C H Heldin

PDGF alpha- and beta-receptors activate unique and common signal transduction pathways.

We have examined the signal transduction pathways of the PDGF alpha‐ and beta‐receptors, in order to characterize the specificity of each receptor type in the signaling. Porcine aortic endothelial cell lines expressing equal levels of either PDGF alpha‐ or beta‐receptors were established. The alpha‐ and beta‐receptor cells responded mitogenically to stimulation with the proper PDGF isoforms. Three aspects of actin reorganization were examined after ligand stimulation: loss of stress fibres, appearance of edge ruffles and formation of circular membrane ruffles. The beta‐receptor cells showed a response to ligand stimulation which included all three features. The alpha‐receptor cells exhibited edge ruffles and loss of stress fibres, but circular ruffles could not be found in several independent alpha‐receptor cell lines. The beta‐receptor cells, but not the alpha‐receptor cells, were able to migrate chemotactically towards a concentration gradient of ligand. The molecular basis for the differences in signalling were explored by comparing the pattern of increased phosphorylation of potential substrates for the alpha‐ and beta‐receptors in [32P]orthophosphate labelled intact cells and using an in vitro kinase assay. Certain of the observed substrates were common for the two receptors, whereas others were specific for either one. We conclude that certain of the known PDGF induced cellular effects are transduced only by the beta‐receptor; the presence of alpha‐receptor‐specific substrates suggests that there are also alpha‐receptor‐specific signals, which have yet to be identified.

Phosphorylation of Ser165 in TGF-beta type I receptor modulates TGF-beta1-induced cellular responses.

Transforming growth factor-beta (TGF-beta) signals via an oligomeric complex of two serine/threonine kinase receptors denoted TGF-beta type I receptor (TbetaR-I) and type II receptor (TbetaR-II). We investigated the in vivo phosphorylation sites in TbetaR-I and TbetaR-II after complex formation. Phosphorylation of TbetaR-II was observed at residues in the C-terminus (Ser549 and Ser551) and at residues in the juxtamembrane domain (Ser223, Ser226 and Ser227). TGF-beta1 induced in vivo phosphorylation of serine and threonine residues in the juxtamembrane domain of TbetaR-I in a region rich in glycine, serine and threonine residues (GS domain; Thr185, Thr186, Ser187, Ser189 and Ser191), and more N-terminal of this region (Ser165). Phosphorylation in the GS domain has been shown previously to be involved in activation of the TbetaR-I kinase. We show here that phosphorylation of TbetaR-I at Ser165 is involved in modulation of TGF-beta1 signaling. Mutations of Ser165 in TbetaR-I led to an increase in TGF-beta1-mediated growth inhibition and extracellular matrix formation, but, in contrast, to decreased TGF-beta1-induced apoptosis. A transcriptional activation signal was not affected. Mutations of Ser165 changed the phosphorylation pattern of TbetaR-I. These observations suggest that TGF-beta receptor signaling specificity is modulated by phosphorylation of Ser165 of TbetaR-I.

Identification of Tyr-762 in the platelet-derived growth factor alpha-receptor as the binding site for Crk proteins.

Tyr-762 is an autophosphorylation site in the human platelet-derived growth factor (PDGF) α-receptor. In order to investigate whether phosphorylated Tyr-762 serves as a docking site for downstream signal transduction molecules, affinity purification using an immobilized synthetic peptide containing phosphorylated Tyr-762 and its surrounding amino acid residues was performed. Proteins in HeLa cell lysate of molecular sizes 27, 38 and 40 kDa bound to the phosphorylated, but not to the unphosphorylated peptide. Analyses of partial amino acid sequences of the purified proteins indicated that they were identical to CrkI, CrkII and CrkL respectively. The wild-type PDGF α-receptor, when expressed in porcine aortic endothelial cells, formed complexes with CrkII and CrkL upon ligand stimulation, which was specifically inhibited by a synthetic peptide containing phosphorylated Tyr-762. Replacement of Tyr-762 with a phenylalanine residue in the PDGF α-receptor abrogated ligand-induced binding of Crk proteins. Tyrosine phosphorylation of CrkII and CrkL increased by 1.8- and 1.3-fold, respectively, upon ligand stimulation of the wild-type α-receptor. In contrast, the Y762F mutant PDGF α-receptor failed to induce tyrosine phosphorylation of Crk proteins. CrkII and CrkL constitutively formed complex with the guanine nucleotide exchange factor C3G, in unstimulated as well as PDGF-stimulated cells. Moreover, the activated wild-type PDGF α-receptor but not the Y762F mutant receptor was found in a C3G immunoprecipitate, suggesting that a ternary complex between the activated PDGF α-receptor, Crk and C3G was formed. DNA synthesis stimulated by PDGF-BB as well as PDGF-induced MAP kinase activation was similar in cells expressing wild-type and mutant receptors. Interestingly, the activated PDGF β-receptor was found not to bind Crk proteins. Instead, Tyr-771 of the β-receptor, which is localized at an analogous position to Tyr-762 in the α-receptor, binds RasGAP. RasGAP is not bound to the α-receptor. Thus, this region in the kinase inserts of the two receptors may be important for the divergency in signaling from the two PDGF receptors.

Bone morphogenetic protein type IB receptor is progressively expressed in malignant glioma tumours.

The distribution of bone morphogenetic protein (BMP) type I receptors and the activin type I receptor (ActR-I) was investigated in 16 cases of human glioma and five cases of non-tumourous gliosis tissue by immunohistochemical technique. Both BMP type IA (BMPR-IA) and the type IB (BMPR-IB) receptors were detected in human glioma cells. A significant increase in BMPR-IB in tumour cells was observed in malignant glioma compared with both low-grade astrocytomas (n=16, P<0.005) and gliosis (n=13, P<0.001). However, enhancement of BMPR-IA staining was moderate and ActR-I staining was only weakly expressed in the malignant glioma tumours. Osteogenic protein (OP)-1/BMP-7, which is known to bind BMPR-IA, BMPR-IB and ActR-I, was expressed in nervous tissue and was also detected in anaplastic areas of malignant glioma. In contrast to the tissue materials, BMPR-IA was expressed to a stronger degree than BMPR-IB in human glioma cell lines; the growth of these cells was suppressed by OP-1. These results suggest the presence of BMP receptors and a functional role for BMPs in malignant glioma.