Platelet-derived growth factor beta receptor regulates interstitial fluid homeostasis through phosphatidylinositol-3' kinase signaling

Ocular side effects associated with imatinib mesylate and perifosine for gastrointestinal stromal tumor

Imatinib mesylate and perifosine are two relatively new drugs that have improved outcomes for patients with gastrointestinal stromal tumors in recent years. The ocular side effects of these two drugs are discussed in this chapter. The most common ocular side effect associated with imatinib mesylate is periorbital edema. Perifosine has been associated with a ring-shaped perilimbal corneal ulceration that can be treated with topical steroids and topical antibiotics.

Two distinct T cell subsets, CD4+ and CD8+CD60+, and their cytokines are required for in vitro induction of human ragweed-specific memory IgE responses

CD8(+)CD60(+) T cells (80-98% CD45RO(+); 20% CD23(+)) are significantly increased in the blood of serum IgE(+) ragweed-sensitized (RS) compared with serum IgE-nonatopic humans (p = 0.001). CD8(+)CD60(+) T cells of the RS patients produced IL-2, IL-4, IL-10, IL-12, IFN-alpha. and IFN-gamma, but not IL-6 or IL-13. When their PBMC were cultured with ragweed Ag (RA), peak IgE responses occurred on day 10; none was induced with non-cross-reacting or without Ag; nonatopic PBMC did not respond to any stimulant. When either CD4(+) or CD8(+)CD60(+) T cells were depleted from RS PBMC before culture with RA, no IgE responses were induced. If purified CD4(+) T cells or low numbers of CD8(+)CD60(+) T cells were added back to the depleted PBMC, IgE responses were restored. However, higher numbers of CD8(+)CD60(+) T cells totally suppressed IgE responses. Total suppression also was obtained when RS PBMC were cultured with RA and either anti-IL-2, IL-4, IL-10, IL-12, IFN-gamma (all concentrations), or IFN-alpha (low concentrations), but not anti-IL-6 or IL-13. Higher concentrations of anti-IFN-alpha potentiated IgE responses.

A gain-of-function mutation in the PDGFR-beta alters the kinetics of injury response in liver and skin

Platelet-derived growth factor (PDGF) isoforms stimulate cell proliferation, migration and survival. We recently generated mice carrying a gain-of-function mutation within the activation loop of PDGF beta-receptor (PDGFR-beta D849N). Embryonic fibroblasts derived from these mice show elevated basal phosphorylation and altered kinetics for ligand-induced activation of PDGFR-beta, as well as enhanced proliferation and migration. To investigate the effect of this mutation in vivo, we used carbon tetrachloride-induced liver injury as a model system. We observed a higher basal activation of mutant PDGFR-beta in unchallenged livers; however, the difference in activation upon carbon tetrachloride stimulation was lower than expected, an effect that might be explained by a delayed response of the mutated receptor toward reactive oxygen species. Mutant mice showed enhanced proliferation of nonparenchymal liver cells and activation of hepatic stellate cells, leading to a small increase in early fibrosis formation. Another mouse strain lacking the binding site for phosphatidylinositol-3' kinase in PDGFR-beta showed the reverse phenotype. These results suggest an important role for PDGFR-beta signaling in the early injury-response. We confirmed this hypothesis with a second injury model, cutaneous wound healing, where we observed earlier proliferation and formation of granulation tissue in D849N-mutant mice.

Role of platelet-derived growth factors in physiology and medicine

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) have served as prototypes for growth factor and receptor tyrosine kinase function for more than 25 years. Studies of PDGFs and PDGFRs in animal development have revealed roles for PDGFR-alpha signaling in gastrulation and in the development of the cranial and cardiac neural crest, gonads, lung, intestine, skin, CNS, and skeleton. Similarly, roles for PDGFR-beta signaling have been established in blood vessel formation and early hematopoiesis. PDGF signaling is implicated in a range of diseases. Autocrine activation of PDGF signaling pathways is involved in certain gliomas, sarcomas, and leukemias. Paracrine PDGF signaling is commonly observed in epithelial cancers, where it triggers stromal recruitment and may be involved in epithelial-mesenchymal transition, thereby affecting tumor growth, angiogenesis, invasion, and metastasis. PDGFs drive pathological mesenchymal responses in vascular disorders such as atherosclerosis, restenosis, pulmonary hypertension, and retinal diseases, as well as in fibrotic diseases, including pulmonary fibrosis, liver cirrhosis, scleroderma, glomerulosclerosis, and cardiac fibrosis. We review basic aspects of the PDGF ligands and receptors, their developmental and pathological functions, principles of their pharmacological inhibition, and results using PDGF pathway-inhibitory or stimulatory drugs in preclinical and clinical contexts.

Integrin alphavbeta3 acts downstream of insulin in normalization of interstitial fluid pressure in sepsis and in cell-mediated collagen gel contraction

The administration of insulin is recommended to patients with severe sepsis and hyperglycemia. Previously, we demonstrated that insulin may have direct anti-inflammatory properties and counteracted fluid losses from the circulation by normalizing the interstitial fluid pressure (P(IF)). P(IF) is one of the Starling forces determining fluid flux over the capillary wall, and a lowered P(IF) is one of the driving forces in early edema formation in inflammatory reactions. Here we demonstrate that insulin restores a lipopolysaccharide (LPS)-lowered P(IF) via a mechanism involving integrin alpha(v)beta(3). In C57 black mice (n = 6), LPS lowered P(IF) from -0.2 +/- 0.2 to -1.6 +/- 0.3 (P < 0.05) and after insulin averaged -0.8 +/- 0.2 mmHg (P = 0.098 compared with after LPS). Corresponding values in wild-type BALB/c mice (n = 5) were -0.8 +/- 0.1, -2.1 +/- 0.3 (P < 0.05), and -0.8 +/- 0.3 mmHg (P < 0.05 compared with LPS) after insulin administration. In BALB/c integrin beta(3)-deficient (beta(3)(-/-)) mice (n = 6), LPS lowered P(IF) from -0.1 +/- 0.2 to -1.5 +/- 0.3 mmHg (P < 0.05). Insulin did not, however, restore P(IF) in these mice (averaged -1.7 +/- 0.3 mmHg after insulin administration). Cell-mediated collagen gel contraction can serve as an in vitro model for in vivo measurements of P(IF). Insulin induced alpha(v)beta(3)-integrin-dependent collagen gel contraction mediated by C2C12 cells. Our findings suggest a beneficiary effect of insulin for patients with sepsis with regard to the fluid balance, and this effect may in part be due to a normalization of P(IF) by a mechanism involving the integrin alpha(v)beta(3).

Safety and pharmacokinetics of dose-intensive imatinib mesylate plus temozolomide: phase 1 trial in adults with malignant glioma

We determined the maximum tolerated dose (MTD) and dose-limiting toxicity (DLT) of imatinib mesylate, an inhibitor of the receptor tyrosine kinases platelet-derived growth factor receptor (PDGFR), the proto-oncogene product c-kit, and the fusion protein Bcr-Abl, when administered for 8 days in combination with temozolomide (TMZ) to malignant glioma (MG) patients. MG patients who had not failed prior TMZ were eligible to receive TMZ at a dose of 150-200 mg/m(2) per day on days 4-8 plus imatinib mesylate administered orally on days 1-8 of each 4-week cycle. Patients were stratified based on concurrent administration of CYP3A4-inducing antiepileptic drugs (EIAEDs). The imatinib dose was escalated in successive cohorts of patients independently for each stratum. Imatinib, at doses ranging from 400 mg to 1,200 mg, was administered with TMZ to 65 patients: 52 (80%) with glioblastoma multiforme (GBM) and 13 (20%) with grade III MG. At enrollment, 34 patients (52%) had stable disease, and 33 (48%) had progressive disease; 30 patients (46%) were on EIAEDs. The MTD of imatinib for patients concurrently receiving or not receiving EIAEDs was 1,000 mg. DLTs were hematologic, gastrointestinal, renal, and hepatic. Pharmacokinetic analyses revealed lowered exposures and enhanced clearance among patients on EIAEDs. Among GBM patients with stable disease at enrollment (n=28), the median progression-free and overall survival times were 41.7 and 56.1 weeks, respectively. Imatinib doses up to 1,000 mg/day for 8 consecutive days are well tolerated when combined with standard TMZ dosing for MG patients. A subsequent phase 2 study is required to further evaluate the efficacy of this regimen for this patient population.

