While nonsense-mediated RNA decay (NMD) is an established mechanism to rapidly degrade select transcripts, the physiological regulation and biological significance of NMD are not well characterized. We previously demonstrated that NMD is inhibited in hypoxic cells. Here we show that the phosphorylation of the ␣ subunit of eukaryotic initiation factor 2 (eIF2␣) translation initiation factor by a variety of cellular stresses leads to the inhibition of NMD and that eIF2␣ phosphorylation and NMD inhibition occur in tumors. To explore the significance of this NMD regulation, we used an unbiased approach to identify approximately 750 NMD-targeted mRNAs and found that these mRNAs are overrepresented in stress response and tumorpromoting pathways. Consistent with these findings, the inhibition of NMD promotes cellular resistance to endoplasmic reticulum stress and encourages tumor formation. The transcriptional and translational regulations of gene expression by the microenvironment are established mechanisms by which tumor cells adapt to stress. These data indicate that NMD inhibition by the tumor microenvironment is also an important mechanism to dynamically regulate genes critical for the response to cellular stress and tumorigenesis.During tumorigenesis, a disorganized vasculature leads to amino acid and glucose deprivation, cellular hypoxia, the accumulation of reactive oxygen species (ROS), and various other stresses (5, 12). Cellular adaptation to the hostile tumor microenvironment requires the regulation of stress-induced genes (reviewed in reference 16). For example, the transcription factor ATF-4, upregulated in human tumors due to the stress-induced phosphorylation of the ␣ subunit of eukaryotic translation initiation factor 2 (eIF2␣), transactivates genes involved in amino acid metabolism, angiogenesis, and ROS attenuation (2,3,33). Cells that cannot phosphorylate eIF2␣ or that are deficient in ATF-4 and other stress-induced transcription factors do not form tumors in vivo (2, 13, 40), and therefore, a major goal in cancer biology has been to better understand and potentially target these adaptive mechanisms. However, while the translational and transcriptional responses that promote adaptation to the tumor microenvironment are well established, the role of mRNA stabilization in the cellular stress response has not been as thoroughly studied.Nonsense-mediated RNA decay (NMD) degrades up to 30% of all mutated protein-coding mRNAs, including those responsible for many genetic disorders, such as thalassemia, cystic fibrosis, and muscular dystrophy (11). During the processing of mammalian pre-mRNA, introns are excised and marked by an exon junction complex, which contains core NMD components. Newly synthesized mRNAs are thought to undergo a pioneering round of translation by a complex that includes eIF2␣ (6). When this translation complex pauses at a premature termination codon (PTC) upstream of an exon junction complex, the RNA helicase UPF1/Rent1, an essential component of the NMD process, is recruited and the...
Sensing invading pathogens early in infection is critical for establishing host defense. Two cytosolic RIG-like RNA helicases, RIG-I and MDA5, are key to type I interferon (IFN) induction in response to viral infection. Mounting evidence suggests that another viral RNA sensor, protein kinase R (PKR), may also be critical for IFN induction during infection, although its exact contribution and mechanism of action are not completely understood. Using PKR-deficient cells, we found that PKR was required for type I IFN induction in response to infection by vaccinia virus lacking the PKR antagonist E3L (VVΔE3L), but not by Sendai virus or influenza A virus lacking the IFN-antagonist NS1 (FluΔNS1). IFN induction required the catalytic activity of PKR, but not the phosphorylation of its principal substrate, eIF2α, or the resulting inhibition of host translation. In the absence of PKR, IRF3 nuclear translocation was impaired in response to MDA5 activators, VVΔE3L and encephalomyocarditis virus, but not during infection with a RIG-I-activating virus. Interestingly, PKR interacted with both RIG-I and MDA5; however, PKR was only required for MDA5-mediated, but not RIG-I-mediated, IFN production. Using an artificially activated form of PKR, we showed that PKR activity alone was sufficient for IFN induction. This effect required MAVS and correlated with IRF3 activation, but no longer required MDA5. Nonetheless, PKR activation during viral infection was enhanced by MDA5, as virus-stimulated catalytic activity was impaired in MDA5-null cells. Taken together, our data describe a critical and non-redundant role for PKR following MDA5, but not RIG-I, activation to mediate MAVS-dependent induction of type I IFN through a kinase-dependent mechanism.
