Platelet-derived growth factor (PDGF) is a critical regulator of mesenchymal cell migration and proliferation. The vital functions of PDGFs for angiogenesis, as well as development of kidney, brain, cardiovascular system and pulmonary alveoli during embryogenesis, have been well demonstrated by gene knock-out approaches. Clinical studies reveal that aberrant expression of PDGF and its receptor is often associated with a variety of disorders including atherosclerosis, fibroproliferative diseases of lungs, kidneys and joints, and neoplasia. PDGF contributes to cancer development and progression by both autocrine and paracrine signaling mechanisms. In this review article, important features of the PDGF isoforms and their cell surface receptor subunits are discussed, with regards to signal transduction, PDGF-isoform specific cellular responses, and involvement in angiogensis, and tumorstromal interactions.

The assembly of the Cbl-SETA/CIN85-endophilin complex at the C terminus of the epidermal growth factor receptor (EGFR) following ligand activation mediates its internalization and ubiquitination. We found that the SETA/CIN85-interacting protein Alix/AIP1, which also binds endophilins, modulates this complex. Alix was found to associate indirectly with EGFR, regardless of its activation state, and with ⌬EGFR, which signals at low intensity and does not bind Cbls or SETA/CIN85. In agreement with this, Alix interaction did not occur via SETA/CIN85. However, SETA/CIN85 and Alix were capable of mutually promoting their interaction with the EGFR. Increasing the level of Alix weakened the interaction between SETA/CIN85 and Cbl and reduced the tyrosine phosphorylation of c-Cbl and the level of ubiquitination of EGFR, SETA/CIN85, and Cbls. This antagonism of the Cbl-SETA/CIN85 complex by Alix was reflected in its diminution of EGFR internalization. In agreement with this, small interfering RNA-mediated knockdown of Alix promoted EGFR internalization and downregulation. It has been suggested that SETA/CIN85 promotes receptor internalization by recruiting endophilins. However, Alix was also capable of increasing the level of endophilin associated with EGFR, implying that this is not sufficient to promote receptor internalization. We propose that Alix inhibits EGFR internalization by attenuating the interaction between Cbl and SETA/CIN85 and by inhibiting Cbl-mediated ubiquitination of the EGFR.Receptor tyrosine kinase signaling plays a central role in cellular growth control and is often deregulated in cancer. Escape from Cbl-mediated ubiquitination and downregulation is one common characteristic of receptor tyrosine kinases that have undergone oncogenic deregulation (8,35). Therefore, understanding how the interaction between the receptors and the Cbl protein complex is regulated is important for the development of strategies aimed at impairing oncogenic signaling in transformed cells. Recent work by several laboratories has demonstrated that normal ligand activation of receptor tyrosine kinases, which has long been recognized to lead to the binding and phosphorylation of Cbl proteins via their PTB domains, also results in the recruitment of the SETA/CIN85-endophilin complex by binding SETA/CIN85's SH3 domains to a PXXXPR motif in the C termini of the Cbls (23,39,41,42). Cbls recruit E2 ubiquitin conjugase in parallel, via their RING finger domains, and so cause the receptor to be ubiquitinated (11-13) and SETA/CIN85 to be monoubiquitinated in its C terminus (15,39,41). The internalization and ubiquitination of the receptor can be mechanistically separated, and the interaction with the SETA/CIN85-Cbl complex may be primarily involved in internalization into clathrin-coated vesicles, while

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Platelet-derived growth factor (PDGF) is a potent mitogen for mesenchymal cells. The PDGF B-chain (c-sis proto-oncogene) homodimer (PDGF BB) and v-sis, its viral counterpart, activate both ␣-and ␤-receptor subunits (␣-PDGFR and ␤-PDGFR) and mediate anchorageindependent growth in NIH3T3 cells. In contrast, the PDGF A chain homodimer (PDGF AA) activates ␣-PDGFR only and fails to induce phenotypic transformation. In the present study, we investigated ␣-and ␤-PDGFR specific signaling pathways that are responsible for the differences between the transforming ability of PDGF AA and BB. To study PDGF BB activation of ␤-PDGFR, we established NIH3T3 clones in which ␣-PDGFR signaling is inhibited by a dominant-negative ␣-PDGFR, or an antisense construct of ␣-PDGFR. Here, we demonstrate that ␤-PDGFR activation alone is sufficient for PDGF BB-mediated anchorage-independent cell growth. More importantly, inhibition of ␣-PDGFR signaling enhanced PDGF BB-mediated phenotypic transformation, suggesting that ␣-PDGFR antagonizes ␤-PDGFR-induced transformation. While both ␣-and ␤-receptors effectively activate ERKs, ␣-PDGFR, but not ␤-PDGFR, activates stress-activated protein kinase-1/cJun NH 2 -terminal kinase-1 (JNK-1). Inhibition of JNK-1 activity using a dominant-negative JNK-1 mutant markedly enhanced PDGF BB-mediated anchorage-independent cell growth, demonstrating an antagonistic role for JNK-1 in PDGF-induced transformation. Consistently, overexpression of wild-type JNK-1 reduced PDGF BBmediated transformation. Taken together, the present study showed that ␣-and ␤-PDGFRs differentially regulate Ras-mitogen-activated protein kinase pathways critical for regulation of cell transformation, and transformation suppressing activity of ␣-PDGFR involves JNK-1 activation.

Platelet-derived growth factor (PDGF) is a potent mitogen for mesenchymal cells. PDGF AA functions as a "competent factor" that stimulates cell cycle entry but requires additional (progression) factors in serum to transit the cell cycle beyond the G 1 /S checkpoint. Unlike PDGF AA, PDGF B-chain (c-sis) homodimer (PDGF BB) and its viral counterpart v-sis can serve as both competent and progression factors. PDGF BB activates ␣-and ␤-receptor subunits (␣-PDGFR and ␤-PDGFR) and induces phenotypic transformation in NIH 3T3 cells, whereas PDGF AA activates ␣-PDGFR only and fails to induce transformation. We showed previously that ␣-PDGFR antagonizes ␤-PDGFR-mediated transformation through activation of stress-activated protein kinase-1/c-Jun NH 2 -terminal kinase-1, whereas both ␣-PDGFR and ␤-PDGFR induce mitogenic signals. These studies revealed a striking feature of PDGF signaling; the specificity and the strength of the PDGF growth signal is modulated by ␣-PDGFR-mediated simultaneous activation of growth stimulatory and inhibitory signals, whereas ␤-PDGFR mainly induces a growthpromoting signal. Here we demonstrate that PDGF BB activation of ␤-PDGFR alone results in more efficient cell cycle transition from G 1 to S phase than PDGF BB activation of both ␣-PDGFR and ␤-PDGFR. PDGF AA activation of ␣-PDGFR or PDGF BB activation of both ␣-and ␤-PDGFRs up-regulates expression of p21, an inhibitor of cell cycle-dependent kinases and a downstream mediator of the tumor suppressor gene product p53. However, ␤-PDGFR activation alone fails to induce p21 WAF1/CIP1 expression. We also demonstrate that ␣-PDGFR-activated JNK-1 is a critical signaling component for PDGF induction of p21 WAF1/CIP1 promoter activity. The ability of PDGF/JNK-1 to induce p21 WAF1/CIP1 promoter activity is independent of p53, although the overall p21 WAF1/CIP1 promoter activities are greatly reduced in the absence of p53. These results provide a molecular basis for differential regulation of the cell cycle and transformation by ␣-and ␤-PDGFRs.