1 Introduction

Hepatocellular carcinoma (HCC) is the most common type of liver cancer, accounting for approximately 90% of cases, and is often accompanied by hepatitis B virus (HBV) infection or non-alcoholic steatohepatitis (NASH) as leading risk factors [1, 2]. Malignant transformation to HCC is associated with intense metabolic reprogramming, particularly related to lipid uptake, biosynthesis, (subcellular) transport, distribution, degradation, and signaling [3]. Compared to non-tumorous liver cells, which largely acquire fatty acids from extracellular sources, HCC cells develop a remarkable ability to synthesize lipids de novo while upregulating fatty acid uptake [4, 28,29,30,31], thereby revealing potential targets for anti-cancer therapy [32, 33]. This multifaceted landscape is tightly controlled by a multitude of enzymes and other factors responsible for lipid biosynthesis, lipid catabolism, and energy balance, which are subject to rigorous transcriptional and post-transcriptional regulation [11].

The transcription factor SREBP-1 serves as a central driver of lipogenesis, regulating the expression of multiple lipogenic enzymes involved in the biosynthesis of cholesterol, fatty acids, and triglycerides. It acts as a hub for myriad physiological and pathophysiological cellular processes, functioning in both transcriptional and post-transcriptional regulation [16, 34]. SREBP-1 promotes cell growth and viability, and loss of SREBP-1 signaling leads to severe lipotoxicity in glioma cells [26]. This lipotoxicity can be mitigated by the addition of monounsaturated fatty acids (MUFAs) and is considered to result from an imbalance between saturated fatty acids and MUFAs due to disturbed desaturation by SCD1 [26]. SREBP-1 is also involved in the regulation of ER stress and cell death. In osteosarcoma cells, the overexpression of SREBP-1 inhibits cell proliferation, upregulates the expression and phosphorylation of protein kinase RNA-like endoplasmic reticulum kinase (PERK), and amplifies the PERK-activated unfolded protein response (UPR), leading to ER stress-induced apoptosis and autophagy [35]. Silencing of SREBP-1 attenuates this stress response [35], whereas SREBP-1 upregulation by high glucose promotes cell growth and inhibits apoptosis and autophagy in pancreatic cancer cells [36]. Furthermore, SREBP-1 decreases ferroptosis sensitivity by inducing the SCD1-MUFA axis and modulates inflammatory reactions, such as those induced by obesity and toll-like receptor 4 (TLR4) [37,247]. The authors propose that both effects contribute to the induction of ferroptosis in HCC cells. Supplementation with SCD1 products, i.e., the MUFAs oleic acid or palmitoleic acid, actually compensates for the cytotoxicity induced by HCAR1/MCT1 suppression [37]. Note that lactate increases the levels of HCAR1, which is upregulated in tumors of HCC patients compared to adjacent tissues [37]. Together, lipid metabolic adaptations through the AMPK-SREBP-1c system essentially regulate ferroptosis sensitivity in HCC, as recently suggested for primary mouse embryonic fibroblasts and several cancer cell lines [37, 240, 248].

Another mechanism by which AMPK enhances ferroptosis sensitivity of HCC is related to branched-chain amino acid aminotransferase 2 (BCAT2), which was identified as a ferroptosis suppressor in a kinome CRISPR/Cas9-based screen in HepG2 cells [98]. BCAT2 catalyzes the reversible transamination of branched-chain amino acids and α-ketoglutarate, converting them to the corresponding α-keto acids and glutamate [98]. Note that the availability of glutamate is essential to keep ferroptosis at bay [238]. Accordingly, HCC cells are protected from lipid peroxidation when cellular glutamate levels and glutamate release are maintained [98]. Feedback loops seem to exist: the induction of ferroptosis in HepG2 cells has been proposed to trigger ferritinophagy and increase ROS levels, thereby activating AMPK, inhibiting SREBP-1, and downregulating BCAT2 [98].