We cloned activated T cells from thyroid tissue of patients with autoimmune thyroid disease. After separation on 40% Percoll gradients, T cells were cultured for 2-7 days with T cell growth factor (interleukin 2; 20 U/mL) and cloned by limiting dilution (0.3 cells/well) in the presence of irradiated autologous peripheral blood mononuclear cells (PMC; 10,000/well) as feeder cells. Fifty-seven clones were successfully expanded and tested for reactivity, cytotoxicity, helper/suppressor function, and phenotype. In the reactivity assays clones were tested for responses to autologous and allogeneic PMC, thyroid cells, human thyroglobulin (hTg), and microsomal antigen. Two distinct patterns of functional T cell clones emerged from these characterization studies. Seventy-five percent of T cell clones recovered from Graves' disease thyroid tissue (n = 21) were of helper-induced (CD4+/4B4+) phenotype, and most were effective immunoglobulin helper clones. Fifty percent of Graves' T cell clones responded to autologous PMC, and 33% had a proliferative response to autologous thyroid cells. No cytotoxic clones were derived from Graves' thyroid tissue. By contrast, intrathyroidal T cell clones from patients with autoimmune thyroiditis (n = 36) were 59% suppressor/cytotoxic (CD8+) phenotype, 17% suppressed immunoglobulin secretion, and 55% were cytotoxic to allogeneic blast cells. Fifty-five percent of clones also responded to autologous PMC, and one clone was nonspecifically autocytotoxic. In the thyroid antigen proliferation assays 11% of thyroiditis clones reacted to human thyroglobulin, but none responded to microsomal antigen. Two clones were cytotoxic to autologous but not allogeneic thyroid cells. These data demonstrate that the majority of intrathyroidal T cells in autoimmune thyroid disease are autoreactive. However, small numbers of thyroid-specific T cell clones are present within the thyroid of such patients; they are principally helper-inducer T cells in Graves' disease thyroid and cytotoxic T cells in autoimmune thyroiditis.
The generation of artificial human thyroid tissues in suspension (low-shear environment, present in simulated microgravity [MG] and generated by a rotary cell culture system [RCCS]), was enhanced by increasing medium kinematic viscosity with a (3% v/v) suspension of extracellular matrix (basement membrane extract [BME]) in serum-free medium to generate artificial human thyroid organoids. Recombinant human keratinocyte growth factor (KGF, 7 ng/mL) facilitated human thyrocyte aggregation and three-dimensional (3-D) differentiation. There was an MG-associated decrease in extractable DNA that was reversed after addition of keratinocyte growth factor (KGF). In simulated MG, the increase in extractable DNA after KGF addition was up to 170% over non-KGF control cultures. In contrast, monolayer cultures in unit gravity showed a maximum DNA increase of 39% after KGF addition. Morphologically, differentiated thyroid neofollicles displayed polarization and were located in close proximity after 2 weeks of culture. Immunogold labeling with antibody to human thyroglobulin (Tg) revealed staining of follicular lumina and secretory vesicles, and a time-dependent increase in human Tg was detected in the culture media. Culture under simulated MG thus allowed direct visualization of KGF-facilitated thyrocyte/extracellular matrix interaction. Such artificial human thyroid organoids-generated in MG and in the presence of KGF-structurally resembled natural thyroid tissue. The above findings may have implications for autoimmune thyroid disease where KGF (if, for example, secreted locally by intraepithelial gammadelta T cells among other cells) may contribute to thyroid cell growth.
We have investigated the TSH responsiveness of normal and abnormal human thyroid cells cultured in the short term with high serum concentrations and for up to 6 months in a low serum, chemically-defined, medium. Cells from normal human thyroid tissue (n = 9), multinodular goitre (n = 6), benign follicular adenomata (n = 6), and differentiated thyroid carcinoma (n = 3) formed confluent monolayers which were sensitive to bovine TSH (bTSH) in concentrations greater than 25 microU/ml when assessed by the intracellular response of cyclic AMP at 7 d of culture. Such sensitivity was less than that observed with a continuously proliferating thyroid cell line (FRTL-5) derived from Fisher rat thyroid and which responded to concentrations of bTSH as low as 5-10 microU/ml. Human cells derived from iodine/antithyroid drug treated Graves' thyroid tissue (n = 6) were less sensitive than normal cells requiring up to 500 microU/ml bTSH to increase intracellular cyclic AMP and poorly differentiated thyroid cancer cells (n = 3) failed to respond to bTSH. Long-term human thyroid cultures of normal and follicular adenoma cells in the chemically-defined medium used for the FRTL-5 cells had absent fibroblast growth and continued in monolayer form without significant follicle formation. These cells remained highly sensitive to bTSH stimulation when tested after 4, 13, and 26 weeks of continuous culture. All such cell preparations failed to proliferate under conditions which favoured the rapid growth of the rat thyroid cells. These data demonstrated that while thyroid cell culture conditions described in the literature do not permit proliferation of human thyroid cells, they do allow an assessment of their functional state in vitro which may lead to a further understanding of thyroid cell pathophysiology.
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