1 phosphorylation.

Identification of a novel sonic hedgehog response element in the chicken ovalbumin upstream promoter-transcription factor II promoter

Sonic hedgehog (Shh) is a secreted morphogen that regulates dorso-ventral patterning within the neural tube during embryonic development. It is well established that Shh can induce motor-neuron differentiation that coincides with the appearance of specific motor-neuron markers including chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) and Isl1. However, the mechanism of Shh-induced signaling pathway in vertebrates is not clearly defined. In this report we have identified COUP-TFII as a target gene for Shh. In addition we have used a 1.6-kb region of the COUP-TFII promoter to identify a target element that mediates the Shh-induced activity. Extensive deletions introduced within this region have further enabled us to identify a novel sonic hedgehog response element (ShhRE) in the COUP-TFII promoter. Point mutations introduced within the ShhRE reveal some key nucleotides that are essential for protein(s)-binding activity. Finally, the ShhRE is capable of functioning as a true enhancer element and can mediate Shh-induced transactivation of reporter gene via a heterologous promoter.

Identification of a novel sonic hedgehog response element in the chicken ovalbumin upstream promoter-transcription factor II promoter

Sonic hedgehog (Shh) is a secreted morphogen that regulates dorso-ventral patterning within the neural tube during embryonic development. It is well established that Shh can induce motor-neuron differentiation that coincides with the appearance of specific motor-neuron markers including chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII) and Isl1. However, the mechanism of Shh-induced signaling pathway in vertebrates is not clearly defined. In this report we have identified COUP-TFII as a target gene for Shh. In addition we have used a 1.6-kb region of the COUP-TFII promoter to identify a target element that mediates the Shh-induced activity. Extensive deletions introduced within this region have further enabled us to identify a novel sonic hedgehog response element (ShhRE) in the COUP-TFII promoter. Point mutations introduced within the ShhRE reveal some key nucleotides that are essential for protein(s)-binding activity. Finally, the ShhRE is capable of functioning as a true enhancer element and can mediate Shh-induced transactivation of reporter gene via a heterologous promoter.

Direct evidence that lactogenic hormones induce homodimerization of membrane-anchored prolactin receptor in intact Nb2-11C rat lymphoma cells

The ability of full-size prolactin receptor (PRLR) from Nb2 rat lymphoma cell line to undergo lactogenic hormone-induced dimerization in intact cells or in a partially purified microsomal fraction was tested. The stoichiometry of ovine placental lactogen (oPL) binding to PRLR was documented by SDS-PAGE of the covalently cross-linked complexes between [125I]oPL and intact Nb2-11C cells. The molecular masses of the specific bands were 82 and 141 kDa, corresponding to PRLR:oPL and (PRLR)2:oPL complexes. These results provide direct evidence for the occurrence of hormone-induced receptor dimerization in intact cells. Gel-filtration studies revealed that under non-denaturing conditions, the purified receptor forms high-molecular-mass aggregates (190 and 540 kDa) composed of receptor dimers and oligomers. Since this aggregation was not dependent on the presence of lactogenic hormone, it is possible that the receptor in the intact cells may already exist as a noncovalent dimer or oligomer and that hormone-induced dimerization stabilizes the complex or changes its conformation.

Antilipolytic actions of vanadate and insulin in rat adipocytes mediated by distinctly different mechanisms

