Escape from stress granule sequestration: another way to drug resistance?

Overexpression of P-glycoprotein, encoded by the MDR1 (multidrug resistance 1) gene, is often responsible for multidrug resistance and chemotherapy failure in cancer. We have demonstrated that, in leukaemic cells, P-glycoprotein expression is regulated at the translational level. More recently, we have shown that in cells overexpressing P-glycoprotein, MDR1 mRNA does not aggregate into translationally silent stress granules. Importantly, this is not unique for MDR1, since other transcripts encoding transmembrane proteins, and which are thus translated at the endoplasmic reticulum, follow the same pattern. By using a series of chimaeric transcripts, we have demonstrated that transcript localization at the endoplasmic reticulum bypasses the signals dictating stress granule sequestration. Polysome profile analyses and protein synthesis experiments indicate that, upon stress withdrawal, endoplasmic-reticulum-bound transcripts resume translation faster than those at the cytosol, which have been sequestered into stress granules. This may represent a novel mechanism by which drug-resistant cells respond quickly to stress, helping them to survive the cytotoxic effect of chemotherapeutic drugs.