The p70 ribosomal protein S6 kinase 1 (S6K1) plays a key role in cell growth and proliferation by regulating insulin sensitivity, metabolism, protein synthesis, and cell cycle. Thus, deregulation of S6K contributes to the progression of type 2 diabetes, obesity, aging, and cancer. Considering the biological and clinical importance of S6K1, a complete understanding of its regulation is critical. One of the key motifs in the activation of S6K1 is a turn motif, but its regulation is not well understood. Here we provide evidence for two mechanisms of modulating turn motif phosphorylation and S6K1 activity. First, mammalian target of rapamycin regulates turn motif phosphorylation by inhibiting its dephosphorylation. Second, we unexpectedly found that glycogen synthase kinase (GSK)-3 promotes turn motif phosphorylation. Our studies show that mammalian target of rapamycin and GSK-3 cooperate to control the activity of S6K1, an important regulator of cell proliferation and growth. Our unexpected results provide a clear rationale for the development and use of drugs targeting GSK-3 to treat diseases such as diabetes, cancer, and age-related diseases that are linked to improper regulation of S6K1. M ammalian target of rapamycin (mTOR) is a central regulator of cell proliferation and growth. mTOR integrates signals from multiple inputs such as growth factors, stress, nutrients, and energy to regulate protein synthesis, cell cycle progression, actin organization, and autophagy (1). Because of the essential roles of mTOR in cell growth and metabolism, deregulation of mTOR is prominent in the development and progression of cancer and in metabolic diseases such as diabetes and obesity. Its deregulation can be caused by overexpression and/or overactivation of upstream effectors, or deletion of negative regulators.The 40S ribosomal protein S6 kinase (S6K) is a major substrate of mTOR and is a crucial effector of mTOR signaling (2). One of the S6K isoforms, S6K1, plays important roles in cell growth, proliferation, and cell differentiation by regulating ribosome biogenesis, protein synthesis, cell cycle progression, and metabolism (3-5). Recent studies suggest that deletion of S6K1 not only increases lifespan but also reduces the incidence of agerelated pathologic processes, including bone, immune, and motor dysfunction and insulin resistance (6, 7). Because of its important role in cell growth and insulin sensitivity, aberrant activation of S6K1 plays a major role in the progression of tumors, diabetes, obesity, and aging (2, 6, 7). Therefore, understanding the mechanism of S6K1 regulation will contribute to the ongoing efforts to develop novel drugs that provide effective treatments to combat diseases that are characterized by deregulation of the S6K signaling pathway.S6K1 belongs to the AGC kinase family, a subgroup of Ser/Thr protein kinases (PKs) that are related to PKA, PKG, and PKC. This group includes S6K1, Akt, p90 ribosomal S6K (RSK), mitogen-and stress-activated protein kinase, and several members of the PKC family, wh...

Insufficient nutrients disrupt physiological homeostasis resulting in diseases and even death. Considering the physiological and pathological consequences of this metabolic stress, the adaptive responses that cells utilize under this condition are of great interest. We show that under low glucose conditions, cells initiate adaptation followed by apoptosis responses using PERK/Akt and MEK1/ERK2 signaling, respectively. For adaptation, cells engage the endoplasmic reticulum stress-induced unfolded protein response, which results in PERK/Akt activation and cell survival. Sustained and extreme energetic stress promotes a switch to isoform-specific MEK1/ERK2 signaling, induction of GCN2/eIF2α phosphorylation and ATF4 expression, which overrides PERK/Akt-mediated adaptation and induces apoptosis through ATF4-dependent expression of pro-apoptotic factors including Bid and Trb3. ERK2 activation during metabolic stress contributes to changes in TCA cycle and amino acid metabolism, and cell death, which is suppressed by glutamate and α-ketoglutarate supplementation. Taken together, our results reveal promising targets to protect cells or tissues from metabolic stress.

Protein synthesis has a key role in the control of cell proliferation, and its deregulation is associated with pathological conditions, notably cancer. Rapamycin, an inhibitor of mammalian target of rapamycin complex 1 (mTORC1), was known to inhibit protein synthesis. However, it does not substantially inhibit protein synthesis and cell proliferation in many cancer types. We were interested in finding a novel target in rapamycin-resistant cancer. The rate-limiting factor for translation is eukaryotic translation initiation factor 4E (eIF4E), which is negatively regulated by eIF4E-binding protein 1 (4E-BP1). Here, we provide evidence that glycogen synthase kinase (GSK)-3β promotes cell proliferation through positive regulation of protein synthesis. We found that GSK-3β phosphorylates and inactivates 4E-BP1, thereby increasing eIF4E-dependent protein synthesis. Considering the clinical relevance of pathways regulating protein synthesis, our study provides a promising new strategy and target for cancer therapy.

Deregulation of translation initiation factors contributes to many pathogenic conditions, including cancer. Here, we report the definition of a novel regulatory pathway for translational initiation with possible therapeutic import in cancer. Specifically, we found that casein kinase 1e (CK1e) is highly expressed in breast tumors and plays a critical role in cancer cell proliferation by controlling mRNA translation. Eukaryotic translation initiation factor eIF4E, an essential component of the translation initiation complex eIF4F, is downregulated by binding the negative-acting factor 4E-BP1. We found that genetic or pharmacologic inhibition of CK1e attenuated 4E-BP1 phosphorylation, thereby increasing 4E-BP1 binding to eIF4E and inhibiting mRNA translation. Mechanistic investigations showed that CK1e interacted with and phosphorylated 4E-BP1 at two novel sites T41 and T50, which were essential for 4E-BP1 inactivation along with increased mRNA translation and cell proliferation. In summary, our work identified CK1e as a pivotal regulator of mRNA translation and cell proliferation that acts by inhibiting 4E-BP1 function. As CK1e is highly expressed in breast tumors, these findings offer an initial rationale to explore CK1e blockade as a therapeutic strategy to treat cancers driven by deregulated mRNA translation. Cancer Res; 74(1); 201-11. Ó2013 AACR.