Vanadate, which mimics the biological effects of insulin, also inhibits lipolysis in rat adipocytes. Here we demonstrate that the antilipolytic effect of vanadate differs from that of insulin at least by the five following criteria: 1) vanadate inhibits lipolysis mediated by high (supraphysiological) concentrations of catecholamines; 2) vanadate antagonizes (Bu)2cAMP-mediated lipolysis; 3) vanadate antagonizes isobutylmethylxanthine-dependent lipolysis, 4) vanadate inhibits lipolysis mediated by okadaic acid; and 5) wortmannin, which blocks the antilipolytic effect of insulin, fails to block vanadate-mediated antilipolysis. Vanadate does activate phosphoinositol 3-kinase, and wortmannin blocks this activation. Our working hypothesis assumes that all of the insulin-like effects of vanadate, including antilipolysis, are initiated by the inhibition of protein phosphotyrosine phosphatases (PTPases). Among documented PTPase inhibitors we found that VOSO4 (oxidation state +4), several organic vanadyl compounds (+4), zinc (Zn2+), tungstate (W), and molybdate (Mo) also had antilipolytic activity. The order of potency was vanadyl acetylacetonate > or = VOSO4 > or = NaVO3 > or = vanadyl-dipicolinate > Zn2+ >> W > Mo, and it correlated better with the inhibition of adipose membranal-PTPases in cell-free experiments. We have concluded that the antilipolytic effect of vanadate is 1) mechanistically distinct from that of insulin, 2) independent of phosphoinositol 3-kinase activation, and 3) independent of the lipolytic cascade. We also strongly suggest that the antilipolytic effect of vanadate emanates from inhibiting adipose membranal, rather than cytosolic PTPases, and present preliminary data showing distinct differences in catalysis between these two PTPase categories. Overall, the study indicates that antilipolysis can be manifested via alternative, insulin-independent, signal-transducing pathways.

Phenylarsine oxide and vanadate: apparent paradox of inhibition of protein phosphotyrosine phosphatases in rat adipocytes

Vanadate mimics, whereas phenylarsine oxide (PAO) antagonizes, the effects of insulin in rat adipocytes. Both vanadate and PAO are documented inhibitors of protein-phosphotyrosine phosphatases. The relationship between the inhibition of 'inhibitory' PTPase and 'stimulatory' PTPase has been studied here in primary rat adipocytes. Low concentrations of PAO (IC50 = 0.6-2.0 microM) blocked the stimulating effects of insulin, vanadate and pervanadate on hexose uptake and glucose metabolism. Inhibition of isoproterenol-mediating lipolysis by vanadate and insulin was not blocked by PAO. The activating effects of okadaic acid on hexose uptake and glucose metabolism, which occur at points downstream to tyrosine phosphorylation, were also not blocked by PAO. Subsequent studies suggested that the PAO-sensitive PTPase comprises a minute fraction of the total adipocytic PTPase activity. To identify its location we applied procedures involving fractionations and activation of non-receptor adipocytic protein tyrosine kinase by PAO and vanadate in cell free assays. We found that the 'inhibitory' PTPase is exclusively associated with the membrane fraction whereas the 'stimulatory' PTPases are present in both the cytosolic and plasma membrane compartments. We next searched for markers, possibly associated with PAO-dependent desensitization and found that several proteins became phosphorylated on tyrosine moieties in the supernatant of PAO but not in vanadate pretreated adipocytes. In summary, we propose the presence of a minute, plasma membrane associated PTPase in primary rat adipocytes, inhibition of which arrests the activation of glucose metabolism. In contrast, inhibition of all the other cellular adipose PTPases, ultimately activates rather than inhibits these same bioeffects.

Evidence for the distinct vanadyl(+4)-dependent activating system for manifesting insulin-like effects

Both exogenously added vanadate (oxidation state +5) and vanadyl (oxidation state +4) mimic the rapid responses of insulin through alternative signaling pathways, not involving insulin receptor activation [reviewed in Shechter et al. (1995) Mol. Cell. Biochem. 153, 39-47]. Vanadium exhibits complex chemistry, fluctuating between vanadate(+5) and vanadyl(+4), according to the prevailing conditions. Using several experimental approaches, we report here on a distinct vanadate(+5)-independent, vanadyl(+4)-dependent activating pathway. The key components of this pathway are membrane protein phosphotyrosine phosphatases (PTPases) and a cytosolic (nonreceptor) protein-tyrosine kinase (CytPTK). We further suggest that vanadate(+5) is not reduced rapidly to vanadyl(+4) inside the cell, and entered vanadyl sulfate(+4) is capable of undergoing spontaneous oxidation to vanadate(+5) in vivo. Finally, we show that the promotion and full expression of a downstream bioeffect such as lipogenesis requires both activation of CytPTK and prolonged stability of vanadyl(+4) against oxidation.

