The discovery of cancer genes has been propelled by genetic analyses and more recently by next generation DNA sequencing technologies. Combined, these studies have been identified 127 significantly mutated cancer genes that cover diverse signaling pathways ^[1][2]. Mutations acting as drivers in cancer are positively selected during tumor growth and constitute solid proof of the involvement of a particular gene as a driver in the disease. Mutations in STAT3 and STAT5 have been reported in patients with solid cancers, but unlike hyperactivation of the JAK/STAT pathway, STAT3/5 mutations in cancer are relatively infrequent and occur mostly in hematological malignancies.

An overview of reported STAT3/5 mutations in solid cancers, based on data collected from the Catalogue of Somatic Mutations in Cancer (COSMIC) database. Mutations in STAT3 are more prevalent than mutations in STAT5A or STAT5B genes. Noticeably, gastrointestinal cancers have the highest rates of STAT3/5 mutations compared with other solid cancers. Missense mutations tend to cluster within the SH2 domain, where gain-of-function mutations were previously characterized ^[2][3][3,4], as well as within the DNA binding domain and to an extent the N-terminal domain. Interestingly, the STAT3 Tyrosine 640 into Phenylalanine (Y640F) hotspot gain-of-function mutation reported in various lymphoid malignancies has also been detected in patients with liver cancer. Nonsense and frameshift mutations are less frequent and more disperse, likely representing loss-of-function events. Notably, a hotspot frameshift mutation at position Q368 within the DNA binding domain of STAT5B has been reported in 24 patients with various types of carcinoma; this frameshift generates a stop codon shortly after the mutation and is therefore likely to be loss-of-function, although characterization of this mutation has not been performed.

As opposed to mutation rates, STAT3/5 activation is very frequent in human cancers, perhaps reflecting increased cytokine signaling or mutations in cytokine receptors or negative regulators. STAT3/5 activation can be detected using antibodies that measure total levels or activation marks in STAT3/5 proteins (e.g. tyrosine phosphorylation). A better assessment of STAT3/5 activation can be obtained by measuring downstream signaling targets (i.e., mRNA levels of STAT3 ^[4][5] and STAT5 ^[5][6] target genes). A recent metanalysis of 63 different onstudies were concluded that STAT3 protein overexpression was significantly associated with a worse 3-year overall survival (OS) (OR = 2.06, 95% CI = 1.57 to 2.71, p < 0.00001) and 5-year OS (OR = 2.00, 95% CI = 1.53 to 2.63, p < 0.00001) in patients with solid tumors ^[6][7]. Elevated STAT3 expression was associated with poor prognosis in gastric cancer, lung cancer, gliomas, hepatic cancer, osteosarcoma, prostate cancer and pancreatic cancer. However, high STAT3 protein expression levels predicted a better prognosis for breast cancer ^[6][7]. ItThis wasstudy mixed data of both STAT3 and phospho-STAT3 (p-STAT3) expression limiting its ability to associate pathway activation to prognosis. It wasHere, we summarized the data linking activation of STAT3/5 to overall survival in several major human solid cancers identifying the biomarkers usedd in each study. Taken together, it wathe results clearly showed that STAT3 and STAT5 are important cancer genes despite their relatively low mutation frequency.

STAT3 activation is clearly a factor linked to bad prognosis in patients with lung cancer, liver cancer, renal cell carcinoma (RCC) and gliomas. In other tumors, the association is not significant. In solid tumors, STAT3 activation is more frequent than STAT5 activation although no explanation for this difference was proposed. In prostate cancer, both STAT3 and STAT5 have been associated with castration-resistant disease and proposed as therapeutic targets ^[7][8][8,9]. In colon cancer, the association between p-STAT3 and survival varies. according to the study, Bbut a high p-STAT3/p-STAT5 ratio indicates bad prognosis ^[9][10]. Also in breast cancer, p-STAT5 levels are clearly associated with better prognosis ^[10][11]. In liver cancer, STAT5 has ambivalent functions that were recently reviewed by Moriggl and colleagues ^[11][12]. Understanding mechanistically how STAT3/5 promote transformation and tumor suppression is important for the eventual design of new treatments. Also, survival data is highly influenced by the response of patients to their treatment and may not always reflect all mechanistic links between STAT3/5 activity and tumor biology. Of note, the effect of any gene is conditioned by the genetic context of gene action. Some genes can clearly exert a tumor suppressor effect in the initial stages of carcinogenesis that is lost when cancer mutations or epigenetic changes inactivate key effectors of these tumor suppressor pathways ^[12][13]. Human studies are usually limited to late stage tumors because it is easier to collect samples at that point. IStudies in model systems, including primary cells, organoids and mouse models are thus required for a full understanding of how cancer genes work specifically at early stages in tumorigenesis.