There are also emerging links between AMPK and the ubiquitin-conjugating system, which interestingly has recently been implicated in the regulation of ferroptosis [249, 250]. In HCC cells, ubiquitin-conjugating enzyme E2 O (UBE2O) has been shown to confer malignant features, such as cell growth, migration, and invasion, by decreasing AMPKα2 stability and promoting the mTORC1 pathway [249]. UBE2O is overexpressed in HCC compared to adjacent normal tissues, and high UBE2O levels correlate with worse clinical outcomes in HCC patients [249]. Future studies are required to elucidate whether the UBE2O-dependent degradation of AMPKα2 along with mTORC1 activation effectively activates SREBP-1 in HCC and whether such regulation is functional in terms of ferroptosis sensitization and tumorigenesis.

Given the important role of inflammation and immune surveillance in HCC initiation and progression [230, 251,252,253], it is also noteworthy that HCC upregulates the expression of pro-inflammatory cytokines [254, 255]. TNFα and related cytokines promote tumor growth under certain conditions by creating a pro-inflammatory tumor microenvironment and/or initiating pro-survival signaling cascades [256, 257] and may also promote lipogenesis by interfering with AMPK activation (and other pathways) and relieving the repression of the mTORC1-SREBP-1c pathway, as shown for HCC cells [258, 259]. SREBP-1 itself has a dual role in inflammation in the tumor microenvironment, acting either to promote inflammation [301].

The functional link between STAT signaling and SREBP-1 is still emerging. Of relevance to HCC, mTORC1 interacts with and phosphorylates STAT5, which then binds to the SREBP-1 promoter and stimulates SREBP-1 transcription [8]. Accordingly, liver-specific activation of mTORC1 increases lipogenic gene expression and spontaneous tumorigenesis in mouse liver [8], and high levels of SREBP-1 and phospho-STAT5 are associated with poor survival in HCC patients [8]. Furthermore, liver-specific STAT5 knockout in mice induces steatosis by upregulating SREBP-1 and PPARγ signaling and simultaneously activating the c-Jun N-terminal kinase 1 (JNK1)-STAT3 pathway [87]. When liver-specific STAT5 deficiency is combined with glucocorticoid receptor deletion, fatty liver develops into HCC [87]. This malignant transformation is ascribed to lipid mobilization from adipose tissue and aggravated hepatic lipid accumulation along with insulin resistance and increased expression of the pro-inflammatory cytokine TNF-α [87]. Another study found that TNF-α and insulin upregulate proprotein convertase subtilisin/kexin type 9 (PCSK9) in HepG2 cells dependent on suppressor of cytokine signaling 3 (SOCS3)-JAK-STAT3 [92]. Overexpression of SOCS3 induces Akt phosphorylation and increases the expression of SREBP-1c, lipogenic genes, and ApoB, without affecting cholesterol biosynthesis [92]. Whether PCSK9 contributes to the regulation of SREBP-1 in addition to its role in insulin resistance has not been investigated. ROS-scavenging and inhibitor studies in high glucose–challenged HepG2 cells suggest that PCSK9 expression is induced by ROS-dependent enrichment of nuclear SREBP-1 (but not SREBP-2) [302]. Indeed, oxidative stress activates SREBP-1c and induces lipid accumulation in HepG2 cells [303], and an SRE binding site exists on the PCSK9 promoter [81]. The authors further propose that PCSK9 elevates extracellular levels of LDL cholesterol (LDLc), which protects against sorafenib-induced HCC cell death [302].

Notably, the hepatoprotective flavonolignan silibinin from Silybum marianum inhibits STAT3 activation and suppresses SREBP-1-mediated lipid accumulation in endometrial carcinoma cells and tumors [304, 305]. Although a functional link between STAT3 and SREBP-1 signaling has not been confirmed for the anti-tumoral effect of silibinin, we consider such a mechanism likely to contribute to the proposed anti-lipogenic and HCC-preventive activities of the natural extracts.