Lactogenic hormones rapidly activate p21( ras )/mitogen-activated protein kinase in Nb2-11C rat lymphoma cells

Lactogenic hormone-dependent Nb2-11C cells proliferate in response to prolactin (PRL) or human growth hormone (hGH). We have investigated the activation of p21( ras ) and mitogen-activated protein kinase (MAP-kinase) by hGH in lactogen-dependent Nb2-11C and in autonomous hormone-independent Nb2-SP rat lymphoma cells. Exposure of Nb2-11C cells to hGH resulted in a dose-dependent activation of p21( ras ) and of MAP-kinase. Activation occurs at physiological hGH concentration and with a rapid onset (∼1 min) reaching maximal level at 10-20 min. In contrast, in Nb2-SP autonomous lactogen-independent cells, p21( ras ) and MAP-kinase are constitutively activated and a challenge with lactogenic hormone had a modest additional activating effect. TPA, an activator of protein kinase C, enhanced p21( ras ) and MAP-kinase activity in Nb2-11C cells but failed to induce proliferation. The mechanism of activation of p21( ras ) in Nb2-11C cells by lactogenic hormones involves both an increased binding of guanine nucleotides to p21( ras ) as well as an increase in GTP/GDP+GTP ratio. In summary, we have demonstrated here that activation of the p21( ras )/MAP-kinase pathway follows PRL receptor activation but is not sufficient for the lactogenic hormone-dependent mitogenesis.

Insulin-like actions of vanadate are mediated in an insulin-receptor-independent manner via non-receptor protein tyrosine kinases and protein phosphotyrosine phosphatases

Most or all mammalian cells contain vanadium at a concentration of 0.1-1.0 microM. The bulk of the vanadium in cells is probably in the reduced vanadyl (IV) form. Although this element is essential and should be present in the diet in minute quantities, no known physiological role for vanadium has been found thus far. In the years 1975-1980 the vanadate ion was shown to act as an efficient inhibitor of Na+,K(+)-ATPase and of other related phosphohydrolyzes as well. In 1980 it was observed that vanadate vanadyl, when added to intact rat adipocytes, mimics the biological actions of insulin in stimulating hexose uptake and glucose oxidation. This initiated a long, currently active, field of research among basic scientists and diabetologists. Several of the aspects studied are reviewed here.

