Targeting carbonic anhydrase IX improves the anti-cancer efficacy of mTOR inhibitors

Mitigating the resistance of MCF-7 cancer cells to Doxorubicin under hypoxic conditions with novel coumarin based carbonic anhydrase IX and XII inhibitors

In the present study, the design and synthesis of novel coumarin derivatives 8a-h, 11a-d and 16a-c as potential selective inhibitors for the tumor associated human carbonic anhydrase isoforms (hCA IX and XII) was reported. All the newly synthesized derivatives showed potent to mild activity against the targeted CA IX (KI = 0.08-9.57 µM), with selectivity indices over CA I (SI = 2.0-21.9) and over CA II (SI = 1.1-15.7). They showed similar activities against CA XII (KI = 0.06-9.48 µM) with selectivity indices over CA I (SI = 1.4-21.2) and CA II (SI = 0.9-15.5). Compound 16b featuring sulfonamide function possessed promising inhibitory activities against the targeted isoforms CA IX and XII with KI values of 0.08 and 0.06 µM, respectively. Interestingly, it was found that using compound 16b at a nontoxic concentration as an adjuvant with Doxorubicin against MCF-7 cells enhanced the cytotoxicity under hypoxia by almost 3.5 folds; IC50 decreased from 25.74 to 7.43 µM. Therefore, compound 16b restored the cytotoxicity of Doxorubicin against MCF-7 cells under hypoxia, almost as normoxia. Furthermore, flow cytometry analysis of a combination treatment of compound 16b and Doxorubicin to the MCF7 cell line revealed an increase in cell cycle arrest at the G2/M phase and a more efficient apoptotic effect than Doxorubicin alone. Furthermore, compound 16b showed no cytotoxicity against normal breast MCF-10A cell line (IC50 = 296.25 µM).

Carbonic anhydrase IX: An atypical target for innovative therapies in cancer

Carbonic anhydrases (CAs), are metallo-enzymes implicated in several pathophysiological processes where tissue pH regulation is required. CA IX is a tumor-associated CA isoform induced by hypoxia and involved in the adaptation of tumor cells to acidosis. Indeed, several tumor-driving pathways can induce CA IX expression, and this in turn has been associated to cancer cells invasion and metastatic features as well as to induction of stem-like features, drug resistance and recurrence. After its functional and structural characterization CA IX targeting approaches have been developed to inhibit its activity in neoplastic tissues, and to date this field has seen an incredible acceleration in terms of therapeutic options and biological readouts. Small molecules inhibitors, hybrid/dual targeting drugs, targeting antibodies and adoptive (CAR-T based) cell therapy have been developed at preclinical level, whereas a sulfonamide CA IX inhibitor and an antibody entered Phase Ib/II clinical trials for the treatment and imaging of different solid tumors. Here recent advances on CA IX biology and pharmacology in cancer, and its therapeutic targeting will be discussed.

Quantitative Imaging of Hypoxic CAIX-Positive Tumor Areas with Low Immune Cell Infiltration in Syngeneic Mouse Tumor Models