13 MYC

The proto-oncogenic transcription factor Myc is induced in response to mitogenic stimuli; regulates genes involved in cell metabolism, growth, and proliferation [306, 307], among them SREBF1; and is frequently overexpressed in cancer, including HCC [306, 307]. Direct binding of Myc to two recognition sites on the SREBP-1 promoter has been confirmed by chromatin immunoprecipitation [247]. ACSL4 is an oncogenic marker of the α-fetoprotein-high subtype of HCC and promotes HCC tumor formation and metastasis in Huh-7-grafted mice [308]. The tumor-promoting activity is partially dependent on SREBP-1, as demonstrated by SREBP-1 overexpression in ACSL4-silenced tumors. ACSL4 enhances SREBP-1 expression by stabilizing c-Myc [311], which binds directly to the SREBP-1 promoter region and activates transcription [308]. In consequence, the expression of the SREBP-1 target genes is induced and triglycerides and cholesterol accumulate. Mechanistically, ACSL4 attenuates proteasomal degradation of c-Myc in an ERK- and FBW7-dependent manner [311]. How ACSL4 modulates the ERK-FBW7-c-Myc axis is not readily understood, though the mechanism likely involves changes in the availability of PUFAs or PUFA-derived metabolites.

Complementary mechanistic insights come from the genetic manipulation of lysophosphatidylcholine acyltransferase 3 (LPCAT3) in primary mouse hepatocytes and liver [347]. The LXR target gene LPCAT3 uses PUFA-CoA (derived from ACSL4) as substrate and incorporates the acyl-chain into phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) [348, 349]. Accordingly, LPCAT3 deletion decreases the proportion of PUFA-containing PC in the ER, reduces SREBP-1 processing to the mature nuclear form, and thereby suppresses lipogenic responses [349]. (i) Feeding, (ii) delivery of exogenous PUFA-containing PC to the ER, and (iii) LXR activation have opposite effects [349]. Dynamic changes in the PUFA composition of the ER membrane influence either the SCAP/SREBP-1 interaction or the transport of SREBP-1 to the Golgi [348]. The detailed mechanisms remain elusive and might involve phospholipid classes other than PC. In support of this hypothesis, PE with saturated fatty acids impairs the processing of the Drosophila SREBP ortholog in S2 cells [350], and inhibition of de novo phospholipid biosynthesis leads to a mislocalization of S1P and S1P to the ER (instead of the Golgi), allowing efficient processing of SREBP-1 [351]. Interestingly, supplementation of PUFA-containing PC and depletion of LPCAT3 have also been reported to decrease Akt phosphorylation by reducing the kinase’s affinity to phosphoinositides at membranes [352, 353]. Whether the associated decrease in long-term Akt signaling affects SREBP-1 expression has not been addressed.

19 Other SREBP-1 regulatory factors and mechanisms

In the following, we summarize diverse mechanisms that have been shown to regulate SREBP-1 in HCC and discuss their (pre-)clinical implications.

The multidomain adaptor protein β2-spectrin (SPTBN1), which is involved in TGFβ-SMAD3 (mothers against decapentaplegic homolog 3) signaling, among others, increases SREBP-1 activity, lipogenesis, and HCC progression in mice fed a high-fat diet or a Western diet [95]. The authors propose that caspase 3 cleaves SPTBN1 and SREBP-1 and that the N-terminal product of SPTBN1 (N-SPTBN1) interacts with cleaved SREBP-1 to stabilize the nuclear form of SREBP-1, thereby inducing the expression of target genes. Accordingly, liver-specific deletion of SPTBN1 (which is overexpressed in NASH along with caspase 3) protects mice against hepatic steatosis, fibrosis, inflammation, tissue damage, and HCC [95].