Non-receptor cytosolic protein tyrosine kinases from various rat tissues

Adipocytic-cytosolic non-receptor protein tyrosine kinase (CytPTK) when activated can substitute for the insulin receptor tyrosine kinase (InsRTK), in manifesting several insulin effects in insulin-receptor independent fashion. Our aims here were to utilize PolyGlu4Tyr, a good experimental exogenous substrate for protein tyrosine kinases (PTKs) in general, for studying qualitative and quantitative parameters of CytPTKs extracted from different tissue cytosols. At the same time, we would search for a unique specific marker specifically characterizing CytPTKs. High speed supernatants of spleen, thymus, smooth muscle, lung and kidney were found to be rich in CytPTK activities. Their specific activities being 6- to 13-fold that of liver or adipose cytosols. Brain, testis and adrenal cytosols were an intermediate source of CytPTK activity, whereas CytPTK activity of heart and skeletal muscle was low. It was also evaluated that the capacity of the cytosol to phosphorylate PolyGlu4Tyr is 15-50% that of the non-stimulated Triton X-100 extractable plasma membrane PTKs. Fractionation of the cytosols on superose 12 column revealed several CytPTKs within the same tissue, their peaks ranging between 30 and 450 kDa. Immunoblotting analysis showed Fyn and Lyn were present in most tissue cytosols. Upon immunoprecipitation, however, with anti-Fyn or anti-Lyn, negligible amounts (< 2%) of the total cellular CytPTK were precipitated. Thus, these general markers of CytPTKs comprise only a minor proportion of the total intracellular PolyGlu4Tyr phosphorylating capacity. To see whether a specific marker for CytPTK could be detected, we also examined the requirement of CytPTKs for divalent ions, their preferred phosphate donor and their sensitivity to inhibition by known PTK inhibitors. We found that the order of reactivity with divalent cations was Co2+ > Mn2+ > Mg2+, while Zn2+ and Ca2+ did not support CytPTK activity. The best phosphate donor was ATP (ED50 = 5 microM), but other nucleoside 3-phosphates could substitute for ATP at high concentrations. With respect to these parameters, no basic difference exists between cytosolic and plasma-membrane PTKs. The PTK inhibitors, genestein and quercetin, inhibited both cytosolic and membranal PTKs at micromolar concentrations. In contrast, staurosporine was a potent inhibitor of CytPTKs (IC50 5-20 nM) and a poor inhibitor of membranal PTKs (IC50 10-40 microM). One of the conclusions we can draw from this study is that tissue cytosols contain PolyGlu4Tyr phosphorylating capacity in quantities greater than previously assumed and that the low level of phosphotyrosine found in cells is not the result of limited intracellular levels of CytPTKs.

A cytosolic protein tyrosine kinase activity is induced by follicle stimulating hormone and insulin like growth factor-I in human granulosa cells

Follicle stimulating hormone (FSH) induces estradiol (E2) production in rat, porcine and human granulosa cells with a concomitant increase in cAMP. In human granulosa cells insulin like growth factor-I (IGF-I) induces E2 production without cAMP accumulation. In the current study we report that IGF-I and FSH effects on aromatase activity both involve activation of a cytosolic soluble protein tyrosine kinase (CytPTK). This FSH and IGF-I stimulated CytPTK activity was blocked by AG-82 (a tyrosine kinase inhibitor) and by staurosporine (STS) (a non specific protein kinase inhibitor) at concentrations which inhibited E2 production. These new findings strengthen the concept of fail-safe mechanism in E2 production in human granulosa cells by an involvement of tyrosine kinase(s) activity downstream of cAMP formation and protein kinase A (PKA) activation.

Insulin-like effects of tungstate and molybdate: mediation through insulin receptor independent pathways

The insulin-like effects of tungstate (W) and molybdate (Mo) were studied in rat adipocytes and compared to those of vanadate. Other than being less potent, W and Mo resembled vanadate in stimulating lipogenesis, in activating glucose oxidation, in enhancing rate of hexose uptake, and in inhibiting lipolysis. Tungstate and molybdate did not activate the insulinreceptor tyrosine kinase (InsRTK). Quercetin which blocks InsRTK activity and insulin stimulation of glucose metabolism, failed to inhibit when these bioeffects were stimulated by W or Mo. The metalooxide, however, activated a staurosporine sensitive non receptor, cytosolic protein tyrosine kinase (CytPTK), and staurosporine blocked W or Mo dependent lipogenesis in rat adipocytes. Staurosporine did not prevent Mo and W either from activating hexose transport, or from inhibiting lipolysis. Tungstate and molybdate were less effective than vanadate in inhibiting adipose PTPases in cell free systems. Membranal PTPases were more sensitive to W and Mo inhibition than cytosolic PTPases. While the presence of a nucleophile such as hydroxylamine reversed inhibition of PTPase by vanadate it did not affect inhibition by W or Mo. In summary, the insulinomimetic effects of W and Mo appear to resemble qualitatively that of vanadate in all respects. Both act in an insulin receptor-independent-fashion, activate CytPTK and trigger additional effects that are not mediated by the InsRTK or by CytPTK. The quantitative differences may be attributed to reduced capacity of W and Mo relative to vanadate to inhibit the relevant PTPases in intact cells.

