The role of protein kinase C in laminin-mediated neurite outgrowth

Morphine inhibits indolactam V-induced U937 cell adhesion and gelatinase secretion

We demonstrate that indolactam V, a non-phorbol protein kinase C activator, promotes U937 cell attachment to fibronectin, type IV collagen and laminin. In the absence of indolactam V, 2-4% of U937 cells attach to all test substrates, however, in the presence of 100 nM indolactam V, 25, 16 and 11% of U937 cells attach to fibronectin, type IV collagen and laminin, respectively. When added concomitantly, 90 microM H-7, a protein kinase C inhibitor, reduces indolactam V-induced U937 cell adhesion to fibronectin by 91%. Monoclonal antibodies directed against both the beta1 and alpha 5 integrin subunits inhibit indolactam V-induced U937 cell adhesion to fibronectin by 62 and 52%, respectively. Indolactam V also promotes homotypic aggregation in U937 cells, which is blocked with either anti-ICAM or anti-LFA-1 antibodies. In addition, indolactam V promotes U937 cell secretion of a 92 kDa gelatinase as demonstrated by zymography. In the presence of low levels of morphine (10 nM-1.0 microM), the U937 cell attachment to matrix proteins was not significantly affected. However, in the presence of 10 microM morphine, the indolactam V treated cells exhibit a 71-74% reduction in cell adhesion to the matrix proteins. Further, 10 microM morphine also blocks indolactam V-induced homotypic aggregation and gelatinase secretion. The inhibitory effect of morphine on cell-matrix adhesion and gelatinase secretion was not inhibited by the opiate receptor antagonist naloxone (1 microM). While 10 microM naloxone did partially counteract the effect of 10 microM morphine on U937 cell attachment, this effect was likely non-specific since 10 microM naloxone alone increased cell adhesion. Supporting this conclusion, PCR analysis revealed that U937 cells do not express the mu high affinity morphine receptor. Also, indolactam V did not induce mu receptor expression, suggesting that morphine acts on U937 cells in a non-specific fashion.

Insulin rescues retinal neurons from apoptosis by a phosphatidylinositol 3-kinase/Akt-mediated mechanism that reduces the activation of caspase-3

The ability of insulin to protect neurons from apoptosis was examined in differentiated R28 cells, a neural cell line derived from the neonatal rat retina. Apoptosis was induced by serum deprivation, and the number of pyknotic cells was counted. p53 and Akt were examined by immunoblotting after serum deprivation and insulin treatment, and caspase-3 activation was examined by immunocytochemistry. Serum deprivation for 24 h caused approximately 20% of R28 cells to undergo apoptosis, detected by both pyknosis and activation of caspase-3. 10 nm insulin maximally reduced the amount of apoptosis with a similar potency as 1.3 nm (10 ng/ml) insulin-like growth factor 1, which acted as a positive control. Insulin induced serine phosphorylation of Akt, through the phosphatidylinositol (PI) 3-kinase pathway. Inhibition of PI 3-kinase with wortmannin or LY294002 blocked the ability of insulin to rescue the cells from apoptosis. SN50, a peptide inhibitor of NF-kappaB nuclear translocation, blocked the rescue effect of insulin, but neither insulin or serum deprivation induced phosphorylation of IkappaB. These results suggest that insulin is a survival factor for retinal neurons by activating the PI 3-kinase/Akt pathway and by reducing caspase-3 activation. The rescue effect of insulin does not appear to be mediated by NF-kappaB or p53. These data suggest that insulin provides trophic support for retinal neurons through a PI 3-kinase/Akt-dependent pathway.

Protein kinase C mediates neurite guidance at an astrocyte boundary

During development, astrocytes play an active role in directing axons to their final targets. This guidance has been attributed in part to the increased expression of guidance molecules, such as tenascin-C and chondroitin sulfate proteoglycans, by boundary-forming astrocytes. We have previously used a culture model of astrocyte boundaries to demonstrate that neurites growing on permissive astrocytes alter their trajectory as they encounter less-permissive astrocytes. The present study investigated the role of the protein kinase C (PKC) family of signal transduction molecules in this form of axonal guidance. Neurons were plated onto mixed astrocyte monolayers in the presence of agents that either downregulate the phorbol ester-sensitive PKC isoforms or inhibit PKC. Both downregulation and inhibition of PKC increased the percentage of neurons that crossed onto the nonpermissive astrocytes. On astrocyte monolayers, phorbol ester modulation of PKC but not PKC inhibitors resulted in a decrease in overall neurite extension. PKC inhibitors also caused a similar alteration in the neuronal response to cell-free boundaries, at concentrations that did not inhibit neurite extension. Thus, phorbol-ester-sensitive PKC isoforms direct the guidance of neurites by astrocyte-derived matrix molecules.

Nitric oxide mediates laminin-induced neurite outgrowth in PC12 cells

Laminin is a potent stimulator of neurite outgrowth in a variety of primary neurons and neuronal cell lines. Here, we investigate the role of nitric oxide in the signaling mechanism of laminin-mediated neurite outgrowth in the PC12 cell line. Within 8 s of exposure to laminin, PC12 cells produce nitric oxide. Peak laminin-induced nitric oxide levels reach 8 nM within 12 s of exposure to laminin and constitutive nitric oxide production is sustained for 1 min. A neurite outgrowth promoting synthetic peptide (AG73), derived from the laminin-1-alpha globular domain, also stimulated nitric oxide release. The nitric oxide synthase inhibitor, 1-NAME, prevents the formation of nitric oxide and here, 1-NAME inhibited both laminin-mediated and AG73-mediated neurite outgrowth by 88 and 95%, respectively. In contrast, C16, a synthetic peptide derived from the laminin-1-gamma chain, is shown here to promote PC12 cell attachment, but not neurite outgrowth. Interestingly, the C16 peptide did not activate nitric oxide release, suggesting that laminin-induced nitric oxide release in PC12 cells is associated only with neurite outgrowth promoting laminin domains and signals. In addition, the data here show that the nitric oxide released by PC12 cells in response to laminin is required as a part of the mechanism of laminin-mediated neurite outgrowth.

Laminin stimulates protein tyrosine dephosphorylation in PC12 cells

Laminin stimulates neurite outgrowth in rat pheochromocytoma cells (PC12 cells). Here, we investigated laminin signal transduction mechanisms by adding the tyrosine kinase/phosphatase modulators, genistein, quercetin, aurin tricarboxylic acid (ATA), and vanadate to PC12 cells. At 10 microM both genistein and quercetin enhanced laminin-mediated neurite outgrowth by 1.7- and 2.3-fold, respectively, while at 10 microM, ATA inhibited laminin-mediated neurite outgrowth by 92%. Vanadate inhibited neurite outgrowth by 63% at 10 microM. Immunoblot analysis revealed four proteins of approximately 240, 22, 110, and 35 kDa, which were dephosphorylated on tyrosine residues in laminin-treated PC12 cells, but not in NIH 3T3 cells. These results demonstrate that laminin-mediated neurite outgrowth involves protein tyrosine dephosphorylation and suggests that this mechanism may have specificity to neuronal cells.