MicroRNA-27a (miR-27a) modulates cancer cell proliferation, apoptosis, migration, and invasion as well as angiogenesis and therapy resistance and has both oncogenic and tumor suppressor functions [354,355,356,357,358]. miR-27a is also preferentially expressed in HCV-infected liver and has in this context been found to repress SREBP-1 expression along with other major lipogenic regulators and enzymes, including RXRα, PPARα, PPARγ, and FASN in human HUH-7.5 hepatocellular carcinoma cells [359]. Repression of miR-27a increases cellular lipid accumulation and HCC infectivity, whereas overexpression has the opposite effect.

The related RNA binding proteins Lin28A and Lin28B (Lin28A/B) are upregulated in HCC and other cancers [360]. They initiate the post-transcriptional repression of the let-7 microRNA family, but also bind various mRNAs with roles in cell metabolism, cell cycle progression, and survival [361,362,363,364,365]. Among these mRNAs are those of SREBP-1 and SCAP, which are positively regulated by Lin28A/B in HCC cell lines [43]. By interacting with SREBP-1 and SCAP mRNA, the proto-oncogenes Lin28A/B enhance the translation and processing of SREBP-1 and stimulate tumor growth in mouse xenografts of human PLC hepatocellular carcinoma cells [43]. Mechanistically, the authors (i) demonstrate that SREBP-1, the SREBP-1 target gene SCD1, and ER stress (or the associated UPR) are involved in HCC progression; (ii) point to the imbalance of fatty acid unsaturation upon Lin28A/B silencing; and (iii) propose that Lin28A/B protects HCC cells from ER lipotoxicity [43]. Whether the recently discovered SCD1-derived lipokine PI(18:1/18:1) is enriched upon SCD1 upregulation and contributes to the tumor-protective activity of Lin28A/B has not been addressed [366].

HDGF has a highly conserved N-terminal PWWP domain and exerts oncogenic activity (related to transformation, survival, metastasis, and angiogenesis) through incompletely understood mechanisms [367,368,369,370,371]. Nuclear-localized HDGF was recently shown to bind to and act as a co-activator of SREBP-1 and to enhance lipogenic gene expression in HepG2 cells by attenuating the recruitment of the transcription repressor C-terminal binding protein 1 (CTBP1) [89]. In support of a functional role of (nuclear) HDGF in regulating SREBP-1 levels, the combined expression of the two factors indicates a poor prognosis in HCC [89]. Sequence variability exists in PWWP, and the A-type variant (in contrast to wildtype HDGF) recruits CTBP1, suppresses lipid biosynthesis, and decreases proliferation of HepG2 cells, both in vitro and in murine xenografts [89].

The transcription factor p53, a major tumor suppressor, and ferredoxin reductase (FDXR), which is involved in steroid biosynthesis, negatively regulate the maturation of SREBP-1/2 and thus keep cellular levels of triglycerides and cholesterol in check, as shown for mouse embryonic fibroblasts (MEFs), HepG2 cells, and/or mouse liver [372]. The authors further suggest, based on correlative data, that the availability of the cholesterol efflux pump ABCA1, which is induced by either p53 [373] or FDXR [372], determines SREBP-1/2 activation [372]. Deletion of p53 or FDXR (as well as the double KO) induces hepatic steatosis, inflammation, and spontaneous tumorigenesis in mice, which is, however, not limited to HCC [372]. On the other hand, p53 binds to the SREBP-1 promoter and induces SREBP-1 transcriptional activity in HepG2 cells [46]. NAD(P)H quinone oxidoreductase-1 (NQO1), which is highly expressed in HCC and associated with poor outcome, induces SREBP-1 transcription in HepG2 cells through this mechanism, specifically by preventing ubiquitination and proteasomal degradation of p53 [46]. Notably, the NQO1-p53-SREBP-1-axis stabilizes the EMT transcription factor Snail, thereby inducing lipid anabolism and EMT of HCC cells, which promotes the progression and metastasis of HCC [46].

20 Inhibitors of SREBP-1 signaling with diverse mechanisms

A large number of small molecules and some oligonucleotide-based approaches have been reported to interfere with SREBP-1 signaling in HCC (respectively HCC-promoting liver pathologies) by different, only partially understood mechanisms, as summarized in Table 1 and discussed below for selected compounds.