Limited diffusion of oxygen in combination with increased oxygen consumption leads to chronic hypoxia in most solid malignancies. This scarcity of oxygen is known to induce radioresistance and leads to an immunosuppressive microenvironment. Carbonic anhydrase IX (CAIX) is an enzyme functioning as a catalyzer for acid export in hypoxic cells and is an endogenous biomarker for chronic hypoxia. The aim of this study is to develop a radiolabeled antibody that recognizes murine CAIX to visualize chronic hypoxia in syngeneic tumor models and to study the immune cell population in these hypoxic areas. An anti-mCAIX antibody (MSC3) was conjugated to diethylenetriaminepentaacetic acid (DTPA) and radiolabeled with indium-111 (¹¹¹In). CAIX expression on murine tumor cells was determined using flow cytometry, and in vitro affinity of [¹¹¹In]In-MSC3 was analyzed in a competitive binding assay. Ex vivo biodistribution studies were performed to determine in vivo radiotracer distribution. CAIX⁺ tumor fractions were determined by mCAIX microSPECT/CT, and the tumor microenvironment was analyzed using immunohistochemistry and autoradiography. We showed that [¹¹¹In]In-MSC3 binds to CAIX-expressing (CAIX⁺) murine cells in vitro and accumulates in CAIX⁺ areas in vivo. We optimized the use of [¹¹¹In]In-MSC3 for preclinical imaging such that it can be applied in syngeneic mouse models and showed that we can quantitatively distinguish between tumor models with varying CAIX⁺ fractions by ex vivo analyses and in vivo mCAIX microSPECT/CT. Analysis of the tumor microenvironment identified these CAIX⁺ areas as less infiltrated by immune cells. Together these data demonstrate that mCAIX microSPECT/CT is a sensitive technique to visualize hypoxic CAIX⁺ tumor areas that exhibit reduced infiltration of immune cells in syngeneic mouse models. In the future, this technique may enable visualization of CAIX expression before or during hypoxia-targeted or hypoxia-reducing treatments. Thereby, it will help optimize immuno- and radiotherapy efficacy in translationally relevant syngeneic mouse tumor models.

Carbonic Anhydrase IX Inhibitors as Candidates for Combination Therapy of Solid Tumors

Combination therapy is becoming imperative for the treatment of many cancers, as it provides a higher chance of avoiding drug resistance and tumor recurrence. Among the resistance-conferring factors, the tumor microenvironment plays a major role, and therefore, represents a viable target for adjuvant therapeutic agents. Thus, hypoxia and extracellular acidosis are known to select for the most aggressive and resilient phenotypes and build poorly responsive regions of the tumor mass. Carbonic anhydrase (CA, EC 4.2.1.1) IX isoform is a surficial zinc metalloenzyme that is proven to play a central role in regulating intra and extracellular pH, as well as modulating invasion and metastasis processes. With its strong association and distribution in various tumor tissues and well-known druggability, this protein holds great promise as a target to pharmacologically interfere with the tumor microenvironment by using drug combination regimens. In the present review, we summarized recent publications revealing the potential of CA IX inhibitors to intensify cancer chemotherapy and overcome drug resistance in preclinical settings.

High in vitro and in vivo synergistic activity between mTORC1 and PLK1 inhibition in adenocarcinoma NSCLC

Significant rational is available for specific targeting of PI3K/AKT/mTOR pathway in the treatment of non-small cell lung cancer (NSCLC). However, almost all clinical trials that have evaluated Pi3K pathway-based monotherapies/combinations did not observe an improvement of patient’s outcome. The aim of our study was therefore to define combination of treatment based on the determination of predictive markers of resistance to the mTORC1 inhibitor RAD001/Everolimus. An in vivo study showed high efficacy of RAD001 in NSCLC Patient-Derived Xenografts (PDXs). When looking at biomarkers of resistance by RT-PCR study, three genes were found to be highly expressed in resistant tumors, i.e., PLK1, CXCR4, and AXL. We have then focused our study on the combination of RAD001 + Volasertib, a PLK1 inhibitor, and observed a high antitumor activity of the combination in comparison to each monotherapy; similarly, a clear synergistic effect between the two compounds was found in an in vitro study. Pharmacodynamics study demonstrated that this synergy was due to (1) tumor vascularization decrease, increase of the HIF1 protein expression and decrease of the intracellular pH, and (2) decrease of the Carbonic Anhydrase 9 (CAIX) protein that could not correct intracellular acidosis. In conclusion, all these preclinical data strongly suggest that the inhibition of mTORC1 and PLK1 proteins may be a promising therapeutic approach for NSCLC patients.

