SYNAPTOPODIN-2 ISOFORM A AND D EXPRESSION IN HT-29 HUMAN COLON ADENOCARCINOMA CELLS

Abstract

Synaptopodin-2 is a largely unfolded actin-binding protein that also possesses the characteristics of a hub protein. Synaptopodin-2 has other known binding partners such as filamin and [alpha]-actinin (Weins, et al. 2001 and Linemann, et al. 2010), as well as zyxin and integrin-linked kinase (Yu, et al. 2006 and 2011). Previous research has shown that synaptopodin-2 promotes the rapid polymerization of actin (Chalovich and Schroeter, 2010). Cell migration, adhesion, division, and development depend on the dynamic remodeling of the actin cytoskeleton and evidence for actin-binding proteins playing tumorigenic roles in human cancers is increasingly accruing. Specifically, synaptopodin-2 has been suggested to act as a tumor suppressor in prostate and bladder cancer where loss of expression by deletion or hypermethylation leads to an increased rate of invasiveness (Lin et al., 2001 and Cebrian et al., 2008). In contrast to evidence supporting the tumor suppressor role of synaptopodin-2, a tumor activator role has also been proposed in which the over-expression of synaptopodin-2 in human endothelial kidney and mouse myoblast cells increased its invasiveness (Van Impe, et al. 2003). We hypothesize that the localization of synaptopodin-2 isoforms affects their function within HT-29 cells. As previous researchers have found synaptopodin-2 to act as a tumor suppressor when in the nucleus and a tumor activator when in the cytoplasm in prostate cancer cells, we expected to find similar results with a colon cancer cell line. Using isoform-specific GFP fusion proteins, the role of synaptopodin-2 was studied in HT-29 human colon adenocarcinoma cells. While five isoforms of synaptopodin-2 have been reported (Chalovich and Schroeter, 2010), little is known about the function of these individual isoforms. This work provides evidence for the existence of two cytoplasmic isoforms of synaptopodin-2. We also observed that these synaptopodin-2 isoforms remain cytoplasmic after differentiation, in contrast to a nuclear isoform that translocates from the nucleus to the cytoplasm after differentiation in multiple cell lines. Ultimately, this research does not support the previous notion that synaptopodin-2 acts as a tumor suppressor in the nucleus and a tumor activator in the cytoplasm, but rather provides evidence that the effects of synaptopodin-2 are isoform dependent.