STAT3 is also phosphorylated by other intracellular kinases, such as the non-receptor tyrosine kinases Src and Abl. STAT3 phosphorylation leads to its homodimerization. The homodimer enters the nucleus, in order to exert its transcriptional activity regarding genes with diverse biological consequences
[16][11]. STAT3 acetylation, by histone acetyltransferase on a lysine residue (K685), is an alternative mechanism of STAT3 activation that stabilizes the STAT3 dimer
[17][12]. On the other hand, STAT3 negative regulation is mainly controlled by tyrosine phosphatases, which dephosphorylate/deactivate STAT3. STAT3 is inhibited by a suppressor of cytokine signaling 3 (SOCS3) at the receptor level, and by E3 SUMO-protein ligase (PIAS3) at the transcriptional level (
Figure 1)
[17,18,19,20,21][12][13][14][15][16].
3. Mechanisms of STAT3 Implication in Tumorigenesis
The canonical STAT3 pathway that functions transiently and is strictly regulated in normal cells, has been documented to be persistently activated in a variety of solid and hematological malignancies [32][17]. STAT3 is also activated by non-canonical pathways in tumorigenesis, which involve STAT3 phosphorylation in a (Ser)727 alternative site of the C-terminus, by mitogen-activated protein kinase (MAPK), c-Jun N-terminal kinase (JNK), or protein kinase C (PKC) pathways. Constitutively activated STAT3 translocates to the nucleus and enhances gene transcription, which favors cell proliferation, resistance to apoptosis, survival, angiogenesis, tumor-promoting inflammation, tumor-mediated immune evasion, and, ultimately, invasion and metastasis. Recent evidence demonstrates that STAT3 promotes tumorigenesis through additional processes, including inflammation-promoting carcinogenesis, obesity and/or metabolism, CSCs, and pre-metastatic niche formation [33][18].
Aberrant STAT3 activation is mainly attributed to the excessive production of cytokines and growth factors in the tumor microenvironment (TM), high expression of protein tyrosine kinases, and epigenetic suppression of negative regulators of STAT3, such as SOCS3 and protein tyrosine phosphatases [34][19]. Particularly, the activation of the IL-6/JAK/STAT3 core axis is observed in many types of cancer. IL-6 is overexpressed in the TM and stimulates the JAK/STAT3 pathway, which is a molecular event that is associated with poor prognosis for cancer patients. JAK2 is specifically activated in myeloproliferative disorders and activates STAT3 [35][20]. IL-6 plays a protective role for cancer cells, against DNA damage, apoptosis, and oxidative stress [36][21]. Regarding angiogenesis, STAT3 is activated in cancer cells by reactive oxygen species (ROS), mechanistic target of rapamycin complex 1 (mTORC1) and/or IL-6, and further induces hypoxia-inducible factor 1-alpha (HIF-1a) activation during hypoxia. HIF-1a induces the transcription of vascular endothelial growth factor (VEGF), which, in turn, affects the surrounding endothelial cells. This event activates, once again, STAT3 via the VEGF receptor (VEGFR). In endothelial cells, STAT3, along with HIF-1a and sp1, promotes the transcription of genes that mediate the cell growth, survival, and migration of endothelial cells, therefore promoting angiogenesis [37][22].
4. Mechanisms of STAT3 Engagement in Colorectal Tumorigenesis
4.1. STAT3 Expression and Regulation in Colorectal Cancer Cells
The immunohistochemical evaluation of p-STAT3 in 108 CRC cases demonstrated cytoplasmic and nuclear expression, positive correlation of pSTAT3 expression, with the depth of the tumor invasion, Dukes, TNM stage, and reduced overall survival [40][23]. STAT3-enhanced activation in CRC occurs from IL-6 in the serum and the tumor microenvironment, from growth factors and growth factor receptors, tyrosine kinases (Src, Bcr-Abl), and loss-of-function mutations for the STAT3 suppressors/inhibitors (phosphatases, SOCS, PIAS) [35][20].
Beyond direct STAT3 regulation by upstream molecules, STAT3 is also activated by the suppression of STAT3 negative regulators, such as PTPRT [41][24]. PTPRT functions as a tumor suppressor gene, and is mutated in colon, lung, stomach, and skin (melanoma) cancers. In CRC, it is the most frequently mutated tyrosine phosphatase among the tyrosine phosphatome that was examined in 18 CRCs. PTPRT presents five mutations that decrease its activity [42,43][25][26]. STAT3 has been identified as a substrate of PTPRT. The epigenetic suppression (promoter hypermethylation) of PTPRT occurs in several human cancers, but most notably in CRC (78.7%, 289/367 tumors analyzed) [44][27].