1-(4-Bromophenyl)-3-(pyridin-3-yl)urea (SI-1), an inhibitor of SREBP-1 activation, decreases the mRNA expression of SREBP-1 target genes in HCC cells more potently than betulin or fatostatin (FASN: IC50 = 0.3, 1.6, and 1.0 μM, respectively), lowers aerobic glycolysis, and potentiates the anti-tumoral efficacy of radiofrequency ablation towards xenograft HCC (at 0.5–5 mg/kg; peroral administration, p.o.) [44].

SREBP decoy oligodeoxynucleotides are short, double-stranded DNA sequences that mimic SREs and compete with them for binding to SREBP without generating a functional response, thereby blocking the expression of SREBP target genes [374]. They have been shown to inhibit the expression of SREBP-1c, ACC1, FASN, SCD1, and HMGCR in hyperlipidemic mice fed a high-fat diet, thereby alleviating the associated inflammation as indicated by the reduction in pro-inflammatory cytokine levels [115].

Scientists at Merck developed siRNA oligonucleotide-lipid nanoparticles (siRNA-LNPs) targeting SCAP and demonstrated that this approach is effective in reducing hepatic SCAP mRNA expression [119]. As a result, hyperlipidemia is attenuated in rhesus monkeys and mice [119, 120], in the latter accompanied by decreased hepatic Ldlr and proprotein convertase subtilisin kexin/type 9 (Pcsk9) expression, repression of Srebp-regulated genes, and inhibition of de novo lipogenesis [119]. The serine kinase PCSK9, which is a therapeutic target for lipid-lowering drugs, is secreted by hepatocytes and subjects LDL receptors to lysosomal degradation [375].

The diarylthiazole fatostatin interacts with SCAP, inhibits its glycosylation, and blocks the transport of SREBP-1 from the ER to the Golgi, thereby suppressing SREBP-1 maturation [117, 376]. Fatostatin attenuates hepatic steatosis in obese mice while reducing body weight and blood glucose levels [117] and exhibits anti-tumoral/growth-retarding [377,378,379] and ferroptosis-inducing activity [379], which has been ascribed to impaired SREBP-1 activation [376]. We expect that fatostatin may also be cytotoxic to HCC cells, although this has not been explicitly demonstrated.

The pentacyclic lupane-type triterpenoid betulin from birch bark has a broad spectrum of pharmacological activities, among others, directed against metabolic disorders and cancer, including HCC [38, 66, 380]. Pleiotropic anti-tumoral mechanisms have been proposed for betulin, including inhibition of SREBP-1/2 maturation, which reduces fatty acid and cholesterol biosynthesis in HCC cells (2.3–100 μM betulin) and DEN-induced HCC in mice (gavage of 100 mg betulin/kg/day) [38, 66, 116]. Mechanistically, betulin physically interacts with SCAP (at 100 μM) and thereby promotes the interaction of SCAP with Insig1/2 to retain SREBPs at the ER [116]. Suppression of SREBP-1/2 and SREBP target genes in experimental murine HCC was accompanied by decreased mRNA expression of pro-inflammatory cytokines, such as TNFα [38]. While the authors confirm a functional link between SREBP processing and pro-inflammatory cytokine expression, the cytokine-lowering mechanism of betulin and its contribution to the anti-HCC activity remain diffuse, especially when considering that betulin may also target the TLR4 and nuclear factor (NF)-κB pathways [39].

