Forskolin-mediated cAMP activation upregulates TNF-α expression despite NF-κB downregulation in LPS-treated Schwann cells

Although Schwann cells have been found to play a key role in inflammation and repair following nerve injury, the exact pathway is still unknown. To explore the mechanism by which Schwann cells exert their effects in the neuron microenvironment, we investigated two main inflammatory pathways: the NF-κB and cAMP pathways, and their downstream signaling molecules. In this study, lipopolysaccharide (LPS), a bacterial endotoxin, was used to activate the NF-κB pathway, and forskolin, a plant extract, was used to activate the cAMP pathway. The rat RT4-D6P2T Schwann cell line was treated with 0.1, 1, or 10 μg/mL of LPS, with or without 2 μM of forskolin, for 1, 3, 12, and 24 hours to determine the effects of elevated cAMP levels on LPS-treated cell viability. To investigate the effects of elevated cAMP levels on the expression of downstream signaling effector proteins, specifically NF-κB, TNF-α, AKAP95, and cyclin D3, as well as TNF-α secretion, RT4-D6P2T cells were incubated in the various treatment combinations for a 3-hour time period. Overall, results from the CellTiter-Glo viability assay revealed that forskolin increased viability in cells treated with smaller doses of LPS for 1 and 24 hours. For all time points, 10 μg/mL of LPS noticeably reduced viability regardless of forskolin treatment. Results from the Western blot analysis revealed that, at 10 μg/mL of LPS, forskolin upregulated the expression of TNF-α despite a downregulation of NF-κB, which was also accompanied by a decrease in TNF-α secretion. These results provide evidence that cAMP might regulate TNF-α expression through alternate pathways. Furthermore, although cAMP activation altered AKAP95 and cyclin D3 expression at different doses of LPS, there does not appear to be an association between the expression of AKAP95 or cyclin D3 and the expression of TNF-α. Exploring the possible interactions between cAMP, NF-κB, and other key inflammatory signaling pathways might reveal a potential therapeutic target for the treatment of nerve injury and inflammation.

Role of A-kinase anchoring proteins in cyclic-AMP-mediated Schwann cell proliferation

Proliferation of Schwann cells during peripheral nerve development is stimulated by the heregulin/neuregulin family of growth factors expressed by neurons. However, for neonatal rat Schwann cells growing in culture, heregulins produce only a weak mitogenic response. Supplementing heregulin with forskolin, an agent that elevates cyclic AMP levels, produces a dramatic increase in the proliferation of cultured Schwann cells. The mechanisms underlying this synergistic effect required for Schwann cell proliferation in vivo is not well established. Characterizing the A-kinase anchoring proteins (AKAPs) in Schwann cells might help identify substrates tethered to and phosphorylated by the cAMP-dependent protein kinase A (PKA). Using an RII overlay assay that detects AKAPs that are bound to the type II regulatory subunits of PKA, we identified AKAP150 in Schwann cells. Western blot analysis revealed that additional AKAPs, specifically AKAP95, and yotiao were also present. Disruption of PKA/AKAP interaction with Ht-31 peptide resulted in an increase in luciferase-conjugated cyclin D3 promoter activity. Transfection with sequence-specific AKAP siRNAs for AKAP150 and AKAP95 produced a marked reduction in cell proliferation. Immunoblot analysis revealed that knock down of AKAP95 protein caused a significant decrease in expression of the cell cycle regulatory proteins cyclin D2, cyclin D3 and the cell survival signal Akt/Protein Kinase B (Akt/PKB). Morphological characterization of Schwann cell AKAPs indicated the presence of nuclear (AKAP95), cytoplasm-associated (AKAP150) and perinuclear (yotiao) A-kinase anchoring proteins. These results indicate a role for AKAP95 and AKAP150 in the synergistic response of Schwann cells to treatment with heregulin and forskolin.

