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INVITED REVIEW

C/EBP transcription factors in the lung epithelium

¹Institute for Cancer Studies, Division of Genomic Medicine, University of Sheffield, Sheffield S10 2RX, United Kingdom; and ²Department of Medical Nutrition, Novum, Karolinska Institutet, SE 141 86 Huddinge, Sweden

	ABSTRACT

TOP ABSTRACT THE C/EBP TRANSCRIPTION FACTOR... REGULATION OF LUNG GENE... UNIQUE ROLES FOR THE... CONCLUSION DISCLOSURES REFERENCES

 
During recent years, the biological roles of CCAAT/enhancer binding proteins (C/EBPs) in the lung have started to be uncovered. C/EBPs form a family within the basic region-leucine zipper class of transcription factors. In the lung epithelium C/EBP, -, and - are expressed. Lung-specific target genes for these transcription factors include the surfactant proteins A and D, the Clara cell secretory protein, and the P450 enzyme CYP2B1. As more information is gathered, a picture is emerging in which C/EBP has a role in regulating proliferation as well as differentiation-dependent gene expression, whereas C/EBP and -, in addition to a partly overlapping role in regulating expression of differentiation markers, also seem to be involved in responses to injury and hormones.

CCAAT/enhancer binding protein; differentiation; gene expression regulation; proliferation regulation

	THE C/EBP TRANSCRIPTION FACTOR FAMILY

TOP ABSTRACT THE C/EBP TRANSCRIPTION FACTOR... REGULATION OF LUNG GENE... UNIQUE ROLES FOR THE... CONCLUSION DISCLOSURES REFERENCES

 
Structure and expression. C/EBP factors belong to the bZIP class of basic domain transcription factors (81, 82). Six members (C/EBP-) constitute the mammalian C/EBP family. Of these, C/EBP was first identified and is the founding member. The basic region of C/EBP factors is a highly positively charged domain that directly interacts with the DNA (see Fig. 1). All members of the C/EBP family have similar basic region DNA-binding motifs except C/EBP (which lacks DNA-binding activity). As a consequence of the high similarity in the basic region, C/EBP, C/EBP, and C/EBP have been shown to interact with virtually identical DNA sequences (6, 53, 80). The leucine zipper region is also conserved between the different family members, whereas the amino-terminal transactivation domain is more diverse. The leucine zipper domain is involved in homo- and heterodimerization, and all proteins in the C/EBP family have been shown to form homo- and heterodimers. Of the tissue-specific C/EBP family members, three have been demonstrated to be expressed in lung, namely C/EBP, -, and - (6, 80). Although C/EBP and - are expressed at highest relative levels in the liver and fat, C/EBP is expressed at highest levels in the lung (6). C/EBP factors have been suggested to play important roles in controlling differentiation. In liver, fat, and white blood cells of the myelomonocytic lineage, C/EBP factors have been demonstrated to be important regulators of different aspects of differentiation, including proliferation, cell cycle arrest, and gene expression (reviewed in Refs. 18, 21, 22, 39, 56).

View larger version (11K): [in this window] [in a new window]   Fig. 1. Schematic representation of the CCAAT/enhancer binding protein (C/EBP) transcription factors. Indicated are the protein domains and their primary functions.

Expression pattern in lung. A similar role in lung cellular differentiation is suggested by the expression of C/EBP in the developing lung; expression is initiated in close temporal proximity to the initiation of cellular differentiation and the appearance of differentiation markers (Ref. 40; T. Berg and M. Nord, unpublished observations; Fig. 2). In the adult lung, C/EBP is expressed in the type II cells of the alveolar epithelium, and lower levels are also seen in the bronchiolar epithelial Clara cells, as revealed by immunohistochemical studies and studies on isolated epithelial cells (9, 40, 51, 69). C/EBP is expressed in type II cells of the alveolar region (61, 69), and expression is also seen in the bronchiolar epithelium (T. Berg, T. N. Cassel, and M. Nord, unpublished observations). C/EBP shows high-level expression in the bronchiolar epithelium and lower levels in alveolar type II cells (9, 37, 61, 69). The developmental expression pattern of C/EBP and - has not been investigated by immunohistochemical methods, but analyses on whole embryonic lung show that these factors also increase late during development (5, 61). However, their temporal expression patterns seem to precede that of C/EBP, similar to what has been demonstrated during adipocyte differentiation (21). An important role of C/EBP in lung cellular differentiation is suggested by the phenotype of the C/EBP(-/-) mouse. The phenotype is lethal, and mice die within 10 h after birth due to severe hypoglycemia (27, 76). However, a fraction of C/EBP(-/-) mice succumbs immediately after birth from apparent respiratory distress (27). Closer examinations revealed impaired lung cellular differentiation with abnormalities in the alveolar epithelium and hyperproliferation of type II cells (27, 68). No lung phenotype has been reported in the C/EBP(-/-), C/EBP(-/-), or C/EBP(-/-)/C/EBP(-/-) double-knockout mice (63, 66, 70).

