
Thioredoxin-like protein 1 (TXNL1), known as thioredoxin-related protein of 32 kDa (TRP32), is a member of the thioredoxin family and is localized in the nucleus and cytoplasm (1, 2). TXNL1 processes two domains, the N-terminal Trx domain and a C-terminal domain which possess the unknown function 1000 (DUF1000) and this domain interacts with the 26S proteasome (3, 4). It is well known that this protein alters the pathogenesis of a number of diseases through their anti-oxidative mechanism. It is reported that TXNL1 protects against glucose deprivation-induced cytotoxicity in human embryonic kidney 293 cells (HEK-293 cells) (5) and overexpression of TXNL1 inhibits mammalian cell proliferation and acts as a transcriptional repressor through direct binding to the transcription factor B-Myb in SNU-1 cells (6). Furthermore overexpression of TXNL1 prevents cell death and cancer progression through the inactivation of oxidative stress-induced phosphatase of regenerating liver (PRL) (7). Xu
Reactive oxygen species (ROS) like H2O2 cause oxidative damage that is highly associated with a wide range of diseases including neuronal diseases and cancers (9, 10). Increased production of ROS induced toxic effects on DNA damage in neuronal cells and high levels of ROS are associated with brain ischemic injury (11). It is well known that Trx1 protein plays important role in reducing oxidative stress in the brain, therefore, it was hypothesized that inhibition or regulation of ROS production through TXNL1 may have a protective effect against brain ischemic injury (12, 13). However, the precise protective mechanism of this protein against ischemic insult is not well studied yet.
Protein transduction domains (PTDs) including trans-acting activator of transcription (Tat) PTD are well described tools for protein delivery into cells. Notably, Tat PTD fusion proteins are not be limited by the protein size of the protein and various cell permeable fusion proteins have been showed protective effects
We showed a diagram of Tat-TXNL1 and TXNL1 protein. After the construction of both proteins, we confirmed the purified proteins. Purified Tat-TXNL1 and TXNL1 protein displayed the expected molecular weights (Supplementary Fig. 1).
The HT-22 cells were exposed to Tat-TXNL1 (0.5-7 μM) for 1 h or Tat-TXNL1 (7 μM) for 5-60 min, confirm the transduced Tat-TXNL1. Tat-TXNL1 transduced into HT-22 cells in a concentration and a time-dependent manner. Also, Transduced Tat-TXNL1 showed for to 12 h and distributed in the cells cytoplasm and nucleus. However, TXNL1 did not transduce into the cells (Supplementary Fig. 2).
To examine the effects of Tat-TXNL1 against oxidative stress-induced HT-22 cell damage; the cells were treated with hydrogen peroxide (H2O2) and assessed for viability, ROS generation, and DNA fragmentation. Tat-TXNL1 transduced HT-22 cells showed markedly increased cell survival compared with only H2O2 treated HT-22 cells (Fig. 1A). Also, transduced Tat-TXNL1 markedly reduced ROS generation and DNA damage (Fig. 1B, C). On the other hand, there was no significant difference between H2O2 alone and TXNL1 treated cells.
Mitogen activated protein kinases (MAPKs) and Akt signaling pathways play crucial roles in cell death regulation and survival on oxidative stress (25-28). We examined whether Tat-TXNL1 inhibited the activation of MAPK and Akt signaling (Fig. 2). In the H2O2 treated cells, phosphorylated MAPKs and Akt levels were increased compared to the control cells; the phosphorylated MAPKs and Akt levels were not changed in the TXNL1 treated cells. However, Tat-TXNL1 treated cells showed markedly reduced the phosphorylated MAPKs and Akt levels.
As shown in Fig. 3, p53 and Bax levels were markedly increased in the H2O2 treatment HT-22 cells while Tat-TXNL1 significantly reduced the phosphorylated p53 and Bax levels. On the other hand, Tat-TXNL1 increased levels of Bcl-2, pro-caspase-3, and pro-caspase-9 levels. There was no significant difference between H2O2 alone and TXNL1 treated cells.
We performed an immunohistochemistry test to determine the effects of Tat-TXNL1 on neuronal damage after ischemic brain injury (Fig. 4). Cresyl violet (CV) staining was used to examine changes in the cellular distribution and morphology in the brain. In the vehicle- and TXNL-ischemia group, a significant loss of CV-positive neuronal cells was observed at 7 days after ischemic brain injury. However, a large number of CV-positive neuronal cells were observed in the Tat-TXNL1-treated ischemia group. Also, Fluoro-Jade B (F-JB) specifically binds to degenerating neurons and is known as a marker for degenerating or dead cells. In this study, after F-JB staining, F-JB positive cells were markedly increased in the vehicle- and TXNL-ischemia group. However, F-JB positive cells were markedly reduced in the Tat-TXNL1-treated ischemia group.
Further, we examined the changes in ionized calcium-binding adaptor molecule 1 (Iba-1; a marker for microglia) and glial fibrillary acidic protein (GFAP; a marker for astrocyte)-positive cells after ischemic brain injury (Fig. 4). In the vehicle- and TXNL-ischemia group, Iba-1, and GFAP-positive cells were significantly increased and hypertrophied compared to that in the sham control group. In the Tat-TXNL1-treated ischemia group, there was no significant change in the morphology of Iba-1 and GFAP-positive cells compared to the sham control group.
