These studies confirmed inferences from microarray analyses (8
) and revealed that specific secretory pathways are induced in the liver in association with the acute-phase response during infection. Moreover, they have provided the first evidence to our knowledge that STAT3 is directly involved in the induction of secretory proteins.
Bacterial pneumonia triggers the activation of both STAT3 and RelA in the liver (7
). These transcription factors in hepatocytes mediate blood proteome remodeling during pneumonia, which curbs bacterial dissemination (8
). The processing of this newly synthesized load of acute-phase proteins would be expected to put significant pressure on the secretory apparatus of hepatocytes. The present studies showed that genes involved in the UPR and in the protein secretion pathway are induced in the liver during severe infections, and this depends on STAT3. After integrating all of these data, we propose that STAT3 is involved in multiple stages of the pathway leading to secretory protein induction during the acute-phase response ().
Fig 9 Proposed model of STAT3 contributions to the acute-phase response elicited by pneumonia. During pneumonia, hepatocyte STAT3 is activated by IL-6 and also by ER stress resulting from the newly synthesized load of acute-phase proteins. STAT3 induces the (more ...)
The transcription of acute-phase proteins driven by STAT3 contributes to the expression of a protein load in the ER that triggers ER stress. Secretory cells such as hepatocytes have a well-developed ER, which facilitates efficient processing of secreted proteins (24
). When this secretory capacity becomes overwhelmed, ER stress induces the UPR, which increases expression of protein folding and secretion machinery (14
). During bacterial pneumonia, IL-6 stimulates STAT3-driven expression of acute-phase transcripts (7
), which are then translated via the translocon into the ER. This initial protein load can likely be handled by the constitutively expressed secretory machinery of hepatocytes, but as the acute-phase response continues to increase through 24 h (7
), the folding and vesicular transport capacity of the ER in hepatocytes becomes overwhelmed. As a result, misfolded proteins accumulate and deplete the reserve of chaperones and foldases in the ER, causing ER stress and triggering the UPR (). The UPR then increases expression of multiple proteins involved in translocation and folding in the ER as well as in vesicular transport between the ER and Golgi complex. Consistent with this, we observed multiple markers of ER stress in the liver during pneumonia, including GRP78, CHOP, ATF4, and ERp72, most of which were diminished by the interruption of STAT3. These data suggest that STAT3 can function upstream of ER stress in the liver during bacterial pneumonia.
In addition, STAT3 may directly drive the transcription of secretory protein genes. STAT3 binds to the promoter regions of secretory protein genes during pneumonia. During pneumonia, STAT3 activation in the liver requires IL-6 (7
), which may form a direct conduit to secretory protein expression (). Furthermore, tunicamycin administration in vivo
led to phosphorylation of STAT3 and the STAT3-dependent expression of secretory proteins in the liver, suggesting that STAT3 can function downstream of ER stress. Autocrine, paracrine, or endocrine signaling from IL-6 or other cytokines may activate STAT3 in this setting. Alternatively, it is conceivable that intracellular pathways triggered by ER stress can directly activate STAT3 in hepatocytes. Prior studies revealed connections between ER resident proteins, such as ERp57, and STAT3 transcriptional activity (25
), although the mechanisms and significance are unclear. An intriguing speculation is that protein kinase R-like ER kinase (PERK), an ER membrane-localized tyrosine kinase that becomes activated in the UPR, might phosphorylate STAT3 and thereby directly link ER stress to STAT3 activity (27
). STAT3 may be activated by multiple different pathways during the acute-phase response, resulting in its association with proximal promoter regions of secretory protein genes with STAT3-dependent induction of these transcripts.
To our knowledge, secretory protein gene induction has previously been demonstrated to be mediated only by traditional transcription factors of the UPR, including XBP-1, ATF6, and ATF4 (14
). As with perhaps any gene, secretory protein gene transcription is certain to be mediated by multiple transcription factors acting cooperatively. The dependence on STAT3 in the present studies does not suggest an independence from other transcription factors previously implicated in the UPR. However, the present data provide the first demonstration, to our knowledge, that STAT3 is involved in either the UPR or secretory protein expression.
Finally, it will be important to determine the functional significance of STAT3-mediated induction of secretory proteins during the acute-phase response. The increased secretory machinery should enhance acute-phase protein translation into the ER, acute-phase protein folding in the ER, and the passage of acute-phase proteins from the ER to the Golgi apparatus. The net effects of these processes should be 2-fold, to promote immunity and to limit injury. Increased acute-phase proteins in the blood enhances humoral and phagocytic defenses against bacteria (8
), and so enhancing delivery of this secreted cargo to the blood compartment is an innate immune response that improves systemic defense. Furthermore, sustained or severe ER stress results in cell death (14
), and the relief of ER stress due to enhanced secretory machinery during the acute-phase response may help prevent liver injury during severe infection. Altogether, these studies identify STAT3 as a novel mediator of the UPR during the acute-phase response. We propose that, in addition to driving the expression of acute-phase proteins, STAT3 functions in the liver during the acute-phase response to enhance the delivery of blood-borne innate immunity proteins and to limit liver injury.