View larger version (13K): [in this window] [in a new window]   Fig. 2. Gene expression in the lung epithelium during development. Indicated are key events in lung development, expression pattern of differentiation markers, and expression of transcription factors. Numbers refer to mouse lung development. CCSP, Clara cell secretory protein; SP-A, surfactant protein A; HNF, hepatocyte nuclear factor; TTF, thyroid transcription factor. (From Refs. 3, 20, 25, 26, 38, 40, 48, 49, 52, 61, 64, 71, 72, 85, and Berg and Nord, unpublished observations.)

Role for C/EBP transcription factors in lung epithelial differentiation? Studies in transgenic and knockout mice have revealed that forkhead box transcription factors, especially FoxA2/hepatocyte nuclear factor (HNF)-3 and the homeodomain transcription factor Nkx2.1/thyroid transcription factor (TTF)-1, play major roles in cellular differentiation of the lung epithelium, as well as specifying developmental processes in the early developing lung (2, 35, 46, 47, 79, 84). This is in agreement with the expression pattern of these transcription factors. They are turned on at an early time point in lung development and are sustained in the epithelium throughout lung development (3, 38, 48; Fig. 2). As the expression of HNF-3 factors and Nkx2.1 is initiated from an early time point of lung development, additional transcription factors are likely to be important to promote the extensive cellular differentiation program that is initiated in late lung development. Several lines of reasoning support the concept that C/EBP factors could be important for cellular differentiation during late lung development; C/EBP transcription factors have been demonstrated to play important roles in cellular differentiation in a variety of different tissues including breast, adipocytes, white blood cells, and liver. Notably, a relationship between lung and liver transcriptional regulation is likely, as suggested by their shared endodermal origin, the important role of HNF-3 factors, and the expression of C/EBP factors in both organs. Moreover, C/EBP transcription factors, including C/EBP, C/EBP, and C/EBP, are expressed in the lung, and their expression is initiated in temporal proximity to the initiation of cellular differentiation and the appearance of differentiation markers. And finally, C/EBP(-/-) mice demonstrate morphological abnormalities with hyperproliferation of alveolar type II cells, indicating a role for C/EBP in the control of proliferation-differentiation processes in the lung epithelium.

	REGULATION OF LUNG GENE EXPRESSION

TOP ABSTRACT THE C/EBP TRANSCRIPTION FACTOR... REGULATION OF LUNG GENE... UNIQUE ROLES FOR THE... CONCLUSION DISCLOSURES REFERENCES

 
Clara cell secretory protein/secretoglobin 1a1. Clara cell secretory protein (CCSP)/secretoglobin 1a1 was the first lung-specific gene demonstrated to be regulated by C/EBP transcription factors in transcription regulation studies (9, 51). On the basis of sequences in the promoter showing similarities to consensus C/EBP-binding sites and correlating expression patterns, C/EBP regulation of the surfactant protein (SP)-A gene has also been proposed (40). CCSP is a small secretory protein abundantly expressed in the Clara cells of the bronchiolar epithelium and in similar cells in the larger airways (50). Although the exact physiological role of CCSP remains to be established, it has been suggested that CCSP plays a role in protection against oxidative stress and/or has an anti-inflammatory function (67, 83). CCSP is expressed in a differentiation-dependent pattern in the developing lung, in temporal proximity to cellular differentiation (50). Expression starts late in lung development in parallel with the appearance of Clara cells. Postnatally, the expression levels of CCSP continue to increase, and adult levels are not reached until the Clara cells are fully differentiated.

