Researchers at the Ludwig Institute for Cancer Research discovered a mechanism that methodically reverts-or de-differentiates-cancer cells of a benign subtype of pancreatic tumors into a progenitor (immature) state of cellular development that proceeds to spawn highly aggressive tumors capable of metastasis to the liver and lymph nodes.
The study, led by Douglas Hanahan, PhD, Distinguished Scholar at the Lausanne Branch, shows that engagement of the mechanism is associated with poorer outcomes in patients diagnosed with pancreatic neuroendocrine tumors (PanNETs).
The hormone-producing cells of the pancreas cluster together in small groups, referred to as islets, throughout the pancreas. When a tumor forms in one of these clusters, PanNETs, or islet cell carcinoma, occur. They can be either benign or malignant.
PanNETs are the rarest form of pancreatic cancer; the more common form of pancreatic cancer is pancreatic adenocarcinoma, which develops in the exocrine tissue of the pancreas, representing more than 90 percent of all pancreatic cancers.
"Developmental biologists have studied the lineage of the particular cells that make insulin and identified different phases, from the earliest progenitor cells and their brethren to more mature beta cells," said Hanahan. "For decades, there has been a myriad of data suggesting cancers alter differentiations, resulting in many manifestations. Multi-step tumor development and evolution is a complex milieu.
"It's hard to separate the individual changes," he noted. "Aberrations and differentiations happen in tumors. The question is, are they separable?"
Dedifferentiation as a Separate Step
"Cancer progenitor cells are immortal, growing like embryonic cells, while evading the killing by the normal immune system," said Hanahan, who is also Professor Emeritus at Ecole Polytechnique Federale de Lausanne.
It is widely known cancer develops in a multi-step process in which cells have several different mechanisms that must fail before a critical mass is reached and cells become cancerous. Dedifferentiation refers to a cellular process in which a mature or differentiated cell loses its special function, then reverts to a less mature or less specialized state.
New insights from the study explain the intentional dedifferentiation process of cancer cells, providing concrete evidence that such cellular dedifferentiation, observed across certain cancer types, is not a random consequence of cancer cells' additional aberrations.
"Our study provides evidence in a single tumor type that dedifferentiation is an independently regulated and separable step in multi-step tumorigenesis," said Hanahan. "Moreover, this is not nonspecific dedifferentiation, but rather the result of a precise reversion of a developmental pathway that generated the mature cell type from which the cancer arose."
Hanahan and colleagues previously reported these tumors as two subtypes: a relatively benign, "well-differentiated" subtype that maintains many features of insulin-producing beta cells, and a more aggressive, poorly differentiated subtype lacking those features.
"Using a PanNET mouse model, the data identified many characteristics of normal islet progenitor cells within the 'poorly differentiated' cancer cells," he said. "Additionally, the progression from benign to aggressive PanNET tumors requires cancer cells to retrace the pathway of beta cell differentiation and maturation to assume the progenitor state."
From the angiogenic switch to the form of a solid tumor, PanNETs range from non-invasive to highly aggressive, invasive and metastatic cancer cells, said Hanahan. "The cells are transformed into malignant cancer and localized into a larger sea within the pancreas. "
RNA Molecule Activates Protein
The most notable results of the study captured when the tumor cells prepared for the dedifferentiation process, which identified the increased production of an RNA molecule. The researchers also discovered a molecular circuit in the cancer cells that governs the dedifferentiation process.
"Tumor cells that are poised to dedifferentiate step up their production of a type of RNA molecule that regulates gene expression, known as microRNA-181 CD," said Hanahan. Notably, microRNA-181 is known to be involved in establishing the progenitor state during development of the islet beta cells, he noted.
"Importantly, microRNA-181 does not have any impact on the rate of proliferation," said Hanahan. "Thus, microRNA-181 is the driving factor in proliferative expansion. This microRNA ultimately causes the activation of HMGB4, a protein that controls the expression of a suite of genes that pushes the cells into a progenitor state."
In mature islet cells, the HMGB3 regulatory factor is not expressed as an intermediary protein, directly regulated by miR-181, which suppresses its expression until microRNA-181 interferes to produce a cascading effect, he noted.
Future Studies & Questions
Asked if there are other types of cancers that revert to a progenitor, to ultimately develop into metastatic cancer in another region of the body, Hanahan noted: "There are clues. We're hoping this publication will stimulate efforts to similarly analyze other forms of cancer.
"For decades, we've known that most cancers develop a series of discrete histological stages. We see the incidence curves as the cancer is a multistep process. As the cell progresses, we notice a sea of changes within the cells, but it's hard to differentiate the colony sub-sections. Mouse models of human cancer such as this one allow us to identify the stages in multi-step tumorigenesis and to investigate how they progress."
Interestingly, Hanahan engineered the particular mouse model for cancer used in this study back in 1984. "I found it interesting that mice were getting tumors," he said. "I never imagined I would be studying cancer biology in this prototypical mouse model 35 years later."
The molecular circuit in cells, which shows the tumor cells step up their production with the microRNA-181, is only partially understood.
"Although we don't have the entire pathway charted, we have a key part of it," said Hanahan. And the most important question for the future: "What mechanism is turning on microRNA-181 to induce this switch to reverse differentiation into a progenitor that facilitates malignancy?"
Amy Gallagher is a contributing writer.