A-kinase anchoring proteins interact with phosphodiesterases in T lymphocyte cell lines

The cAMP protein kinase A (PKA) pathway in T cells conveys an inhibitory signal to suppress inflammation. This study was performed to understand the mechanisms involved in cAMP-mediated signaling in T lymphocytes. A-kinase anchoring proteins (AKAPs) bind and target PKA to various subcellular locations. AKAPs also bind other signaling molecules such as cyclic nucleotide phosphodiesterases (PDEs) that hydrolyze cAMP in the cell. PDE4 and PDE7 have important roles in T cell activation. Based on this information, we hypothesized that AKAPs associate with PDEs in T lymphocytes. Immunoprecipitation of Jurkat cell lysates with Abs against both the regulatory subunit of PKA (RIIalpha) and specific AKAPs resulted in increased PDE activity associated with RIIalpha and AKAP95, AKAP149, and myeloid translocation gene (MTG) compared with control (IgG). Immunoprecipitation and pull-down analyses demonstrate that PDE4A binds to AKAP149, AKAP95, and MTG, but not AKAP79, whereas PDE7A was found to bind only MTG. Further analysis of MTG/PDE association illustrated that PDE4A and PDE7A bind residues 1-344 of MTG16b. Confocal analysis of HuT 78 cells stained with anti-PDE7A showed overlapping staining patterns with the Golgi marker GM130, suggesting that PDE7A is located in the Golgi. The staining pattern of PDE7A also showed similarity to the staining pattern of MTG, supporting the immunoprecipitation data and suggesting that MTG may interact with PDE7A in the Golgi. In summary, these data suggest that AKAPs interact with both PKA and PDE in T lymphocytes and thus are a key component of the signaling complex regulating T cell activation.

Cell-cell interactions in the testis of adjuvant-induced arthritic rat

Rats with adjuvant-induced arthritis (AA) have low levels of serum testosterone, and production of testosterone reportedly is influenced by macrophage secretory products. This study was undertaken to understand the mechanism mediating this hypoandrogenism. Testicular macrophages from AA and nonarthritic (NA)rats were cultured, and conditioned media was added to testicular interstitial cells and Percoll-purified cells from NA rats. Testosterone production by interstitial cells stimulated with luteinizing hormone (LH) and incubated with adjuvant-induced arthritic macrophage conditioned medium (AAMCM) was significantly lower than in cells incubated with nonarthritic macrophage conditioned medium (NAMCM). However, there was no difference in testosterone production by Percoll-purified Leydig cells and those stimulated with LH when incubated with AAMCM or NAMCM. To determine whether an intermediary cell type was involved in mediating inhibition of testosterone production, AAMCM and NAMCM were added to a reconstituted preparation of testicular interstitial cells. Addition of AAMCM restored the inhibitory effect, suggesting that arthritic hypoandrogenism is mediated by cell-cell interaction. These results suggest that a factor produced by macrophages from AA rats appears to mediate testosterone production by acting in conjunction with other cells in the testicular interstitium.

Immunization of mice against a synthetic N-terminal extracellular domain gonadotropin-releasing hormone receptor peptide: evidence for a direct uterine effect

PROBLEM

Immature male and female mice were immunized with a synthetic peptide corresponding to amino acids 5-17 (ASLEQDPNHCSAI) of the mouse hypophyseal gonadotropin-releasing hormone (GnRH) receptor.

METHOD

Effect of immunization (postpuberal) was restricted to the uterus. Pituitary-gonadal functions were not altered.

RESULTS

The endometrial lining of immunized females was thin and lacked glandular development. These observations were corroborated in actively immunized and passively immunized adult females.

CONCLUSIONS

Apparently endometrial cells express a unique surface antigen, though reactive with antipeptide antibodies, that differs from the prototype pituitary GnRH receptor. Antibodies that selectively inhibit endometrial maturation might be used to treat proliferative diseases of the uterus.

Induction of apoptotic or lytic death in an ovarian adenocarcinoma cell line by antibodies generated against a synthetic N-terminal extracellular domain gonadotropin-releasing hormone receptor peptide

A polyclonal antiserum was generated in ovariectomized sheep against a synthetic peptide corresponding to amino acids 5-17 of the deduced mouse pituitary gonadotropin-releasing hormone (GnRH) receptor. Antipeptide antibodies did not bind native cells, but did react strongly with a human ovarian cancer cell line (OVCAR-3) reportedly sensitive to GnRH. Growth of cultured OVCAR-3 cells was specifically suppressed by antipeptide serum. This was attributed in part to programmed death (chromatin condensation and DNA fragmentation) of cells by antibody-induced apoptosis. Antibodies also exhibited a cytolytic effect (lactate dehydrogenase release) toward OVCAR-3 cells in the presence of the complement. Endometria of passively immunized mice lacked development; thus, antipeptide antibodies evidently recognize Mullerian duct derivatives. Experiments are in progress to determine whether the putative antigen is a variant of the pituitary GnRH receptor or a largely dissimilar protein. Effector-functional antibodies could be useful in the management of ovarian or uterine neoplasia.