Disruption of PDGFRalpha-initiated PI3K activation and migration of somite derivatives leads to spina bifida

Spina bifida, or failure of the vertebrae to close at the midline, is a common congenital malformation in humans that is often synonymous with neural tube defects (NTDs). However, it is likely that other etiologies exist. Genetic disruption of platelet-derived growth factor receptor (PDGFR) alpha results in spina bifida, but the underlying mechanism has not been identified. To elucidate the cause of this birth defect in PDGFRalpha mutant embryos, we examined the developmental processes involved in vertebrae formation. Exposure of chick embryos to the PDGFR inhibitor imatinib mesylate resulted in spina bifida in the absence of NTDs. We next examined embryos with a tissue-specific deletion of the receptor. We found that loss of the receptor from chondrocytes did not recapitulate the spina bifida phenotype. By contrast, loss of the receptor from all sclerotome and dermatome derivatives or disruption of PDGFRalpha-driven phosphatidyl-inositol 3' kinase (PI3K) activity resulted in spina bifida. Furthermore, we identified a migration defect in the sclerotome as the cause of the abnormal vertebral development. We found that primary cells from these mice exhibited defects in PAK1 activation and paxillin localization. Taken together, these results indicate that PDGFRalpha downstream effectors, especially PI3K, are essential for cell migration of a somite-derived dorsal mesenchyme and disruption of receptor signaling in these cells leads to spina bifida.

Mitogenic signaling via platelet-derived growth factor beta in metanephric mesenchymal cells

Mice deficient in either platelet-derived growth factor (PDGF) B chain or PDGF receptor (PDGFR) beta lack mesangial cells. PDGF stimulates proliferation and migration of metanephric mesenchymal cells, from which mesangial cells are derived. Binding of PDGF to PDGFR-beta induces autophosphorylation at specific tyrosine residues and activates various effector proteins, including phosphatidylinositol-3-kinase (PI3-K). This study explored the role of PI 3-K and reactive oxygen species (ROS) in PDGF-mediated signaling using cells established from wild-type and PDGFR-beta -/- metanephric blastemas at 11.5 days post-conception. PDGF-induced effects that were dependent on PI3-K activation were determined using PDGFR-beta -/- cells made to express "add-back" mutant PDGFR-beta capable of binding PI3-K. We found that PDGF is mitogenic for mesenchymal cells expressing PDGFR-beta, and PI3-K is an important regulator of PDGF-induced DNA synthesis. Activation of ERK1/2 is partially dependent on PI3-K, and both the PI3-K and MEK-ERK1/2 pathways contribute to PI3-K-dependent mitogenesis. In addition, PDGF-induced DNA synthesis in wild-type cells was found to be dependent on ROS that are generated downstream of PI3-K activation. Using antisense oligonucleotides and small interfering RNA, we determined that the NAD(P)H oxidase Nox4 produces these ROS that activate Akt and the MEK-ERK1/2 mitogenic cascade. In conclusion, the present study demonstrates Nox4 involvement in PDGF-induced DNA synthesis in metanephric mesenchymal cells and provides the first evidence that PDGF-induced PI3-K activity enhances production of ROS by Nox4.

Pericytes: gatekeepers in tumour cell metastasis?

Tumour cells use two major routes to spread during metastasis, e.g. lymph vessels and blood vessels within or surrounding the primary tumour. The growth rate of the primary tumour often correlates with the quantity of new blood vessels that form within the tumour. However, qualitative abnormalities of the tumour vasculature profoundly affect the perfusion of the primary tumour and the escape of tumour cells into the circulation. In this paper, we review recent evidence for a novel role of the supporting mural cells in limiting blood-borne metastasis.

Treatment with imatinib improves drug delivery and efficacy in NSCLC xenografts

Imatinib, an inhibitor of PDGF-Rβ and other tyrosine kinase receptors, has been shown to decrease microvessel density and interstitial fluid pressure in solid tumours, thereby improving subsequent delivery of small molecules. The purpose of this study was to test whether pretreatment with imatinib increases the efficacy of traditional chemotherapy in mice bearing non-small cell lung carcinoma xenografts, and to investigate the effects of imatinib on liposomal drug delivery. Efficacy treatment groups included (n=9–10): saline control, imatinib alone (oral gavage, 100 mg kg⁻¹ × 7 days), docetaxel alone (10 mg kg⁻¹ i.p. 2 × /week until killing), and imatinib plus docetaxel (started on day 7 of imatinib). Tumours were monitored until they reached four times the initial treatment volume (4 × V) or 28 days. A separate experiment compared tumour doxorubicin concentrations (using high performance liquid chromatography) 24 h after treatment with liposomal doxorubicin alone (6 mg kg⁻¹ i.v., n=9) or imatinib plus liposomal doxorubicin (n=16). Imatinib plus docetaxel resulted in significantly improved antitumour efficacy (0/10 animals reached 4 × V by 28 days) when compared to docetaxel alone (3/9 reached 4 × V, P=0.014) or imatinib alone (9/10 reached 4 × V, P=0.025). Pretreatment with imatinib also significantly increased tumour concentrations of liposomal doxorubicin. Overall, these preclinical studies emphasise the potential of imatinib as an adjunct to small molecule or liposomal chemotherapy.

Cooperation between the PDGF receptors in cardiac neural crest cell migration

Neural crest cells (NCCs) are essential components of the sympathetic nervous system, skin, craniofacial skeleton, and aortic arch. It has been known for many years that perturbation of migration, proliferation, and/or differentiation of these cells leads to birth defects such as cleft palate and persistent truncus arteriosus (PTA). Previously, we had shown that disruption of the platelet-derived growth factor receptor (PDGFR) alpha in NCCs resulted in defects in craniofacial and aortic arch development, the latter with variable penetrance. Because we observed ventricular septal defects in embryos that are null for the PDGFRbeta, we hypothesized that both PDGF receptors are involved in NCC formation. Here, we show that both receptors are expressed in cardiac NCCs and that the combined loss of the PDGFRalpha and PDGFRbeta in NCCs resulted in NCC-related heart abnormalities, including PTA and a ventricular septal defect (VSD). Using NCC lineage tracing, we observed that loss of PDGF receptor signaling resulted in reduced NCCs in the conotruncus region, leading to defects in aortic arch septation. These results indicate that while PDGFRalpha plays a predominant role in NCC development, the PDGFRbeta is expressed by and functions in cardiac NCCs. Combined PDGF receptor signaling is required for sufficient recruitment of cardiac NCCs into the conotruncal region and for formation of the aortico-pulmonary and ventricular septum.

The platelet-derived growth factor receptor as a therapeutic target

Several small molecule tyrosine kinase inhibitors that block the epidermal growth factor receptor and vascular endothelial growth factor receptor function are among the many recently developed targeted anticancer therapeutic agents. Increasing evidence indicates that inhibition of other tumor stromal targets could provide additional and possibly synergistic antitumor effects. This article focuses on the platelet-derived growth factor receptor as one such potential target.

Emerging role of platelet-derived growth factor receptor-beta inhibition in radioimmunotherapy of experimental pancreatic cancer

PURPOSE

Thus far, the therapy of pancreatic cancer remains an insurmountable challenge. Not a solitary therapeutic modality in the battery of available therapeutic options is capable to cure or, at the very least, stop the progression of this disease in any meaningful way. The purpose of reported here studies was to implement a multimodality approach to radioimmunotherapy of pancreatic cancer and, ultimately, to develop a course of therapy with the clinical value.

