Describe three mechanisms of methotrexate drug resistance in human cancer cells.

Understanding how cancer cells develop resistance to chemotherapy drugs like methotrexate is critical in the fight against cancer. Methotrexate resistance due to decreased drug uptake is a significant barrier to its effectiveness. Normally, methotrexate enters cancer cells through a special channel called the Reduced Folate Carrier (RFC). When cancer cells reduce the number of these RFC channels on their surfaces or alter their structure, less methotrexate gets into the cells.

Imagine if methotrexate were a key and the RFC its specific lock; if the lock is changed (altered function) or there are fewer locks available (reduced expression), then fewer keys can get in to do their job. This reduced drug availability within the cell results in decreased inhibition of the enzyme DHFR, which is essential for DNA synthesis and cell growth. This escape tactic used by cancer cells necessitates the development of new therapeutic strategies to counteract the downturn in drug uptake.

Another cunning mechanism by which cancer cells resist methotrexate is increased drug efflux. This involves the upregulation of specific proteins known as ATP-binding cassette (ABC) transporters. These proteins act like pumps, removing substances, including drugs like methotrexate, from the inside of the cell to the outside.

To visualize this, think of ABC transporters as busy little workers who continually throw buckets of water (the drug) out of a sinking boat (the cancer cell), preventing the boat from being submerged. The overexpression of transporters such as P-glycoprotein and Multidrug Resistance Protein 1 leads to a rapid expulsion of methotrexate. Consequently, despite the drug being administered to the patient, its intracellular concentration decreases, undermining its ability to suppress the enzyme DHFR. The challenge for cancer therapy lies in finding ways to inhibit these efflux pumps, thereby keeping the methotrexate concentration within the cell at a lethal level for the cancer cells.

Alterations in the target enzyme, dihydrofolate reductase (DHFR), is one more sophisticated strategy utilized by cancer cells to resist methotrexate. These alterations can happen in two ways: mutation of the DHFR gene or overexpression of the enzyme. A mutation could change the shape of the DHFR enzyme so much that methotrexate can no longer effectively bind to it. Imagine changing the shape of a lock: even if you have the right key (methotrexate), it won’t fit anymore.

Alternatively, cancer cells can simply produce more DHFR than usual, a classic case of 'if you can't beat them, outnumber them.' With extra copies of the enzyme, even if methotrexate binds to some, there are still enough DHFR molecules free to carry on the cell's work. As DHFR is essential for making the DNA bases necessary for cell replication, its continued activity allows cancer cells to keep dividing. Treatment strategies may include developing drugs that can bind to mutated forms of DHFR or counteract its overexpression to maintain the therapeutic efficacy of methotrexate.