4.2. STAT3 in Invasion and Metastasis
STAT3 is implicated in the invasion and metastasis of CRC cells, through various mechanisms. In CRC cells, the IL-6/STAT3 axis promotes EMT and, therefore, the CRC aggressive phenotype via the STAT3-dependent transcriptional upregulation of Fos-related antigen-1 (Fra-1) TF. Fra-1 belongs to the Fos family of TFs, and is associated with phenotype invasive transformation and EMT in various types of cancer. In a 229 CRC patient cohort, Fra-1 expression was positively correlated with an increased depth of invasion, lymph node, and liver metastases. Fra-1 was also expressed according to IL-6 and pSTAT3 expression. In CRC cells, STAT3 needs to be both phosphorylated and acetylated to bind to the Fra-1 promoter and induce Fra-1 transcription [48][28].
MiR-34 is known to suppress EMT. The MiR-34 gene is a target of activated STAT3, which causes transcriptional repression of miR-34, and therefore induces EMT promotion in CRC cells and tumors [49][29]. Furthermore, the p53-inducible miR-34a control of the tyrosine kinase colony-stimulating factor 1 receptor is alleviated in CRC, through a feedback loop that is mediated by STAT3 [50][30]. STAT3 is also involved by enhancing the acquisition of EMT and cancer stem cells traits by CRC cells, through transcriptional induction of NANOG TF.
4.3. STAT3 in Angiogenesis
During hypoxia, STAT3 is activated in cancer cells by ROS, mTORC1, and/or IL-6, and further induces HIF-1a. This transcription factor induces the transcription of VEGF. This event affects the surrounding endothelial cells, in order to activate, once again, STAT3 via VEGFR. In endothelial cells, STAT3, along with HIF-1a and sp1, promotes the transcription of genes that mediate cell growth, survival, and migration of ECS, therefore angiogenesis [37][22]. Corticotrophin-releasing hormone (CRH) and its receptor CRHR contribute to CRC cells proliferation, and promote angiogenesis.
4.4. STAT3 in Tumor-Promoting Inflammation
Chronic inflammation is involved in several processes during tumorigenesis, such as proliferation, survival, transformation, invasion, angiogenesis, and metastasis
[57][31]. Regarding CRC, in particular, patients with inflammatory bowel diseases (ulcerative colitis—Crohn’s disease) have increased risk of developing CRC, and colitis-associated CRC (CAC) presents poor prognosis
[57,58][31][32]. Studies that have employed gain-of-function and loss-of-function in vivo models show that STAT3 mediates the impact of IL-6 and IL-11 on the initiation and development of CAC. STAT3 activity is considerably reduced in the colon and intestinal epithelial cells of
IL-6-/- mice, whereas STAT3 silencing in intestinal epithelial cells has a profound effect on CRC tumorigenesis in vivo
[8][4]. IL-6 and IL-11 activate STAT3 through gp130, thus establishing an IL-6, IL-11/gp130/STAT3 signaling axis that prevents enterocytes from apoptosis, and facilitates cell survival and cell proliferation, ultimately promoting tumor growth
[59][33].
STAT3 is a critical regulator of inflammatory processes. Therefore, it has also emerged as key molecule for inflammation-associated processes, during tumor development of both CAC and sporadic CRC
[60][34]. STAT3 is a core mediator of CAC, since it facilitates IL-6-induced proliferating and anti-apoptotic effects on intestinal epithelial cells
[8][4]. Furthermore, the status of the gut microbiome and the proportion of certain gut microbiota are also important during inflammation-associated CRC, with certain bacteria either promoting or suppressing tumorigenesis
[61,62][35][36]. Therefore, the abundance of
Streptococcus bovis/gallolyticus, enterotoxigenic
Bacteroides fragilis, and
Escherichia coli NC101 have been suggested as risk factors for CRC
[63][37].
