ARS | Publication request: Homotypic gap junctional communication associated with metastasis increases suppression increases with PKA kinase activity and is unaffected by P13K inhibition

Submitted to: Cancer Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 28, 2010
Publication Date: December 1, 2010
Citation: Bodenstin, T.M., Vaidya, K.S., Ismail, A., Beck, B.H., Cook, L.M., Diers, A.R., Landar, A., Welch, D.R. 2010. Homotypic gap junctional communication associated with metastasis increases suppression increases with PKA kinase activity and is unaffected by P13K inhibition. Cancer Research. 70(23):10002-10011.

Interpretive Summary: Gap junctional intercellular communication (GJIC) is a process that allows cells to directly transfer molecules and nutrients between cells that are in physical contact through cell membrane pores known as connexons. Interestingly, GJIC is often dysregulated in cancer cells, suggesting that this process is important for cellular homeostasis. Connexins, the functional units of connexons, are regulated within the cell by multiple signaling cascades that transport them from the cytosol to the plasma membrane. Many reports have linked a decrease in connexin protein expression to decreases in GJIC in cancer cells. However, we report here that restoring signaling through the Protein Kinase A (PKA) pathway in multiple cancer cell lines is capable of increasing GJIC without inducing the expression of a connexin gene. These results suggest that cancer cells may often cause a disruption in signals related to connexin intracellular transport, rather than simply reducing the expression of connexin genes. Importantly, we show that these signaling defects can be restored with the use of cell permeable signaling inhibitor compounds, proving potential for development of molecules to modulate GJIC in the clinical setting.

Technical Abstract: Loss of gap junctional intercellular communication (GJIC) between cancer cells is a common characteristic of malignant transformation. This communication is mediated by connexin proteins that make up the functional units of gap junctions. Connexins are highly regulated at the protein level and phosphorylation events play a key role in their trafficking and degradation. The metastasis suppressor breast cancer metastasis suppressor 1 (BRMS1) upregulates GJIC and decreases phosphoinositide-3-kinase (PI3K) signaling. On the basis of these observations, we set out to determine whether there was a link between PI3K and GJIC in tumorigenic and metastatic cell lines. Treatment of cells with the well-known PI3K inhibitor LY294002, and its structural analogue LY303511, which does not inhibit PI3K, increased homotypic GJIC; however, we found the effect to be independent of PI3K/AKTinhibition. We show in multiple cancer cell lines of varying metastatic capability that GJIC can be restored without enforced expression of a connexin gene. In addition, while levels of connexin 43 remained unchanged, its relocalization from the cytosol to the plasma membrane was observed. Both LY294002 and LY303511 increased the activity of protein kinase A (PKA). Moreover, PKA blockade by the small molecule inhibitor H89 decreased the LY294002/LY303511-mediated increase in GJIC. Collectively, our findings show a connection between PKA activity and GJIC mediated by PI3K-independent mechanisms of LY294002 and LY303511. Manipulation of these signaling pathways could prove useful for antimetastatic therapy.