S100A4 is a Ca2+-binding protein promoting metastasis in several types of tumors. The protein possesses a wide variety of biological functions, and locates to different subcellular compartments including the nucleus, cytoplasm and extracellular space. Even though S100A4 is mainly localized in the latter two compartments, we have previosusly shown that the protein is localized to the nucleus in more aggressive forms of colorectal cancer (CRC) and that nuclear S100A4 has prognostic impact. This may suggest a function of nuclear S100A4 in the regulation of tumor progression and metastasis formation.
The CRC cell line SW620 shows heterogeneity in nuclear S100A4 expression and in preliminary experiments we sorted cells into three distinct phases of the cell cycle using Fluorescent-activated cell sorting (FACS). When comparing the phases, the G2/M cells showed increased accumulation of nuclear S100A4 compared to cells in the G1 and S phase, respectively. Further validation was done using treatment with the cell cycle inhibitors nocodazole or thymidine, followed by subsequent release of cells. Protein expression of S100A4 at selected time points after release did also show accumulation in the nucleus around the G2/M transition making this subcellular translocation interesting for further investigation. Cancer cells are known to have a dysfunctional cell division apparatus and these results made it tempting to speculate whether S100A4 may be involved in cell cycle regulation. To further study the functional importance of the nuclear expression two different approaches were followed.
First, CRC cells exclusively expressing S100A4 in the nucleus were made and used to identify downstream targets of nuclear S100A4. At the moment potential targets are being validated.
Furthermore, we searched for factors known to be associated with the G2/M phase. Of great interest, the Proximity Ligation Assay (PLA) demonstrated that S100A4 and Cyclin B1, an important regulator of the G2/M transition, are within the same protein complex, and we are currently in the process of further characterizing this interaction.
With clinical data linking nuclear expression of S100A4 with aggressive CRC, elucidation of the biological processes involved might unravel novel therapeutic possibilities.
Citation Format: Eivind V. Egeland, Kjetil Boye, Kjersti Flatmark, Solveig J. Pettersen, Tove Oyjord, Gunhild M. Maelandsmo. S100A4 in colorectal cancer - biological function of nuclear localization. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2004. doi:10.1158/1538-7445.AM2014-2004