1. Introduction
The cell stress response is an intrinsic system in all cells responding and adapting to environmental stimulations. One of the representative stress response systems is the heat shock factor (HSF)–heat shock protein (HSP) program that maintains proteostasis in cells [
1,
2,
3,
4] and promotes cancer progression [
5,
6,
7]. The HSF–HSP system was originally found to be activated in response to heat shock stress (HSS) but was subsequently shown to also be induced by oxidative stress, heavy metals, toxins, bacterial infections, and other stresses [
1]. Such proteotoxic stresses cause protein misfolding and thus activate the HSF–HSP system. Of note, the HSF–HSP system is often activated in cancer [
8,
9,
10].
Heat shock protein 90 (HSP90) members are stress-inducible protein chaperones that assist protein folding and re-folding to give their clients functionality in the intracellular space. As HSP90 has several hundred protein substrates (called ‘clients’), it is involved in many cellular processes beyond protein folding, which include DNA repair, development, the immune response, and neurodegeneration [
11,
12,
13,
14,
15]. Elevated expression of HSP90 has been observed in many cancer types and correlates with poor prognosis, increased metastatic potential, and resistance to therapy [
16,
17,
18,
19]. Moreover, the HSP90 alpha and beta isoforms are often released with extracellular vesicles (EV), including exosomes, by cancer cells and trigger cancer initiation and progression, as well as the polarization of tumor-associated macrophages (TAM) to an immunosuppressive M2 subtype [
6,
17,
20,
21,
22]. In addition, HSP90 is produced and released by immunocytes, such as macrophages, and plays a key role in antigen cross-presentation [
14,
15,
23,
24]. HSF1 is the master regulator of the protein quality control machinery in response to proteotoxic stress conditions [
2,
3,
25] and enhances cancer progression [
5,
7]. Upon proteotoxic stress, HSF1 binds to heat shock elements (HSE) in the promoter regions of HSP genes and other stress-inducible genes [
2,
3,
25]. HSF1 drives oncogenesis in many ways beyond inducing the gene expression of chaperones [
7,
26,
27,
28], co-chaperones [
6], and non-chaperone target genes [
9]. However, less is known about how HSP90 genes are attenuated by alternative transcription factors.
The SCAN domain-containing transcription factors (SCAN-TF) contain the SREZBP-CTfin51-AW1-Number 18 cDNA domain (SCAND), a leucine-rich oligomerization domain highly conserved among the SCAN-TF family (
Figure S1). This family contains more than 50 members, most of which contain a zinc finger (ZF) domain to scan DNA sequences for binding; hence, they are called SCAN-ZF factors [
29,
30,
31,
32,
33]. Myeloid zinc finger 1 (MZF1), also known as ZSCAN6 or ZNF42, is a prototypical SCAN-ZF that contains an N-terminal SCAN domain, a linker region, and a C-terminal DNA binding domain [
34,
35,
36]. Many studies have identified MZF1 as an oncogenic transcription factor [
34,
37,
38,
39,
40] and cancer stemness factor [
41,
42]. However, depending on the context, MZF1 can also function as a tumor suppressor [
43,
44,
45]. While there are more than 50 types of SCAN-TFs, only 6 zinc-fingerless SCAND-only proteins exist [
30,
31]. SCAND1 is a SCAN domain-only protein that can hetero-oligomerize with other SCAN-ZFs, including MZF1, through inter-SCAN domain interactions to repress transcription [
32,
33,
37,
43,
46]. Thus, hetero-oligomerization between SCAND molecules and SCAN-ZF molecules can transform their roles, forming a transcriptional repressor complex [
32,
33,
37,
43,
46]. Indeed, SCAND1 represses the co-chaperone
CDC37 gene (encoding cell division control 37) by interacting with MZF1 and suppressing prostate cancer [
37]. Moreover, SCAND1 and MZF1 are mutually inducible and form oligomers that can reverse epithelial-to-mesenchymal transition (EMT), tumor growth, and migration by repressing EMT driver genes and mitogenic protein kinase (MAPK) genes [
43]. High expression of MZF1 correlated with poor prognoses in prostate cancer and kidney cancer, whereas
SCAND1 and
MZF1 expression correlate with better prognoses in pancreatic cancer and stage III head and neck cancers [
43]. These suggest that MZF1 alone is oncogenic, whereas repressing complexes of SCAND1 and MZF1 is tumor suppressor, depending on their gene expression in cancer cases. SCAND2 is another member of SCAND factors with high homologies. Of note, SCAND2 RNA has been registered as
SCAND2P, a pseudogene for long noncoding RNA (lncRNA), and protein-coding SCAND2 mRNA in the NCBI database, although it has not been biologically investigated.