5. STAT3 in Colorectal Cancer Treatment
We already know that STAT3 inhibition in vitro suppresses tumor growth and promotes apoptosis in cancer cells. STAT3 integrates the oncogenic signaling of multiple upstream kinase molecules. Therefore, STAT3 inhibition is a promising therapeutic strategy that could abrogate the oncogenic impact of several upstream activated kinase molecules. Although JAK1/2 inhibition has favorable effects in JAK/STAT-driven hematological malignancies, agents that target STAT3 are still in the early phases of development. Strategies targeting STAT3 are categorized into the following groups: (1) inhibitors of the SH2 domain or STAT3 dimerization, (2) upstream tyrosine kinase inhibitors, (3) STAT3 pathway oligonucleotides, (4) inhibitors of STAT3–DNA binding, and (5) mimics of STAT3 negative regulators
[32][17]. There is a number of STAT3 inhibitors with promising efficacy in preclinical studies, which have entered clinical trials (
Table 1)
[10][6].
Table 1.
Clinical trials of CRC patients including STAT3 inhibitors (
).
Identifier |
Status |
Title |
Condition |
Phase |
Drug |
NCT03647839 |
Active, not recruiting |
Modulation Of The Tumour Microenvironment Using Either Vascular Disrupting Agents or STAT3 Inhibition in Order to Synergise With PD1 Inhibition in Microsatellite Stable, Refractory Colorectal Cancer |
Colorectal Cancer Metastatic |
II |
BBI608 |
NCT02983578 |
Active, not recruiting |
Danvatirsen and Durvalumab in Treating Patients With Advanced and Refractory Pancreatic, non-small cell Lung Cancer, and Mismatch Repair Deficient Colorectal Cancer |
Advanced Colorectal Carcinoma
Mismatch Repair Deficiency
Refractory Colorectal Carcinoma
Stage III Colorectal Cancer AJCC v8
Stage IIIA Colorectal Cancer AJCC v8
Stage IIIB Colorectal Cancer AJCC v8
Stage IIIC Colorectal Cancer AJCC v8
Stage IV Colorectal Cancer AJCC v8
Stage IVA Colorectal Cancer AJCC v8
Stage IVB Colorectal Cancer AJCC v8
Stage IVC Colorectal Cancer AJCC v8 |
II |
Danvatirsen |
NCT03195699 |
Recruiting |
Oral STAT3 Inhibitor, TTI-101, in Patients With Advanced Cancers |
Colorectal Cancer
Advanced Cancer |
I |
TTI-101 |
NCT03522649 |
Recruiting |
A Phase III Clinical Study of Napabucasin (GB201) Plus FOLFIRI in Adult Patients With Metastatic Colorectal Cancer |
Previously Treated Metastatic Colorectal Cancer |
III |
BBI608 |
NCT02753127 |
Completed |
A Study of Napabucasin (BBI-608) in Combination With FOLFIRI in Adult Patients With Previously Treated Metastatic Colorectal Cancer (CanStem303C) |
Colorectal cancer |
III |
BBI608 |
NCT01776307 |
Completed |
A Study of BBI608 in Adult Patients With Advanced Colorectal Cancer |
Colorectal Cancer |
II |
BBI608 |
NCT03522649 |
Recruiting |
A Phase III Clinical Study of Napabucasin (GB201) Plus FOLFIRI in Adult Patients With Metastatic Colorectal Cancer |
Previously Treated Metastatic Colorectal Cancer |
III |
BBI608 |
NCT02641873 |
Completed |
A Study of BBI608 Administrated With FOLFIRI + Bevacizumab in Adult Patients With Metastatic Colorectal Cancer |
Metastatic Colorectal Cancer |
I |
BBI608 |
NCT03647839 |
Active, not recruiting |
Modulation Of The Tumour Microenvironment Using Either Vascular Disrupting Agents or STAT3 Inhibition in Order to Synergise With PD1 Inhibition in Microsatellite Stable, Refractory Colorectal Cancer (MODULATE) |
Colorectal Cancer Metastatic |
II |
BBI608 |
NCT01830621 |
Completed |
BBI608 and Best Supportive Care vs. Placebo and Best Supportive Care in Pretreated Advanced Colorectal Carcinoma |
Colorectal Carcinoma |
III |
BBI608 |
6. Conclusions
STAT3 has been established as a critical regulator of context-dependent processes in cancer cells and their surrounding stromal cells. Although there are findings that attribute STAT3 either an oncogenic or a tumor-suppressive role in solid tumors and CRC, it has been widely accepted that STAT3 activity mainly lies on STAT3 homodimerization and activation for gene transcription, which regulates proliferation, apoptosis, invasion and metastasis, angiogenesis, and TM immune status in CRC. Therefore, STAT3 inhibition is a promising therapeutic strategy, with a footnote regarding proper context-dependent application.