Improving chemotherapeutic drug penetration in melanoma by imatinib mesylate.

BACKGROUND: Imatinib mesylate has specific activity in inhibiting select tyrosine kinase receptors, including platelet-derived growth factor receptors (PDGFRs) and c-kit. In general, melanomas widely express PDGFR and c-kit, and their in vivo resistance to chemotherapy is attributable to high tumor interstitial fluid pressure (IFP). Recent studies have suggested that PDGFR-beta inhibition reduces tumor IFP, and thus increases the uptake of concomitantly administered drugs. OBJECTIVE: The present study was designed to investigate the potential of imatinib mesylate as a therapy for melanoma or as an adjuvant to chemotherapeutics. METHODS: Using in vivo mouse models, the effect of imatinib mesylate on the growth of melanoma with or without dacarbazine was studied. RESULTS: Imatinib mesylate enhanced the antitumor effect of dacarbazine on in vivo growth and lung metastases of melanoma cells, although treatment with only imatinib mesylate had no effect. We could detect perivascular expression of PDGF beta-receptor in melanoma tumors. Interestingly, dacarbazine uptake in melanoma was more than three-times increased by treatment with imatinib mesylate, while its uptake in serum or bone marrow was not affected by imatinib mesylate. CONCLUSIONS: These data suggest interference with PDGF receptors, or their ligands, as a novel strategy to increase drug uptake and therapeutic effectiveness of chemotherapy for melanoma.

Identification of a subset of pericytes that respond to combination therapy targeting PDGF and VEGF signaling.

The aim of our study was to further explore the use of anti-angiogenic therapy targeting the vascular endothelial growth factor receptor (VEGFR) on endothelial cells while simultaneously targeting platelet-derived growth factor receptors (PDGFRs) on adjacent pericytes. B16 mouse melanoma tumors exogenously expressing PDGF-BB (B16/PDGF-BB) display higher pericyte coverage on the vasculature compared to the parental B16 tumors (B16/mock). These models were used to investigate the effects of combination therapy targeting VEGFR and PDGFR signaling on size-matched tumors. Combination therapy using 25 mg/kg/day of the VEGFR inhibitor PTK787 and 100 mg/kg/day of the PDGFR inhibitor STI571 decreased the tumor growth rate of both tumor types, but the inhibition was only significant in the B16/PDGF-BB tumors. Combination therapy induced vessel remodeling, primarily by reducing the vessel density in B16/mock tumors, and by reducing the vessel size in B16/PDGF-BB tumors. When analyzing the effects of combination therapy on tumor vessel pericytes, it was found to primarily reduce the subpopulation of alpha-smooth muscle actin and PDGFRbeta-positive pericytes partly detached from the tumor vessels, without affecting the number of pericytes closely attached to the endothelium, which also express desmin. Taken together, these data demonstrate an increased benefit of targeting both VEGFR and PDGFR pathways in B16/PDGF-BB tumors, and demonstrates that the increased tumor growth inhibition in this model is accompanied by a reduction in a specific subset of pericytes, characterized by being loosely attached to endothelial cells and negative for the pericyte marker desmin.

Lymphoid enhancer factor 1 makes cells resistant to transforming growth factor beta-induced repression of c-myc.

The activation of lymphoid enhancer factor (LEF)/T-cell factor (TCF)-mediated transcription by sustained expression of beta-catenin and the loss of transforming growth factor beta (TGF-beta) signaling are essential steps in carcinogenesis, particularly for cancers of the colon, breast, and liver. The oncogene c-myc is a common target of both of these signaling pathways and a key regulator of cell cycle progression. Here we have identified a novel LEF/TCF-responsive element in the promoter of the human c-myc gene. beta-Catenin activated the transcriptional activity of the c-myc promoter by binding to this element in various cell lines. When TCF-4 was bound to this element, TGF-beta dissociated beta-catenin and repressed the transcriptional activity of the c-myc promoter. However, TGF-beta could not dissociate beta-catenin and could not repress c-myc transcription when LEF-1 was bound to the element instead of TCF-4. These findings suggest that enhanced expression of LEF-1, which occurs frequently in colon cancer, may make cells refractory to the down-regulation of c-myc and the subsequent growth arrest induced by TGF-beta.

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.

Highly active antiretroviral therapy attenuates re-endothelialization and alters neointima formation in the rat carotid artery after balloon injury.

Highly active antiretroviral therapy (HAART) has led to a sustained decline of HIV-associated morbidity and mortality. HAART exhibits significant side effects, however, such as hyperlipidemia and hyperglycemia, which possibly contribute to accelerated atherosclerosis in HAART-treated patients. In addition, direct effects of HAART on vascular cells have been described, which may promote atherosclerotic lesion formation. The effects of HAART on balloon-induced neointima formation have not been studied previously. The rat carotid artery balloon model was used to evaluate the effects of HAART (lopinavir, ritonavir, lamivudine, and zidovudine) on neointima formation and endothelial recovery. Furthermore, the effects of concomitant administration of the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor rosuvastatin were investigated. HAART-treated animals displayed an increase in lesion size (neointima/media ratio: 1.14 +/- 0.32 vs. 1.31 +/- 0.20 in control vs. HAART; P < 0.05) and an impaired regenerative capacity of the endothelium, as indicated by reduction in endothelial regrowth from an adjacent undilated vessel segment 14 days after injury (re-endothelialization area: 8.29 +/- 1.45 mm vs. 5.09 +/- 0.53 mm in control vs. HAART; P < 0.05). When rosuvastatin was given in addition to HAART, these effects were not observed. In conclusion, HAART inhibited endothelial cell-mediated healing and promoted neointima formation after angioplasty in rats. These deleterious effects were attenuated by cotreatment with rosuvastatin, however. Our studies suggest that currently used drug regimens against HIV infection may lead to an increased risk for restenosis after percutaneous vascular interventions. Moreover, the findings indicate that the additional treatment with statins might counteract these adverse effects by HAART.

c-myc is a downstream target of the Smad pathway.

c-Myc is one of the most potent regulators of cell cycle progression in higher eukaryotes. Down-regulation of c-Myc is a critical event for growth inhibition induced by transforming growth factor-beta (TGF-beta) and is frequently impaired in cancer cells. We determined a Smad-responsive element in the c-myc promoter. This element is a complex of the TGF-beta1 inhibitory element (TIE) originally identified in the transin/stromelysin promoter and an E2F site responsible for transcriptional activation of the c-myc promoter. Smad3 and E2F-4 directly bound to the element (TIE/E2F), and substitution of two nucleotides in TIE/E2F impaired binding of both Smad3 and E2F-4 as well as serum-induced activation and TGF-beta-induced suppression of the c-myc promoter activity. Smads bound TIE/E2F within 1 h after stimulation with TGF-beta, before the suppression of c-myc transcription, whereas binding of p130 to TIE/E2F became augmented later than 12 h. TGF-beta signaling did not compete with E2F-4 for binding to TIE/E2F, but reduced p300 co-immunoprecipitating with E2F-4. Therefore, TGF-beta signaling may suppress c-myc promoter activity by dissociating p300 from E2F-4.