Proximal protein tyrosine kinases in immunoreceptor signaling

The oxygen-substituted palmitic acid analogue, 13-oxypalmitic acid, inhibits Lck localization to lipid rafts and T cell signaling

Palmitoylation of cysteines 3 and 5 is necessary for targeting Lck to lipid rafts and is needed for Lck function in T cell receptor (TCR) signaling. Point mutations of cysteines 3 and 5 result in a form of Lck that fails to associate with the plasma membrane, which limits the usefulness of this genetic approach to address the role of palmitoylation in the distribution of Lck within the plasma membrane. To circumvent this problem, we sought to identify a palmitic acid analogue that would enable plasma membrane association of Lck, but not facilitate its localization within lipid rafts. Here we examined the effects of the heteroatom-substituted analogue of palmitic acid, 13-oxypalmitic acid (13-OP), on Lck subcellular localization and function. 13-OP is similar in chain length to palmitic acid, but possesses reduced hydrophobicity. We found that treatment of cells with 13-OP inhibited incorporation of omega-[(125)I]iodopalmitate into Lck. 13-OP inhibited localization of Lck to lipid rafts without altering its membrane localization. Consistent with the dissociation of Lck from rafts, treatment with 13-OP abolished Lck association with the GPI-anchored protein, CD48, but not the transmembrane glycoprotein CD4. Jurkat T cells treated with 13-OP showed marked reduction in tyrosine phosphorylation and activation of mitogen-activated protein kinase upon TCR stimulation. In conclusion, the less hydrophobic analogue of palmitate, 13-OP, alters the normal acylation of Lck that provides Lck with the necessary hydrophobicity and tight packing order required for inclusion in lipid rafts.

Negative regulation of Lck by Cbl ubiquitin ligase

The Cbl-family ubiquitin ligases function as negative regulators of activated receptor tyrosine kinases by facilitating their ubiquitination and subsequent targeting to lysosomes. Cbl associates with the lymphoid-restricted nonreceptor tyrosine kinase Lck, but the functional relevance of this interaction remains unknown. Here, we demonstrate that T cell receptor and CD4 coligation on human T cells results in enhanced association between Cbl and Lck, together with Lck ubiquitination and degradation. A Cbl(-/-) T cell line showed a marked deficiency in Lck ubiquitination and increased levels of kinase-active Lck. Coexpression in 293T cells demonstrated that Lck kinase activity and Cbl ubiquitin ligase activity were essential for Lck ubiquitination and negative regulation of Lck-dependent serum response element-luciferase reporter activity. The Lck SH3 domain was pivotal for Cbl-Lck association and Cbl-mediated Lck degradation, with a smaller role for interactions mediated by the Cbl tyrosine kinase-binding domain. Finally, analysis of a ZAP-70-deficient T cell line revealed that Cbl inhibited Lck-dependent mitogen-activated protein kinase activation, and an intact Cbl RING finger domain was required for this functional effect. Our results demonstrate a direct, ubiquitination-dependent, negative regulatory role of Cbl for Lck in T cells, independent of Cbl-mediated regulation of ZAP-70.

Fluorescent histochemical techniques for analysis of intracellular signaling

Intracellular signaling relies on the orchestrated cooperation of signaling proteins and modules, their intracellular localization, and membrane trafficking. Recently, a repertoire of fluorescence-based techniques, which significantly increases our potential for detailed studies of the involved mechanisms, has been introduced. Microscopic techniques with increased resolution have been combined with improved techniques for detection of signaling proteins. Transfections of fluorescently tagged proteins have allowed in vivo microscopy of their trafficking and interactions with other proteins and intracellular structures. We present an overview of general signaling principles and a description of techniques based on fluorescent microscopy suited for studies of signaling mechanisms.

CXCR3 expression on CD34(+) hemopoietic progenitors induced by granulocyte-macrophage colony-stimulating factor: II. Signaling pathways involved

CXCR3, known to have four ligands (IFN-gamma inducible protein 10 (gamma IP-10), monokine induced by IFN-gamma (Mig), I-TAC, and 6Ckine), is predominantly expressed on memory/activated T lymphocytes. We recently reported that GM-CSF induces CXCR3 expression on CD34(+) hemopoietic progenitors, in which gamma IP-10 and Mig induce chemotaxis and adhesion. Here we further report that stimulation with GM-CSF causes phosphorylation of Syk protein kinase, but neither Casitas B-lineage lymphoma (Cbl) nor Cbl-b in CD34(+) hemopoietic progenitors can be blocked by anti-CD116 mAb. Specific Syk blocking generated by PNA antisense completely inhibits GM-CSF-induced CXCR3 expression in CD34(+) progenitors at both mRNA and protein as well as at functional levels (chemotaxis and adhesion). Cbl and Cbl-b blocking have no such effects. Thus, GM-CSF binds to its receptor CD116, and consequently activates Syk phosphorylation, which leads to induce CXCR3 expression. gamma IP-10 and Mig can induce Syk, Cbl, and Cbl-b phosphorylation in CD34(+) progenitors by means of CXCR3. gamma IP-10 or Mig has induced neither chemotaxis nor adhesion in GM-CSF-stimulated Cbl-b-blocked CD34(+) hemopoietic progenitors, whereas SDF-1alpha induces both chemotaxis and adhesion in these cells. Interestingly, gamma IP-10 and Mig can induce chemotaxis and adhesion in GM-CSF-stimulated Syk- or Cbl-blocked CD34(+) hemopoietic progenitors. Thus, Cbl-b, but not Syk and Cbl phosphorylation, is essential for gamma IP-10- and Mig-induced chemotaxis and adhesion in CD34(+) hemopoietic progenitors. This study provides a useful insight into novel signaling transduction pathways of the functions of CXCR3/gamma IP-10 and Mig, which may be especially important in the cytokine/chemokine environment for mobilization, homing, and recruitment during proliferation, differentiation, and maturation of hemopoietic progenitor cells.