Ribosomal S6 Kinase Signaling Research Articles

Integrins promote cytokinesis through the RSK signaling axis.

Cytokinesis is the final stage in cell division. Although integrins can regulate cytokinesis, the mechanisms involved are not fully understood. In this study, we demonstrate that integrin-regulated ERK (extracellular signal-related kinase) and RSK (p90 ribosomal S6 kinase) signaling promotes successful cytokinesis. Inhibiting the activation of ERK and RSK in CHO cells by a mutation in the integrin β1 cytoplasmic tail or with pharmacological inhibitors results in the accumulation of cells with midbodies and the formation of binucleated cells. Activation of ERK and RSK signaling by the expression of constitutively active RAF1 suppresses the mutant phenotype in a RSK-dependent manner. Constitutively active RSK2 also restores cytokinesis inhibited by the mutant integrin. Importantly, the regulatory role of the RSK pathway is not specific to CHO cells. MCF-10A human mammary epithelial cells and HPNE human pancreatic ductal epithelial cells exhibit a similar dependence on RSK for successful cytokinesis. In addition, depriving mitotic MCF10A cells of integrin-mediated adhesion by incubating them in suspension suppressed ERK and RSK activation and resulted in a failure of cytokinesis. Furthermore, inhibition of RSK or integrins within the 3D context of a developing salivary gland organ explant also leads to an accumulation of epithelial cells with midbodies, suggesting a similar defect in cytokinesis. Interestingly, neither ERK nor RSK regulates cytokinesis in human fibroblasts, suggesting cell-type specificity. Taken together, our results identify the integrin-RSK signaling axis as an important regulator of cytokinesis in epithelial cells. We propose that the proper interaction of cells with their microenvironment through integrins contributes to the maintenance of genomic stability by promoting the successful completion of cytokinesis.

The Src homology 2 domain-containing adapter protein B (SHB) regulates mouse oocyte maturation.

SHB (Src homology 2 domain-containing adapter protein B) is involved in receptor tyrosine kinase signaling. Mice deficient in the Shb gene have been found to exhibit a transmission ratio distortion with respect to inheritance of the Shb null allele among offspring and this phenomenon was linked to female gamete production. Consequently, we postulated that Shb plays a role for oocyte biology and thus decided to investigate oocyte formation, meiotic maturation, and early embryo development in relation to absence of the Shb gene. Oogenesis was apparently accelerated judging from the stages of oocyte development on fetal day 18.5 and one week postnatally in Shb −/− mice; but in adulthood ovarian follicle maturation was impaired in these mice. Completion of meiosis I (first polar body extrusion) was less synchronized, with a fraction of oocytes showing premature polar body extrusion in the absence of Shb. In vitro fertilization of mature oocytes isolated from Shb +/+, +/− and −/− mice revealed impaired early embryo development in the −/− embryos. Moreover, the absence of Shb enhanced ERK (extracellular-signal regulated kinase) and RSK (ribosomal S6 kinase) signaling in oocytes and these effects were paralleled by an increased ribosomal protein S6 phosphorylation and activation. It is concluded that SHB regulates normal oocyte and follicle development and that perturbation of SHB signaling causes defective meiosis I and early embryo development.

Targeting RSK: An Overview of Small Molecule Inhibitors

Ribosomal S6 kinase (RSK) is a family of serine/threonine kinases that has been identified as a promising anti-cancer target. While a number of protein kinase inhibitors that have potent activity against other serine/threonine kinases were shown to also inactivate RSK, there is keen interest in the three different inhibitor chemotypes that were shown to be RSK specific, since these compounds have tremendous utility as chemical probes in elucidating the biochemistry of the RSK signaling cascade and unraveling the molecular basis of cancer. Because each compound may have therapeutic potential, the nonspecific kinase inhibitors as well as the RSK specific inhibitors will be discussed.

Dissociation of Akt/PKB and ribosomal S6 kinase signaling markers in a transgenic mouse model of Alzheimer’s disease

Previous studies demonstrated that the PKR (double-stranded RNA-activated protein kinase) pathway was activated while the mTOR (mammalian target of rapamycin) pathway was inhibited in Alzheimer’s disease (AD). Here, we analysed upstream and downstream factors of mTOR in brain of APP SL/PS1 KI mice displaying a massive neuronal loss in hippocampus. While mTOR levels were not modified, we found a great activation of Akt with a robust accumulation of P-Akt (T308) in non-apoptotic neurons at 6 months of age. At the opposite, a significant decrease of the p70/85S6K activation was observed in brain of PS1 KI and APP SL/PS1 KI mice with a very weak or no nucleocytoplasmic P-p70/85S6K (T389) staining in apoptotic neurons of APP SL/PS1 KI mice. Furthermore, the activation of Erk1/2, 4E-BP1 and p70S6K (T421/S424) (substrate of Erk1/2), except eIF4E, was not modified. These findings demonstrate a clear dissociation between Akt and ribosomal S6K signaling markers in these mice which could be involved in the AD pathological process.

Hypomorphic Mutation of PDK1 Suppresses Tumorigenesis in PTEN+/− Mice

Many cancers possess elevated levels of PtdIns(3,4,5)P(3), the second messenger that induces activation of the protein kinases PKB/Akt and S6K and thereby stimulates cell proliferation, growth, and survival. The importance of this pathway in tumorigenesis has been highlighted by the finding that PTEN, the lipid phosphatase that breaks down PtdIns(3,4,5)P(3) to PtdIns(4,5)P(2), is frequently mutated in human cancer. Cells lacking PTEN possess elevated levels of PtdIns(3,4,5)P(3), PKB, and S6K activity and heterozygous PTEN(+/-) mice develop a variety of tumors. Knockout of PKBalpha in PTEN-deficient cells reduces aggressive growth and promotes apoptosis, whereas treatment of PTEN(+/-) mice with rapamycin, an inhibitor of the activation of S6K, reduces neoplasia. We explored the importance of PDK1, the protein kinase that activates PKB and S6K, in mediating tumorigenesis caused by the deletion of PTEN. We demonstrate that reducing the expression of PDK1 in PTEN(+/-) mice, markedly protects these animals from developing a wide range of tumors. Our findings provide genetic evidence that PDK1 is a key effector in mediating neoplasia resulting from loss of PTEN and also validate PDK1 as a promising anticancer target for the prevention of tumors that possess elevated PKB and S6K activity.