PDGF Translational Research Group
Group moved from 1/9-2011
New adress:
School of Biosciences
University of Birmingham
Edgbaston
Birmingham, B15 2TT
UK
Email: c.hellberg@bham.ac.uk
Group leader
Carina Hellberg
Carina.Hellberg(AT)LICR.uu.se
Phone +46 18 160403
The research in the Translational research section is focused on the following three aims:
• Regulation of PDGF ß-receptor signal transduction by tyrosine phosphatases
• Regulation of the intracellular sorting of PDGF ß-receptors
• PDGF receptors as cancer drug target
Biological background
Tyrosine phosphorylation of proteins is an essential component of signal transduction pathways that regulate cell growth, survival and death, as well as adhesion, migration and differentiation. Both protein tyrosine kinases and protein tyrosine phosphatases (PTPs) control cellular phosphotyrosine levels. Protein tyrosine phosphatases are natural receptor tyrosine kinase antagonists and serve as regulators of both non-receptor and receptor tyrosine kinases.
The amplification of genes encoding tyrosine kinase receptors have been described in several forms of tumors. The increased expression of both ligand and receptor within a tumor suggests that tumor growth is at least partly due to autocrine receptor activation. It is also clear that stromal cells participate in the growth and survival of tumor cells, a process that is dependent on growth factor signaling both in tumor- and stroma cells. In addition, tyrosine kinase receptors are involved in the signals inducing tumor vascularization, a process that is necessary for both growth and metastasis of tumors.
The appreciation of the role of tyrosine kinases in the generation and progression of cancer has lead to development of a number of anti-tumor drugs that specifically targets tyrosine kinases. For example Gleevec, which targets the Bcr-Abl kinase, the PDGF receptors and c-Kit, has been successfully used in treatment of CML and gastrointestinal stromal tumors. Further understanding of the molecular mechanisms underlying tumor formation should generate a new wave of target-specific drugs. This, in combination with improved screening of patient material to determine the pathogenic alterations in the cancer cell signaling network for each tumor, should lead to treatments in which the available drugs are applied in specific combinations depending on the outcome of the tumor analysis.
Ongoing projects
Identification of tyrosine phosphatases that regulate PDGF ß-receptor phosphorylation
The PDGF ß-receptor contains eleven tyrosine residues that are autophosphorylated following receptor activation. Several tyrosine phosphatases that dephosphorylate specific tyrosine residues on the PDGF ß-receptor have been identified. This suggests the exciting possibility that the activity of a phosphatase could modulate specific signaling pathways induced by receptor activation, rather than turning off the signaling completely. By regulating the expression and activation of tyrosine phosphatases, the cell could consequently modulate growth factor-induced signals, and fine-tune its cellular response to surrounding environment.
We have identified the T-cell protein tyrosine phosphatase as a site-selective regulator of PDGF ß-receptor phosphorylation. The screening for tyrosine phosphatases that are regulating PDGF ß-receptor phosphorylation and signal transduction is being continued, with a current focus on the LAR phosphatase.
It has become increasingly clear that the intracellular sorting of receptor tyrosine kinases regulates the quality and duration of their signals. Following its activation, the PDGF ß-receptor is internalized and sorted towards degradation. We found that the increased PDGF ß-receptor phosphorylation observed in T-cell phosphatase -/- fibroblasts is accompanied by an induction of receptor recycling, which occurs through Rab 4 positive recycling endosomes
(Figure 1; Karlsson et al., Mol. Biol. Cell 2006).
The induction of recycling is specific for the PDGF ß-receptor, since neither the PDGF alpha-receptor nor the IGF-1 receptor display increased recycling in T-cell phosphatase -/- fibroblasts. Since PDGF ß-receptors do not normally recycle, these fibroblasts provide a unique model system for studying the regulation of the intracellular trafficking of the PDGF ß-receptor. We are currently investigating the molecular mechanisms whereby the T-cell phosphatase regulates PDGF ß-receptor trafficking, and have identified activation of phospholipase C-gamma and protein kinase C-alpha as a critical components in the sorting of the PDGF ß-receptor into Rab 4 positive recycling endosomes. We are currently working to identify the substrate for protein kinase C-alpha that regulates receptor sorting.
Regulation of receptor trafficking appears to determine the duration of receptor signaling, since recycling of the PDGF ß-receptor lead to slower receptor degradation. The physiological response of cells in vivo is dependent on crosstalk between different signaling pathways. To study if activation of protein kinase C by other receptor types affects PDGF ß-receptor sorting and signaling, fibroblasts were treated with lysophosphatidic acid. In cells treated with lysophosphatidic acid, the PDGF ß-receptor recycled following ligand binding. Lysophospatidic acid also sensitized fibroblasts to low concentrations of PDGF-BB. Therefore, a current goal in the lab is to investigate how changes in the subcellular localization of receptor tyrosine kinases regulate their signal transduction.
Figure 1. Co-localization between the PDGF ß-receptor (red) and Rab4a (green) in T-cell phosphatase -/- fibroblasts.
PDGF receptors as cancer drug targets
PDGF receptor signaling is implicated in various cancer associated processes, including autocrine stimulation of growth, stimulation of tumor fibroblasts and promotion of tumor angiogenesis.
During angiogenesis, PDGF receptor signaling is important for the recruitment of pericytes to the newly formed tumor vessels, a process needed for the stabilization and function of the newly formed vasculature. The presence of pericytes on tumor vessels has been proposed to protect endothelial cells from anti-angiogenic therapy targeting the VEGF pathway. Thus, PDGF receptors on pericytes could provide a novel target for anti-angiogenic therapy. To investigate, this we use several experimental carcinoma tumor model where PDGF-dependent pericyte recruitment contributes to tumor growth.
Treatment of tumors with a combination of PDGF- and VEGF receptor inhibitors, but not single agent treatment, significantly reduced tumor growth. This effect was more pronounced in the fast-growing tumors with a pericyte-rich vasculature, and coincided with a reduction in immature pericytes. Pericytes expressing the differentiation marker desmin were unaffected, identifying these cells as a subset of pericytes resistant to anti-angiogenic therapy. The growth inhibition was associated with vascular remodeling, and an increase in tumor cell apoptosis and a decrease in proliferation. This suggests that PDGF receptor antagonists could potentiate anti-angiogenic treatment in mature tumors, which are relatively insensitive to VEGF receptor inhibitors.
Figure 2. 3D rendering of tumor vessels perfused with dye
In experimental models, inhibition of PDGF receptors on stromal cells increases tumor uptake of low molecular weight chemotherapy drugs by lowering tumor interstitial fluid pressure (IFP). VEGF receptor inhibitors lower tumor IFP by decreasing vessel permeability, and we recently showed that the combination of PDGF- and VEGF receptor inhibitors can act together to additively reduce tumor IFP. In these experiments, the timing of the therapies was critical, which has prompted us to further investigate the role of these inhibitors on tumor vessel function.
| Group Members: | ||
| Position | Name | |
| Group leader | Carina Hellberg | |
| Senior Technician | Aive Ågren | |
| Postdoc | Tijs van Wieringen | |
| PhD student | Wei Zheng | |
| PhD student | Haisha Ma | |
Supplementary Figure Melanoma Research 2010.pdf
| Selected Publications | |
| Theses: | |
Defended 091002 |
|
Defended 100409 |
|
| Books: | |
| Original papers: | |
|
|
|
|
|
|