The Crosslinks between the Sensor Proteins
Although the three branches of the UPR pathway have been investigated extensively, the interconnections between these three branches are yet to be studied more thoroughly. But it has been found that IRE1α and ATF6 pathways pose a close relationship. In attempts of analyzing the UPR pathway using cis -acting elements and trans -acting elements involved with genes associated with the UPR pathway, Yoshidaet al. 2000 has reported that overexpression of soluble ATF6 activates transcription of the CHOP, XBP1 genes, and ER chaperone genes constitutively, whereas overexpression of a dominant negative mutant of ATF6 blocks the induction by ER stress[19]. Yoshida et al. 2001 proposed a mechanism for ER stress response activation through ATF6, with their findings. They proposed that in response to ER stress ATF6 initiates and induces the expression of ER chaperones and the XBP1 gene by directly binding to ER stress response elements. Then the spliced XBP1 produced by the activation of IRE1 induces the transcription of ER chaperons[20].
The cis -acting unfolded protein response elements (UPRE) is playing a significant role in UPR. Yamamoto et al. 2007 demonstrated that ER stress-mediated transactivation through UPRE and expression of some of the ER quality control proteins diminish in ATF6α knockout cells even in the presence of XBP1. They further reported that ATF6 cannot directly bind with UPRE to execute the UPR, suggesting that ATF6 and XBP1 form a heterodimerized ATF6-XBP1 complex to bind with UPREs and this complex has shown an eight-fold higher affinity to UPRE than the XBP1 homodimer, indicating the importance of the crosstalk between the two branches. Additionally, they demonstrated that ATF6 plays a crucial role in ER quality control process as EDEM and HRD1, two proteins involved in the degradation branch of ER quality control, both depend on XBP1 and ATF6[21].
Upon the induction of ER stress, splicing of XBP1 is induced. Because of this splicing, the C-terminal region of XBP1 is switched, resulting in the production of both unspliced and spliced mRNA forms, which will then lead to the production of pXBP1(U) and pXBP1(S) respectively. pXBP1(S) functions as the transcription factor with its specific C terminal region while pXBP1(U) acts as a shuttle between the cytoplasm and the nucleus[16], [22], [23]. The pXBP1(U) and pXBP1(S) get together and form the pXBP1(U) - pXBP1(S) complex which is subjected to the proteasome, because of the presence of degradation domain on the C-terminal of pXBP1(U)[16], [24]. Furthermore, it has been reported by Yoshida et al. 2009 that pXBP1(U) prefers to bind with pATF6α(N), making it susceptible to the proteasome,suggesting that pXBP1(U) has a negative effect on ATF6[24].
Moreover, Tsuru et al. 2016, reported a novel mechanism that explains the interconnection between IRE1α expression and PERK-ATF4, under ER stress. Their experiments showed that the splicing ratio of XBP1 mRNA in PERK knockout cells was increased by treatment with tunicamycin but decreased thereafter, whereas PERK-expressing cells maintained the ratio for several hours. Therefore, they suggested that PERK affects on IRE1α -XBP1 pathway under ER stress. Additionally, they demonstrated that the effect of PERK on the IRE1α -XBP1 pathway occurs in a different manner to that of ATF6 on IRE1α -XBP1 pathway[25].