The Carbonic Anhydrase IX inhibitor SLC-0111 as emerging agent against the mesenchymal stem cell-derived pro-survival effects on melanoma cells

Mesenchymal stem cells (MSC) take part to solid tumour-associated stroma and critically influence progression of malignancy. Our study represents a striking example of melanoma progression to a more malignant and resistant phenotype promoted by MSC and the possibility to contrast this diabolic liaison using CAIX inhibitors. In particular, we demonstrated that melanoma cells exposed to a MSC-conditioned medium switch to a more malignant phenotype, characterised by resistance to programmed cell death and endowed with an epithelial-to-mesenchymal transition and stem cell characteristics. These effects were reversed abrogating MSC CAIX activity using SLC-0111, a CAIX inhibitor. Moreover, the acquisition by melanoma cells of a Vemurafenib-resistant phenotype upon MSC-conditioned medium exposure was removed when MSC were treated with SLC-0111. Therefore, MSC may profoundly reprogramme melanoma cells towards a wide resistant phenotype through CAIX involvement, as the use of SLC-0111 is able to contrast the development of this highly risky adaptation for disease progression.

Clinical and Pre-Clinical Evidence of Carbonic Anhydrase IX in Pancreatic Cancer and Its High Expression in Pre-Cancerous Lesions

Hypoxia is a common phenomenon that occurs in most solid tumors. Regardless of tumor origin, the evolution of a hypoxia-adapted phenotype is critical for invasive cancer development. Pancreatic ductal adenocarcinoma is also characterized by hypoxia, desmoplasia, and the presence of necrosis, predicting poor outcome. Carbonic anhydrase IX (CAIX) is one of the most strict hypoxia regulated genes which plays a key role in the adaptation of cancer cells to hypoxia and acidosis. Here, we summarize clinical data showing that CAIX expression is associated with tumor necrosis, vascularization, expression of Frizzled-1, mucins, or proteins involved in glycolysis, and inevitably, poor prognosis of pancreatic cancer patients. We also describe the transcriptional regulation of CAIX in relation to signaling pathways activated in pancreatic cancers. A large part deals with the preclinical evidence supporting the relevance of CAIX in processes leading to the aggressive behavior of pancreatic tumors. Furthermore, we focus on CAIX occurrence in pre-cancerous lesions, and for the first time, we describe CAIX expression within intraductal papillary mucinous neoplasia. Our review concludes with a detailed account of clinical trials implicating that treatment consisting of conventionally used therapies combined with CAIX targeting could result in an improved anti-cancer response in pancreatic cancer patients.

CAIX-Mediated Control of LIN28/let-7 Axis Contributes to Metabolic Adaptation of Breast Cancer Cells to Hypoxia

Solid tumors, including breast cancer, are characterized by the hypoxic microenvironment, extracellular acidosis, and chemoresistance. Hypoxia marker, carbonic anhydrase IX (CAIX), is a pH regulator providing a selective survival advantage to cancer cells through intracellular neutralization while facilitating tumor invasion by extracellular acidification. The expression of CAIX in breast cancer patients is associated with poor prognosis and metastases. Importantly, CAIX-positive hypoxic tumor regions are enriched in cancer stem cells (CSCs). Here we investigated the biological effects of CA9-silencing in breast cancer cell lines. We found that CAIX-downregulation in hypoxia led to increased levels of let-7 (lethal-7) family members. Simultaneously with the increase of let-7 miRNAs in CAIX-suppressed cells, LIN28 protein levels decreased, along with downstream metabolic pathways: pyruvate dehydrogenase kinase 1 (PDK1) and phosphorylation of its substrate, pyruvate dehydrogenase (PDH) at Ser-232, causing attenuation of glycolysis. In addition to perturbed glycolysis, CAIX-knockouts, in correlation with decreased LIN28 (as CSC reprogramming factor), also exhibit reduction of the further CSC-associated markers NANOG (Homeobox protein NANOG) and ALDH1 (Aldehyde dehydrogenase isoform 1). Oppositely, overexpression of CAIX leads to the enhancement of LIN28, ALDH1, and NANOG. In conclusion, CAIX-driven regulation of the LIN28/let-7 axis augments glycolytic metabolism and enhances stem cell markers expression during CAIX-mediated adaptation to hypoxia and acidosis in carcinogenesis.