STAT3 and STAT5 promote tumor progression by regulating the expression of cell cycle, survival and pro-inflammatory genes. In addition, they control mitochondrial functions, metabolism and stemness, as discussed below .

As transcription factors, STAT3 and STAT5 regulate many genes required for cell cycle progression and cell survival. A major target of the transcriptional control of the mammalian cell cycle is cyclin D. STAT3 regulates cyclin D expression in a complex with CD44 and the acetyltransferase p300. The latter acetylates STAT3 promoting its dimerization, nuclear translocation and binding to the cyclin D promoter ^[13][25]. Other cell cycle and survival genes regulated by STAT3 include c-MYC (myc proto-oncogene), B-cell lymphoma 2 (BCL2), BCL2L1/BCL-XL (B-cell lymphoma-extra large), MCL1 (Myeloid Cell Leukemia Sequence 1) and BIRC5/survivin ^[14][26]. IRecent wastudies combined ChIPSeq with whole transcriptome profiling in ABC DLBCL (activated B cell-like diffuse large B cell lymphoma) cell lines and revealed that STAT3 activates genes in the Phosphoinositide 3-Kinase (PI3K)/AKT/Mammalian Target of Rapamycin (mTOR) pathway, the Nuclear Factor Kappa-Light-Chain Enhancer of Activated B-Cells (NF-κB) pathway and the cell cycle regulation pathway, while repressing type I interferon signaling genes ^[15][27]. STAT5 also regulates the expression of cell cycle and cell survival genes ^[12][13] including AKT1 ^[16][28], which encodes a pro-survival kinase.

Pharmacological agents that limit inflammation have been proposed for cancer prevention ^[25][37]. The use of metformin, a drug widely used to control diabetes, has been associated with a dramatic reduction in cancer incidence in many tissues ^[26][38]. Although the primary site of action of this drug is in mitochondria, a consequence of its effects is a potent reduction in the activation of NF-κB and STAT3, suggesting that the promising anticancer actions of metformin are related to its ability to curtail pro-inflammatory gene expression ^[27][28][39,40]. In contrast to STAT3, STAT5B inhibits NF-κB activity in the kidney fibroblast cell line COS by competing with coactivators of transcription ^[29][41], while it stimulates NF-κB in leukemia cells ^[30][42]. These results suggest the involvement of different regulatory mechanisms of STAT5 in hematopoietic cancers compared with solid cancers.

In addition to their canonical roles in inflammation and immunity, STAT3 and STAT5 have been shown to localize to mitochondria. The mitochondrial localization of STAT3 is required for its ability to support malignant transformation in murine embryonic fibroblasts and breast cancer cells ^{[31][32][33][34]}[43,44,45,46], and mito-STAT3 regulates mitochondrial metabolism and mitochondrial gene expression ^{[33][35][36][37][38][39]}[45,47,48,49,50,51]. Several reports have been suggested that STAT3 can be imported to mitochondria after phosphorylation on S727 ^[32][33][44,45] or upon acetylation ^[40][41][52,53]. Other studies have revealed that STAT3 mitochondrial translocation is mediated by interactions with Heat Shock Protein 22 (HSP22), Gene Associated with Retinoic and Interferon-Induced Mortality 19 (GRIM-19) or Translocase of Outer Mitochondrial Membrane 20 (TOM20) ^[42][43][44][54,55,56]. The mRNAs coding for some mitochondrial proteins are translated close to or in physical interaction with the import complex TOM ^[45][46][57,58]. The structural motifs mediating those interactions are located in the 3′ and 5′ UTRs of the mRNAs ^[47][48][59,60] and it will be interesting to investigate whether the mRNA of STAT3 also possesses RNA localization signals (zip codes) to localize in close proximity to mitochondria.

