This is a guest post by Christine Gould, a postdoctoral researcher in Sanford-Burnham’s NCI-designated Cancer Center.
Researchers in our NCI-designated Cancer Center identified that hypoxia, a low-oxygen condition, induced the formation of invadopodia in human cancer cells through signals created by adjacent cells. Invadopodia are protruding structures in cancer cells that degrade the surrounding extracellular matrix to promote invasion into the surrounding tissue.
Neighboring cells promoted invasive behavior in cancer cells by direct contact with other cells (cell contact-dependent signaling) and by releasing factors that altered the cell’s behavior (paracrine signaling). These findings, published on April 15 in the Journal of Cell Biology and recently highlighted as article of the month on Cell Migration Gateway, reveal how hypoxic conditions in the cell combines different cellular signals to facilitate invasive behavior in both neighboring and distant cells.
“The goal of this study was to understand the mechanisms that regulate cancer-cell invasion, and in particular those operating under hypoxic conditions,” said Begoña Diaz, Ph.D., Sanford-Burnham research assistant professor in the laboratory of Sara Courtneidge, Ph.D., and the lead author of the paper.
Hypoxia and invadopodia
Tumor cells are often deprived of oxygen in their environment. One of the key transcription factors that control gene expression in response to hypoxic conditions is HIF1-a. Diaz and collaborators found that HIF1-a, whose expression is maintained under low-oxygen conditions, was required for hypoxia-induced formation of invadopodia in several cancer cell lines. How could this transcription factor control invadopodia in cancer cells?
Hypoxia and cell contact-dependent signaling
Hypoxia can turn on the notch signaling pathway, which is an important means of cell-cell communication. Cancer cells contain both the notch ligand and the receptor on the cell surface, so two cells that are touching can communicate. When the notch ligand binds to a notch receptor on a neighboring cell, the notch receptor is cleaved inside the cell to generate a response. When experiments were performed that blocked this pathway or depleted cancer cells of the notch receptor, invadopodia did not form under hypoxia. Diaz was then curious what would happen to cells under normoxia (standard levels of oxygen) when the notch pathway was turned on. Indeed, these cells formed more invadopodia, but how?
These cancer cells were communicating by releasing some factor in the environment that altered their behavior. If cell growth medium from hypoxic cells was added to normoxic cells, more invadopodia formed. However, the medium from normoxic cells had no effect. The researchers concluded that there was something in the media from hypoxic cells that caused invadopodia formation. This factor was, in fact, a substrate of the enzyme ADAM12.
ADAM12, an effector of invadopodia signaling
ADAM12 functions in the cell by cutting other proteins. Hypoxia and notch signaling increased the amount of ADAM12 in the cell. Media obtained from hypoxic cells but not cells that lacked ADAM12-induced invadopodia formation in cells growing under normal oxygen levels. Diaz concluded that ADAM12 activity released an invadopodia-promoting factor. One of the potential candidates was heparin-binding epidermal growth factor (HB-EGF).
Crosstalk between notch and EGFR signaling linked by ADAM12
HB-EGF, which can be released from the cell by ADAM12’s activity, activates the epidermal growth factor receptor (EGFR). Adding HB-EGF to normoxic cells promoted invadopodia formation, while inhibiting HB-EGF decreased invadopodia in hypoxic cells. Inhibiting any component of the upstream pathway, Notch or ADAM12, prevented cells from releasing HB-EGF in response to hypoxia.
Now, what relevance does this pathway have for human cancer? ADAM12 levels were increased in lung tumors, corresponding with HIF1-a expression, suggesting that this mechanism could work in vivo.
“This work uncovers a novel mechanism by which cancer cells are more invasive under hypoxia,” Diaz said. “Since intratumoral hypoxia is an important determinant of tumor malignancy, understanding the underlying mechanism may help designing novel therapeutic approaches to limit cancer cell invasion and metastasis.”
Begoña Díaz, Angela Yuen, Shinji Iizuka, Shigeki Higashiyama, and Sara A. Courtneidge (2013). Notch increases the shedding of HB-EGF by ADAM12 to potentiate invadopodia formation in hypoxia The Journal of Cell Biology DOI: 10.1083/jcb.201209151