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Smad4/DPC4-mediated tumor suppression through suppression of angiogenesis
I Schwarte-Waldhoff1, O V Volpert, N P Bouck
1Immunologisch-Molekularbiologisches Labor, Medizinische Universitätsklinik Bochum, Knappschaftskrankenhaus, Ruhr-Universität Bochum, Germany.
Abstract:
Smad4/DPC4 (deleted in pancreatic carcinoma, locus 4) is a tumor suppressor gene lost at high frequency in cancers of the pancreas and other gastrointestinal organs. Smad4 encodes a key intracellular messenger in the transforming growth factor beta (TGF-beta) signaling cascade. TGF-beta is a potent inhibitor of the growth of epithelial cells; thus, it has been assumed that loss of Smad4 during tumor progression relieves this inhibition. Herein, we show that restoration of Smad4 to human pancreatic carcinoma cells suppressed tumor formation in vivo, yet it did not restore sensitivity to TGF-beta. Rather, Smad4 restoration influenced angiogenesis, decreasing expression of vascular endothelial growth factor and increasing expression of thrombospondin-1. In contrast to the parental cell line and to control transfectants that produced rapidly growing tumors in vivo, Smad4 revertants induced small nonprogressive tumors with reduced vascular density. These data define the control of an angiogenic switch as an alternative, previously unknown mechanism of tumor suppression for Smad4 and identify the angiogenic mediators vascular endothelial growth factor and thrombospondin-1 as key target genes.
Insights
Restoring the Smad4 tumor suppressor gene suppressed pancreatic cancer growth by controlling angiogenesis, not by restoring TGF-beta signaling. Smad4 targets key angiogenic factors, offering a novel mechanism for tumor suppression.
Area of Science:
- Oncology
- Molecular Biology
- Cancer Genetics
Background:
- Smad4 (deleted in pancreatic carcinoma, locus 4) is a tumor suppressor gene frequently lost in pancreatic and gastrointestinal cancers.
- Smad4 is a critical component of the transforming growth factor beta (TGF-beta) signaling pathway, which normally inhibits epithelial cell growth.
- Loss of Smad4 is presumed to relieve TGF-beta-mediated growth inhibition, contributing to tumor progression.
Purpose of the Study:
- To investigate the mechanism by which Smad4 suppresses tumor formation in pancreatic cancer.
- To determine if Smad4 restoration re-sensitizes cancer cells to TGF-beta.
- To identify the molecular pathways regulated by Smad4 in tumor suppression.
Main Methods:
- Restoration of Smad4 expression in human pancreatic carcinoma cells.
- In vivo tumor formation assays in immunocompromised mice.
- Analysis of TGF-beta signaling pathway components.
- Measurement of angiogenic factors: vascular endothelial growth factor (VEGF) and thrombospondin-1 (TSP-1).
Main Results:
- Smad4 restoration suppressed tumor formation in vivo but did not restore TGF-beta sensitivity.
- Smad4-reinstated cells exhibited reduced expression of vascular endothelial growth factor (VEGF) and increased expression of thrombospondin-1 (TSP-1).
- Tumors formed by Smad4-reinstated cells were smaller, nonprogressive, and showed reduced vascular density compared to controls.
Conclusions:
- Smad4 suppresses tumor growth through a novel mechanism involving the control of the angiogenic switch, independent of TGF-beta signaling.
- Smad4 regulates tumor angiogenesis by modulating VEGF and TSP-1 expression.
- Targeting Smad4-mediated angiogenic pathways presents a potential therapeutic strategy for pancreatic cancer.