IL-6-induced stimulation of c-myc translation in multiple myeloma cells is mediated by myc internal ribosome entry site function and the RNA-binding protein, hnRNP A1.

Prior work indicates that c-myc translation is up-regulated in multiple myeloma cells. To test a role for interleukin (IL)-6 in myc translation, we studied the IL-6-responsive ANBL-6 and IL-6-autocrine U266 cell lines as well as primary patient samples. IL-6 increased c-myc translation, which was resistant to rapamycin, indicating a mechanism independent of mammalian target of rapamycin (mTOR) and cap-dependent translation. In contrast, the cytokine enhanced cap-independent translation via a stimulatory effect on the myc internal ribosome entry site (IRES). As known IRES-trans-activating factors (ITAF) were unaffected by IL-6, we used a yeast-three-hybrid screen to identify novel ITAFs and identified hnRNP A1 (A1) as a mediator of the IL-6 effect. A1 specifically interacted with the myc IRES in filter binding assays as well as EMSAs. Treatment of myeloma cells with IL-6 induced serine phosphorylation of A1 and increased its binding to the myc IRES in vivo in myeloma cells. Primary patient samples also showed binding between A1 and the IRES. RNA interference to knock down hnRNP A1 prevented an IL-6 increase in myc protein expression, myc IRES activity, and cell growth. These data point to hnRNP A1 as a critical regulator of c-myc translation and a potential therapeutic target in multiple myeloma.

ROCK-II-induced membrane blebbing and chromatin condensation require actin cytoskeleton.

Ectopic expression of ROCK II (Rho kinase II or ROKalpha), an effector of Rho GTPase, induces membrane blebbing and chromatin condensation. ROCK II can induce membrane blebbing in the presence of the caspase inhibitor z-VAD-fmk or in caspase-3-deficient MCF-7 cells, indicating that the activation of caspases is not required. ROCK-II-induced membrane blebbing, however, is reversed by the myosin light chain kinase inhibitor ML-7 or cytochalasin D. In addition, the expression of a constitutively activated form of cofilin (S3A-cofilin) suppresses both membrane blebbing and chromatin condensation in ROCK II expressing cells. These findings suggest that the activation of actin-myosin contractility is responsible for membrane blebbing and chromatin condensation and implicate ROCK II as a potential mediator of the morphological changes associated with apoptosis.

Molecular basis for interaction between Icap1 alpha PTB domain and beta 1 integrin.

Icap1 alpha is a 200-amino acid protein that binds to the COOH-terminal 13 amino acids ((786)AVTTVVNPKYEGK(798)) of the integrin beta(1) subunit. Alanine scanning mutagenesis of this region revealed that Val(787), Val(790), and (792)NPKY(795) are critical for Icap1 alpha binding. The NPXY motif is a known binding substrate for phosphotyrosine binding (PTB) domain proteins. The sequences of Icap1 alpha, residues 58--200, and the beta(1) integrin, residues 786-797, were aligned to the available PTB-peptide structures to generate a high quality structural model. Site-directed mutagenesis showed that Leu(135), Ile(138), and Ile(139) of Icap1 alpha, residues predicted by the model to be in close proximity to (792)NPKY(795), and Leu(82) and Tyr(144), residues expected to form a hydrophobic pocket near Val(787), are required for the Icap1 alpha-beta(1) integrin interaction. These findings indicate that Icap1 alpha is a PTB domain protein, which recognizes the NPXY motif of beta(1) integrin. Furthermore, our date suggest that an interaction between Val(787) and the hydrophobic pocket created by Leu(82) and Tyr(144) of Icap1 alpha forms the basis for the specificity of Icap1 alpha for the beta(1) integrin subunit.