Permolybdate and pertungstate--potent stimulators of insulin effects in rat adipocytes: mechanism of action

In previous studies, tungstate and molybdate were found to mimic the biological actions of insulin. It was suggested that these metallooxides initially inhibit vanadate-sensitive protein phosphotyrosine phosphatase (PTPase). This, in turn, stimulates a staurosporine-sensitive cytosolic protein tyrosine kinase (cytPTK), which activates several insulin bioeffects via insulin-independent pathways (Shisheva & Shechter, 1991, 1993; Elberg et al., 1994). Tungstate and molybdate, however, facilitate bioeffects in rat adipocytes only at high (millimolar) concentrations (Goto et al., 1992). We report here that incubations of tungstate or molybdate with hydrogen peroxide (H2O2) result in the formation of pertungstate (pW, peroxide of tungstate) or permolybdate (pMo, peroxide of molybdate). Pertungstate and permolybdate were found to stimulate all or most of the insulin bioeffects in rat adipocytes. Moreover, these permetallooxides are 80-180-fold more potent stimulators than the corresponding metallooxides. This shift in potency resembles that of pervanadate relative to vanadate in stimulating the same effect in rat adipocytes (Fantus et al., 1989). pW and pMo are also active in normalizing blood glucose levels in streptozotocin-induced diabetic rats. Further studies aimed at understanding the higher efficacy of this permetallooxide revealed the following: (a) All three permetallooxides (pV, pW, pMo) are oxidizing agents relative to reduced glutathione (GSH). They oxidize stoichiometric amounts of GSH to GSSG. (b) All three metallooxides do not oxidize GSH to GSSG. (c) Both metallooxides and permetallooxides inhibit rat adipocytic PTPase at micromolar quantities (IC50 = 3-10 microM). Permetallooxides, however, inhibited a larger PTPase fraction (80-100%) compared to metallooxides (40-70% of the total).(ABSTRACT TRUNCATED AT 250 WORDS)

Vanadium activates or inhibits receptor and non-receptor protein tyrosine kinases in cell-free experiments, depending on its oxidation state. Possible role of endogenous vanadium in controlling cellular protein tyrosine kinase activity

We have shown that vanadium mimics several insulin effects in rat adipocytes, via a staurosporine sensitive cytosolic protein tyrosine kinase (CytPTK; Shisheva, A., and Shechter, Y. (1993) J. Biol. Chem. 268, 6463). Here we demonstrate that vanadium effects on protein tyrosine kinases are preserved after cell disintegration. Vanadium inhibits or activates protein tyrosine kinases depending on its oxidation state and the tyrosine kinase studied. Vanadyl (4+) but not vanadate (5+) inhibits receptor tyrosine kinases such as the insulin receptor (IC50 value = 23 +/- 4 microM) and the insulin-like growth factor-I receptor (IC50 = 19 +/- 3 microM). Inhibition is non-competitive with respect to ATP, Mn2+, or substrate concentrations. Preincubation of adipocytes with vanadyl (0.4 mM), and staurosporine (which arrests the cytosolic enzyme) substantially inhibited insulin-stimulated lipogenesis. Vanadyl is readily oxidized to vanadate by hydrogen peroxide. In contrast, CytPTKs were poorly inhibited by vanadyl, and vanadate stimulated several CytPTKs 2-6 fold. CytPTK derived from rat adipocytes, liver and brain were activated, and CytPTK from Nb2 lymphoma cells was not affected. CytPTK extracted from insulin-responsive tissues are more sensitive to vanadate activation (ED50 = 3 +/- 0.7 microM), whereas the brain enzyme is less sensitive (ED50 = 27 +/- 3 microM). Tungstate, molybdate, and phenylarsine oxide also stimulate CytPTK, suggesting that the vanadate effect is secondary to inhibiting protein phosphotyrosine phosphatases. This study supports a working hypothesis implicating the intracellular vanadyl pool in modulating CytPTK activity. Any physiological conditions converting vanadyl to vanadate (i.e. H2O2 production) will activate CytPTK and consequently CytPTK-dependent bioeffects.