EXPERIMENTAL DESIGN

Animal model was NCr-nu/nu mouse bearing s.c. xenografts of SW1990 pancreatic adenocarcinoma. Radioimmunotherapy based on (131)ICC49, a TAG-72-targeting monoclonal antibody, was augmented with imatinib, a potent inhibitor of platelet-derived growth factor receptor-beta. The postulated interactions between these two modalities depended on the imatinib-induced drop in the tumor interstitial fluid pressure and the subsequent increase of (131)ICC49 uptake into the tumor, resulting in improved tumor responses to radioimmunotherapy.

RESULTS

Biodistribution studies revealed a 50% improvement in the tumor uptake of (131)ICC49 in mice treated with imatinib. Tumor development was practically arrested for approximately 3 weeks in response to the treatment composed of (131)ICC49 and imatinib with tumor quadrupling time (T(Q)) of 40.8 days. (131)ICC49 alone and imatinib alone also delayed the tumor growth to T(Q) of 30.2 and 31.2 days, respectively. Unanticipated was the significant response of SW1990 to a brief treatment with imatinib given i.p. at 100 mg/kg b.i.d. for 3 days. Xenografts in control mice receiving injection of PBS had T(Q) of 23 days.

CONCLUSIONS

The inclusion of imatinib in the radioimmunotherapy regimen is beneficial and it does not produce any overt side effects. The improved responses of pancreatic cancer xenografts to the multimodality treatment comprising radioimmunotherapy and platelet-derived growth factor receptor-beta inhibition suggest that this approach to therapy of pancreatic cancer may also be successful in patients.

c-Src couples PI 3 kinase/Akt and MAPK signaling to PDGF-induced DNA synthesis in mesangial cells

Platelet-derived growth factor BB (PDGF) and PDGF receptor-beta (PDGFR) play critical roles in mesangial cell proliferation during embryonic development and in mesangioproliferative glomerulonephritis. We have shown previously that phosphatidylinositol (PI) 3 kinase/Akt and Erk1/2 mitogen-activated protein kinase (MAPK) contribute to PDGF-dependent proliferation of mesangial cells, but the mechanism by which these two enzyme cascades are activated by PDGFR signaling is not precisely known. We examined the role of c-Src tyrosine kinase in this process. PDGF increased phosphorylation of c-Src in a time-dependent manner indicating its activation. A pharmacologic inhibitor of c-Src, PP1, blocked PDGF-induced DNA synthesis with concomitant inhibition of c-Src phosphorylation. Immune-complex kinase assays of c-Src and PDGFR demonstrated inhibition of c-Src tyrosine kinase activity by PP1, without an effect on PDGFR tyrosine phosphorylation. Both PP1 and expression of dominant negative c-Src inhibited PDGF-induced PI 3 kinase, resulting in attenuation of Akt kinase activity. Expression of constitutively active c-Src increased Akt activity to the same extent as with PDGF. Constitutively active c-Src augmented PDGF-induced Akt activity, thus contributing to Akt signaling. Inhibition of c-Src tyrosine kinase blocked PDGF-stimulated MAPK activity and resulted in attenuation of c-fos gene transcription with concomitant prevention of Elk-1 transactivation. Furthermore, inhibition of c-Src increased p27(Kip1) cyclin kinase inhibitor, and attenuated PDGF-induced pRb phosphorylation and CDK2 activity. These data provide the first evidence in mesangial cells that PDGF-activated c-Src tyrosine kinase relays signals to PI 3 kinase/Akt and MAPK. Furthermore our results demonstrate that c-Src integrates signals into the nucleus to activate CDK2, which is required for DNA synthesis.

Lowering of interstitial fluid pressure after neurogenic inflammation in mouse skin is partly dependent on mast cells

Neurogenic inflammation is known to induce lowering of interstitial fluid pressure (Pif) in mouse skin. This study examined the possible role of mast cell activation secondary to neuropeptide release in lowering of Pifby using KitW/ KitW-vmice, which are devoid of mast cells, including connective tissue mast cells (CTMCs). Pifwas measured in paw skin of anesthetized (fentanyl-fluanison and midazolam, 1:1) mice with glass capillaries connected to a servo-controlled counterpressure system. In contrast to wild-type mice, intravenous administration of mast cell-activating compound 48/80 induced no lowering of Pifin KitW/ KitW-vmice. Intravenous challenge with substance P (SP), calcitonin gene-related peptide (CGRP), or capsaicin induced a significant ( P < 0.05) lowering of Pifin wild-type mice to −2.16 ± 0.28, −1.96 ± 0.11, and −2.22 ± 0.19 mmHg, respectively, compared with vehicle (−0.49 ± 0.11 mmHg). In KitW/ KitW-vmice the Pifresponse to SP was completely abolished (−0.53 ± 0.32 mmHg) while the response to CGRP and capsaicin was attenuated (−1.33 ± 0.13 and −1.42 ± 0.13 mmHg, respectively) although significantly ( P < 0.05) lowered compared with vehicle. Immunohistochemical analysis revealed no difference in distribution or density of SP- and CGRP-immunoreactive fibers in paws of KitW/ KitW-vcompared with wild-type mice. We conclude that lowering of Pifnormally depends on mast cells. However, the sensory nerves can also elicit a lowering of Pifthat is independent of mast cells.

Imaging the modulation of adenoviral kinetics and biodistribution for cancer gene therapy

To explore systemic utilization of Epstein-Barr virus (EBV)-specific transcriptionally targeted adenoviruses, three vectors were constructed to examine kinetics, specificity, and biodistribution: adv.oriP.luc, expressing luciferase under EBV-specific control; adv.SV40luc, expressing luciferase constitutively; and adv.oriP.E1A.oriP.luc, a conditionally replicating adenovirus, expressing both luciferase and E1A. Bioluminescence imaging (BLI) was conducted on tumor-bearing severe combined immunodeficient (SCID) mice (C666-1, EBV-positive human nasopharyngeal cancer) treated intravenously (i.v.) with 3 x 10(8) infectious units (ifu) of the adenoviral vectors. At 72 hours, adv.oriPluc demonstrated an 8.4-fold higher tumor signal than adv.SV40luc; adv.oriP.E1A.oriP.luc was 26.7-fold higher; however, a significant liver signal was also observed, necessitating further action to improve biodistribution. Several compounds were examined to this end, including norepinephrine, serotonin, clodronate liposomes, and STI571, to determine whether any of these measures could improve adenoviral biodistribution. Each of these interventions was assessed using BLI in mice i.v. injected with adv.oriP.luc. STI571 achieved the highest increase in tumor-to-liver ratio (TLR; 6.6-fold), which was associated with a 59% reduction in tumor interstitial fluid pressure (IFP) along with a decrease in platelet-derived growth factor-beta receptor (PDGF beta R) activation. This study reports the favorable modulation by STI571 of the biodistribution of adenoviral vectors, providing a potential approach to improving therapeutic outcome.

Emerging Safety Issues with Imatinib and Other Abl Tyrosine Kinase Inhibitors

Imatinib and other Abl tyrosine kinase inhibitors (TKIs), such as dasatinib and nilotinib, have significantly improved the outcome of patients with chronic myeloid leukemia. Imatinib and dasatinib are currently Food and Drug Administration (FDA) approved, and nilotinib is expected to gain FDA approval soon. In addition, several other Abl TKIs are being evaluated in various clinical trials. Imatinib has also shown efficacy in the therapy of gastrointestinal stromal tumors, Philadelphia chromosome-positive acute lymphocytic leukemia and hypereosinophilic syndrome. Because of their efficacy, more patients will receive Abl TKIs for a longer period of time. Imatinib was FDA approved after a short follow-up because of its exceptional efficacy and safety profile. The most common adverse events reported included fluid retention, fatigue, diarrhea, and muscle cramps. With longer follow-up, issues related to the long-term use of imatinib have been discussed. Our aim is to review the emerging safety issues of Abl TKIs after a longer follow-up.

