The 90 kDa ribosomal S6 kinase-2 (RSK2) is a growth factor-stimulated protein kinase with two kinase domains. The C-terminal kinase of RSK2 is activated by ERK-type MAP kinases, leading to autophosphorylation of RSK2 at Ser386 in a hydrophobic motif. The N-terminal kinase is activated by 3-phosphoinositidedependent protein kinase-1 (PDK1) through phosphorylation of Ser227, and phosphorylates the substrates of RSK. Here, we identify Ser386 in the hydrophobic motif of RSK2 as a phosphorylationdependent docking site and activator of PDK1. Treatment of cells with growth factor induced recruitment of PDK1 to the Ser386-phosphorylated hydrophobic motif and phosphorylation of RSK2 at Ser227. A RSK2-S386K mutant showed no interaction with PDK1 or phosphorylation at Ser227. Interaction with Ser386-phosphorylated RSK2 induced autophosphorylation of PDK1. Addition of a synthetic phosphoSer386 peptide (RSK2 373±396 ) increased PDK1 activity 6-fold in vitro. Finally, mutants of RSK2 and MSK1, a RSK-related kinase, with increased af®nity for PDK1, were constitutively active in vivo and phosphorylated histone H3. Our results suggest a novel regulatory mechanism based on phosphoserine-mediated recruitment of PDK1 to RSK2, leading to coordinated phosphorylation and activation of PDK1 and RSK2.

90-kDa ribosomal S6 kinase-2 (RSK2) belongs to a family of growth factor-activated serine/threonine kinases. Our study extend recent findings which implicate PDK1 in the activation of protein kinases B and C and p70 S6K , suggesting that PDK1 controls several major growth factor-activated signal transduction pathways.The 90-kDa ribosomal S6 kinases (RSK1-3) 1 are a family of broadly expressed serine/threonine kinases that are activated by extracellular signal-regulated protein kinases (ERK1 and -2) in response to many growth factors, polypeptide hormones, and neurotransmitters (Refs. 1-3; reviewed in Refs. 4 and 5). Inactivating mutations in the RSK2 gene are responsible for the human Coffin-Lowry syndrome, which is characterized by severe mental retardation and progressive skeletal deformations (6, 7). At the cellular level, RSK2 has been proposed to regulate the activity of the transcription factor cAMP response element-binding protein (CREB) (8, 9) and the transcriptional co-activators p300 and CREB-binding protein (10). RSK1 can phosphorylate the estrogen receptor and enhance its transcriptional activity (11) and may also be an activator of the transcription factor NFB through phosphorylation of IB␣ (12, 13). Besides a role in transcriptional control, findings in Xenopus laevis oocytes implicate RSK in stimulation of meiosis via inactivation of the p34 cdc2 -inhibitory kinase Myt1 (14). Finally, RSK can phosphorylate the Ras GTP-exchange molecule SOS and may thereby exert negative feedback of the Ras-ERK pathway (15). Recently, a family of two mitogen-and stress-activated protein kinases (MSK) has been discovered, which resembles RSK in having two kinase domains and other structural hallmarks (16,17). MSK is activated by ERK as well as by p38 mitogen-activated protein kinase in response to growth factors and various cellular stress stimuli.The two kinase domains of RSK are connected by a ϳ100-amino acid sequence, referred to here as the linker. The substrates of RSK identified so far are phosphorylated by the N-terminal kinase (NTK) (18 -20), whereas the C-terminal kinase (CTK) and the linker participate in the regulation of the NTK (18,20,21). The mechanism of activation of RSK is complex and involves phosphorylation of at least four sites (Fig. 1), as demonstrated with RSK1 in cells treated with phorbol ester, a potent activator of ERK (21). As a probable sequence of events, ERK phosphorylates two sites, one in the linker and one in the activation loop of the CTK, leading to its activation (20 -22). The CTK then phosphorylates an additional site in the linker (21,23). Dual phosphorylation of the linker leads to increased phosphorylation of a serine residue in the activation loop of the NTK and full kinase activity (21). The critical role of this serine is indicated by the finding that its mutation to alanine abolishes the ability of all three RSK isotypes to phosphorylate exogenous substrates in vitro (6,18,21) and that this mutation in RSK2 can cause the Coffin-Lowry syndrome (6). The phosphorylation sit...

SUMMARY The RAS-stimulated RAF-MEK-ERK pathway confers epithelial cells with critical motile and invasive capacities during embryonic development, tissue regeneration and carcinoma progression. Yet many mechanisms by which ERK exerts this control remain elusive. Here, we demonstrate that the ERK-activated kinase RSK is necessary to induce motility and invasive capacities in non-transformed epithelial cells and carcinoma cells. RSK is moreover sufficient to induce certain motile responses. Expression profiling analysis revealed that a primary role of RSK is to induce transcription of potent pro-motile/invasive gene program by FRA1-dependent and independent mechanisms. Strikingly, the program enables RSK to coordinately modulate the extracellular environment, the intracellular motility apparatus, and receptors mediating communication between these compartments to stimulate motility and invasion. These findings uncover a general mechanism whereby the RAS-ERK pathway controls epithelial cell motility by identifying RSK as a key effector, from which emanates multiple highly coordinate transcription-dependent mechanisms for stimulation of motility and invasive properties.

As we found an effect of ear piercing before but not after 1992, this study strongly suggests that implementation of the nickel-exposure regulation in 1992 in Denmark has had the intended effect of protecting the female population from becoming allergic to nickel.

Systemic contact dermatitis is usually seen as flare-up of previous dermatitis or de novo dermatitis similar to allergic contact dermatitis. Although systemic contact dermatitis from medicaments is a well-established entity, the existence of clinically relevant systemic reactions to oral nickel exposure, in particular systemic reactions to nickel in the daily diet, remains controversial. Several studies have shown that oral exposure to nickel can induce systemic contact dermatitis in nickel-sensitive individuals. In most of these studies, however, the exposure dose of nickel used has been considerably higher than the nickel content in the normal daily diet. The aim of the current investigation was to study dose-response dependency of oral exposure to nickel. In a double-blind, placebo-controlled oral exposure trial, 40 nickel-sensitive persons and 20 healthy (non-nickel-sensitive) controls were given nickel sulfate hexahydrate in doses similar to and greater than the amount of nickel ingested in the normal Danish daily diet. The nickel content in urine and serum before and after oral exposure was measured to determine nickel uptake and excretion. The influence of the amount of nickel ingested on the clinical reactions to oral exposure and on nickel concentrations in serum and urine was evaluated. Among nickel-sensitive individuals, a definite dose-response dependency was seen, following oral exposure to nickel. 7 of 10 nickel-sensitive individuals had cutaneous reactions to oral exposure to 4.0 mg nickel, an amount approximately 10 times greater than the estimated normal daily dietary intake of nickel. 4 of 10 nickel-sensitive individuals had cutaneous reactions to 1.0 mg nickel, a dose which is close to the estimated maximum amount of nickel contained in the daily diet. 4 of 10 nickel-sensitive individuals reacted to 0.3 mg nickel or to the amount equivalent to that contained in a normal daily diet, and 1 of 10 reacted to a placebo. None of the 20 healthy controls had cutaneous reactions to 4.0 mg nickel or to a placebo. Prior to oral exposure, there was no measurable difference in the amount of nickel in the urine or serum of nickel-sensitive persons and healthy controls. Following the oral challenge, the nickel content in the urine and serum of both nickel-sensitive and healthy control individuals was directly related to the dose of nickel ingested.