Resistance to anticancer drugs is a major problem in oncology affecting a large number of cancer patients. Now, researchers at the CSIR-Institute of Genomics and Integrative Biology (CSIR-IGIB), New Delhi have found a way to make cancer cells that are resistant to two commonly used anticancer drugs — doxorubicin and topotecan — to once again become sensitive to the drugs. Improving or regaining the sensitivity of existing anticancer drugs is a quicker way to address the problem of cancer drug resistance than developing new drugs. The results were published in the journal Scientific Reports .
Chemotherapeutic drugs like doxorubicin and topotecan act by inducing DNA damage. Once the DNA damage gets induced it leads to the activation of an important protein called p21, which gets produced in larger quantities. The p21 protein helps stop the growth of cells and triggers senescence or apoptosis in cancer cells thereby killing them. However, in many drug-resistant cancer cells the production of p21 is compromised, thereby preventing the destruction of cancer cells even in the presence of these drugs.
“A few years ago we and others groups noted the telomere repeat factor 2 (TRF2), which protects the end of human chromosomes called telomeres (much like small clips at the end of shoelaces that keep the ends from fraying), can bind to the genome outside the telomeres,” says Dr. Shantanu Chowdhury from CSIR-IGIB who led the team. “So we wanted to find out where else the TRF2 binds in the genome.”
That search led the team to the p21 protein and they found that the promoter of p21 protein has a TRF2 binding site. The TRF2 specifically binds to a DNA structure called G-quadruplex (G4) which is present in the p21 promoter.
“Once we found that TRF2 binds to the p21 promoter, we wanted to know if it also controls how p21 mRNA is made [mRNA produces the p21 protein]. And that led to the basic finding that TRF2 is a repressor and inhibits the expression of p21 mRNA in multiple cell types,” says Dr. Chowdhury.
Once the researchers understood the mechanism by which the TRF2 binds to p21, they used small molecules that were available (from other researchers) to disrupt the binding of TRF2 to the p21 promoter site. “The small molecules were able to disrupt the binding of TRF2 to the p21 promoter. And when TRF2 is not able to bind to the p21 promoter the expression of p21 does not get compromised,” he says.
“When the small molecules are given along with the anticancer drug doxorubicin there is increased amount of p21 produced and cancer cells that were unresponsive to doxorubicin once again become sensitive to the drug,” says Dr. Chowdhury.
The researchers used fibrosarcoma and breast cancer cell lines to test the combination of small molecules and doxorubicin in reversing cancer drug resistance. “The drug sensitivity increases by over 50% when we use small molecules along with doxorubicin. Drug sensitivity becomes as high as over 80% depending on the dosage of small molecules,” he adds.
“This is a proof-of-concept study to show that cancer cell sensitivity to existing drugs can be regained by using small molecules. This way the existing cancer drugs can be used instead of discovering new drugs,” he says.
Since existing small molecules were used for the study, the researchers do not rule out the possibility of the small molecules binding to other G4 sites in the genome. So the focus of the team is to design specific small molecules that bind only to the G4 site in the p21 promoter.