PDGF Receptors as Targets in Tumor Treatment

Signaling through platelet-derived growth factor (PDGF) receptors contributes to multiple tumor-associated processes. The recent introduction of clinically useful PDGF inhibitors have the last years validated PDGF receptors in malignant and stromal cells as relevant cancer drug targets. Mutational activation of PDGF receptor signaling in malignant cells has been described in some rare tumor types such as dermatofibrosarcoma protuberans, a subset of GISTs, and some hematologic malignancies. Furthermore, expression of PDGF receptors on pericytes is a common characteristic of solid tumors. The clinical efficacy of novel multikinase inhibitors, such as sunitinib and sorafenib, most likely involves targeting of PDGF receptor-dependent pericytes. Preclinical studies suggest that targeting of stromal PDGF receptors might also constitute a novel strategy to enhance tumor drug uptake. Finally, recent studies have implied both pro- and antimetastatic effects of PDGF receptors on malignant and stromal cells. The studies on the roles of PDGF receptors in cancer signaling are thus presently in a dynamic phase where collaborations between oncologists, pathologists, and tumor biologists are predicted to be highly productive.

Expression of activated platelet-derived growth factor receptor in stromal cells of human colon carcinomas is associated with metastatic potential

Platelet-derived growth factor receptor (PDGF-R) expression has been reported in a variety of cancers, including colorectal, breast, lung, ovarian and pancreatic cancers, but the role of PDGF-R expression in the development and progression of colon carcinoma has not yet been elucidated. The purpose of this study was to examine the expression of PDGF and PDGF-R in human colon carcinomas. The expression of PDGF, PDGF-R and phosphorylated PDGF-R (p-PDGF-R) was examined by immunofluorescence in 12 surgical specimens of colon carcinoma and in human colon carcinoma cells growing in the subcutis (ectopic site) and the cecal wall (orthotopic site) of nude mice. In most surgical specimens, tumor cells expressed PDGF-A and -B subunits, without corresponding levels of PDGF-Ralpha and PDGF-Rbeta. PDGF-Rbeta was predominantly expressed by tumor-associated stromal cells and pericytes of tumor vasculature. The expression of PDGF-Rbeta in the stroma was associated with advanced stage disease. Under culture conditions, human colon carcinoma cell lines expressed PDGF-A and -B, but not PDGF-R. In orthotopic tumors, the KM12 cells (Duke's stage B) expressed PDGF-A and -B, but PDGF-Rbeta was expressed only by stromal cells and pericytes in the tumor vasculature. This expression of PDGF-Rbeta by stromal cells and pericytes was higher in tumors growing at the orthotopic site than in those at the ectopic site. The expression of PDGF-Rbeta in the stroma was higher in highly metastatic KM12SM tumors than in low metastatic KM12C tumors. In conclusion, the expression of PDGF-Rbeta in stromal cells is influenced by the organ-specific microenvironment and is associated with metastatic potential.

Expression of platelet derived growth factor family members and the potential role of imatinib mesylate for cervical cancer

BACKGROUND

Despite significant achievements in the treatment of cervical cancer, it is still a deadly disease; hence newer therapeutical modalities are needed. Preliminary investigations suggest that platelet-derived growth factor (PDGF) might have a role in the development of cervical cancer, therefore it is important to determine whether this growth factor pathway is functional and its targeting with imatinib mesylate leads to growth inhibition of cervical cancer cells.

RESULTS

PDGF receptors (PDGFR) and their ligands are frequently expressed in cervical cancer and the majority exhibited a combination of family members co-expression. A number of intronic and exonic variations but no known mutations in the coding sequence of the PDGFRalpha gene were found in cancer cell lines and primary tumors. Growth assays demonstrated that PDGFBB induces growth stimulation that can be blocked by imatinib and that this tyrosine kinase inhibitor on its own inhibits cell growth. These effects were associated with the phosphorylation status of the receptor.

CONCLUSION

The PDGFR system may have a role in the pathogenesis of cervical cancer as their members are frequently expressed in this tumor and cervical cancer lines are growth inhibited by the PDGFR antagonist imatinib.

Treatment with Imatinib in NSCLC is associated with decrease of phosphorylated PDGFR-beta and VEGF expression, decrease in interstitial fluid pressure and improvement of oxygenation

Elevated intratumoral interstitial fluid pressure (IFP) and tumour hypoxia are independent predictive factors for poor survival and poor treatment response in cancer patients. However, the relationship between IFP and tumour hypoxia has not yet been clearly established. Preclinical studies have shown that lowering IFP improves treatment response to cytotoxic therapy. Interstitial fluid pressure can be reduced by inhibition of phosphorylated platelet-derived growth factor receptor-β (p-PDGFR-β), a tyrosine kinase receptor frequently overexpressed in cancer stroma, and/or by inhibition of VEGF, a growth factor commonly overexpressed in tumours overexpressing p-PDGFR-β. We hypothesised that Imatinib, a specific PDGFR-β inhibitor will, in addition to p-PDGFR-β inhibition, downregulate VEGF, decrease IFP and improve tumour oxygenation. A549 human lung adenocarcinoma xenografts overexpressing PDGFR-β were grown in nude mice. Tumour-bearing animals were randomised to control and treatment groups (Imatinib 50 mg kg⁻¹ via gavage for 4 days). Interstitial fluid pressure was measured in both groups before and after treatment. EF5, a hypoxia marker, was administered 3 h before being killed. Tumours were sectioned and stained for p-PDGFR-β, VEGF and EF5 binding. Stained sections were viewed with a fluorescence microscope and image analysis was performed. Imatinib treatment resulted in significant reduction of p-PDGFR-β, VEGF and IFP. Tumour oxygenation was also significantly improved. This study shows that p-PDGFR-β-overexpressing tumours can be effectively treated with Imatinib to decrease tumour IFP. Importantly, this is the first study demonstrating that Imatinib treatment improves tumour oxygenation and downregulates tumour VEGF expression.

Targeting platelet-derived growth factor receptor on endothelial cells of multidrug-resistant prostate cancer

BACKGROUND

Inhibiting phosphorylation of platelet-derived growth factor receptor (PDGFR) by treatment with the PDGFR kinase inhibitor imatinib and the chemotherapeutic agent paclitaxel reduces the incidence and size of human prostate cancer bone lesions in nude mice. Because tumor cells and tumor-associated endothelial cells express activated PDGFR, the primary target for imatinib has been unclear.

METHODS

We selected multidrug-resistant human PC-3MM2 prostate cancer cells (termed PC-3MM2-MDR cells) by culturing them in increasing concentrations of paclitaxel. PC-3MM2-MDR cells were implanted into one tibia of 80 nude mice. Two weeks later, the mice were randomly assigned to receive distilled water (control group), paclitaxel, imatinib, or imatinib plus paclitaxel for 10 weeks (20 mice per group). Tumor incidence and weight, bone structure preservation and osteolysis, and the incidence of lymph node metastasis were determined. The phosphorylation status of PDGFR on tumor cells and tumor-associated endothelial cells and levels of apoptosis were examined with immunohistochemical analyses. Microvessel density was assessed as the number of cells expressing CD31/platelet endothelial cell adhesion molecule 1 (PECAM-1). All statistical tests were two-sided.

RESULTS

PC-3MM2-MDR cells were resistant to paclitaxel and imatinib in vitro. Treatment of implanted mice with imatinib plus paclitaxel led to statistically significant decreases in bone tumor incidence (control = 19 mice with tumors of 19 mice total; imatinib plus paclitaxel = four of 18 mice; P < .001), median tumor weight (control = 1.3 g, interquartile range [IQR] = 1.0-1.9; imatinib plus paclitaxel = 0.1 g, IQR = 0-0.3; P < .001), bone lysis, and the incidence of lymph node metastasis (control = 19 of 19 mice total; imatinib plus paclitaxel = three of 18 mice; P < .001). Treatment with imatinib alone had similar effects, and imatinib treatment also inhibited phosphorylation of PDGFR on tumor cells and tumor-associated endothelial cells and increased the level of apoptosis of endothelial cells, but not tumor cells. Treatment with imatinib and more so with imatinib and paclitaxel decreased mean vessel density (three CD31/PECAM-1-positive cells, 95% confidence interval [CI] = 0 to 9; and control group = 38 CD31/PECAM-1-positive cells, 95% CI = 17 to 59) (P < .001), which was followed by apoptosis of tumor cells.

