The cationic amino acid transporter, Cat-1, facilitates the uptake of the essential amino acids arginine and lysine. Amino acid starvation causes accumulation and increased translation of cat-1 mRNA, resulting in a 58-fold increase in protein levels and increased arginine uptake. A bicistronic mRNA expression system was used to demonstrate the presence of an internal ribosomal entry sequence (IRES) within the 5-untranslated region of the cat-1 mRNA. This study shows that IRES-mediated translation of the cat-1 mRNA is regulated by amino acid availability. This IRES causes an increase in translation under conditions of amino acid starvation. In contrast, cap-dependent protein synthesis is inhibited during amino acid starvation, which is well correlated with decreased phosphorylation of the cap-binding protein, eIF4E. These findings reveal a new aspect of mammalian gene expression and regulation that provides a cellular stress response; when the nutrient supply is limited, the activation of IRES-mediated translation of mammalian mRNAs results in the synthesis of proteins essential for cell survival.Amino acids are essential nutrients for cell growth and maintenance. Mammalian cells have developed an adaptive response to changes in amino acid availability (1). When the amino acid supply is limited, protein synthesis decreases, and there are increases in catabolism of cellular proteins, amino acid biosynthesis, and amino acid transport across the plasma membrane. Together these responses provide cells with amino acids needed for survival. A significant part of this adaptive response is the increased expression of the cat-1 gene, which encodes the transporter for the essential cationic amino acids, lysine and arginine (2). We have shown that the level of the Cat-1 protein and the transport of cationic amino acids increase in amino acid-depleted cells (3). Because amino acid starvation inhibits cap-dependent initiation of protein synthesis (4), we hypothesized that the cat-1 mRNA is translated in amino aciddepleted cells through a cap-independent mechanism. This translation would involve an internal ribosomal entry sequence (IRES 1 ) within the 5Ј-untranslated region (5Ј-UTR). IRES sequences have been implicated in the translation of viral mRNAs in infected cells, where cap-dependent translation of cellular mRNAs is inhibited (5). These sequences are also important in the translation of several mammalian mRNAs encoding regulatory proteins involved in growth and differentiation (6 -11). Furthermore, a role of IRES sequences in cell cycle-dependent translation of mammalian mRNAs has been demonstrated recently (6,(12)(13)(14). This study provides support for our hypothesis by demonstrating that the 5Ј-UTR of the cat-1 mRNA contains an IRES sequence. Moreover, translation from this IRES is stimulated in amino acid-starved cells, when cap-dependent translation is decreased. These findings suggest a mechanism for synthesis of proteins required for amino acid accumulation in starved cells when total protein synthesis is inhib...

These findings support the hypothesis that hyperglycemia evokes an intrinsic pathway of proapoptotic signaling in mesangial cells. In addition, these results point to an important role for the intrinsic pathway in microvascular injury in the diabetic kidney in vivo.

The mechanisms behind the loss of epithelial barrier function leading to alveolar flooding in acute lung injury (ALI) are incompletely understood. We hypothesized that the tyrosine kinase receptor human epidermal growth factor receptor-2 (HER2) would be activated in an inflammatory setting and participate in ALI. Interleukin-1␤ (IL-1␤) exposure resulted in HER2 activation in human epithelial cells and markedly increased conductance across a monolayer of airway epithelial cells. Upon HER2 blockade, conductance changes were significantly decreased. Mechanistic studies revealed that HER2 trans-activation by IL-1␤ required a disintegrin and metalloprotease 17 (ADAM17)-dependent shedding of the ligand neuregulin-1 (NRG-1). In murine models of ALI, NRG-1-HER2 signaling was activated, and ADAM17 blockade resulted in decreased NRG-1 shedding, HER2 activation, and lung injury in vivo. Finally, NRG-1 was detectable and elevated in pulmonary edema fluid from patients with ALI. These results suggest that the ADAM17-NRG-1-HER2 axis modulates the alveolar epithelial barrier and contributes to the pathophysiology of ALI. Acute lung injury (ALI)3 is a severe clinical disorder with an annual incidence of ϳ200,000 and a mortality of 40% in the United States (1). Most commonly seen in the setting of sepsis (2-4), ALI is marked by disruption of the alveolar barrier, leukocyte activation, release of inflammatory cytokines, and hypercoagulability. The net effect is an increase in alveolar epithelial permeability, resulting in alveolar flooding with proteinrich edema and life-threatening hypoxemia (5). An intact epithelial barrier is essential to maintaining normal pulmonary fluid balance. Indeed, damage to the endothelium alone is insufficient to cause pulmonary edema, whereas epithelial injury results in severe lung injury (6 -8). The epithelium provides a greater resistance to proteins and fluid than the capillary endothelium and is responsible for the active ion transport-dependent removal of edema fluid from the distal air spaces of the lung (9 -11).The tyrosine kinase receptor human epidermal growth factor receptor-2 (HER2) is expressed by pulmonary bronchial epithelial cells and is involved in multiple physiologic processes, including cell proliferation and wound repair. The HER receptor family consists of four type 1, membrane-bound tyrosine kinase receptors: HER1 or epidermal growth factor receptor (EGFR), HER2, HER3, and HER4 (12). HER2 has no known ligand and requires partnering with another HER family member for activation. HER2 and 3 are highly expressed in pulmonary bronchial epithelial cells (compared with HER1 and 4) (13). HER3 is the receptor for the ligand neuregulin-1 (NRG-1), but HER3 has no intrinsic signaling properties (14). Upon NRG-1 binding, HER3 heterodimerizes with HER2, resulting in activation of the HER2 tyrosine kinase domain, HER2 autophosphorylation, and initiation of downstream intracellular signaling cascades (13). NRG-1 is expressed in bronchial epithelial cells (13,15,16) and is shed from the cell ...