The Unfolded Protein Response and Human Health

The Unfolded Protein Response and Human Health

A single UPR sensor, ATF6, detects lipotoxic and proteoxic stress independently

Despite extensive headway towards understanding how UPR components respond to the protein-folding demands of the ER after proteotoxic stress, little is known about how the UPR recognizes increased demand for ER membrane expansion, termed lipotoxic stress. Recently, we made the following key discoveries: (1) The levels of two early biosynthetic sphingolipids, dihydroceramide (DHC) and dihydrosphingosine (DHS), increase in response to ER stress. (2) Both DHS and DHC robustly activate ATF6, a key initiator of the UPR in mammals, by directly binding to a unique motif that we identified within ATF6’s transmembrane domain (Tam et al., Developmental Cell, 2018). This was the first demonstration that a UPR component binds to a specific form of a lipid to activate a pathway designed to deal with lipotoxic stress. Our findings revealed that while ER proteotoxic stress activates ATF6 via the ER luminal domain, these different modes of ATF6 activation induce different sets of transcriptional targets. Using RNA sequencing, proteomics and lipidomics, we plan to expand these initial discoveries to investigate the full scope of the UPR response to lipotoxic stress.

Connecting the conserved UPR sensor, IRE1 to  cancer and therapeutics

IRE1 is an ER transmembrane UPR initiator with both riboendonuclease (RNase) and kinase activities. IRE1 cleaves the XBP1/HAC1 intron to generate spliced mRNAs that are translated into potent transcription factors. Furthermore, we reported that IRE1 can also cleave ER-associated RNA, leading to their decay, an activity known as regulated IRE1-dependent decay. Recently, we found that two different activation modes of IRE1 switch between the two RNase types (Tam et al., Cell Reports 2014). We are interested in dissecting the molecular mechanisms using protein biochemistry and kinetic, molecular, biological, and cellular biological approaches

Ire1’s unique structure also provides an opportunity to develop highly specific drugs for either inhibiting or activating Ire1. Such drugs could be promising treatments for various cancers caused, in part, by the deregulation of Ire1 (e.g., multiple myeloma). In collaboration with Dr. Albert Koong (MD Anderson Cancer Center), we have identified and characterized the molecular mechanism of a new Ire1 inhibitor, Irestatin, which inhibits cleavage of the intron of XBP1 mRNA that codes for a UPR-specific transcription factor. Importantly, we found that Irestatin is a potent inhibitor of an in vivo multiple myeloma model experimental system and a T-antigen-driven tumor transplant model in nude mice. Thus, Irestatin not only represents a unique tool for dissecting the molecular mechanism of Ire1 RNase activity, but also holds promise for the development of a new class of anti-tumor drugs. We are also interested in further studies of Irestatin and its derivatives in the progression of other cancers and the role of IRE1 in the tumor microenvironment.

The pictures show that Ire1-GFP becomes big foci at 2 & 4 hrs time points after ER stress induced by thapsigargin (Tg), but at the later time point (8 hrs), Ire1-GFP goes back to be dispersed. The kinetics of XBP1-mRNA splicing and RIDD are shown on the right.

COVID activates the UPR: What are the cellular targets?

Since the end of 2019, SARS-CoV2 has disrupted public health and all aspects of our lives. COVID-19 is a positive-strand RNA virus with unusual characteristics. Currently, neither an effective vaccine nor a treatment has been available. Upon entry into the host cells, one of the unique features of COVID-19 is to cause extensive structural rearrangement of the host cell ER membrane, generating a unique double-membrane structure, called the ER replication platform. Inside this structure, the host cell replication and transcription complexes are placed to concentrate the activities while protecting the viral RNA genome from the host cell defense mechanisms. Upon translation, COVID-19 viral proteins are directly translocated into the lumen of the ER. Certain studies show the activation of the UPR target genes. Using our extensive background of the UPR sensors and roles of individual UPR signaling branch, we are investigating the roles of the UPR signaling branches on COVID-19 production.