CONCLUSION

Tumor-associated endothelial cells, rather than tumor cells themselves, appear to be the target for imatinib in prostate cancer bone metastasis.

Platelet-Derived Growth Factor BB–Mediated Normalization of Dermal Interstitial Fluid Pressure After Mast Cell Degranulation Depends on β3 but Not β1 Integrins

Interstitial fluid pressure (P IF ) is one of the determinants of transcapillary fluid flux and thereby interstitial fluid volume. Cell-mediated control of P IF regulates fluid content in the loose interstitial connective tissues that surround the capillary bed. To maintain a normal P IF in dermis, β1 integrins mediate the tensile strength applied by connective tissue cells on the extracellular matrix. Platelet-derived growth factor (PDGF)-BB normalizes anaphylaxis-induced reduction of P IF . Anti–β3 integrin IgG and a cyclic RGD peptide that inhibits the αVβ3 integrin blocked the ability of PDGF-BB to normalize the lowered P IF resulting from mast cell degranulation. PDGF-BB was unable to normalize P IF lowered as a result of mast cell degranulation in β3-negative mice. Monoclonal anti–β3 integrin IgG had no effect on P IF in normal mouse dermis. In contrast, administration of anti–β1 integrin IgM lowered P IF in normal dermis but had no effect on PDGF-BB–induced normalization of P IF after anaphylaxis. Furthermore, collagen gel contraction mediated by wild-type mouse embryonal fibroblasts were only marginally affected by function-blocking anti–β1 integrin antibodies, especially in the presence of PDGF-BB. In contrast, contraction mediated by αV-negative mouse embryonic fibroblasts was completely blocked by anti–β1 integrin antibodies, even after stimulation with PDGF-BB. These results show a previously unrecognized in vivo function for the αVβ3 integrin, as a participant in the control of P IF during inflammatory reactions. Furthermore, our data demonstrate that PDGF-BB induces connective tissue cells to generate tensile forces via αVβ3 during such reactions.

Phase II study of imatinib mesylate plus hydroxyurea in adults with recurrent glioblastoma multiforme

PURPOSE

We performed a phase II study to evaluate the combination of imatinib mesylate, an adenosine triphosphate mimetic, tyrosine kinase inhibitor, plus hydroxyurea, a ribonucleotide reductase inhibitor, in patients with recurrent glioblastoma multiforme (GBM).

PATIENTS AND METHODS

Patients with GBM at any recurrence received imatinib mesylate plus hydroxyurea (500 mg twice a day) orally on a continuous, daily schedule. The imatinib mesylate dose was 500 mg twice a day for patients on enzyme-inducing antiepileptic drugs (EIAEDs) and 400 mg once a day for those not on EIAEDs. Assessments were performed every 28 days. The primary end point was 6-month progression-free survival (PFS).

RESULTS

Thirty-three patients enrolled with progressive disease after prior radiotherapy and at least temozolomide-based chemotherapy. With a median follow-up of 58 weeks, 27% of patients were progression-free at 6 months, and the median PFS was 14.4 weeks. Three patients (9%) achieved radiographic response, and 14 (42%) achieved stable disease. Cox regression analysis identified concurrent EIAED use and no more than one prior progression as independent positive prognostic factors of PFS. The most common toxicities included grade 3 neutropenia (16%), thrombocytopenia (6%), and edema (6%). There were no grade 4 or 5 events. Concurrent EIAED use lowered imatinib mesylate exposure. Imatinib mesylate clearance was decreased at day 28 compared with day 1 in all patients, suggesting an effect of hydroxyurea.

CONCLUSION

Imatinib mesylate plus hydroxyurea is well tolerated and associated with durable antitumor activity in some patients with recurrent GBM.

Deterministic model of dermal wound invasion incorporating receptor-mediated signal transduction and spatial gradient sensing

During dermal wound healing, platelet-derived growth factor (PDGF) serves as both a chemoattractant and mitogen for fibroblasts, potently stimulating their invasion of the fibrin clot over a period of several days. A mathematical model of this process is presented, which accurately accounts for the sensitivity of PDGF gradient sensing through PDGF receptor/phosphoinositide 3-kinase-mediated signal transduction. Analysis of the model suggests that PDGF receptor-mediated endocytosis and degradation of PDGF allows a constant PDGF concentration profile to be maintained at the leading front of the fibroblast density profile as it propagates, at a constant rate, into the clot. Thus, the constant PDGF gradient can span the optimal concentration range for asymmetric phosphoinositide 3-kinase signaling and fibroblast chemotaxis, with near-maximal invasion rates elicited over a relatively broad range of PDGF secretion rates. A somewhat surprising finding was that extremely sharp PDGF gradients do not necessarily stimulate faster progression through the clot, because maintaining such a gradient through PDGF consumption is a potentially rate-limiting process.

An interstitial network of podoplanin-expressing cells in the human endolymphatic duct

The human endolymphatic duct (ED) with encompassing interstitial connective tissue (CT) is believed to be important for endolymph resorption and fluid pressure regulation of the inner ear. The periductal CT cells are interconnected via numerous cellular extensions, but do not form vessel structures. Here we report that the periductal CT is populated by two distinct cell phenotypes; one expressing podoplanin, a protein otherwise found on lymph endothelia and on epithelia involved in fluid fluxes, and a second expressing a fibroblast marker. A majority of the interstitial cells expressed podoplanin but not the lymphatic endothelial cell markers hyaluronan receptor (LYVE-1) or vascular endothelial growth factor receptor-3 (VEGFR-3). The fibroblast marker positive cells were found close to the ED epithelium. In the mid- and distal parts of the ED, these cells were enriched under folded epithelia. Furthermore, subepithelial CT cells were found to express activated platelet derived growth factor (PDGF)-beta receptors. Cultured CT cells from human inner ear periductal and perisaccular explant tissues were identified as fibroblasts. These cells compacted a three-dimensional collagen lattice by a process that could be promoted by PDGF-BB, a factor involved in interstitial fluid pressure regulation. Our results are compatible with the notion that the periductal CT cells are involved in the regulation of inner ear fluid pressure. By active compaction of the periductal CT and by the formation of villous structures, the CT cells could modulate fluid fluxes over the ED epithelium as well as the longitudinal flow of endolymph in the ED.

Effect of platelet-derived growth factor receptor-beta inhibition with STI571 on radioimmunotherapy

Whereas radioimmunotherapy of hematologic malignancies has evolved into a viable treatment option, the responses of solid tumors to radioimmunotherapy are discouraging. The likely cause of this problem is the interstitial hypertension inherent to all solid tumors. Remarkable improvements in tumor responses to radioimmunotherapy were discovered after the inclusion of STI571 in the therapy regimen. A combination of the tumor stroma-reactive STI571, a potent platelet-derived growth factor receptor-beta (PDGFr-beta) antagonist, and the tumor-seeking radiolabeled antibody B72.3 yielded long-lasting growth arrest of the human colorectal adenocarcinoma LS174T grown as s.c. xenografts in athymic mice. The interaction of STI571 with the stromal PDGFr-beta reduced tumor interstitial fluid pressure (P(IF)) by >50% and in so doing improved the uptake of B72.3. The attenuation of P(IF) also had a positive effect on the homogeneity of antibody distribution. These effects were dose-dependent and under optimized dosing conditions allowed for a 2.45 times increase in the tumor uptake of B72.3 as determined in the biodistribution studies. Single-photon emission computed tomography imaging studies substantiated these results and indicated that the homogeneity of the radioisotope distribution was also much improved when compared with the control mice. The increased uptake of radioimmunotherapy into the tumor resulted in >400% increase in the tumor absorbed radiation doses in STI571 + radioimmunotherapy-treated mice compared with PBS + radioimmunotherapy-treated mice. The improved antibody uptake in response to the attenuation of tumor P(IF) was identified as the primary reason for the growth arrest of the STI571 + radioimmunotherapy-treated tumors. Two related causes were also identified: (a) the improved homogeneity of monoclonal antibody distribution in tumor and (b) the increased tumor radiosensitivity resulting from the improved tumor oxygenation.

