Glioblastoma (GBM; grade IV glioma, WHO) is a primary brain tumor that mainly occurs in adults. GBM-patients have a very dismal prognosis with a median survival of ~15 months despite aggressive radiochemotherapy after maximally possible surgery [1]. Since GBMs are characterized by their diffuse infiltrative growth, a complete surgical resection is nearly impossible and the tumors quickly recur, often with an even more aggressive growth [2]. Different molecular subtypes of GBM have been described based on the genetic profiles of the tumors. Although their nomenclature is not uniformly used, the general consensus agrees on three GBM subtypes: proneural, classical and mesenchymal ^[3][4][3,4], of which the latter one is the most aggressive. Signal Transducer and Activator of Transcription (STAT3) is a molecular hub-like oncoprotein that is highly overactivated in GBM and is associated with the mesenchymal subtype ^[5][6][5,6]. STAT3 is known to regulate multiple hallmarks of cancer such as proliferation, cell survival, angiogenesis and immune evasion (reviewed in ^[7][8][7,8]). The activation of STAT3 is mediated by multiple upstream kinases [9] which phosphorylate the protein at two different sites (reviewed in [10], Tyrosine705 (pY705Stat3) and Serine727 (pS727Stat3). Upon activation, STAT3 forms homodimers or heterodimers with other proteins of the STAT-family, translocates into the nucleus and induces pronounced changes in gene expression. Known targets include genes responsible for various hallmarks of cancer like migration/invasion (MMP2, MMP9; ^[11][12][11,12]) or EMT-like features and immune evasion/suppression (e.g., SNAI1; ^[13][14][13,14]). Targeting STAT3 by upstream signaling inhibitors of the STAT3 pathway or by genetic depletion decreases glioma cell proliferation and migration in vitro and prolongs overall survival of tumor-bearing mice in vivo ^{[15][16][17][18][19][20][21]}[15,16,17,18,19,20,21].

An underappreciated function of STAT3 is its involvement in autophagy and lysosomal function. Recent studies could already show that STAT3 can transcriptionally regulate several genes relevant for autophagy ^{[22][23][24][25][26][27]}[22,23,24,25,26,27]. In addition, unphosphorylated, cytoplasmic STAT3 can have direct, transcription-independent effects on the autophagy pathway [28]. Furthermore, upon translocation to the mitochondria STAT3 was found to decrease mitochondrial reactive oxygen species (ROS) that in turn may trigger autophagy ^{[22][29][30][31][32]}[22,29,30,31,32]. Notably, STAT3 has also been reported to be important for regulating lysosomal membrane permeabilization (LMP) and lysosome-dependent cell death (LDCD) during post-lactational regression of the mammary gland epithelium ^[33][34][33,34] and in breast cancer cells [35]. Curiously, under these circumstances STAT3 rather promotes cell death, a function that is in stark contrast to its canonical anti-apoptotic pro-survival role. The lysosomes have been fittingly described as the “suicide bags” of the cells, because of their low pH and content of acidic hydrolases which are able to degrade macromolecules [36]. Damage to the lysosomal membrane, for example through cationic amphiphilic drugs ^[37][38][37,38], can trigger lysosomal membrane permeabilization (LMP), causing lysosomal rupture and release of its content into the cytosol [39]. The concomitant release of lysosomal cathepsin proteases and hydrolytic enzymes can lead to a specific type of cell death termed lysosome-dependent cell death (LDCD), that depending on the extent of LMP can have necrotic, apoptotic or apoptosis-like features [39]. LDCD has been observed in several pathophysiological conditions such as inflammation, aging, neurodegeneration and cardiovascular disorders ^{[37][39][40][41]}[37,39,40,41]. It has been reported that cancer cells hijack lysosomes to remodel the extracellular matrix in order to increase tissue invasion and metastasis, while at the same time destabilizing them [42]. Accordingly, an increased expression, activity and secretion of cathepsins, as well as changes in lysosomal volume, composition and cellular distribution have been observed in different tumors [43]. Based on the findings suggesting both an increased lysosomal activity and concomitant lysosomal destabilization in certain tumors, these organelles might represent a double-edged sword and render cancer cells more prone to lysosome-targeting agents and LDCD via excessive activation of LMP [42].