Whereas the role of mitochondrial STAT3 has been extensively learnstudied, the role of STAT5 in mitochondria is less clear. The import of STAT5 to mitochondria is regulated by cytokines ^[31][43]. Once imported into the mitochondria, STAT5 binds the D-loop of mitochondrial DNA, although no increase in transcription of mitochondrial genes was observed ^[49][61]. Mito-STAT5 is also able to interact with the Pyruvate Dehydrogenase Complex (PDC) and was shown to regulate metabolism towards glycolysis, as observed in cells treated with cytokines ^[31][49][43,61]. In the same line, STAT3 was also shown to interact with the PDC in mitochondria ^[41][53].

The role of STAT3 in stem cell biology was initially recognized due to the requirement for the cytokine LIF to maintain pluripotency in cultures of mouse embryonic stem (ES) cells. STAT3 activation mediates the induction or repression of several genes in mouse ES cells including the pluripotency factors Oct4, Klf4, Tfcp2l1 and polycomb proteins ^[50][51][52][62,63,64]. Many pluripotency factors, such as Homeobox Protein NANOG, are short-lived proteins. STAT3 controls protein stability by inducing the expression of the deubiquitinase Ubiquitin Specific Peptidase 21 (USP21), stabilizing NANOG in mouse ES cells. Induction of ES cell differentiation promotes the Extracellular Signal-Regulated Kinase (ERK)-dependent phosphorylation of USP21 and its dissociation from NANOG, leading to NANOG degradation ^[53][65]. STAT3 also plays a role in the reprogramming of somatic cells into induced pluripotent stem (IPS) cells ^[54][66] and it has been suggested that its effects depend on the demethylation of pluripotency factor promoters ^[55][67]. STAT3 also activates mitochondrial DNA transcription, promoting oxidative phosphorylation during maintenance and induction of pluripotency ^[56][68]. It is thus likely that the ability of STAT3 to stimulate stemness also plays a role in its oncogenic activity.

In many tumors, a subpopulation of cells possess a higher malignant capacity. These so-called tumor-initiating cells are suspected to regenerate the tumor after cancer chemotherapy and express many genes commonly expressed in ES cells ^[57][69]. It has been shown that STAT3 is required for the formation of tumorspheres and the viability of the cancer stem cell pool in many different tumors ^{[27][28][58][59][60][61][62][63][64][65][66][67][68][69][70][71]}[39,40,70,71,72,73,74,75,76,77,78,79,80,81,82,83]. At least in breast cancer, a critical mechanism stimulated by STAT3 to regulate stemness involves genes in fatty acid oxidation ^[66][67][78,79] and the ability of STAT3 to adjust the levels of reactive oxygen species (ROS) produced in mitochondria ^[67][79]. In colorectal cancer cells, STAT3 forms a complex with the stem cell marker CD44 and the p300 acetyltransferase. Acetylation of STAT3 by this complex allows dimerization, nuclear translocation and binding to the promoters of genes required for stemness such as c-MYC and TWIST1 ^[72][84].

The role of STAT5 in promoting cancer stemness does not affect many cell types and is mostly confined to hematopoietic cancers ^[73][85]. However, Nevalainen and colleagues reported that STAT5B induces stem cell properties in prostate cancer cells ^[74][86] in line with the increase in nuclear STAT5A/B observed in these tumors in correlation with bad prognosis ^[8][9]. Furthermore, transgenic mice with increased expression of prolactin in prostate epithelial cells displayed increases in the basal/stem cell compartment in association with activation of STAT5. This enrichment of stem cells was partially reversed by depletion of Stat5a/b ^[75][87]. The pro-stem cell oncogenic effect of STAT5 in the prostate contrasts with its effects in the mammary gland where STAT5 induces cell differentiation ^[76][88]. The ETS transcription factor Elf5 (E74-like factor 5) is a target of the prolactin-STAT5 axis and promotes mammary cell differentiation ^[77][78][79][89,90,91], supporting the tumor suppressive role of STAT5 in the mammary gland.