Freshly isolated primary Clara cells express high levels of CCSP. During culture in vitro, expression decreases. The expression of transcription factors such as HNF-3 and Nkx2.1, known to regulate CCSP gene expression, does not parallel the rapidly declining CCSP levels during culture in vitro. In contrast, C/EBP expression is rapidly lost. The decrease in C/EBP levels correlates to dedifferentiation of the cells and declining CCSP levels, suggesting a potential role of C/EBP in the regulation of CCSP (51). In addition to C/EBP, C/EBP is expressed in the bronchiolar epithelium and also in isolated Clara cells (8, 9, 37). A functional importance of both C/EBP and C/EBP for CCSP gene regulation has been demonstrated in transient transfection studies in lung epithelial cell lines. These studies showed that both C/EBP and - promote transcription and that this occurs through interaction with two C/EBP-binding sites in the proximal promoter (Ref. 9; Fig. 3). Mutation of either C/EBP-binding site resulted in abolished or strikingly reduced C/EBP- and C/EBP-mediated transactivation of the CCSP promoter, as well as impaired binding of both factors in DNase I footprint analysis. These results indicate that the two C/EBP-binding sites form a compound response element. Electrophoretic mobility shift assays showed that C/EBP and - can bind to both C/EBP-binding sites, whereas in DNase I footprint analyses, the interaction of C/EBP with the proximal site was weak. To summarize, C/EBP and C/EBP activate the CCSP promoter via a compound response element in the proximal promoter. This element consists of two adjacent C/EBP-binding sites (Fig. 3) that individually can bind C/EBP and C/EBP with equal efficiency, but not in the context of the intact promoter. Together, this renders the C/EBP-mediated regulation of the CCSP gene a complicated process.

View larger version (5K): [in this window] [in a new window]   Fig. 3. Schematic representation of the proximal CCSP/secretoglobin 1a1 (Scgb1a1) promoter. Indicated are binding sites for transcription factors regulating the CCSP/Scgb1a1 promoter.

Combinatorial action of C/EBPs and other transcription factors. In the proximal CCSP promoter, located in a region between +1 and -130 bp of the 5' flanking sequence, reside multiple cis-acting elements of potential importance to the promotion of the tissue and cell-specific expression of CCSP. In addition to the two adjacent C/EBP-binding sites, several functional binding sites for Nkx2.1/TTF-1 and FoxA/HNF-3 factors exist within this short region (Fig. 3). The close proximity of binding sites for multiple transcription factors suggests combinatorial action in the regulation of the CCSP gene (50). And indeed, in studies of transient transfection, a strong synergistic transactivation was observed between C/EBP and Nkx2.1 on the CCSP promoter. The Nkx2.1-C/EBP synergism was specific, as other combinations of transcription factors tested resulted in levels of transactivation that were approximately additive. The synergism is mediated through interaction with elements in the proximal CCSP promoter (Fig. 3) and seems to be dependent on cooperative binding of the transcription factors to the promoter (7). These results demonstrate that C/EBP acts together with another lung-enriched transcription factor, Nkx2.1, in the regulation of CCSP. The strong synergistic activity, together with the temporal expression patterns of CCSP, C/EBP, and Nkx2.1, suggests that the appearance of C/EBP in the developing lung is a key event to initiate high-level expression of CCSP in late lung development (Fig. 2). With these results, together with the studies of targeted inactivation of C/EBP and Nkx2.1 (27, 35), the following model concerning the roles of C/EBP and Nkx2.1 in the developing lung epithelium can be formulated: Nkx2.1 serves to specify lung epithelial cell lineage early in lung development and thus regulates lung-specific gene expression. C/EBP has its main role in late development and acts as a regulator of differentiation-dependent gene expression. A gene such as CCSP, which is specifically expressed in the lung epithelium and exhibits a differentiation-dependent expression pattern, would consequently be predicted to require simultaneous presence of Nkx2.1 and C/EBP for high-level expression (Figs. 2 and 3). A role for C/EBP in late lung development is also supported by the findings that C/EBP expression is first detected when differentiation of secretory epithelial cells is initiated: differentiated type II cells appear in the future alveolar epithelium, and differentiation of the bronchiolar Clara cells starts. This is in close connection to the onset of high-level expression of genes with a differentiation-dependent expression pattern such as CCSP (Fig. 2). However, full differentiation of Clara cells is attained postnatally (45, 71, 72). In the postulated model, the appearance of C/EBP in the developing lung epithelium is a key event to initiate expression of differentiated gene products, in a similar fashion as has been demonstrated in hepatocytes and adipocytes (6, 27, 32, 59, 74, 76).

