The chemokine stromal cell-derived factor (SDF-1alpha), the ligand for the CXCR4 receptor, induces a wide variety of effects that include calcium mobilization, chemotactic responses, bone marrow myelopoiesis, neuronal patterning, and prevention of HIV-1 infection. Nonetheless, little is known of the biochemical pathways required to achieve this variety of responses triggered after receptor-chemokine interaction. We developed a set of monoclonal antibodies that specifically recognize the CXCR4 receptor and used them to identify the signaling pathway activated after SDF-1alpha binding in human T cell lines. Here we demonstrate that SDF-1alpha activation promotes the physical association of Galpha(i) with the CXCR4. Furthermore, within seconds of SDF-1alpha activation, the CXCR4 receptor becomes tyrosine phosphorylated through the activation and association with the receptor of JAK2 and JAK3 kinases. After SDF-1alpha binding, JAK2 and JAK3 associate with CXCR4 and are activated, probably by transphosphorylation, in a Galpha(i)-independent manner. This activation enables the recruitment and tyrosine phosphorylation of several members of the STAT family of transcription factors. Finally, we have also observed SDF-1alpha-induced activation and association of the tyrosine phosphatase Shp1 with the CXCR4 in a Galpha(i)-dependent manner. As occurs with the cytokine receptors in response to cytokines, the CXCR4 undergoes receptor dimerization after SDF-1alpha binding and is a critical step in triggering biological responses. We present compelling evidence that the chemokines signal through mechanisms similar to those activated by cytokines.

A broad array of biological responses, including cell polarization, movement, immune and inflammatory responses, and prevention of HIV-1 infection, are triggered by the chemokines, a family of structurally related chemoattractant proteins that bind to specific seven-transmembrane receptors linked to G proteins. Here we discuss one of the early signaling pathways activated by chemokines, the JAK/STAT pathway. Through this pathway, and possibly in conjunction with other signaling pathways, the chemokines promote changes in cellular morphology, collectively known as polarization, required for chemotactic responses. The polarized cell expresses the chemokine receptors at the leading cell edge, to which they are conveyed by rafts, a cholesterol-enriched membrane fraction fundamental to the lateral organization of the plasma membrane. Finally, the mechanisms through which the chemokines promote their effect are discussed in the context of the prevention of HIV-1 infection.

Cell migration represents an important cellular response that utilizes cytoskeletal reorganization as its driving force. Here, we describe a new signaling cascade linking PDGF receptor stimulation to actin rearrangements and cell migration. We demonstrate that PDGF activates Cdc42 and its downstream effector N-WASP to mediate filopodia formation, actin stress fiber disassembly, and a reduction in focal adhesion complexes. Induction of the Cdc42 pathway is independent of phosphoinositide 3-kinase (PI3K) enzymatic activity, but it is dependent on the p85α regulatory subunit of PI3K. Finally, data are provided showing that activation of this pathway is required for PDGF-induced cell migration on collagen. These observations show the essential role of the PI3K regulatory subunit p85α in controlling PDGF receptor–induced cytoskeletal changes and cell migration, illustrating a novel signaling pathway that links receptor stimulation at the cell membrane with actin dynamics.