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Smad4/DPC4 silencing and hyperactive Ras jointly disrupt transforming growth factor-beta antiproliferative responses
1Cell Biology Program, Howard Hughes Medical Institute, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
Abstract:
Smad4/DPC4 is a tumor suppressor gene frequently mutated or deleted in pancreatic and metastatic colon cancers. Smad4 acts as a cofactor that binds transforming growth factor-beta (TGF-beta) receptor-activated Smad2 and Smad3 generating transcriptional complexes. Using SW480.7 colon carcinoma cells, defective in Smad4 function, we have investigated whether this loss plays a role in the resistance of colon cancer cells to the antiproliferative effects of TGF-beta. SW480.7 cells contain only one Smad4 allele, which we found encodes a wild type protein that is not expressed. We generated SW480.7 cells conditionally expressing Smad4 via an ecdysone-inducible system. Smad4 expression in these cells failed to rescue TGF-beta antiproliferative and gene responses (c-myc down-regulation and induction of p21/Cip1 and plasminogen activator inhibitor-1). SW480.7 cells contain an activated Ki-ras oncogene. Hyperactivation of Ras can inhibit Smad nuclear accumulation by their phosphorylation at mitogen-activated protein kinase sites. Co-transfection into SW480.7 cells of Smad4 together with a Ras phosphorylation-resistant Smad3 (but not with wild type Smad2, Smad3, adenomatous polyposis coli (APC), or TGF-beta type II receptor) restored the TGF-beta antiproliferative response. These results suggest that loss of Smad4 function by both deletion and silencing and inhibition of Smad2/3 function by a hyperactive Ras pathway jointly prevent TGF-beta antiproliferative responses in SW480.7 colon cancer cells.
Insights
Loss of Smad4 and Ras pathway activation in colon cancer cells prevents TGF-beta
Area of Science:
- Oncology
- Molecular Biology
- Cell Signaling
Background:
- Smad4 (DPC4) is a tumor suppressor gene crucial in pancreatic and colon cancers.
- Smad4 functions as a cofactor for transforming growth factor-beta (TGF-beta) signaling.
- Colon cancer cells can develop resistance to TGF-beta's antiproliferative effects.
Purpose of the Study:
- Investigate the role of Smad4 loss in TGF-beta resistance in colon cancer.
- Determine if Smad4 re-expression can restore TGF-beta sensitivity.
- Elucidate the interplay between Smad4, Ras signaling, and TGF-beta response.
Main Methods:
- Utilized SW480.7 colon carcinoma cells with defective Smad4 function.
- Employed an ecdysone-inducible system for conditional Smad4 expression.
- Performed co-transfection experiments with Smad4 and various signaling components.
Main Results:
- Smad4 re-expression in SW480.7 cells did not restore TGF-beta antiproliferative responses.
- Activated Ki-ras oncogene in SW480.7 cells was found to inhibit Smad nuclear accumulation.
- Restoration of TGF-beta response was achieved by co-expressing Smad4 with a Ras-resistant Smad3 mutant.
Conclusions:
- Loss of Smad4 function and Ras pathway hyperactivation synergistically inhibit TGF-beta antiproliferative responses in colon cancer.
- Targeting both Smad4 loss and Ras pathway activation may be a strategy to overcome TGF-beta resistance.
- Understanding these complex interactions is vital for developing effective colon cancer therapies.