SP-A. SP-A is one of the hydrophilic surfactant proteins and is expressed in the alveolar type II cells as well as in bronchiolar Clara cells (36). SP-A expression is initiated late in lung development and increases in concert with the appearance of differentiated type II cells (Fig. 2). In the adult lung, SP-A is highly expressed in alveolar type II cells and to a lesser extent in Clara cells (45). On the basis of sequences in the SP-A promoter showing similarities to consensus C/EBP-binding sites and correlations between SP-A and C/EBP expression, regulation of this gene by C/EBP transcription factors has been proposed (40). And recently, a functional C/EBP-binding site was demonstrated in the proximal rat SP-A gene at 180 bp upstream of the start site of transcription (42, 61). This element is necessary for full C/EBP responsiveness of the promoter, and antisense knock-down of C/EBP decreased expression of the endogenous SP-A gene in a human lung cancer cell line. These results demonstrate that C/EBP transcription factors regulate SP-A expression. Likewise, the expression patterns of SP-A and C/EBP factors suggest that C/EBP factors have a role in the differentiation-dependent expression of SP-A in the developing lung (Fig. 2). Together, these studies suggest that C/EBP factors are involved in the regulation of differentiation-dependent gene expression in the alveolar epithelium as well.

SP-D. A second hydrophilic surfactant protein found in lung lavage is SP-D. It is expressed in alveolar type II cells, in Clara cells, and in the tracheobronchial glands of the lung, but unlike other surfactant proteins, SP-D does not localize to lamellar bodies (36). SP-D starts being expressed late in lung development, and expression continues to increase postnatally (Ref. 45; Fig. 2). Transfection studies in human lung cancer cells have revealed the presence of three functional C/EBP response elements in the promoter of the human SP-D gene (31). Two are located close together at 340 and 319 bp upstream of the start site of transcription and the third further upstream at -432. The two sites positioned in tandem at -340 and -319 seem not to bind C/EBP factors simultaneously. However, these binding sites interact with the -432 site to fully transactivate the SP-D promoter. The presence of several interacting C/EBP-binding sites forms a common theme together with the CCSP promoter, which also contains two C/EBP-binding sites acting together. However, in contrast to the SP-D promoter, where the sites are situated nearly 100 bp apart, the cooperating C/EBP-binding sites in the CCSP promoter are separated by only 20 bp. From these results it seems that, similar to SP-A, C/EBP-factors are likely to be key regulators underlying the developmental expression of SP-D. In addition, both SP-A and -D are upregulated in response to acute-phase stimuli such as instillation of lipopolysaccharide (43). As in the liver, C/EBP and - are increased in lung after acute-phase stimuli (1, 14). This suggests that C/EBP and - could be involved in mediating the acute-phase response in lung, similar to what has been described in the liver.

CYP2B1. The P450 enzyme CYP2B1 is constitutively expressed in the lung epithelium, both in the Clara cells of the conducting airway epithelium and in the alveolar type II cells. In the lung epithelium, it also exhibits an expression pattern correlating to the extensive cellular differentiation occurring around the time of birth (Ref. 25; Fig. 2). Studies in cells of liver origin have demonstrated that the CYP2B1 gene is transactivated by C/EBP and C/EBP through a C/EBP-binding site in the proximal promoter (55), and transgenic studies have revealed that 1.3 kb of the 5' flanking region of the CYP2B1 promoter is sufficient to promote lung-specific expression (65). In transient transfections in lung epithelial cells, reporter gene activity driven by 1,350 bp of the CYP2B1 promoter was increased by both C/EBP and C/EBP, and inactivation of the C/EBP-binding site caused a significant reduction of the transactivation efficiency from both factors (8). These results indicate that C/EBP and C/EBP are important in controlling CYP2B1 gene expression in lung epithelial cells and show that the proximal C/EBP-binding site confers C/EBP transactivation.

