Progestin resistance is a major obstacle to treating early stage, well-differentiated
endometrial cancer as well as recurrent
endometrial cancer. The mechanism behind the suboptimal response to
progestin is not well understood. The PTEN tumor suppressor gene is frequently mutated in type I
endometrial cancers and this mutation results in hyperactivation of the PI3K/AKT pathway. We hypothesized that increased activation of AKT promotes an inadequate response to
progestins in
endometrial cancer cells. Ishikawa cells stably transfected with
progesterone receptor B (PRB23 cells) were treated with the AKT inhibitor,
MK-2206, which effectively decreased levels of p(Ser473)-AKT in a dose-dependent (10 nM to 1 uM) and time-dependent manner (0.5 h to 24 h).
MK-2206 inhibited levels of p(Thr308)-AKT and a downstream target, p(Thr246)-PRAS40, but did not change levels of p(Thr202/Tyr204)ERK or p(Thr13/Tyr185)SAPK/JNK, demonstrating specificity of
MK-2206 for AKT. Additionally,
MK-2206 treatment of PRB23 cells resulted in a significant increase in levels of
progesterone receptor B (PRB)
protein. Microarray analysis of PRB23 cells identified PDK4 as the most highly upregulated gene among 70 upregulated genes in response to
R5020. Inhibition of AKT further upregulated
progestin-mediated expression of PDK4 but did not affect another
progestin-responsive gene, SGK1. Treatment of PRB23 cells with
R5020 and
MK-2206 independently decreased viability of cells while the combination of
R5020 and
MK-2206 caused the greatest decrease in cell viability. Furthermore, mice with xenografted
tumors treated with
MK-2206 alone or with
progesterone alone exhibited modest reductions in their
tumor volume. The largest decrease in
tumor size was observed in the mice treated with both
MK-2206 and
progesterone; these
tumors exhibited the least proliferation (Ki67) and the most apoptosis (cleaved caspase-3) of all the treatment groups. In summary, inhibition of AKT stabilizes the
Progesterone Receptor B and augments
progesterone response in
endometrial cancer cells that have hyperactivated AKT.