Sphingosine 1-phosphate and platelet-derived growth factor (pdgf) act via pdgf beta receptor-sphingosine 1-phosphate receptor complexes in airway smooth muscle cells

Platelet-derived growth factor (PDGF) and sphingosine 1-phosphate (SIP) act via PDGFbeta receptor-S1P(1) receptor complexes in airway smooth muscle cells to promote mitogenic signaling. Several lines of evidence support this conclusion. First, both receptors were co-immunoprecipitated from cell lysates with specific anti-SIP, antibodies, indicating that they form a complex. Second, treatment of airway smooth muscle cells with PDGF stimulated the phosphorylation of p42/p44 MAPK, and this phosphorylated p42/p44 MAPK associates with the PDGFbeta receptor-S1P(1) receptor complex. Third, treatment of cells with antisense S1P(1) receptor plasmid construct reduced the PDGF- and S1P-dependent activation of p42/p44 MAPK. Fourth, SIP and/or PDGF induced the formation of endocytic vesicles containing both PDGFbeta receptors and S1P(1) receptors, which was required for activation of the p42/ p44 MAPK pathway. PDGF does not induce the release of S1P(1) suggesting the absence of a sequential mechanism. However, sphingosine kinase 1 is constitutively exported from cells and supports activation of p42/p44 MAPK by exogenous sphingosine. Thus, the presentation of sphingosine from other cell types and its conversion to SIP by the kinase exported from airway smooth muscle cells might enable SIP to act with PDGF on the PDGFbeta receptor-S1P(1) receptor complex to induce biological responses in vivo. These data provide further evidence for a novel mechanism for G-protein-coupled receptor and receptor tyrosine kinase signal integration that is distinct from the transactivation of receptor tyrosine kinases by G-protein-coupled receptor agonists and/or sequential release and action of SIP in response to PDGF.