Researchers headed by a team at NYU Langone Health have discovered that a feature of pancreatic cancer cells’ surroundings determines whether they grow fast or become resistant to chemotherapy. The researchers say the ability of these cancer cells to adapt quickly and toggle between biological responses makes them more likely to survive and harder to treat.
The study revealed a new facet of how pancreatic ductal adenocarcinoma (PDA) cells regulate their levels of autophagy, a “self-eating” process in which they break down their own components into nutrients to survive. When autophagy is turned on, cancer cells focus on surviving, instead of dividing and multiplying, which protects them from chemotherapies designed to attack fast-dividing cells. When autophagy is low, cells multiply faster.
The new work shows that a main factor determining whether pancreatic cancer cells increase autophagy levels is their ability to detect extracellular matrix (ECM) components that surround cancer cells in a tumor, and that are linked with worse outcomes for patients.
The investigators say that both normal and cancer cells grow best when anchored to a specific, guiding ECM. Unanchored cancer cells that fail to detect the ECM increase their autophagy levels. Specifically, the team found that cancer cells detect certain ECM structural proteins, such as laminin (Lam), through a protein on their surfaces called integrin subunit α3 (integrinα3).
“Our findings show that the sensing of the ECM by pancreatic cancer cells enables them to switch between states of active growth and autophagic survival,” said Mohamad Assi, PhD, a postdoctoral fellow in the department of radiation oncology at NYU Langone.
Assi is first author of the researchers’ published paper in Cell, titled “Extracellular matrix sensing regulates intratumoral heterogeneity of autophagic flux,” in which they said, “In summary, we have characterized a non-metabolic regulation of autophagy through ECM sensing, opening the possibility to investigate and target ECM-specific outputs in diseases.”
Autophagy is a cellular process that effectively captures and degrades intracellular proteins and organelles to maintain cellular metabolism and homeostasis, the authors explained. “Autophagy, a programmed self-eating process, underlies the progression of multifactorial diseases like pancreatic ductal adenocarcinoma (PDA),” they noted, commenting that “PDA is a multifactorial disease whose growth and progression are highly dependent on autophagy.”
While the regulation of autophagy levels has historically been linked primarily to nutrient availability and energy sensing, the team continued, “… the complex biology of PDA suggests that other factors, including oxygen levels, pH, and the extracellular matrix (ECM), might also control autophagy levels.”
For their reported study, the research team grew clusters of pancreatic cancer cells in three-dimensional spheres embedded in gel-like substances, which mimic how tumors grow in the body. Using a fluorescent protein, the researchers tracked which cells had high or low autophagy levels.
They found that in pancreatic cells, autophagy levels—historically known to be regulated by nutrient availability—can also be regulated by sensing local changes related to ECM type or structure. “Here, we discovered that human PDA cells can selectively sense key ECM components, such as Lam, to control their intracellular autophagy levels, independently of the canonical energy stress-related pathways,” they stated. In pancreatic tumors, the researchers found that the distance of cancer cells from the ECM creates two distinct populations in the same tumor. One group detects the ECM components, and so has low autophagy levels and a high growth rate. A second population, more distant from the ECM, has high autophagy levels and can better survive chemotherapy.
“The spatial proximity of PDA cells to the ECM shapes their intracellular autophagy levels, leading to heterogeneous biological responses,” the authors stated. “Specifically, PDA cells with low autophagy levels are proliferative, whereas those with high autophagy levels display better tolerance to chemotherapies.” This makes it very unlikely that a single drug could successfully target most cancer cells in a pancreatic tumor, the authors further suggested.
Hydroxychloroquine is to date the only drug approved by the FDA to block autophagy in patients, but has had limited success as a single agent. This is likely because only small amounts of it can reach the tumors, and because not all cancer cells are present in their high-autophagy mode, the authors commented.
With an eye on future treatment design, the team genetically suppressed integrinα3 in spheroid cultures, which forced nearly all of the cancer cells into their high-autophagy mode. This made hydroxychloroquine much more effective at killing the cells. The team also found that removing integrinα3 led to a 50% reduction in cancer cell survival compared to hydroxychloroquine alone.
In another set of experiments, the researchers engineered cancer cells to lack the protein NF2, which passes on the message inside a cancer cell when the activity of integrinα3 is changed. NF2 hinders the integrinα3 signal, so knocking it out significantly reduces autophagy in the cell. Importantly, it does so by slowing down the function of cellular structures called lysosomes, which are critical to the autophagic process, as well as to other survival pathways used by cancer cells. NF2-knockout-driven autophagic and lysosomal inhibition drastically reduced pancreatic tumor growth and triggered cancer cell death.
“Through integrating functional genomics and tumor-like 3D cultures, we show that human PDA cells regulate their autophagy levels by sensing the extracellular matrix (ECM) via the integrinα3-Hippo-YAP1 axis,” the authors stated. They also noted that current strategies designed to block autophagy are effective for a short time but then fail as cancer cells adapt. The newly reported results suggest that targeting both the ECM-mediated regulation of autophagy levels and lysosomal function might provide longer-lasting antitumor responses. “Overall, targeting the ECM-mediated regulation of autophagy levels could be leveraged to sensitize PDA to FDA-approved therapies,” the team stated.