In summary, the results showing C/EBP regulation of the CCSP, SP-A and -D, and CYP2B1 genes indicate an important role for C/EBP transcription factors in regulating lung epithelial gene expression. All these genes show a similar differentiation-dependent expression pattern during lung development with an onset of high-level expression late in lung development (Fig. 2), suggesting that they are regulated in a similar fashion. The expression pattern of C/EBPs during lung development parallels cellular differentiation, and C/EBP expression is initiated in close proximity to the onset of high-level expression of these genes (Fig. 2). Together, this indicates that C/EBPs have a main role late in lung development as a regulator of differentiation-dependent expression of this group of genes.

	UNIQUE ROLES FOR THE DIFFERENT C/EBP FAMILY MEMBERS?

TOP ABSTRACT THE C/EBP TRANSCRIPTION FACTOR... REGULATION OF LUNG GENE... UNIQUE ROLES FOR THE... CONCLUSION DISCLOSURES REFERENCES

 
C/EBP. Results from targeted inactivation of the gene for C/EBP in mice indicate a specific role for this transcription factor in regulating subsets of genes, especially in the liver, where genes involved in gluconeogenesis and glycogen synthesis exhibit specific decreases (18, 76). Increased proliferation is also evident in the livers of C/EBP(-/-) mice (27), and liver regeneration is associated with decreased C/EBP expression. Moreover, forced expression of C/EBP in liver cell lines inhibits proliferation. Together, this suggests a role for C/EBP in regulating proliferation in the liver (22). In the lung, knockout of C/EBP has revealed a critical role in regulating proliferation of the alveolar epithelium as well (27, 68). In contrast, expression of genes regulated by C/EBP factors is seemingly unaffected in the lungs of C/EBP(-/-) mice. This is most probably explained by other C/EBPs present in lung substituting for C/EBP in the transactivation of target genes. An ability of C/EBP to substitute for C/EBP has indeed been demonstrated (12, 18, 19). However, the idea that other C/EBPs can fully substitute for C/EBP in transcriptional regulation in the lung is challenged by the finding of efficient synergism observed specifically between C/EBP and Nkx2.1.

Our understanding of how C/EBP regulates cellular proliferation was recently advanced by several studies; C/EBP has been demonstrated to interact directly with the cyclin-dependent kinases (cdks) 2 and 4, two critical regulators of cell cycle progression (75). Interaction occurs through a short region of the C/EBP polypeptide and prevents interaction of these kinases with their cognate cyclins. As the cyclin-cdk interaction is necessary for activity of the cdks and progression through the cell cycle, this provides a potential mechanism for the growth arrest mediated by C/EBP. The closely related family members C/EBP and -, which are expressed in proliferating cells, do not share this cdk-interacting region. In a separate study, in vivo experiments demonstrated that C/EBP also represses the E2F complex and that this repression is necessary for terminal differentiation of fat cells and granulocytes (57). Activation of the E2F complex is necessary for passage through the restriction point of the cell cycle and is thus central in regulation of proliferation. The E2F repression is mediated by regions of C/EBP separate from the cdk-interacting region, suggesting the existence of two separate pathways for C/EBP to inhibit proliferation. Notably, neither of these regions is involved in activating the conventional array of target genes for C/EBP (44). An additional mechanism by which C/EBP might inhibit proliferation is via transcriptional induction of the cdk inhibitor p21^WAF1/Cip1. This mechanism seems to be especially important for the antiproliferative effects of glucocorticoids (10, 17). Interestingly, in human lung mesenchymal cells, this effect has recently been demonstrated to be mediated by formation of a complex between the glucocorticoid receptor and C/EBP (62). All together, these mechanisms establish the means by which a single factor such as C/EBP couples growth arrest to gene expression in the process referred to as differentiation and indicate that this dual activity is unique for C/EBP (44).

