Dickkopf-1 mediated tumor suppression in human breast carcinoma cells
Andrei M Mikheev1, Svetlana A Mikheeva, John-Patrick Maxwell
1Department of Neurological Surgery, University of Washington, Seattle, WA 98195, USA. amikheev@u.washington.edu
Abstract:
Dickkopf-1 (DKK-1) is a secreted inhibitor of the Wnt signaling pathway. We previously identified DKK-1 as a candidate tumor suppressor and demonstrated that ectopic expression of the DKK-1 suppressed the tumorigenicity of HeLa cells in vitro and in vivo. Since suppression of tumorigenicity of HeLa cells by DKK-1 overexpression was not mediated by effects on beta-catenin dependent transcription, we hypothesized that DKK-1 might also inhibit tumorigenicity of breast carcinoma cell lines lacking an activated canonical Wnt pathway. In the present study we show that ectopic expression of DKK-1 in various breast cancer cell lines resulted in a change in the cell phenotype, increased sensitivity to apoptosis, inhibition of anchorage independent growth in vitro, and suppression of tumorigenicity in vivo. Consistent with known effects of DKK-1 on the canonical Wnt signaling pathway, ectopic expression of DKK-1 in breast carcinoma cells was associated with increased phosphorylation and degradation of beta-catenin. However, none of the breast tumor cells used in this study showed detectable levels of beta-catenin dependent activation of TCF/Lef promoter activity measured by reporter constructs. Consistent with the results of these transient transfection assays, we were unable to demonstrate the expected beta-catenin dependent, TCF/Lef mediated inhibition of cyclin D1 and c-myc gene transcription in breast cells overexpressing DKK-1. However, we found that cells with DKK-1 overexpression have increased activity of CamKII pathway. Overexpression of the constitutively active form of CamKII (T286D) resulted in inhibition of breast cancer cell tumorigenicity. Thus, our study supports the hypothesis that DKK-1 mediated tumor suppressor effect is independent of beta-catenin dependent transcription and identified the CamKII pathway that contributes into DKK-1 signaling.
Insights
Dickkopf-1 (DKK-1) suppresses breast cancer growth independently of Wnt signaling. DKK-1 activates the CamKII pathway, revealing a new mechanism for its tumor suppressor activity.
Area of Science:
- Oncology
- Molecular Biology
- Cell Biology
Background:
- Dickkopf-1 (DKK-1) is a secreted Wnt signaling inhibitor.
- Previous studies identified DKK-1 as a tumor suppressor, inhibiting HeLa cell tumorigenicity.
- The mechanism of DKK-1's tumor suppressor function in breast cancer, particularly in Wnt-independent contexts, requires further elucidation.
Purpose of the Study:
- To investigate the role of DKK-1 in breast carcinoma cell lines lacking canonical Wnt pathway activation.
- To determine if DKK-1 exhibits tumor suppressor effects in breast cancer cells.
- To identify the signaling pathways involved in DKK-1-mediated tumor suppression in breast cancer.
Main Methods:
- Ectopic expression of DKK-1 in various breast cancer cell lines.
- Assessment of cell phenotype, apoptosis sensitivity, and anchorage-independent growth in vitro.
- Tumorigenicity assays in vivo.
- Analysis of beta-catenin signaling, TCF/Lef promoter activity, and gene transcription (cyclin D1, c-myc).
- Investigation of the Calcium/calmodulin-dependent protein kinase II (CamKII) pathway.
Main Results:
- Ectopic DKK-1 expression altered breast cancer cell phenotype, increased apoptosis sensitivity, and inhibited anchorage-independent growth.
- DKK-1 overexpression suppressed breast cancer cell tumorigenicity in vivo.
- While beta-catenin phosphorylation and degradation increased, beta-catenin-dependent TCF/Lef transcription was not activated.
- DKK-1 overexpression led to increased activity of the CamKII pathway.
- Overexpression of a constitutively active CamKII form inhibited breast cancer cell tumorigenicity.
Conclusions:
- DKK-1 exerts tumor suppressor effects in breast cancer cells independently of canonical Wnt/beta-catenin signaling.
- The CamKII pathway is identified as a key mediator of DKK-1's tumor suppressor activity in breast cancer.
- These findings suggest DKK-1 as a potential therapeutic target for breast cancer, acting through non-canonical pathways.
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