It has been unclear whether the SCAND factors and MZF1 are involved in proteotoxic stress response in cancer. Here, we show that the SCANDs and MZF1 are stress-inducible factors and can attenuate HSP90 gene expression in prostate cancer cells. We also show that cell stress alters the transcript variants of protein-coding and noncoding RNA of SCAND2. Moreover, we show that high expression levels of these SCAN-TF RNA can be predictive biomarkers of better prognoses in several cancer types, indicating potential tumor suppressor roles.
3. Discussion
We have shown that the cell stress-inducible SCAND1 and MZF1 genes repress the stress response of the HSF–HSP system (
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5 and
Figure 6). SCAND1, SCAND2, and MZF1/ ZSCAN6 are heat-inducible and could form repressing complexes on
HSP90 genes (
Figure 10) [
37,
43]. These findings were consistent with the data from clinical tumor specimens. SCAND2 and MZF1 RNA were expressed at higher levels in normal tissues than in paired tumor tissues (
Figure 7). In contrast, HSP90 RNA was expressed at higher levels in tumor tissues than in paired normal tissues in many cancer types (
Figure 7). These data suggest that SCAND2/MZF1 hetero-oligomers could inhibit the excess stress response of HSP90 expression in normal tissues, whereas loss of expression of these SCAN-TFs could result in the gain of HSP90 in tumor tissues. We showed that high expression of SCAN-TFs (SCAND1, SCAND2, and MZF1) were predictive biomarkers of enhanced prognoses for patients suffering from pancreatic cancer and head and neck cancers (
Figure 8 and
Figure 9). Moreover, high expression of SCAND2 (and/or lncRNA-SCAND2P) was a predictive biomarker of enhanced prognoses for patients suffering from lung adenocarcinoma, sarcoma, and cervical cancer (
Table 7,
Figure S6). These data indicate that SCAND/MZF1 repressing complexes are potentially tumor suppressing, contributing to better prognoses of patients suffering from several cancer types.
Our data, for the first time, indicate that SCAND2 RNA expression is a novel biomarker of better prognoses in cancer patients (
Table 7,
Figure 8 and
Figure 9). Only one group has previously reported the existence of the
SCAND2 gene [
47]. Moreover, SCAND2 has been registered as
SCAND2P, a pseudogene for lncRNA (Ref seq ID: NR_004859.1 and NR_003654.2). Gene expression data of
SCAND2 (or
SCAND2P) were found in many databases. Of note, the protein structure of SCAND2 found in Phosphosite plus is more conserved with the N-terminal region of MZF1(ZSCAN6) than SCAND1 (
Figure S1). Moreover, complete coding DNA sequences of SCAND2 mRNA are found in the NCBI database (GenBank ID: AF229246.1 (coding 306 aa, AAG33966.1), AK022844.1 (coding 152 aa, BAB14268.1), and AK290489.1 (coding 152 aa, BAF83178.1)) (
Figure 2 and
Figure S2). Our research highlights that HSS can shift the transcript balance from the lncRNA to the protein-coding mRNA of SCAND2 (
Figure 3 and
Figure S3), potentially via changing alternative splicing. SCAND2 and MZF1 RNA were each expressed in normal tissue at higher levels than in tumor tissues (
Figure 7), whereas SCAND1 RNA expression did not show this pattern. Therefore, SCAND2 may form more stable hetero-oligomers with MZF1(ZSCAN6) than SCAND1 to repress oncogenic gene expression in tumors. Further functional analysis of SCAND2 is required for this novel gene.