The hyperproliferative nature of the alveolar epithelium in mice lacking C/EBP indicates that C/EBP has a central role in controlling proliferation in the lung epithelium (27, 68). Findings of dominant-negative mutations in the gene encoding C/EBP in acute myeloid leukemias show that C/EBP is a tumor suppressor gene (54), and recent loss, or decreased expression, of C/EBP in lung cancers has been demonstrated (30). Decreased levels were more commonly seen in adenocarcinomas and correlated to poor differentiation and a more advanced stage of the tumors. Moreover, forced expression of C/EBP in lung cancer cell lines reduced proliferation and induced morphological changes characteristic of differentiation. All together, this demonstrates a role for C/EBP in regulating cellular proliferation in the lung as well. Together with the role of C/EBP in controlling differentiation-dependent gene expression, this indicates a function for C/EBP as a coordinating regulator of two central parts of differentiation of the lung epithelium: expression of differentiated gene products and induction of growth arrest.

C/EBP and -. Present evidence points toward a unique role for C/EBP in regulating proliferation in addition to gene expression. Do the close relatives C/EBP and - have roles of their own in the lung? Certainly they are not vital to baseline lung function and development, as suggested by the findings in mice lacking both C/EBP and -, which exhibit no histological abnormalities of the lung (70). However, when challenged with acute-phase stimuli, C/EBP and - levels are increased in the lung (1). This is reminiscent of the liver, where C/EBP and - are both important regulators of acute-phase genes (56). Several of the acute-phase protein genes are also expressed in the lung and elevated after lung injury. In addition, collectins expressed in the lung such as SP-A and SP-D are increased after lung injury and, on the basis of this, have been suggested as lung-specific acute-phase proteins (43). As outlined above, SP-A and SP-D are regulated by C/EBP factors, and this suggests that C/EBPs regulate acute-phase proteins in the lung as well. After lung injury, early inflammatory mediators such as IL-1 and TNF- as well as IL-6 and the adhesion molecule ICAM-1 are elevated. Also these genes are C/EBP regulated, further adding to the group of C/EBP-regulated genes that are upregulated in lung injury (13, 24, 56). Agents apart from classical acute-phase stimuli but also causing acute-lung injury, such as oxidative stress and bleomycin, also increase the levels of C/EBP and - in the lung (1, 14, 69). This points toward a specific role for C/EBP and - in the reaction to lung injury and the subsequent activation of the innate immune system and inflammatory response.

Glucocorticoids are steroid hormones required for normal lung development. In the absence of glucocorticoid activity, mice die shortly after birth due to respiratory failure. This was clearly demonstrated in knockout mice lacking the glucocorticoid receptor. The lungs of these mice are highly immature with increased proliferation of both mesenchymal and epithelial cells (15). Treatment with glucocorticoids during embryonic development leads to enhanced lung maturation and increased expression of lung epithelial proteins exhibiting a differentiation-dependent expression pattern. Such proteins include the surfactant proteins and CCSP (11, 45). Glucocorticoids act through the glucocorticoid receptor, yet the specific molecular mechanisms for the effects of glucocorticoids on lung development are unclear. Direct effects on the lung epithelium are likely, though paracrine signaling between mesenchymal and epithelial cells has also been suggested (45). Our knowledge of this field was advanced by findings in transgenic mice harboring a mutation in the dimerization domain of the glucocorticoid receptor. This mutation prevents transcriptional regulation by DNA binding but allows regulation by protein-protein interactions. In contrast to the glucocorticoid receptor knockout mice, these mice are viable and do not suffer from respiratory problems (60). This suggests that direct transcriptional activation by the glucocorticoid receptor is of less importance during lung development and that protein-protein interactions play a more prominent role. In line with this, no functional binding sites for the glucocorticoid receptor have been demonstrated in the promoters of the glucocorticoid-regulated surfactant protein and CCSP genes (33). In summary, even though glucocorticoids have a central role in lung development, unclear issues still remain about the mechanisms of glucocorticoids signaling in the lung epithelium. Recent observations suggesting a role for C/EBP and - in glucocorticoid signaling in the lung might shed light on this. In a lung epithelial cell line, glucocorticoid stimulation of the CCSP and CYP2B1 promoters was dependent on the integrity of C/EBP-binding sites in the respective promoter. This is similar to findings in liver and adipose tissue, where glucocorticoid hormones also play an important role during cellular differentiation. In these organs, some glucocorticoid effects on gene expression are mediated via C/EBP and - (18, 28, 41). However, in contrast to these organs, glucocorticoids do not increase the protein levels of C/EBP and - in lung cells. Instead, they enhance DNA-binding activity without affecting protein levels, suggesting glucocorticoid-induced posttranslational modifications that stimulate binding of C/EBP and - to their cognate binding sites in target promoters (4). Furthermore, in human fetal lung explants, glucocorticoids have been shown to increase C/EBP levels (5). Together, these results indicate a previously unknown role for C/EBP and - in glucocorticoid signal transduction in the lung.

