Combating drug resistance in cancer cells

19 Sep 2017

Combating drug resistance in cancer cells

By Dr Shantanu Chowdhury, Senior Fellow

CSIR-IGIB, New Delhi


Like resistance to antibiotics, some cancers are, or become, resistant to anticancer drugs. This baffles clinicians and cancer researchers alike. More so because questions about why, how and when anticancer drug resistance sets in has remained difficult to understand - and counter. 

Therefore, it was quite exciting when initial results showed we could re-sensitise drug resistant cancer cells. And, when we understood how this happens through the mechanisms that we were focused on, the plot became more exciting.

But, first let me talk about combating drug resistance in aggressive metastatic cancer cells. 

Anticancer drugs like doxorubicin work by inducing damage to DNA, which triggers the damage response machinery of cells. Once damage response gets activated cells are forced to stop replicating and eventually perish. In essence, doxorubicin therefore uses mechanisms that are in place to ensure that damaged cells are not allowed to grow unhindered. 

Many of the damage response signals get channelled through an important protein commonly called p21, which on sensing DNA damage is produced in larger amounts. Increased p21 levels in turn lock the gears of the engineering that helps cells to cycle through different stages of growth. In drug resistant cancer cells somehow, we noticed, the levels of p21 do not increase adequately when triggered by doxorubicin - and, therefore, doxorubicin treatment cannot produce the desired effect by killing or maiming cells from growing further.

Now, how did our findings impact the engineering of the gear box? Based on insights from mechanistic work described below we used designer small molecules that were able to increase p21 levels following DNA damage. This was particularly effective in aggressive resistant breast cancer cells that are otherwise unable to induce required amounts of p21 when treated with doxorubicin. Our results show that when made-to-order small molecules are given along with doxorubicin - first, p21 levels jump up, and in turn the gear locks set in ensuring marked reduction in growth of cancer cells. Though further studies remain, what appears to be exciting is that the drug resistant cancer cells are once again susceptible to doxorubicin treatment.

The most fundamental point we learnt was that making of the p21 RNA (which produces p21 protein) is controlled by a well-known but seemingly unconnected protein called TRF2, short for telomere-repeat binding factor 2. As the name suggests, for years scientists have studied TRF2 to understand how it protects the end of human chromosomes called telomeres - much like small clips at the end of shoelaces that keep the ends from fraying. So controlling p21 was as if TRF2 had a new job away from home. 

However, there was a twist.  We started noticing that TRF2 associates with a typical structure formed by DNA - somewhat like a knot - called G-quadruplex within the promoter (a stretch of DNA responsible for managing RNA production) of p21. Though this seemed completely out of the blue first - on second thoughts started making sense as similar G-quadruplex structures are believed to form at TRF2's home ground telomeres. Apart from the basic understanding, TRF2 binding to the p21 promoter G-quadruplex made us think of the problem in a completely different fashion.

Let me go back one step. TRF2 controls p21. Suppressed levels of p21 weaken effect of doxorubicin in cancer cells. Therefore, loosening the control of TRF2 over p21 could be key to regain p21 levels. This was what we therefore put our heads to. The most important clue came from the knowledge that TRF2 required a G-quadruplex structure in the p21 promoter - a structural architecture, in principle, is targetable using small molecules as commonly exploited by pharmacologists for drug molecules. Small molecules tailored to bind the G-quadruplex structure solved the puzzle! In presence of small molecules known to bind G-quadruplex structures TRF2 is unable to control p21 levels tightly in cancer cells. So, when given with doxorubicin these molecules produce increased amounts of p21 and this leads to higher doxorubicin sensitivity, in cells which were otherwise unresponsive to doxorubicin.

Transcription regulation of CDKN1A (p21/CIP1/WAF1) by TRF2 is epigenetically controlled through the REST repressor complex. Hussain T, Saha D, Purohit G, Kar A, Kishore Mukherjee A, Sharma S, Sengupta S, Dhapola P, Maji B, Vedagopuram S, Horikoshi NT, Horikoshi N, Pandita RK, Bhattacharya S, Bajaj A, Riou JF, Pandita TK, Chowdhury S. Scientific Report. September 2017

Banner Image Credit Dr Khuloud T. Al-Jamal, Wellcome Images