Our data also suggested that cell stress regulates RNA variants of the
SCAND2 gene, potentially via alternative RNA splicing, alternative RNA polyadenylation, and/or protein translational control (
Figure 2,
Figure 3 and
Figures S2–S4). Cell stresses, including oxidative stress and cancer therapy-induced stress, have been reported to regulate alternative RNA splicing via the Hu antigen R (HuR), also known as ElavL1 [
9,
48,
49]. We have reported that HSF1 regulates β-catenin RNA, which contains many AU-rich sequences, in mammary cancer cells by controlling HuR/ElavL1 expression [
9]. The Hu/Elav RNA-binding protein family, composed of HuR (also known as HuA), HuB, HuC, and HuD), regulate alternative splicing [
8,
50,
51,
52], while HuR is the most investigated member that binds to AU-rich sequences of RNA. Cell stress also modulates the function of the splicing regulatory protein RBM4 in translation control [
53]. Therefore, alternative expression of SCAND2 RNA variants, including lncRNA-SCAND2P and SCAND2 mRNA, could be regulated by Hu and/or RBM4 RNA-binding proteins under cell-stressed conditions.
Our study also revealed a striking correlation between the expression of HSF1 and the SCAN-TF genes (SCAND1 and MZF1) (
Figure 3). Furthermore, we have identified HSF4 as a potential inducer of SCAN-TF gene expression, including SCAND1, SCAND2 and MZF1. HSF4 lacks a leucine zipper 4 (LZ4) domain, resulting in its constitutive trimerization and DNA-binding activity [
54]. Several HSF4-BSs were found in the promoter regions of
SCAND1, SCAND2, and
MZF1 genes (
Table 1). Thus, the expression of HSF4 could result in the constitutive expression of
SCAND2, SCAND1, and
MZF1 without requiring cellular stress. While HSF4 is known to be oncogenic in several cancer types, such as colorectal cancer, hepatocellular carcinoma, and lymphoma [
55,
56,
57], our data suggested that HSF-dependent expression of SCAN-TFs could actually reduce oncogenic gene expression in tumors.
Moreover, our data suggested that the stress-inducible SCAND–MZF1 complex represses the
HSP90AA1 gene while also repressing other HSPs and many more stress-responsive genes (
Table 6 and
Table S2). We have recently shown SCAND1 and MZF1 expression to negatively correlate with EMT driver genes, including
ZEB1, CTNNB1 and
TGFBR1/2/3, and mitogenic genes encoding kinases in the MEKK–MEK–ERK signaling pathway [
43]. Moreover, SCAN-only family genes and MZF1 expression were negatively correlated with the expression of NF-κB signaling molecules and PI3K-AKT signaling molecules. Thus, we have shown that EMT, some oncogenic signaling pathways, and the HSF–HSP gene expression system are all key targets of the SCAND–MZF1 repression complexes.
Our data also suggested that tumors’ stress levels differs among clinical cases (
Figure 7). Tumor cells are characteristically exposed to various stresses from the microenvironment, such as immune/inflammatory stress [
19], therapeutics [
18], hypoxia [
22,
58,
59,
60,
61], acidification [
62,
63], hyperthermia [
64,
65] or heat stress [
4,
6,
19,
25,
28], endoplasmic reticulum stress [
66], nuclear envelope stress [
67,
68], replication stress [
69], oxidative stress [
70], mechanical stress, osmotic stress, and genotoxic (DNA damage) [
71,
72] and proteotoxic stress [
1,
2,
4,
73]. Therefore, it might be difficult to determine the types and levels of stresses in each tumor. However, there were strong correlations between the RNA expression of SCAN-TFs and HSP90AA1 in clinical tumor specimens (
Figure 6,
Figure 7,
Figure 8 and
Figure 9). These clinical data support the hypothesis that the SCAN-TF complexes repress excessive HSP gene expression and suppress tumors.
In conclusion, we have demonstrated that the cell stress-inducible SCAND and MZF1 repress the stress response in cancer. MZF1 and SCAND1 are mutually inducible and can form a repressive complex on the HSP90 gene promoters. Moreover, cell stress changed the transcript variants from the lncRNA-SCAND2P into protein-coding SCAND2 mRNA. Nevertheless, elevated levels of SCAND2 RNA are novel potential markers of better prognoses in several cancer types, including pancreatic cancer, head and neck cancers, lung adenocarcinoma, sarcoma, and cervical cancer. This effect may ensue from the findings that the SCAND–MZF1 repressive system is important for preventing cancer-related gene expression physiologically while playing a key role in tumor suppression.