Sulforaphane, an isothiocyanate from cruciferous vegetables, and the desaturated analog sulforaphene (30–100 μM, each) inhibit lipogenic enzyme expression in human Huh-7 hepatoma cells by promoting the ubiquitination and proteasomal degradation of pSREBP-1/2 in a SCAP-independent manner [122]. Central to the SREBP-1a-degrading activity of sulforaphane and sulforaphene is the SREPB-1a amino acid sequence from 595 to 784. The direct molecular target of sulforaphane responsible for the induction of SREBP-1/2 degradation remains elusive. Neither does sulforaphane interact directly with SREBPs, nor are known targets of sulforaphane, i.e., Kelch-like ECH-associated protein 1/nuclear factor erythroid 2-related factor 2 (KEAP1/NRF2) and heat shock protein 27 (HSP27), involved in pSREBP degradation [122]. Another study proposed that sulforaphane (1–20 μM; 5–20 mg/kg/day, p.o.) reduces hepatic lipogenic gene expression in rats on a high-fat diet by repressing the ER stress sensor protein kinase-like ER kinase (PERK) and decreasing SREPB-1 expression [123].

The bufadienolide cinobufotalin (0.1–0.4 μM) from the skin secretion of the giant toad inhibits both SREBP-1 expression and the binding of SREBP-1 to SREs in HepG2 cells, seemingly by interacting directly with the transcription factor, which together markedly represses the expression of lipogenic enzymes [61]. Cinobufotalin (2.5–5 mg/kg, i.p.) is effective in vivo and reduces lipogenesis and HCC tumor growth in grafted mice, the latter likely by inducing G2/M cell cycle arrest and apoptosis [61]. A meta-analysis of 27 clinical trials involving 2079 advanced HCC patients indicates that the combination of hepatic arterial chemoembolization (TACE) with adjuvant cinobufotalin injection is safe and more effective than TACE alone for the treatment of end-stage HCC [381].

Diverse small molecules and complex mixtures have been shown to decrease SREBP-1 expression or maturation in HCC cell lines, though the mode of action has often remained elusive (Table 1). These include pyridine co-ligand functionalized Pt(II) complexes [190]; the retinoic acid receptor β2 agonists AC261066 and AC55649 [176]; GPR40 agonists, such as GW9508, AMG-1638, and docosahexaenoic acid [179, 180]; and ethanolic extracts of several herbs, such as Zhiheshouwu (Polygoni multiflori Radix Praeparata) and Liriope platyphylla root [382, 383].

21 Conclusion and perspective

Malignant transformation to HCC induces SREBP-1 signaling through a broad spectrum of regulatory mechanisms that enhance SREBP-1 expression, maturation, protein stability, and nuclear activity. This metabolic reprogramming confers advantages in survival, growth, proliferation, and dissemination to HCC, but also renders tumors sensitive to anti-lipogenic treatment. While the mechanistic insights into SREBP-1 regulation and function are rapidly increasing, pharmacological strategies that selectively target SREBP-1 signaling are still in their infancy. The main reasons for this are the limited availability of high-throughput screening assays for SREBP-1-interacting small molecules, the lack of obvious ligand binding pockets, and incomplete structural information. Crystal structures of SREBP-1 that could aid in the rational design of respective ligands have been solved, but are limited to the yeast ortholog and the N-terminal bHLH-Zip domain of the human transcription factor [384,385,386].

The vast majority of agents targeting SREBP-1 signaling are non-selective, either because of polypharmacological activities or because the upstream targets regulate multiple signaling pathways in addition to SREBP-1 activation/induction. Furthermore, many pathways, including kinase cascades (e.g., PI3K-Akt, mTORC1, AMPK) and transcription factors/co-activators (e.g., LXR), indirectly regulate SREBP-1 signaling. Because these pathways regulate multiple other cellular processes besides SREBP-1 that are involved in tumorigenesis, it is difficult to assess from genetic manipulation studies the extent to which the interference with SREBP-1 contributes to HCC suppression. Thus, many of the agents listed in Table 1 have been evaluated in pre-clinical studies and some in clinical trials, with the effective doses (in mg/kg body weight) used in (pre-)clinical studies reported whenever available. For example, the pan-kinase inhibitor sorafenib is a first-line treatment for HCC [387, 388], and the red wine stilbene resveratrol has been the subject of numerous clinical and epidemiological studies and meta-analyses (including a phase I trial in patients with liver metastases) [389]. AMPK activators have also been intensively studied in recent years for the treatment of metabolic diseases, including cancer, and several compounds have entered clinical evaluation [353]. While these compounds have been reported to interfere with SREBP-1 signaling and it is likely that this effect contributes to their overall efficacy, clinical and pre-clinical data do not allow conclusions as to whether the interference with SREBP-1 signaling mediates the observed beneficial and adverse effects.