Endothelial/pericyte interactions

Interactions between endothelial cells and mural cells (pericytes and vascular smooth muscle cells) in the blood vessel wall have recently come into focus as central processes in the regulation of vascular formation, stabilization, remodeling, and function. Failure of the interactions between the 2 cell types, as seen in numerous genetic mouse models, results in severe and often lethal cardiovascular defects. Abnormal interactions between the 2 cell types are also implicated in a number of human pathological conditions, including tumor angiogenesis, diabetic microangiopathy, ectopic tissue calcification, and stroke and dementia syndrome CADASIL. In the present review, we summarize current knowledge concerning the identity, characteristics, diversity, ontogeny, and plasticity of pericytes. We focus on the advancement in recent years of the understanding of intercellular communication between endothelial and mural cells with a focus on transforming growth factor beta, angiopoietins, platelet-derived growth factor, spingosine-1-phosphate, and Notch ligands and their respective receptors. We finally highlight recent important data contributing to the understanding of the role of pericytes in tumor angiogenesis, diabetic retinopathy, and hereditary lymphedema.

Cutaneous side-effects of kinase inhibitors and blocking antibodies

Although kinase inhibitors raise hope for people with cancer, patients and their clinicians are commonly confronted with the cutaneous side-effects that are associated with the use of these drugs. This review is the result of collaborations between dermatologists, medical oncologists, and pathologists, and discusses the cutaneous side-effects seen after treatment with the inhibitors of epidermal-growth-factor receptor (EGFR), imatinib, sorafenib, and sunitinib. Some of the side-effects caused by these agents are very distressing, partly because they are chronic owing to the long duration of treatment. Therefore, patients need early and appropriate dermatological management. Moreover, several studies have reported a link between the antitumour efficacy of EGFR inhibitors and cutaneous side-effects. Elucidation of this connection could lead to the identification of crucial predictive factors for tumour response.

Modulation of bone microenvironment with zoledronate enhances the therapeutic effects of STI571 and paclitaxel against experimental bone metastasis of human prostate cancer

Prostate cancer cells metastasize to the bone where their interaction with osteoclasts and osteoblasts can lead to alterations in the structure of the bone. We determined whether the systemic administration of the bisphosphonate, zoledronate, could prevent bone lysis and halt the proliferation of human prostate cancer cells injected into the tibia of nude mice. Zoledronate did not affect the in vitro proliferation of human prostate cancer PC-3MM2 cells. The in vivo administration of zoledronate produced significant bone preservation but did not inhibit the progressive growth of PC-3MM2 cells. The systemic administration of STI571 (imatinib mesylate, Gleevec), an inhibitor of phosphorylation of the platelet-derived growth factor receptor, in combination with paclitaxel, produced apoptosis of tumor cells and bone- and tumor-associated endothelial cells. The systemic administration of zoledronate with STI571 and paclitaxel produced a significant preservation of bone structure, a decrease in tumor incidence and weight, and a decrease in incidence of lymph node metastasis. This therapeutic activity was correlated with inhibition of osteoclast function, inhibition of tumor cell proliferation, and induction of apoptosis in tumor-associated endothelial cells and tumor cells. Cancer is a heterogeneous disease that requires multimodality therapy. The present data recommend the combination of a bisphosphonate agent with protein tyrosine kinase inhibitor and an anticycling drug for the treatment of prostate cancer bone metastasis.

Chemosensitization by STI571 targeting the platelet-derived growth factor/platelet-derived growth factor receptor-signaling pathway in the tumor progression and angiogenesis of gastric carcinoma

BACKGROUND

Autocrine and paracrine growth mediated by the platelet-derived growth factor (PDGF)/PDGF receptor (PDGFR)-signaling pathway plays an important role in the progression of solid tumors. The authors assessed the effect of STI571 on the tumor growth of gastric carcinoma in combination with 5-fluorouracil (5-FU) or paclitaxel targeting the PDGF/PDGFR-signaling pathway.

METHODS

In MKN-45 gastric carcinoma cells, the cytotoxic effect was evaluated by 3-(4,5 dimethiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay, and the in vivo antitumor effect was evaluated in a nude mouse xenograft. Both STI571 and an antitumor drug were administered intraperitoneally. Gene expression was assessed by Western blot analysis and immunohistochemical staining. Apoptotic cell death was evaluated by the terminal deoxyuridine triphosphate-biotin nick-end labeling assay, and tumor angiogenesis was evaluated by microvessel density analysis.

RESULTS

Treatment with STI571 alone was not effective in vitro, as assessed by a 50% inhibitory concentration value of 24.3 microM. Combination treatment with STI571 and 5-FU or paclitaxel enhanced the cytotoxic effect somewhat when the concentration of STI571 was increased to 10 microM. Combination treatment with STI571 and 5-FU or paclitaxel enhanced the antitumor effect of the antitumor drug significantly in vivo. The enhanced antitumor effect was associated with increased apoptotic cell death and inhibition of tumor angiogenesis. Treatment with STI571 down-regulated the expression of PDGF-BB and PDGFR-beta in tumor cells and decreased the production of phosphorylated PDGFR-beta and phosphorylated Akt. Furthermore, treatment with STI571 inhibited the expression of PDGFR-beta in stromal cells.

CONCLUSIONS

STI571 was an effective chemosensitizer of antitumor drugs, such as 5-FU and paclitaxel for gastric carcinoma, targeting the PDGF/PDGFR-signaling pathway of tumor cells and stromal cells in disease progression and angiogenesis.

Concurrent male gynecomastia and testicular hydrocele after imatinib mesylate treatment of a gastrointestinal stromal tumor

We report a gastrointestinal stromal tumor (GIST) patient with male gynecomastia and testicular hydrocele after treatment with imatinib mesylate. A 42 yr-old male patient presented for management of hepatic masses. Two years earlier, he had undergone a small bowel resection to remove an intraabdominal mass later shown to be a GIST, followed by adjuvant radiation therapy. At presentation, CT scan revealed multiple hepatic masses, which were compatible with metastatic GIST, and he was prescribed imatinib 400 mg/day. During treatment, he experienced painful enlargement of the left breast and scrotal swelling. Three months after cessation of imatinib treatment, the tumors recurred, and, upon recommencing imatinib, he experienced painful enlargement of the right breast and scrotal swelling. He was diagnosed with male gynecomastia caused by decreased testosterone and noncommunicative testicular hydrocele. He was given androgen support and a hydrocelectomy, which improved his gynecomastia. The mechanism by which imatinib induces gynecomastia and hydrocele is thought to be associated with an inhibition of c-KIT and platelet-derive growth factor. This is the first report, to our knowledge, describing concurrent male gynecomastia and testicular hydrocele after imatinib treatment of a patient with GIST.

Improving delivery of antineoplastic agents with anti-vascular endothelial growth factor therapy

It is believed that impairments in delivery of antineoplastic agents to solid tumors result from abnormalities of the tumor microenvironment. Vascular endothelial growth factor (VEGF), the prototypical angiogenic molecule, is one of the main factors responsible for the development and maintenance of the aberrant tumor vascular network, which is characterized by chaotic, leaky blood vessels with high interstitial fluid pressure and inefficient blood flow. The authors proposed that anti-VEGF therapy would reduce the elevated interstitial fluid pressure in tumors, thereby improving blood flow and potentially improving delivery of cytotoxic agents to tumor cells. For the current report, the authors reviewed characteristics of the abnormal tumor vasculature created under the influence of VEGF, the resulting tumor microenvironment, how the tumor microenvironment may impede delivery of antineoplastic agents, and how the combination of anti-VEGF and cytotoxic therapy may maximize the efficacy of antineoplastic treatment regimens.