Biological activity of bovine placental lactogen in 3T3-F442A preadipocytes is mediated through a somatogenic receptor

Bovine placental lactogen (bPL) exhibited antimitogenic differentiation-promoting biological activity in 3T3-F442A preadipocytes. Competitive binding studies and affinity labelling revealed bPL activity to be mediated through a somatogenic type of receptor that recognizes human growth hormone (hGH) and bovine GH, but not ovine prolactin or human PL. The bioactivity of bPL was sixfold lower than that of hGH despite that bPL is binding to the somatogenic receptors with fivefold higher affinity. This discrepancy may result from the relatively low ability of bPL to induce post-receptoral effects such as receptor dimerization.

Distinct placental lactogen and prolactin (lactogen) receptors in bovine endometrium

Specific binding sites for bovine placental lactogen (bPL) and the lactogenic hormone, prolactin, have been detected in endometrial membranes isolated from uteri of mid-pregnant heifers. The specific binding of human growth hormone (hGH) (used to monitor the presence of lactogenic binding sites) and of bPL was increased approximately 4-fold following treatment of the membranes with 4 M MgCl2. Binding was found to be ligand specific, membrane protein concentration-, time- and temperature-dependent and reversible. Scatchard analysis of bPL and hGH competition binding data revealed curvilinear plots with dissociation constants for the high affinity sites of 4.1 x 10(-11) M and 6.4 x 10(-11) M, respectively. The maximum capacity of binding of bPL at the high affinity site was 21 fmol/mg). membrane protein while approximately twice the level of binding was measured for hGH (39 fmol/mg). Both hGH and bGH, but not ovine prolactin, competed with [125I]bPL for binding. The concentrations of hGH and bGH needed to effectively compete were however 100-fold higher than those required for unlabeled bPL. No specific binding of radiolabeled bGH was detected in endometrial tissue suggesting the absence of bGH receptors. Preferential competition of [125I]hGH binding was observed by prolactin and bPL. From these data it may be inferred that hGH binding is indicative of the presence of both lactogenic (prolactin) and bPL binding sites in endometrial tissue. The presence of distinct bPL receptors in the endometrium from mid-pregnant cows suggests a possible role for bPL in the maintenance of pregnancy.

Mitogenic and binding properties of monoclonal antibodies to the prolactin receptor in Nb2 rat lymphoma cells. Selective enhancement by anti-mouse IgG

Three monoclonal antibodies (mAbs) (T6, U5, and U6) against prolactin (PRL) receptors in rat liver were studied in the rat lymphoma lactogen-dependent (Nb2-11C) and autonomous (Nb2-SP) cell lines. The mAbs had strong affinity for lactogen receptors (Ka = 12-14 nM-1), similar to that of human growth hormone (hGH) which is a lactogenic hormone. T6 and hGH competed for the same binding site, while U5 and U6 interacted with another epitope. The 125I-hGH-receptor complex could be immunoprecipitated by either U5 or U6, but not by T6. Affinity labeling and immunoblotting revealed that hGH and U6 bind to a protein of 63-65 kDa. T6, U5, and U6 were mitogenic in Nb2-11C cells but their respective potencies were 185-, 70-, and 4700-fold lower than that of hGH. Anti-mouse IgG enhanced the mitogenic effect of all three mAbs and almost completely abolished the differences between them, although their mitogenic activity was still 60-120-fold lower than hGH. Des-13-hGH, a competitive antagonist of hGH which hardly effected the binding of 125I-U5, inhibited the U5-stimulated proliferation of Nb2-11C cells in a noncompetitive manner, indicating that simultaneous binding of both ligands fixed the receptor in a nonactive conformation. A Fab fragment of T6 was not mitogenic, and inhibited the hGH-induced mitogenesis in a competitive manner, but its mitogenicity could be restored by anti-mouse IgG. We suggest that the dimerization or oligomerization of the lactogen receptor in Nb2-11C cells is an obligatory step in the transduction of the mitogenic signal. It may be induced by binding of the mAb to a site, which can be either identical or may even be distinct from that which binds the lactogenic hormone.