Results from pharmacological approaches are even more difficult to interpret when considering that the vast majority of SREBP-1 modulators have been shown to have off-target effects; many other compounds have not been adequately studied. Available selective approaches to inhibit SREBP-1 signaling are largely limited to (i) the interaction of SREBP-1 and the upstream kinase PKM2 via peptide ligands (P8) [181], (ii) an apparent direct interaction of cinobufotalin with SREBP-1 (although little is known about potential other targets for this traditional Chinese medicine) [61], and (iii) oligonucleotide-based approaches via siRNA or decoy oligonucleotides [115, 119, 120]. Biopharmaceuticals such as therapeutic antibodies or SREBP-1-interacting proteins, which are expected to provide superior SREBP-1 selectivity, have not been explored so far. Of these more selective approaches, only cinobufotalin has been tested and shown to be effective in an in vivo HCC model [61] and clinical trials [381], while oligonucleotide approaches have been studied in hyperlipidemic animals (where they reduced lipogenesis and hepatic LDL uptake) [119, 120], and knowledge of P8 is limited to cell-based studies [181]. Such strategies may have the potential to achieve selectivity for SREBP-1 targeting. Other promising strategies to narrow down putative side effects (which we have not or only partially addressed here) include (i) selective inhibition of SREBP-1 target genes, such as ACC, FASN, SCD1, and ACLY [32, 390,391,392]; (ii) interference with defined, context-specific SREBP-1 regulators, e.g., PKD3 or CRTC2 [128, 198, 204]; (iii) functionalized nanoparticles for HCC targeting [393,394,395]; and (iv) tumor-specific gene therapy [223, 396].

Based on the above, the (still incompletely pharmacologically characterized) cinobufotalin seems to be currently at the forefront of agents that target SREBP-1 signaling with some selectivity. Nanoparticle-based approaches that deliver SREBP-1-interacting peptides or siRNA to target SREBP-1, SCAP, or other signaling molecules, as well as (not yet explored) therapeutic antibodies directed against these factors, hold great promise for the future. On the other hand, it is questionable whether selectivity is actually desirable to achieve efficacy against a complex disease such as HCC. Therefore, polypharmacological approaches based on the rational inhibition of SREBP-1, as already realized in several drugs and drug candidates, may pave the way to an improved clinical efficacy of rationally designed anti-cancer drugs, particularly in the treatment of HCC.

In addition, interesting new links between SREBP-1 and resistance to therapy (including chemo- and radioresistance) are emerging, driven in part by multiomics approaches [397,398,399,400], but the number of such studies exploring the role of SREBP-1 in therapy-resistant HCC is still very limited [27, 37, 44, 66, 71,72,73,74], and further research in this area is urgently needed. In this context, the bivalent role of SREBP-1 in conveying resistance in cancer (HCC) should be mentioned. High SREBP-1 levels increase HCC aggressiveness and resistance to classical chemotherapeutics, while sensitizing tumors to anti-lipogenic strategies and alternative forms of programmed cell death, such as ferroptosis [27, 42, 74, 98].

In summary, selective SREBP-1 inhibitors are in high demand to investigate the pleiotropic, context-dependent functions of SREBP-1. Current anti-HCC strategies are instead dominated by multitarget small molecules that exhibit (subordinate) SREBP-1 inhibition, broadly interfere with lipogenesis, and may target additional cancer-promoting pathways. Whether they are inferior or superior towards monopharmacological approaches in terms of efficacy and safety remains to be determined in future studies.