2. Research Findings

The oncogenic transcription factor STAT3 acts as a signaling hub molecule involved in regulation of most, if not all hallmarks of cancer, including proliferation, tumor invasion, altered cellular metabolism, angiogenesis, immune evasion and cell survival. Overactivation of STAT3 is frequently observed in malignant gliomas and correlated to the mesenchymal subtype of GBM, a particularly aggressive and treatment-resistant molecular subgroup of these tumors ^[9][44][45][9,65,66]. Therefore, STAT3 represents a very interesting target for GBM therapy. However, STAT3 and other oncogenic transcription factors are a unique class of targets that are very difficult to drug. Therefore, many of the current efforts are focusing on the pharmacological inhibition of upstream JAK kinases in the JAK/Stat pathway, including a phase I clinical trial with the JAK inhibitor WP1066 (NCT01904123), although it should be noted that non-receptor tyrosine kinases such as bone marrow and X-linked (BMX) and members of the Src family can bypass JAK-dependent activation of STAT3 in GBM ^[9][46][9,67]. An alternative approach to target STAT3-driven GBMs is to identify and exploit particular vulnerabilities of these tumors. Extensive and prolonged overactivation of autophagy and the concomitant induction of ACD represents a particular sensitivity of GBM cells and an interesting new therapeutic approach ^{[47][48][49][50][51]}[48,57,68,69,70]. While STAT3 has been found to be important for the (positive and negative) regulation of autophagy [32], the exact role of STAT3 in autophagy is highly dependent on the subcellular localization of STAT3 and the cellular context [32]. Mechanistically, this death-sensitizing role of STAT3 appears to be independent from alterations in the early steps of the autophagy pathway, because STAT3-deficient and proficient cell lines displayed a very similar extent of LC-3-conversion and an unchanged autophagic flux in theour experiments. These findings indicate that STAT3 may rather interfere at the later steps of the autophagosomal/lysosomal pathway, in particular at the lysosomal level. Consistent with this hypothesis, an LMP-promoting role of STAT3 has been documented in several previous studies, for example during mammary gland involution ^[33][34][33,34] as well as in breast cancer cells treated with a derivative of the natural molecule riccardin D [35]. As already outlined above, IM, TIC and Pimo are FIASMAs that accumulate within the lysosomes and induce the detachment and inactivation of acid sphingomyelinase from the lysosomal membrane ^[52][71], thereby enhancing deregulation of lipid homeostasis. This event has in turn been correlated to lysosomal stress and LMP ^[53][54][55][72,73,74].  In addition to enhanced lysosomal activation, the demand for cholesterol may be a second vulnerability of (STAT3-driven) tumors, in particular GBMs that are incapable of de novo cholesterol synthesis and rely on exogenous cholesterol ^[56][57][58][77,78,79]. Hence, the combined targeting of cholesterol trafficking and lysosomal function, as demonstrated to occur with the drugs used in this entry [38], appears to be an interesting strategy for the treatment of GBM.

3. Conclusions

Here, the entry presents a novel pro-death function of the oncoprotein STAT3, which is frequently overactivated in multiple human cancers including glioblastoma and is associated with the most aggressive mesenchymal subtype. Using isogenic Crispr/Cas9-Knockouts and stable shRNA-mediated STAT3-KD cells the loss of STAT3 leads to reduced sensitivity to known ACD-inducers was shown. It is shown further that this is accompanied by reduced lysosomal lipid accumulation in STAT3-deficient cells and therefore less leakage of lysosomal contents in the cytosol, which causes LDCD in STAT3-proficient cells. In conclusion, these findings offer new research directions to develop targeted therapies of STAT3-activated, autophagy-proficient tumors such as glioblastoma.