Role of phosphoinositide 3-kinase regulatory isoforms in development and actin rearrangement

Class Ia phosphoinositide 3-kinases (PI3Ks) are heterodimers of p110 catalytic and p85 regulatory subunits that mediate a variety of cellular responses to growth and differentiation factors. Although embryonic development is not impaired in mice lacking all isoforms of the p85alpha gene (p85alpha-/- p55alpha-/- p50alpha-/-) or in mice lacking the p85beta gene (p85beta-/-) (D. A. Fruman, F. Mauvais-Jarvis, D. A. Pollard, C. M. Yballe, D. Brazil, R. T. Bronson, C. R. Kahn, and L. C. Cantley, Nat Genet. 26:379-382, 2000; K. Ueki, C. M. Yballe, S. M. Brachmann, D. Vicent, J. M. Watt, C. R. Kahn, and L. C. Cantley, Proc. Natl. Acad. Sci. USA 99:419-424, 2002), we show here that loss of both genes results in lethality at embryonic day 12.5 (E12.5). The phenotypes of these embryos, including subepidermal blebs flanking the neural tube at E8 and bleeding into the blebs during the turning process, are similar to defects observed in platelet-derived growth factor receptor alpha null (PDGFRalpha-/-) mice (P. Soriano, Development 124:2691-2700, 1997), suggesting that PI3K is an essential mediator of PDGFRalpha signaling at this developmental stage. p85alpha-/- p55alpha+/+ p50alpha+/+ p85beta-/- mice had similar but less severe defects, indicating that p85alpha and p85beta have a critical and redundant function in development. Mouse embryo fibroblasts deficient in all p85alpha and p85beta gene products (p85alpha-/- p55alpha-/- p50alpha-/- p85beta-/-) are defective in PDGF-induced membrane ruffling. Overexpression of the Rac-specific GDP-GTP exchange factor Vav2 or reintroduction of p85alpha or p85beta rescues the membrane ruffling defect. Surprisingly, reintroduction of p50alpha also restored PDGF-dependent membrane ruffling. These results indicate that class Ia PI3K is critical for PDGF-dependent actin rearrangement but that the SH3 domain and the Rho/Rac/Cdc42-interacting domain of p85, which lacks p50alpha, are not required for this response.

Functions and mechanisms of receptor tyrosine kinase Torso signaling: lessons from Drosophila embryonic terminal development

The Torso receptor tyrosine kinase (RTK) is required for cell fate specification in the terminal regions (head and tail) of the early Drosophila embryo. Torso contains a split tyrosine kinase domain and belongs to the type III subgroup of the RTK superfamily that also includes the platelet-derived growth factor receptors, stem cell or steel factor receptor c-Kit proto-oncoprotein, colony-stimulating factor-1 receptor, and vascular endothelial growth factor receptor. The Torso pathway has been a model system for studying RTK signal transduction. Genetic and biochemical studies of Torso signaling have provided valuable insights into the biological functions and mechanisms of RTK signaling during early Drosophila embryogenesis.

PDGF receptors-mediators of autocrine tumor growth and regulators of tumor vasculature and stroma

PDGFs and their cognate tyrosine kinase alpha- and beta-receptors are involved in multiple tumor-associated processes including autocrine growth stimulation of tumor cells, stimulation of tumor angiogenesis and recruitment and regulation of tumor fibroblasts. The recent development of clinically useful PDGF antagonists, like STI571/Glivec, has increased the interest in PDGF receptors as cancer drug targets. Autocrine PDGF receptor signaling occurs in certain malignancies characterized by mutational activation of PDGF or PDGF receptors, for instance, dermatofibrosaracoma protuberans, gastrointestinal stromal tumors, and hypereosinophilic syndrome. The roles of PDGF in regulation of tumor angiogenesis and tumor fibroblasts are more general, and probably occur in most common solid tumors. Concerning tumor angiogenesis recent studies have predominantly focused on the importance of PDGF receptor signaling for tumor pericyte recruitment. PDGF receptors in the tumor stroma have also attracted attention as interesting drug targets because of their function as regulators of tumor interstitial fluid pressure, tumor transvascular transport and tumor drug uptake. In summary, the improved understanding of the role of PDGF signaling in tumor biology, and the introduction of PDGF antagonists, has set the stage for a continued development of PDGF antagonists as novel cancer drugs.

PDGF signaling in cells and mice

Since its discovery over three decades ago, platelet-derived growth factor (PDGF) has been a model system for learning how growth factors regulate biological processes. For the first several decades investigators used cells grown in tissue culture. More recently, PDGF signaling has also been investigated in mice. This review outlines the advances in these two systems, and highlights some of the directions for future investigation.

Insight into the physiological functions of PDGF through genetic studies in mice

Genetic analyses in mice have contributed significantly to the understanding of the physiological functions of platelet-derived growth factors (PDGFs) and their receptors. Phenotypic analyses of gene knockouts of PDGF-A, PDGF-B, PDGF alpha-receptors (PDGFRalpha) and beta-receptors (PDGFRbeta) have shown that these ligands and receptors play major roles during embryonic development. Conditional and subtle mutations in the same genes and analysis of chimeric mice have provided additional information about the roles of these genes in postnatal development. Transgenic over-expression studies have also demonstrated that PDGF ligands are capable of inducing pathological cell proliferation in a number of different organs. The present review summarizes these findings and discusses their implications for mammalian development and disease.

Development of hygromas or severe edema during treatment with the tyrosine kinase inhibitor STI571 is not associated with platelet-derived growth factor receptor (PDGFR) gene polymorphisms

STI571 is active against Bcr/Abl-, c-kit- and platelet-derived growth factor receptor (PDGFR)-driven malignancies. Mild to moderate edema is common, whereas severe edema, body cavity effusions and subdural hygromas are rarely observed. These effects have been suggested to involve inhibition of PDGFR signaling, but predisposing factors are unknown. We examined SNPs in the PDGFR alpha and beta gene regions in STI571-treated patients with and without life-threatening edema or cerebral hygromas, and in healthy volunteers. By RFLP analysis of 15 SNPs, the frequencies of genotypes did not differ between the three groups. SNPs of PDGFR genes do not appear to play a role in patient's susceptibility to clinically severe edema formation during treatment with STI571.

Role of platelet-derived growth factor in mesangium development and vasculopathies: lessons from platelet-derived growth factor and platelet-derived growth factor receptor mutations in mice

PURPOSE OF REVIEW

The phenotypic consequences of null mutations in the platelet-derived growth factor-B and the platelet-derived growth factor beta-receptor genes in mice have demonstrated that these proteins play pivotal roles in the development of the vascular smooth muscle cell lineage, including pericytes and mesangial cells.

RECENT FINDINGS

The lethality of these mutants has precluded analysis of the physiological and pathophysiological consequences of platelet-derived growth factor-B and platelet-derived growth factor beta-receptor deficiency in adults. This review summarizes and discusses recent data from certain tissue-specific and subtle mutations in the platelet-derived growth factor-B and platelet-derived growth factor beta-receptor genes that are compatible with postnatal viability in spite of severe developmental deficits in pericyte and mesangial cell recruitment. In the postnatal period, the animals studied developed a characteristic set of pathological changes to small blood vessels of the retina and the kidney glomerulus, which sheds light on the importance of pericytes and mesangial cells for vascular integrity and function after birth.

SUMMARY

These microvascular abnormalities and their consequences bear a resemblance to diabetic microangiopathy and nephropathy. The platelet-derived growth factor-B and platelet-derived growth factor beta-receptor mutant mouse models, therefore, might serve as valuable tools in the dissection of some of the pathogenic events in diabetic microangiopathy.

Blockade of platelet-derived growth factor receptor-beta by CDP860, a humanized, PEGylated di-Fab', leads to fluid accumulation and is associated with increased tumor vascularized volume

PURPOSE

CDP860 is an engineered Fab' fragment-polyethylene glycol conjugate, which binds to and blocks the activity of the beta-subunit of the platelet-derived growth factor receptor (PDGFR-beta). Studies in animals have suggested that PDGFR-beta inhibition reduces tumor interstitial fluid pressure, and thus increases the uptake of concomitantly administered drugs. The purpose of this study was to determine whether changes in tumor vascular parameters could be detected in humans, and to assess whether CDP860 would be likely to increase the uptake of a concurrently administered small molecule in future studies.

