VAV-1, PLC-gamma 1, MKK7 (MAP2K7), LAT, GSK3 beta, IKK-gamma, c-Jun, 188.8.131.52, Rac1, NF-kB, Bcl-XL, TCR alpha/beta, Calmodulin, PAK1, CD28, AKT(PKB), 184.108.40.206, I-kB, MEKK1(MAP3K1), IKK (cat), ITK, 220.127.116.11, ZAP70, PtdIns(3,4,5)P3, NF-AT2(NFATC1), Ca('2+) cytosol, Slp76, NF-AT, Ca('2+) endoplasmic reticulum lumen, IKK-beta, CD86, PI3K cat class IA, Fyn, JNK(MAPK8-10), 1-(1,2-diacyl-glycerol 3-phospho)- inositol 4-phosphate, CD80, PIP5KI, PtdIns(4,5)P2, <endoplasmic reticulum lumen> Ca('2+) = <cytosol> Ca('2+), PI3K reg class IA, MEK4 (MAP2K4), Lck, NF-AT3(NFATC4), NF-AT1(NFATC2), BAD, GRAP2, IP3, CD3, DAG, GRB2, IP3 receptor, PKC-theta, IKK-alpha, Calcineurin A (catalytic)
CD 28 signaling
Induction of immune response requires T cells to receive two sets of signals from antigen-presenting cells. The first signal is delivered via T-cell receptor complex ( TCR ), while the second one proceeds via co-receptor CD28. TCR and CD28 are independent signaling units. However, Cd28 amplifies signal triggered by TCR ligation .
CD28 is a T-cell surface protein activated by interacting with the B-cell activation antigens CD80 and CD86 .
In response to ligand activation, CD28 binds with the regulatory subunits of phosphatidylinositol kinase 1 ( PIK3R1A ), adaptor proteins GRB2 and GRB2-related adaptor protein ( GRAP2 ) and T cell-specific tyrosine kinase ITK , .
Lck and Fyn are critical for the CD28 co-stimulation. These kinases phosphorylate motifs present in the cytoplasmic tail of CD28, thus enabling it to bind p85 subunit of PI3K and GRB2 . Lck and Fyn also phosphorylate and consequently activate Itk and Vav-1 respectively , .
CB28 binds with SLP-76 via adaptor proteins GRB2 and GRAP2. SLP-76 recruits VAV-1, linking CD28 to VAV effectors. Mechanism of Vav-1 activation upon CD28 ligation is unclear. Putatively, TCR selectively induces ZAP-70 activation, followed by phosphorylation of the scaffold proteins LAT and SLP-76 . LAT, in turn, can bind to the SLP-76 adaptor via GRB2 and GRAP .
Vav-1 exerts pleiotropic effects mediated by the Rho family of guanosine triphosphatases (GTPases). Rac1 is a target for VAV and participates in actin cytoskeletal remodeling.
Additionally, VAV-1 regulates activity of PLC-gamma 1 by facilitating availability of PtdIns(4,5)P2, the PLC-gamma 1 substrate via stimulation of PIP-5 kinase, a Rac1 downstream element .
Itk also phosphorylates and activates PLC-gamma 1 .
The activated PCL-gamma 1 is responsible for synthesis of second messengers Diacylglycerol ( DAG ) and Inositol 1,4,5-triphosphate ( IP 3 ) by cleaving Phosphatidylinositol 4,5 bisphosphate ( PtdIns(4,5)P2 ) at the plasma membrane. DAG activates a number of proteins, including various isoforms of protein kinase C (PKC). PKC-theta activates kinase IKK, which phosphorylates serine residues on I-kappa-B proteins, thus marking them for destruction via ubiquitination, and, thereby, enabling activation of the NF-kappa-B complex .
IP3 binds IP3 Receptor (IP3R ), which is localized primarily on the endoplasmic reticulum where it stimulates release of calcium from intracellular stores. Calcium-bound Calmodulin associates with and activates serine/threonine phosphatase Calcineurin. Calcineurin dephosphorylates NF-AT family of transcription factors leading to theirs translocation to the nucleus .
Activated PI3K converts PtdIns(4,5)P2 into Phosphatidylinositol 3,4,5-triphosphate ( PtdIns(3,4,5)P 3 ) . CD28 ligation stimulates generation of PtdIns(3,4,5)P 3. PtdIns(3,4,5)P 3 associates with the inner face of the plasma membrane promoting recruitment of proteins with pleckstrin homology (PH) domains, such as ITK, VAV1, and Akt . Akt blocks Glycogen synthase kinase-3 ( GSK-3 ), which phosphorilates NF-AT and, thereby, prevents its nuclear translocation .
Additionally, CD28 activates JNK cascade via VAV-1 activation. VAV-1 activates Rac1 and CDC24, which activate JNK via MEKK1 and MKK4/7. Activated JNK phosphorylates transcription factors such as JUN, thereby activating AP1 complex, involved in regulation of cell proliferation .