Researchers at IISER Pune have found a particular protein (DNA-dependent protein kinase, DNA-PK), which normally repairs any damage to cell DNA, playing a central role in the transformation of normal cells into cancerous cells. The activated DNA-PK disrupts the structure and function of the Golgi, trafficking and polarity of epithelial cells, leading to the transformation.
Researchers from the Indian Institute of Science Education and Research (IISER) Pune, have for the first time studied the early stages of normal cells transforming into cancerous cells. Using breast epithelial cells grown in 3D cultures, a team led by Mayurika Lahiri at IISER Pune also found a particular protein (DNA-dependent protein kinase, DNA-PK), which normally repairs any damage to cell DNA, playing a central role in the transformation process. The results of the study were published in the Journal of Cell Science.
While cells are continually exposed to DNA-damaging agents, the surveillance system in place in cells checks for any errors on the DNA and immediately repairs them. But when the surveillance system or the DNA repair mechanism gets compromised, the errors on the DNA tend to accumulate in the genome. After a while, the cells (with accumulated errors) appear abnormal and have most of the characteristics — morphology and behaviour — of cancerous cells.
In the study, an alkylating agent (a drug used in chemotherapy for treating cancer) was used for inducing the transformation of breast epithelial cells into cancerous cells by activating the DNA-PK gene. The activated DNA-PK was found to disrupt the structure and function of the Golgi, an organelle found in the cell but outside the nucleus.
“Observing the abnormal Golgi was shear serendipity. Cancer-causing agents cause damage to the DNA, which is found inside the nucleus. But in this case, we found the alkylating agent to also disrupt the Golgi, which is found outside the nucleus,” says Prof. Lahiri.
As a result of the disruption of the Golgi morphology, the movement of proteins (trafficking) from the endoplasmic reticulum to the cell membrane via Golgi was impaired. As a result, the polarity — ability to distinguish which side of the epithelial cell is the top and which side is the bottom — was disrupted and the cells were no longer epithelial cells, thus resulting in the transformation to cancerous cells. “Disruption of polarity is one of the hallmarks of cancerous cells,” she says.
The cells treated with the alkylating agent were found to be forming colonies; the ability of cells to form colonies is one of the important characteristics of transformation. Also, the cells changed in shape and became more spindle-like. They also became motile resulting in cells migrating to other regions, and invasive too. “Because of their invasiveness, the cells were able to penetrate the 3D substrate used for growing the cells,” she says.
To confirm that activation of DNA-PK was causing the phenomena of transformation, the researchers used a small molecule to inhibit the activity of DNA-PK. “The inhibitor was able to partially reverse the polarity disruption and the Golgi regained its normal morphology. But the trafficking could not be reversed at all,” says Prof. Lahiri. “This helped confirm the key role played by DNA-PK in transformation.”