Integrated RNA-seq and ATAC-seq profiling for assessing chemically induced inflammatory responses in human TK6 cells

Cells are constantly under siege. They face a barrage of stressors from the outside world, from pollutants to radiation, as well as internal pressures like metabolic shifts or DNA damage. When a cell encounters such a threat, it does not simply sit idle; it launches a complex defense program. One of the most critical responses is inflammation, a biological alarm system designed to repair damage and fight off invaders. However, when this alarm rings too loudly or for too long, it can turn against the body, contributing to chronic diseases and even cancer. For decades, scientists have understood that chemicals can trigger these inflammatory responses, but the full picture of how a cell "remembers" a chemical exposure after the substance is gone has remained hazy. We know that chemicals can alter the DNA sequence itself, causing mutations, but they can also change how genes are read without touching the code. This second layer of control involves the physical packaging of DNA. Inside the nucleus, DNA is wrapped around protein spools, and the tightness of this wrapping determines which genes are accessible to the cell's machinery. When the packaging loosens, genes can be turned on; when it tightens, they are silenced. Understanding how a chemical exposure reshapes this packaging, and how that reshaping persists over time, is essential for predicting long-term health risks.

In a recent study, researchers set out to map this hidden landscape of chemical-induced inflammation. They focused on a specific type of human cell line, TK6, which is widely used in safety testing because it behaves much like the immune cells found in the human body. The team exposed these cells to a chemical called TPA, a substance known to trigger strong inflammatory reactions but not to damage DNA directly. The experiment was designed to look at time in a way that is often overlooked: they did not just check the cells while the chemical was present. Instead, they washed the chemical away and watched what happened over the next week. They collected samples immediately after the exposure, and then again three and seven days later, while the cells were growing in a clean environment. To see what was happening, the researchers used two powerful tools. The first, RNA sequencing, acted like a census, counting every active gene to see which ones were being turned on or off. The second tool, known as ATAC sequencing, acted like a surveyor, mapping the physical accessibility of the DNA to see which regions were loosened up and ready for action. By combining these two views, the team could see not just what the cell was saying, but how the cell's internal structure had changed to allow those words to be spoken.

The results revealed a story of persistence. When the cells were exposed to TPA, they immediately began to change their gene expression, turning up the volume on genes related to immune activation and inflammation. This was expected. However, the surprise came after the chemical was removed. The changes did not fade away as the cells returned to a normal state. Instead, the transcriptional activity continued to evolve. Three and seven days after the exposure ended, the cells were still showing distinct patterns of gene activity that were different from their pre-exposure state. The number of genes that were turned up increased over time, while the number of genes turned down decreased, suggesting a shift toward a sustained, active immune program. This indicated that the initial chemical hit had set off a chain reaction that the cells were still processing long after the threat was gone.

To understand why this was happening, the researchers looked at the chromatin, the physical structure of the DNA. They found that the chemical exposure had caused a massive and progressive remodeling of this structure. Immediately after the treatment, thousands of regions of the genome had become more accessible, meaning the DNA had loosened up in those spots. But the story did not stop there. As the days passed, the number of these altered regions grew significantly. By the third day, the number of accessible regions had nearly doubled compared to the immediate post-exposure count, and by the seventh day, it had grown even larger. The cells were not just reacting to the chemical; they were actively rewriting their own regulatory landscape. This progressive change in the physical state of the DNA suggested that the cell was locking in a new, long-term state of alertness.

The most revealing part of the study came when the team overlaid the gene activity data with the chromatin accessibility data. They looked for genes where the change in activity matched the change in the physical structure. On the first day, only about one-third of the genes that were turned up had a corresponding loosening of the DNA nearby. Many genes were active, but their physical packaging hadn't visibly changed yet. However, as time went on, this connection strengthened. By the third and seventh days, nearly half of the upregulated genes showed a matching increase in accessibility. The match was even stronger for genes that were turned down; the proportion of these genes that had a corresponding tightening of the DNA rose from less than ten percent on day zero to nearly forty percent by day seven. This growing alignment suggests that the cell is gradually cementing its new state, ensuring that the inflammatory response is supported by a stable, physical change in the genome's architecture.

The researchers then zoomed in on specific genes that are known to be linked to human inflammatory diseases. They found that genes associated with conditions like lupus and rheumatoid arthritis showed this coordinated behavior. For instance, a gene involved in cell signaling that is often linked to autoimmune disease showed both increased activity and increased accessibility in the days following the chemical exposure. Similarly, a gene that helps white blood cells stick to surfaces and move through the body showed a coordinated decrease in both activity and accessibility. These were not random fluctuations; they were precise, structural shifts in the genome that mirrored the cell's functional response. The study concludes that by looking at both the gene activity and the physical state of the DNA, scientists can identify a more robust signature of chemical stress. This dual-layer signature, where the cell's output matches its internal structure, offers a clearer window into how chemicals might trigger long-lasting inflammatory effects, providing a potential new way to assess safety risks that goes beyond simply checking for DNA damage.