PATIENTS AND METHODS

Patients with advanced ovarian or colorectal cancer and good performance status received intravenous infusions of CDP860 on days 0 and 28. Patients had serial dynamic contrast-enhanced magnetic resonance imaging studies to measure changes in tumor vascular parameters.

RESULTS

Three of eight patients developed significant ascites, and seven of eight showed evidence of fluid retention. In some patients, the ratio of vascular volume to total tumor volume increased significantly (P < .001) within 24 hours following CDP860 administration, an effect suggestive of recruitment of previously non-functioning vessels.

CONCLUSION

These observations suggest that inhibition of PDGFR-beta might improve delivery of a concurrently administered therapy. However, in cancer patients, further exploration of the dosing regimen of CDP860 is required to dissociate adverse effects from beneficial effects. The findings challenge the view that inhibition of PDGF alone is beneficial, and confirm that effects of PDGFR kinase inhibition mediate, to some extent, the fluid retention observed in patients treated with mixed tyrosine kinase inhibitors.

High interstitial fluid pressure - an obstacle in cancer therapy

Many solid tumours show an increased interstitial fluid pressure (IFP), which forms a barrier to transcapillary transport. This barrier is an obstacle in tumour treatment, as it results in inefficient uptake of therapeutic agents. There are a number of factors that contribute to increased IFP in the tumour, such as vessel abnormalities, fibrosis and contraction of the interstitial matrix. Lowering the tumour IFP with specific signal-transduction antagonists might be a useful approach to improving anticancer drug efficacy.

A Gain of Function Mutation in the Activation Loop of Plateletderived Growth Factor β-Receptor Deregulates Its Kinase Activity

The platelet-derived growth factor receptors (PDGFRs) are receptor tyrosine kinases implicated in multiple aspects of cell growth, differentiation, and survival. Recently, a gain of function mutation in the activation loop of the human PDGFRalpha has been found in patients with gastrointestinal stromal tumors. Here we show that a mutation in the corresponding codon in the activation loop of the murine PDGFRbeta, namely an exchange of asparagine for aspartic acid at amino acid position 849 (D849N), confers transforming characteristics to embryonic fibroblasts from mutant mice, generated by a knock-in strategy. By comparing the enzymatic properties of the wild-type versus the mutant receptor protein, we demonstrate that the D849N mutation lowers the threshold for kinase activation, causes a dramatic alteration in the pattern of tyrosine phosphorylation kinetics following ligand stimulation, and induces a ligand-independent phosphorylation of several tyrosine residues. These changes result in deregulated recruitment of specific signal transducers. The GTPase-activating protein for Ras (RasGAP), a negative regulator of the Ras mitogenic pathway, displayed a delayed binding to the mutant receptor. Moreover, we have observed enhanced ligand-independent ERK1/2 activation and an increased proliferation of mutant cells. The p85 regulatory subunit of the phosphatidylinositol 3 '-kinase was constitutively associated with the mutant receptor, and this ligand-independent activation of the phosphatidylinositol 3'-kinase pathway may explain the observed strong protection against apoptosis and increased motility in cellular wounding assays. Our findings support a model whereby an activating point mutation results in a deregulated PDGFRbeta with oncogenic predisposition.

A novel function of insulin in rat dermis

In this study we present a novel function of insulin in rat dermis. We investigated local effects of insulin on interstitial fluid pressure (Pif), and capillary albumin leakage and pro-inflammatory cytokine production in skin and serum after intravenous lipopolysaccharide (LPS), tumour necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) challenge treated with a glucose-insulin-potassium regimen (GIK). The main objective for this study was to investigate anti-inflammatory effects of insulin. Work by others shows that insulin stimulates cell adhesion, and that this effect is dependent upon phosphatidylinositol 3-kinase (PI3K) activity. Cytokines like platelet-derived growth factor BB (PDGF-BB) attenuate lowering of Pif, possibly via PI3K. LPS and pro-inflammatory cytokines contribute to oedema development during acute inflammation by lowering the Pif. Intravenous injection of LPS, TNF-alpha or IL-1beta to Wistar Møller rats caused a lowering of Pif, but after local injection of insulin in the paw, Pif increased back to control values. IL-1beta caused a lowering in control from -0.5 +/- 0.2 mmHg to -3.0 +/- 0.2 mmHg after 20 min (mean +/- S.E.M.) (P < 0.05). Within 50 min after insulin injection the pressure was increased to -0.6 +/- 0.2 mmHg (P > 0.05 compared with control). Insulin was given together with a PI3K inhibitor (wortmannin) locally in the skin, almost abolishing the effect of insulin on Pif. A GIK regimen was given as a continuous intravenous infusion, significantly attenuating the oedema formation after LPS or TNF-alpha/IL-1beta challenge. The same GIK regimen caused a significant reduction in pro-inflammatory cytokines in serum and in interstitial fluid in skin of endotoxaemic rats. These experiments show a possible role for insulin in the interstitium during inflammation induced by LPS and TNF-alpha/IL-1beta. Insulin can attenuate a lowering of Pif possibly via PI3K, and it has an anti-inflammatory effect by inhibiting production of pro-inflammatory cytokines.

Simultaneous blockade of platelet-derived growth factor-receptor and epidermal growth factor-receptor signaling and systemic administration of paclitaxel as therapy for human prostate cancer metastasis in bone of nude mice

Once prostate cancer metastasizes to bone, conventional chemotherapy is largely ineffective. We hypothesized that inhibition of phosphorylation of the epidermal growth factor receptor (EGF-R) and platelet-derived growth factor receptor (PDGF-R) expressed on tumor cells and tumor-associated endothelial cells, which is associated with tumor progression, in combination with paclitaxel would inhibit experimental prostate cancer bone metastasis and preserve bone structure. We tested this hypothesis in nude mice, using human PC-3MM2 prostate cancer cells. PC-3MM2 cells growing adjacent to bone tissue and endothelial cells within these lesions expressed phosphorylated EGF-R and PDGF-R alpha and -beta on their surfaces. The percentage of positive endothelial cells and the intensity of receptor expression directly correlated with proximity to bone tissue. Oral administration of PKI166 inhibited the phosphorylation of EGF-R but not PDGF-R, whereas oral administration of STI571 inhibited the phosphorylation of PDGF-R but not EGF-R. Combination therapy using oral PKI166 and STI571 with i.p. injections of paclitaxel induced a high level of apoptosis in tumor vascular endothelial cells and tumor cells in parallel with inhibition of tumor growth in the bone, preservation of bone structure, and reduction of lymph node metastasis. Collectively, these data demonstrate that blockade of phosphorylation of EGF-R and PDGF-R coupled with administration of paclitaxel significantly suppresses experimental human prostate cancer bone metastasis.

Platelet-derived growth factor production by B16 melanoma cells leads to increased pericyte abundance in tumors and an associated increase in tumor growth rate

Platelet-derived growth factor (PDGF) receptor signaling participates in different processes in solid tumors, including autocrine stimulation of tumor cell growth, recruitment of tumor stroma fibroblasts, and stimulation of tumor angiogenesis. In the present study, the B16 mouse melanoma tumor model was used to investigate the functional consequences of paracrine PDGF stimulation of host-derived cells. Production of PDGF-BB or PDGF-DD by tumor cells was associated with an increased tumor growth rate. Characterization of tumors revealed an increase in pericyte abundance in tumors derived from B16 cells producing PDGF-BB or PDGF-DD. The increased tumor growth rate associated with PDGF-DD production was not seen in mice expressing an attenuated PDGF beta-receptor and was thus dependent on host PDGF beta-receptor signaling. The increased pericyte abundance was not associated with an increased tumor vessel density. However, tumor cell apoptosis, but not proliferation, was reduced in tumors displaying PDGF-induced increased pericyte coverage. Our findings thus demonstrate that paracrine PDGF production stimulates pericyte recruitment to tumor vessels and suggest that pericyte abundance influences tumor cell apoptosis and tumor growth.