As expected, therapy with all PI3K pathway inhibitors com pletely inhibited the proliferation potential of GFP expressing handle cells. However, RSK3 and RSK4 overexpression in MCF7 cells counteracted the development inhibitory properties of all PI3K pathway inhibitors tested. In contrast, whereas AKT1 expressing cells have been resistant to the PI3K mTOR targeted agents, they remained sensitive to treatment using the AKT inhibi tor MK2206. The RSK family of proteins comprises a group of very connected serine threonine kinases that regulate cell development, survival, and cellular proliferation downstream on the RAS RAF MEK ERK pathway. To elucidate the mechanisms behind PI3K inhibitor resistance in RSK overexpressing cells, we sought to uncover dif ferences in cellular responses to PI3K mTOR inhibition between control and RSK overexpressing cells.
Earlier selleck chemical studies have estab lished that BEZ235 induces apoptosis in cell lines sensitive to PI3K mTOR inhibition. Because each RSK and AKT overexpres sion bring about decreased sensitivity to PI3K inhibitors, we reasoned that these attenuated responses might be as a result of the inhibition of apoptosis. As expected, the addition of either BEZ235 or BKM120 substantially enhanced PARP and caspase 7 cleavage, indica tive of apoptosis, in GFP expressing handle cells. In contrast, we observed decreased cleaved PARP and cleaved caspase 7 in RSK3 four Vor AKT1 overexpressing cells upon remedy with BEZ235 or BKM120. Moreover, remedy of control cells with BEZ235 led to increased PARP cleavage in a dose dependent man ner, which was once more attenuated in cells expressing RSK or AKT1. We also observed a marked decrease within the accumulation of cells in sub G1 in the RSK4 overexpressing cells compared with manage cells upon therapy with BEZ235.
Related findings had been observed in RSK overexpressing cells treated together with the pan PI3K inhibitors BKM120 and GDC0941. Taken collectively, these information recommend that RSK over expressing cells are resistant to PI3K NVP-BKM120 price mTOR inhibition at the very least in aspect by means of decreased induction of apoptosis. Many recent reports have demonstrated that the anti tumor effects of PI3K inhibition might be lowered by the activation with the ERK signaling pathway or by upregulation of protein trans lation. Likewise, we investigated the regulation of protein translation in our RSK or AKT1 overexpressing cells. In manage cells, PI3K pathway blockade together with the PI3K inhibitor BKM120, the dual PI3K mTOR inhibitor BEZ235, or the catalytic mTOR inhibitor pp242 markedly lowered eIF4B and rpS6 phosphorylation, 2 essential regulators of cap dependent translation. In contrast, dephosphorylation of ribosomal protein S6 and eIF4B by PI3K, mTOR, or dual PI3K mTOR inhibitors was abrogated within the RSK overexpressing cells. We extended these analyses to other RSK loved ones members.