Pancreatic Cancer Metastasis Through Amoeboid Movement
How Amoeboid Behaviors and Epithelial-to-Mesenchymal Transitions Impact Pancreatic Ductal Adenocarcinoma Virulence
Background
Pancreatic ductal adenocarcinoma (PDAC) is a cancer in which gland-forming epithelial tissue forms a malignant tumor. Adenocarcinomas are the most prevalent type of human tumor, and PDAC is the 12th most common cancer worldwide. It is estimated to become the second leading cause of cancer-related death by 2030. Part of the reason this cancer is so deadly is because in over 50% of those diagnosed, metastasis has already occurred. Metastasis means that the cancer has spread from the site of the primary tumor to another location within the body.
While part of the cell maturation cycle requires cells to migrate, once they have differentiated into a mature cell they usually are not supposed to migrate anymore. Cancer cells exhibit an epithelial-to-mesenchymal transition (EMT), which means the cells change from epithelial (growing in layers with an attached surface and a free surface) cells to mesenchymal (free-floating, easy migration, non-epithelial) cells that have properties like increased migration abilities, invasion, and avoiding cell death. Cells that have undergone this transition also display amoeboid behavior when grown in a 3D cell culture. Amoeboid behavior includes a rounded shape for easy cell travel, and an increase in the Rho-ROCK-Myosin II pathway, which is responsible for regulating cellular migration and is heavily involved in metastasis in many kinds of cancer. This study looked at whether amoeboid behavior is a characteristic of pancreatic cancers and how those tumors interact with their environment to grow and metastasize.
Summary
The scientists used eight human PDAC cell lines to determine the relationship between EMT protein markers and cytoskeleton characteristics, and amoeboid or epithelial expression. They did this by measuring the levels of EMT markers and myosin II activity and classifying the cells grown in 3D cell cultures according to their epithelial or amoeboid presentation. They found that cells that grew individually were more likely to be amoeboid, and cells that grew in colonies were mostly epithelial. The amoeboid cells were found to be more invasive than the epithelial cells due to their rounded shape for easy travel and the loss of the basement membrane to hold them in place. These amoeboid cancer cells were also later found in early-stage PDAC human tissue biopsies, indicating that these findings occur within the organism as well as within the cell lines studied here.
Next PDAC cells were treated with a ROCK inhibitor to block ROCK from its job regulating myosin II. This inhibition was able to minimize the number of individual/amoeboid cells and return cells to groups to resemble a more epithelial state, and this change to groups of cells was accompanied by a decrease in the invasion capabilities of these cells. After using ROCK inhibition to determine the effects of myosin II regulation in vitro (outside the organism, in this case within a collagen matrix) the researchers next looked at high myosin II levels in vivo (within the organism) by examining invasive fronts (IFs) in mouse tumors and metastatic areas. Imaging showed that in addition to high myosin II levels at these IFs, cells displaying a previously epithelial phenotype changed to amoeboid presentation once they reached the edge of the IFs, which correlates to their increased migration and invasion abilities discussed above.
In addition to increased invasion and metastasis capabilities, cancer cells are also immune resistant. CD73 is an enzyme and a membrane-bound immune checkpoint that helps convert adenosine monophosphate (AMP) into adenosine, which is an immunosuppressant. Overexpression of this enzyme has been linked to EMT expression, metastasis, and chemoresistance, as well as solid tumors such as pancreatic cancer. In the mouse model, the scientists were able to use antibodies to block CD73 on the membrane and this blockage decreased myosin II activity and metastasis, but it did not affect the primary tumor. This could suggest chemotherapeutic treatments to address the primary tumor, and the use of CD73 blocking antibodies to prevent metastasis.
This article demonstrates that amoeboid behaviors are characteristics of PDAC tumors and that these features help drive the disease’s virulent metastasis, invasion, and immune suppression mechanisms, and with further study could prove targets for therapeutic treatments capable of treating primary tumors and metastatic disease.