Thioredoxin plays a critical role in several biological processes including the maintenance of cellular homeostatic and redox balance (29-31) and TXNL1, a member of the thioredoxin family, is present in the cytoplasm, and the nucleus of the cells (1, 2). TXNL1 is involved in the function of the regulatory particle non-ATPase 11, a subunit of the 26S proteasome, translation elongation factor 1A, and in the transfer of misfolded protein degradation (32-34). Other studies have shown that overexpression of TXNL1 protected cell death by reducing cellular cytotoxicity (5, 7). Recent study reported that recombinant seahorse TXNL1 has antioxidant and free radical scavenging activity (35) and several studies have reported that excessive ROS enhances the modification of macromolecules and cell death (36) and involved in a wide range of diseases including neural diseases like brain ischemic injury (9-11). Although Sugawara and Chan reported that inhibition of excessive ROS production can protect against ischemic brain injury (13), they didn’t show the precise protective mechanism. Therefore, in the present study, we investigated protective mechanism of TXNL1 protein against hippocampal neuronal cell damage induced by oxidative stress by using the cell permeable Tat-TXNL1 fusion protein.
It has been reported that excessive ROS increases phosphorylation of MAPKs leading to neuronal cell death (26, 37, 38) and overexpression of TXNL1 shows reduction of neuronal cell damage by regulation of MAPKs signaling pathways and plays an anti-apoptotic effect
Since it is well known that apoptosis plays a pivotal role in ischemic brain injury, it is necessary to investigate the proteins such as Bax and Bcl-2 involved in the apoptotic signaling pathways. Bax and Bcl-2 play a key role in the apoptosis because overexpression of Bax is primarily executed apoptosis and down-regulation of Bcl-2 initiates caspase-dependent cell death and it was reported that the regulation of Bax and Bcl-2 expressions has been suggested as a therapeutic strategy in apoptosis-related disorders (42-44). Other studies have reported that treatment with coenzyme Q10 (CoQ10), a strong anti-oxidant agent, markedly increased Bcl-2 levels in cerebral ischemic injury, suggesting that CoQ10 protects ischemic injury by inhibiting apoptosis (45).
We have shown that Tat-TXNL1 increased Bcl-2, pro-caspase-3, and pro-caspase-9 levels, whereas Bax levels were reduced in H2O2-exposed HT-22 cells. These findings indicate that transduced Tat-TXNL1 suppressed ROS-induced apoptosis through the regulation of pro-apoptotic proteins and anti-apoptotic proteins. We confirmed the Yoo
At high levels of ROS, p53 protein plays an important role in the progression of apoptosis through interaction with the mitochondrial apoptotic proteins Bax and Bcl-2 which are known as signaling proteins that regulate the direct activation of pro-caspase-9 and pro-caspase-3 (49-51). A potent ROS inducer H2O2 reduces pro-caspase-9 and pro-caspase-3 in PC12 cells in a time-dependent manner and promotes phosphorylation of p53 through activation of pro-caspase-9 and pro-caspase-3 signaling by Bax and Bcl-2 (52). Recently, there are several reports involved in the relationship between ischemic neuronal cell damage and apoptotic signaling. Tan
Since the oxidative stress is significantly involved in ischemia, inhibition of oxidative stress is a potential target for neuroprotection in ischemic injury (11-13, 56). Activation of astrocytes and microglia in the brain plays crucial roles in the pathogenesis of cerebral ischemic injury and some studies suggested that the inhibition of astrocytes and microglia activations is a potential target for neuroprotective strategies in ischemic injury (57-61). In a previous study, we have reported that Tat fused glyoxalase and CIAPIN1 (Tat-glyoxalase and Tat-CIAPIN1) protein reduced astrocytes and microglia activation in an ischemic animal model (19, 22). We showed that Tat-TXNL1 markedly increased CV-positive neuronal cells whereas F-JB-positive neuronal cells were reduced in the ischemic animal model. We further have shown that Iba-1 and GFAP-positive cells were significantly increased in the ischemic injury animal model. However, Tat-TXNL1 reduced the Iba-1 and GFAP-positive cells and no significant change in the morphology compared to the normal animal. These results indicate that Tat-TXNL1 reduced neuronal cell death and activation of astrocytes and microglia in the ischemia animal model.
In summary, we demonstrated that Tat-TXNL1 transduced into HT-22 cells and showed protective effects against H2O2-induced cytotoxicity by inhibiting excessive ROS production through MAPK and apoptotic signaling pathways. Increasing intracellular ROS can lead to MAPK activation and apoptotic cell signaling and transduced Tat-TXNL1 prevented hippocampal neuronal cell death and this fusion protein plays a protective role in ischemic brain injury. Those results indicate that TXNL1 protein can be a putative therapeutic agent for brain ischemic injury.
See supplementary information for this section.
This research was supported by Basic Science Research Program (2018R1D1A3B07049265 & 2019R1A6A1A11036849) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.
The authors have no conflicting interests.
![]() |
![]() |