GSK3ß phosphorylates Six1 transcription factor and regulates its APC/CCdh1 mediated proteosomal degradation.

Sine oculis homeobox homolog 1 (Six1) is a developmentally important transcription factor that regulates cellular proliferation, apoptosis, and dissemination during embryogenesis. Six1 overexpression as reported in multiple cancers modulates expression of a repertoire of its target genes causing an increase in proliferation, metastasis and survival of cancer cells. Six1 exists as a cell cycle regulated nuclear phosphoprotein and its cellular turnover is regulated by APC/C (Anaphase promoting complex / Cyclosome) complex mediated proteolysis. However, the kinases that regulate Six1 proteolysis have not been identified and the mechanistic details that cause its overproduction in various cancers are lacking. Here, we report that Six1 is a physiological GSK3ß substrate. GSK3ß interacts with Six1 and phosphorylates it at Ser221 within the conserved consensus sequence in its carboxy terminus. Using pharmacological inhibition, siRNA mediated knockdown and protein overexpression of GSK3ß; we show that GSK3ß regulates Six1 protein stability. Pulse chase analysis of Six1 revealed that GSK3ß regulates its ubiquitin proteolysis such that Six1 phosphomimicking mutant (Six1S221E) for Ser221 site had dramatically increased half-life than its phosphodeficient (Six1S221A) and wild type variants. Furthermore, we demonstrate that GSK3ß rescues Six1 from APC dependent proteolysis by regulating its binding with APC/C co-activator protein Cdh1. Importantly, strong positive correlation exists between GSK3ß and Six1 protein levels throughout the cell cycle and in multiple cancers indicating that GSK3ß activation may in part contribute to Six1 overproduction in a subset of human cancers.

TGF-ß signaling: A recap of SMAD-independent and SMAD-dependent pathways.

Transforming growth factor-ß (TGF-ß) is a proinflammatory cytokine known to control a diverse array of pathological and physiological conditions during normal development and tumorigenesis. TGF-ß-mediated physiological effects are heterogeneous and vary among different types of cells and environmental conditions. TGF-ß serves as an antiproliferative agent and inhibits tumor development during primary stages of tumor progression; however, during the later stages, it encourages tumor development and mediates metastatic progression and chemoresistance. The fundamental elements of TGF-ß signaling have been divulged more than a decade ago; however, the process by which the signals are relayed from cell surface to nucleus is very complex with additional layers added in tumor cell niches. Although the intricate understanding of TGF-ß-mediated signaling pathways and their regulation are still evolving, we tried to make an attempt to summarize the TGF-ß-mediated SMAD-dependent andSMAD-independent pathways. This manuscript emphasizes the functions of TGF-ß as a metastatic promoter and tumor suppressor during the later and initial phases of tumor progression respectively.

SIX1 transcription factor: A review of cellular functions and regulatory dynamics.

Sine Oculis Homeobox 1 (SIX1) is a member of homeobox transcription factor family having pivotal roles in organismal development and differentiation. This protein functionally acts to regulate the expression of different proteins that are involved in organ development during embryogenesis and in disorders like cancer. Aberrant expression of this homeoprotein has therefore been reported in multiple pathological complexities like hearing impairment and renal anomalies during development and tumorigenesis in adult life. Most of the cellular effects mediated by it are mostly due to its role as a transcription factor. This review presents a concise narrative of its structure, interaction partners and cellular functions vis a vis its role in cancer. We thoroughly discuss the reported molecular mechanisms that govern its function in cellular milieu. Its post-translational regulation by phosphorylation and ubiquitination are also discussed with an emphasis on yet to be explored mechanistic insights regulating its molecular dynamics to fully comprehend its role in development and disease.

Eukaryotic Initiation Factor 4E phosphorylation acts a switch for its binding to 4E-BP1 and mRNA cap assembly.

Translational regulation has invited considerable interest consequent of its circumstantial dysregulation during cancer genesis. eIF4E (Eukaryotic Initiation Factor 4E) has been identified as an important factor involved in tumor progression by way of instrumenting the convergence of oncogenic signals for up-regulation of Cap-dependent translation. In the backdrop of dramatic eIF4E over-expression in a large population of human cancers, we suggest that the tumorigenic property of eIF4E is strictly attributed to its phosphorylation state. We provide evidence that while phosphorylated eIF4E fails to be sequestered by 4E-BP1, its dephosphorylated form shows overwhelming binding with 4E-BP1 without any consideration to the state of 4E-BP1 phosphorylation to suggest that eIF4E-4EBP1 binding is governed by eIF4E phosphorylation instead of 4E-BP1. We also show that eIF4E engages in Cap-assembly formation preferably in a phosphorylation-dependent manner to suggest that eIF4E phosphorylation rather than 4E-BP1 regulates its availability for Cap-assembly.