Site-directed mutations of human growth hormone that selectively modify its lactogenic activity and binding properties

Two novel analogs of human (h) GH, 1) Des-7-hGH (Arg8Met, Asp11Ala) in which the Arg8 was substituted by Met and Asp11 by Ala, and 2) bovine (b) GH/hGH hybrid II (MetAla 1-13/14-191, Ala11Asp) composed of 13 N-terminal amino acid of bGH and elongated by two amino acids (Met-Ala-1-13) and 14-191 amino acids of hGH, were constructed and expressed in Escherichia coli. CD spectra indicated that the alpha-helix content of the purified proteins was similar to that of the native hormone. Both analogs retained their full ability to stimulate the proliferation of Nb2 lymphoma cells, and their binding to the lactogen receptors in homogenate of Nb2 cells and in microsomal fraction from bovine lactating mammary gland was only slightly reduced. However, their ability to bind to the somatogen receptors in intact IM-9 lymphocytes and bovine liver was reduced by 7- to 11-fold (bGH/hGH hybrid II) and 20- to 30-fold (Des-7-hGH). Both analogs were able to down-regulate the respective lactogen and somatogen receptors in intact Nb2 and IM-9 cells. The galactopoietic activity of both analogs in the lactating bovine mammary explants bioassay was almost completely abolished, and the bGH/hGH hybrid II exhibited a remarkable antagonistic activity. These results further indicate that the lactogen receptors in different species or organs are not identical. We have shown that the new recombinant analogs of hGH that recognize both somatogen and lactogen receptors but have modified postreceptor effects are helpful in elucidating these differences.

Indication of different lactogen and somatogen binding sites in the human growth hormone molecule as probed with monoclonal antibodies

The relationship between the structure of human growth hormone (hGH) and the hormone-receptor interaction has been investigated using as probes monoclonal antibodies (Mabs) to hGH of defined epitope specificity profile. Seven high affinity Mabs were studied for their ability (i) to inhibit the binding of 125I-hGH to Nb2-SP rat lymphoma cells and to IM-9 human lymphocytes possessing lactogen and somatogen type receptors, respectively; and (ii) to interfere with the hormone (hGH or Met8Leu hGH)-induced proliferation in Nb2-11C lymphoma cells. The ability of these Mabs to inhibit the 125I-hGH binding and the hormone-induced proliferation in Nb2-11C cells was negatively correlated with the ability of these Mabs to cross-react with met14 hGH. Furthermore, Mabs Nos. 3 and 7, which cross-reacted minimally (0.2-0.4%) with Met8Leu hGH, were unable to interfere with the mitogenic activity of Met8Leu hGH in Nb2-11C cells. These results indicate that the first 13 amino acids of the N-terminal region of hGH are necessary for its lactogen activity. The inhibition of 125I-hGH binding to IM-9 cells by these Mabs was similar to those observed in Nb2-SP cells, except for Mabs Nos. 19 and 1. These Mabs inhibited more strongly the binding of 125I-hGH to IM-9 than to Nb2 cells and recognized antigenic epitopes close to the C-terminal part of the molecule. These results suggest that the somatogen receptor binding site of hGH may be located on two sites, one at the N-terminal and the other one close to the C-terminal, while the lactogen receptor is mainly confined to the N-terminal part.

