AKT-independent Phosphorylation of TSC2 and Activation of mTOR and Ribosomal Protein S6 Kinase Signaling by Prostaglandin F2α

Abstract

The corpus luteum (CL)2 is a transient endocrine gland derived from an ovulated follicle within the ovary. The steroidogenic cells of the CL produce progesterone, a prerequisite for normal maintenance of pregnancy in mammals. In the event of pregnancy, the CL retains its role in progesterone synthesis in support of early pregnancy. In the absence of pregnancy luteolysis or corpus luteum regression occurs, a physiologic process associated with a reduction in progesterone secretion (functional regression) followed by death of endothelial and steroidogenic cells (structural regression) (1-4). Prostaglandin F2␣ (PGF2␣) was identified as a luteolytic factor over 35 years ago (5, 6). Recent genetic studies in mice lacking the PGF2␣ receptor (FP) further highlight the role for PGF2␣ in CL regression. Mice lacking FP receptors experience defects in CL regression and consequently parturition does not occur because of the maintenance of progesterone secretion at the end of pregnancy (7).The FP receptor is a member of the large class of heterotrimeric G-protein-coupled receptors (GPCRs) (8) and is present on the surface of steroidogenic luteal cells. PGF2␣ binding to its G q -coupled receptor results in activation of phospholipase ␤ (PLC␤) and consequent generation of the second messengers; diacylglycerol and inositol trisphosphate (9). The resultant increase in protein kinase C (PKC) activity in luteal cells contributes to the activation of other downstream protein kinases. PGF2␣ stimulates the activity of the extracellular signal-regulated kinase (Erk) family of mitogen-activated protein kinases (MAPK) through a mechanism that involves the PKC-dependent phosphorylation and activation of Raf (10, 11). These events result in the induction of early response genes that code for transcription factors that in turn, alter the synthesis of proteins that regulate progesterone synthesis (12-14). Little is understood regarding other signaling mechanisms initiated by PGF2␣ that account for the actions of PGF2␣ in the CL.The mammalian target of rapamycin (mTOR) protein is a key regulator of protein translation via mechanisms involving the phosphorylation of the translation regulator eukaryotic initiation factor 4E (eIF4E) -binding protein (4EBP1) and the 70-kDa ribosomal protein S6 protein kinase 1 (S6K1) (15,16 receptor; GPCR, G-protein-coupled receptor; mTOR, mammalian target of rapamycin; m 7 G, 7-methyl-guanosine; PI3K, phosphatidylinositol-3-kinase; PGF2␣, prostaglandin F2␣; S6K1, p70 ribosomal S6 kinase; TSC2, tuberous sclerosis complex 2; FBS, fetal bovine serum; FCS, fetal calf serum; JNK, c-Jun N-terminal kinase; PMA, phorbol 12-myristate 13-acetate; PKC, protein kinase C.

Novel pathway in Bcr-Abl signal transduction involves Akt-independent, PLC-γ1-driven activation of mTOR/p70S6-kinase pathway

“…For instance, FGF-9 could induce a PLC-g1-controlled mTOR and p70S6-kinase activation, although PI3K and Akt were not at all activated in the analyzed human endometrial stroma cells (Wing et al, 2005). Two other examples of Aktindependent mTOR/p70S6-kinase activation are provided from transformed B lymphocytes (Wlodarski et al, 2005) and from prostaglandin-F2a-treated bovine luteal cells (Arvisais et al, 2006), even though the mechanisms involved here might be different.…”

Oncogenic tyrosine kinase NPM/ALK induces activation of the rapamycin-sensitive mTOR signaling pathway

“…As the MEK/ERK signaling pathway is induced by NPM/ALK (Marzec et al, 2007) and can activate mTOR (Tee et al, 2003;Roux et al, 2004;Ma et al, 2005;Arvisais et al, 2006), we examined whether inhibition of MEK would impact mTOR activation in ALK þ TCL cells. Treatment of Sudhl-1 cells with two different MEK inhibitors, U0126 and PD98059, markedly suppressed phosphorylation of not only ERK1/2 but also the mTOR target S6rp (Figure 4a).…”

“…As MEK/ERK was found to act on mTOR by phosphorylating TSC2 (Tee et al, 2003;Roux et al, 2004;Ma et al, 2005;Arvisais et al, 2006), we tested the impact of MEK inhibition on TSC2 phosphorylation. In addition to serine 939, the target of Akt (Manning et al, 2002) and possibly other kinases, we evaluated serine 1798 targeted by the ERK1/2 complex (Tee et al, 2003;Roux et al, 2004 U0126 and PD98059 had no appreciable effect on TSC2 phosphorylation on serine 939, they profoundly inhibited serine 1798 phosphorylation (Figure 4c).…”

“…Accordingly, Aktmediated TCS phosphorylation is not critical for mTOR activity during Drosophila development (Dong and Pan, 2004). Furthermore, at least two different ligands -TSH acting on thyroid epithelial cells (Suh et al, 2003) and prostaglandin F2a in luteal cells (Arvisais et al, 2006) can activate mTOR without exerting any appreciable effect on Akt. Finally, in transformed B lymphocytes mTOR activation is mostly, if not completely, PI3K/ Akt-independent .…”

“…Although insulin represents the best characterized factor, a few other mTOR stimulators have been recently described. Accordingly, such diverse cell surface receptors and receptor ligands as CD40 (Sakata et al, 1999), polycystein-1 (Shillingford et al, 2006) and prostaglandin F2a (Arvisais et al, 2006) can also activate mTOR. Based on our findings we can now add NPM/ALK to this expanding list.…”