As expected, remedy with all PI3K pathway inhibitors com pletely inhibited the proliferation prospective of GFP expressing control cells. Nevertheless, RSK3 and RSK4 overexpression in MCF7 cells counteracted the development inhibitory properties of all PI3K pathway inhibitors tested. In contrast, while AKT1 expressing cells had been resistant towards the PI3K mTOR targeted agents, they remained sensitive to therapy together with the AKT inhibi tor MK2206. The RSK household of proteins comprises a group of highly associated serine threonine kinases that regulate cell development, survival, and cellular proliferation downstream with 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 involving handle and RSK overexpressing cells.
Prior selleck chemicals research have estab lished that BEZ235 induces apoptosis in cell lines sensitive to PI3K mTOR inhibition. Since both RSK and AKT overexpres sion cause decreased sensitivity to PI3K inhibitors, we reasoned that these attenuated responses could be on account 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 control cells. In contrast, we observed reduced cleaved PARP and cleaved caspase 7 in RSK3 4 Vor AKT1 overexpressing cells upon therapy with BEZ235 or BKM120. Furthermore, remedy of control cells with BEZ235 led to improved PARP cleavage within a dose dependent man ner, which was once again attenuated in cells expressing RSK or AKT1. We also observed a marked decrease in the accumulation of cells in sub G1 in the RSK4 overexpressing cells compared with control cells upon therapy with BEZ235.
Related findings have been observed in RSK overexpressing cells treated using the pan PI3K inhibitors BKM120 and GDC0941. Taken together, these data recommend that RSK over expressing cells are resistant to PI3K selleckchem Bortezomib mTOR inhibition a minimum of in element by means of decreased induction of apoptosis. A number of recent reports have demonstrated that the anti tumor effects of PI3K inhibition may be lowered by the activation in 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 handle cells, PI3K pathway blockade together with the PI3K inhibitor BKM120, the dual PI3K mTOR inhibitor BEZ235, or the catalytic mTOR inhibitor pp242 markedly reduced eIF4B and rpS6 phosphorylation, two important regulators of cap dependent translation. In contrast, dephosphorylation of ribosomal protein S6 and eIF4B by PI3K, mTOR, or dual PI3K mTOR inhibitors was abrogated in the RSK overexpressing cells. We extended these analyses to other RSK family members.