Chimeric bovine-human growth hormone prepared by recombinant DNA technology: binding properties and biological activity

A chimeric bovine GH (amino acids Met-Asp-Gln-greater than 1-23) and human GH (hGH) (amino acids 24-191) plasmid was constructed and expressed in Escherichia coli. The purified protein (chimeric GH) exhibited a 2-3 order of magnitude lower affinity toward lactogenic receptors in Nb2 lymphoma cells, microsomal fractions from bovine mammary gland and male rat liver. The affinity towards somatogenic receptors in IM-9 human lymphocytes and male rat liver was decreased to a much lesser degree. This diminished affinity towards lactogenic receptors was accompanied by a parallel decrease in the ability of the chimeric GH to stimulate proliferation of Nb2-11C lymphoma cells and the lipogenesis in bovine mammary gland. This implies that occupation of the respective receptors by either chimeric GH or hGH leads to identical postreceptoral effects. The chimeric GH was also capable of down-regulating the lactogenic receptors in Nb2 lymphoma cells and was recognized by three anti-hGH monoclonal antibodies. These and previously published results indicate that the N-terminal part of hGH is essential for the high affinity binding to lactogenic receptors and subsequent biological effect. Removal or replacement by a corresponding part of bovine GH converts the hormone, respectively to weak antagonist or agonists. Analysis of our data, based on hydropathy index leads us to suggest that the high affinity binding site of the hGH towards lactogenic receptors is mainly confined to amino acids nos. 8-18.

Solubilization and characterization of lactogenic hormone receptor from kidney of lactating cow

Solubilization of the microsomal fraction from bovine kidney by Triton X-100 or by 3-[(3-cholamidopropyl)-dimethylammonio] 1-propanesulfonate (CHAPS) increased 2-fold the thermodynamic association constant for hGH. While solubilization with CHAPS did not change the 13-fold preferential binding of human growth hormone (hGH) over ovine prolactin (oPRL), solubilization with Triton X-100 increased this preference to 47-fold. The binding was optimal at pH 7-7.5 in the presence of 10 mM of MgCl2 or CaCl2. The association rate with hGH was identical in the microsomal and Triton X-100 solubilized fractions but the dissociation was slower in the latter. Only partial dissociation was observed at neutral pH. Full dissociation was, however, achieved by lowering the pH to 4-5, indicating that the binding was not covalent. Gel filtration studies of the Triton X-100 solubilized fraction after preincubation in the presence of reducing agent revealed two sharp peaks of activity, one having Mr of greater than 700 kDa that represented the aggregated receptor, and the second, with Mr 110-115 kDa. The specificity of the partially purified receptors clearly shows that they are lactogenic and not somatogenic. They resemble lactogenic receptors found in other bovine organs, but differ from other species particularly in their differential affinities of PRL and hGH.

A comparative study of lactogenic hormone binding sites in the adrenal gland, ovary and kidney of the lactating cow

The specific binding of 125I-oPRL to microsomal fractions from the adrenal gland, ovary and kidney of the lactating cow was significantly lower than binding of iodinated hGH. In addition, the ability of oPRL to compete with iodinated hGH as compared to hGH, was 50-fold lower in the adrenal gland 35-fold lower in the ovary and 18-fold lower in the kidney. These results are similar to those obtained in the mammary gland and liver, whereas the ability of oPRL to compete with iodinated hGH was 90-fold lower, as compared to hGH. In the kidney the difference between the binding of iodinated hGH and iodinated oPRL was smaller. Results obtained with a solubilized kidney microsomal fraction also show a slightly higher affinity for oPRL than in other tissues. Thus the phenomena of differential affinities of oPRL and hGH to lactogenic hormone binding sites, characterizes most lactogenic hormone target tissues in the lactating cow. The distribution of these sites in different parts of the tissues was also studied. In the adrenal gland, the binding capacity in the cortex was 8-fold higher than in the medulla. In the ovary most of the binding sites were found in the corpus luteum, while in the kidney the binding capacity was higher in the cortex as compared to the medulla.