Eukaryotic initiation factor 4E is a novel effector of mTORC1 signaling pathway in cross talk with Mnk1.

Cellular signals that influence Cap-dependent translation have assumed significant relevance in the backdrop of their enforced dysregulation during oncogenesis. Eukaryotic initiation factor 4E(eIF4E), the mRNA cap-binding protein, has emerged as a key player to facilitate tumor progression through upregulated cap-dependent translation synchronized with enhanced cell division. We provide evidence that eIF4E phosphorylation is regulated by mTORC1 by virtue of its interaction with Raptor through a novel TPTPNPP motif and consequent phosphorylation invitro and in vivo in a Rapamycin-sensitive manner. While we show that phosphorylation pattern of eIF4E responds faithfully to Rapamycin inhibition, the prolonged exposure to Rapamycin rescues the loss of eIF4E phosphorylation through Mnk1 activation. We also present evidence that eIF4E interacts with the amino terminal domain of S6K1 in a phospho-dependent manner, and this interaction is instrumental in overriding Rapamycin inhibition of S6K1. The data endorses eIF4E as a regulatory subunit that modulates the functional attributes of mTOR effectors to synchronize cap-dependent translation with growth assertion.

Eukaryotic initiation factor 4E (eIF4E): A recap of the cap-binding protein.

Eukaryotic initiation factor 4E (eIF4E), a fundamental effector and rate limiting element of protein synthesis, binds the 7-methylguanosine cap at the 5' end of eukaryotic messenger RNA (mRNA) specifically as a constituent of eIF4F translation initiation complex thus facilitating the recruitment of mRNA to the ribosomes. This review focusses on the engagement of signals contributing to growth factor originated maxim and their role in the activation of eIF4E to achieve a collective influence on cellular growth, with a key focus on conjuring vital processes like protein synthesis. The review invites considerable interest in elevating the appeal of eIF4E beyond its role in regulating translation viz a viz cancer genesis, attributed to its phosphorylation state that improves the prospect for the growth of the cancerous cell. This review highlights the latest studies that have envisioned to target these pathways and ultimately the translational machinery for therapeutic intervention. The review also brings forward the prospect of eIF4E to act as a converging juncture for signaling pathways like mTOR/PI3K and Mnk/MAPK to promote tumorigenesis.

Eukaryotic Initiation Factor 4E (eIF4E) sequestration mediates 4E-BP1 response to rapamycin.

The cap dependent translation initiation is a tightly controlled process of cooperative ternary complex formation by 4E-BP1, eIF4E and the 5' cap of eukaryotic mRNA in response to environmental cues like glucose, nutrients and growth factor levels. Based on the well-described effects of mTORC1/rapamycin complex on 4E-BP1 phosphorylation/s, it is generally accepted that rapamycin is a global inhibitor of cap-dependent translation. We have previously shown that 4E-BP1 resistance to rapamycin was overcome by the stoichiometric abundance of S6K1. Now we present evidence that the TOS-bearing amino terminal domain of S6K1 is sufficient to relieve the rapamycin resistance of 4E-BP1 as TOS deleted variants of S6K1, active or inactive with regard to S6K1 activity failed to bring about relief of 4E-BP1 resistance to rapamycin. We also show that the reciprocal inactivation of S6K1 by abundance of 4E-BP1 gets accomplished only with intact TOS motif in the protein. The data presented in this study identifies eIF4E and not Raptor as a cellular factor responsible to regulate rapamycin sensitivity of 4E-BP1 suggesting that the phosphorylation dynamics and rapamycin sensitivity of 4E-BP1 and S6K1 are regulated independently.

Reappraisal to the study of 4E-BP1 as an mTOR substrate - A normative critique.

mTOR-4E-BP1 axis is regarded as the best oncogenic circuitry impinging on translational control whereby mTORC1 dictates post-translational regulation of 4E-BP1. This review provides new insights into the molecular network of signalling pathways highlighting the recent explosion of studies in respect to the deviant behaviour of 4E-BP1 towards mTORC1. Despite the striking conservation of mTOR nexus, the eccentric phosphorylation dynamics of 4E-BP1 negate the apparent linear architecture of mTORC1 attesting the importance of other kinases that may evoke cross-talks with the conventional frame, most of which are enlisted in the manuscript. We also throw light on the tenuous role of rapamycin in 4E-BP1 regulation, which further necessitates the evaluation of 4E-BP1 to envisage the underlying molecular mechanisms in the discovery of novel drugs of 4E-BP1 for new treatment strategies. Finally, the review brings forward comprehensive studies delineating the redundancy of 4E-BP isoforms in regulating translational control.