How could the same factors (C/EBP and -) be involved in the response to both inflammatory stimuli and a strong anti-inflammatory agent such as glucocorticoids? The findings that C/EBPs are targets for posttranslational modifications such as phosphorylation and sumoylation, which subsequently affect DNA binding and/or transactivation potential, could be part of the answer (34, 58). Posttranslational modifications have been most studied in C/EBP. This protein carries multiple phosphorylation sites that have been demonstrated as targets for a diverse set of signal transduction pathways. For instance, stress-activated protein kinases, such as p38, which are activated by cellular stress and involved in the acute-phase reaction, cause C/EBP phosphorylation on Ser105, inducing the transactivation potential (23). The same amino acid residue is also a target for PKC phosphorylation (73). Other described phosphorylation events can decrease transactivation potential, inhibit DNA binding, or affect nucleo-cytoplasmic shuttling of C/EBP. Also, C/EBP is a target for regulated phosphorylation, but this has not been studied in the same detail as for C/EBP (58). The ability of C/EBP and - to undergo posttranslational modifications suggests a potential scenario in which glucocorticoids cause specific posttranslational modifications of C/EBP and - other than those caused by inflammatory stimuli. These events could then target C/EBP and - toward glucocorticoid-regulated lung epithelium-specific genes and away from inflammatory genes.

	CONCLUSION

TOP ABSTRACT THE C/EBP TRANSCRIPTION FACTOR... REGULATION OF LUNG GENE... UNIQUE ROLES FOR THE... CONCLUSION DISCLOSURES REFERENCES

 
Even though we are far from a complete understanding, a picture is emerging in which C/EBP has a role in directly regulating proliferation as well as differentiation-dependent gene expression. C/EBP and - have a partly overlapping role in regulating expression of differentiation markers but are also involved in responses to different stimuli such as acute lung injury and glucocorticoid hormones.

	DISCLOSURES

TOP ABSTRACT THE C/EBP TRANSCRIPTION FACTOR... REGULATION OF LUNG GENE... UNIQUE ROLES FOR THE... CONCLUSION DISCLOSURES REFERENCES

 
This work was supported by Swedish Research Council Grants 14677 and 14678, Swedish Heart-Lung Foundation Grant 20020602, the Swedish Medical Society, Swedish Institute, the following research foundations: Harald Jeanssons Stiftelse, Harald och Greta Jeanssons Stiftelse, Magnus Bergvalls Stiftelse, Robert Lundbergs Minnesstiftelse, Anders Otto Svärds och Ulrika Ekelunds Stiftelse, Stiftelsen Sigurd och Elsa Goljes minne, Ollie och Elof Ericssons Stiftelse för Vetenskaplig Forskning, and the Research Foundations of the Karolinska Institutet.

	FOOTNOTES

 

Address for reprint requests and other correspondence: M. Nord, Dept. of Medical Nutrition, Karolinska Institutet, Novum, Huddinge Univ. Hospital, SE 141 86 Huddinge, Sweden (E-mail: magnus.nord{at}mednut.ki.se).

	REFERENCES

TOP ABSTRACT THE C/EBP TRANSCRIPTION FACTOR... REGULATION OF LUNG GENE... UNIQUE ROLES FOR THE... CONCLUSION DISCLOSURES REFERENCES

 

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