The Interplay of Dysregulated pH and Electrolyte Imbalance in Cancer

Cancer cells and tissues have an aberrant regulation of hydrogen ion dynamics driven by a combination of poor vascular perfusion, regional hypoxia, and increased the flux of carbons through fermentative glycolysis. This leads to extracellular acidosis and intracellular alkalinization. Dysregulated pH dynamics influence cancer cell biology, from cell transformation and tumorigenesis to proliferation, local growth, invasion, and metastasis. Moreover, this dysregulated intracellular pH (pHi) drives a metabolic shift to increased aerobic glycolysis and reduced mitochondrial oxidative phosphorylation, referred to as the Warburg effect, or Warburg metabolism, which is a selective feature of cancer. This metabolic reprogramming confers a thermodynamic advantage on cancer cells and tissues by protecting them against oxidative stress, enhancing their resistance to hypoxia, and allowing a rapid conversion of nutrients into biomass to enable cell proliferation. Indeed, most cancers have increased glucose uptake and lactic acid production. Furthermore, cancer cells have very dysregulated electrolyte balances, and in the interaction of the pH dynamics with electrolyte, dynamics is less well known. In this review, we highlight the interconnected roles of dysregulated pH dynamics and electrolytes imbalance in cancer initiation, progression, adaptation, and in determining the programming and reprogramming of tumor cell metabolism.

Assessment of the antiproliferative and apoptotic roles of sulfonamide carbonic anhydrase IX inhibitors in HeLa cancer cell line

Carbonic anhydrase IX (CA IX) has recently been validated as an antitumor/antimetastatic drug target. In this study, we examined the underlying molecular mechanisms and the anticancer activity of sulfonamide CA IX inhibitors against cervical cancer cell lines. The effects of several sulfonamides on HeLa, MDA-MB-231, HT-29 cancer cell lines, and normal cell lines (HEK-293, PNT-1A) viability were determined. The compounds showed high cytotoxic and apoptotic activities, mainly against HeLa cells overexpressing CA IX. We were also examined for intracellular reactive oxygen species (ROS) production; intra-/extracellular pH changes, for inhibition of cell proliferation, cellular mitochondrial membrane potential change and for the detection of caspase 3, 8, 9, and CA IX protein levels. Of the investigated sulfonamides, one compound was found to possess high cytotoxic and anti-proliferative effects in HeLa cells. The cytotoxic effect occurred via apoptosis, being accompanied by a return of pHe/pHi towards normal values as for other CA IX inhibitors investigated earlier.

Targeting Tumor Microenvironment by Small-Molecule Inhibitors

The tumor microenvironment (TME) is a hypoxic, acidic, and immune/inflammatory cell–enriched milieu that plays crucial roles in tumor development, growth, progression, and therapy resistance. Targeting TME is an attractive strategy for the treatment of solid tumors. Conventional cancer chemotherapies are mostly designed to directly kill cancer cells, and the effectiveness is always compromised by their penetration and accessibility to cancer cells. Small-molecule inhibitors, which exhibit good penetration and accessibility, are widely studied, and many of them have been successfully applied in clinics for cancer treatment. As TME is more penetrable and accessible than tumor cells, a lot of efforts have recently been made to generate small-molecule inhibitors that specifically target TME or the components of TME or develop special drug-delivery systems that release the cytotoxic drugs specifically in TME. In this review, we briefly summarize the recent advances of small-molecule inhibitors that target TME for the tumor treatment.

•

Tumor microenvironment (TME) is an indispensable part of tumor and is an important therapeutic target.

•

TME is more penetrable and accessible than tumor cell area.

•

Small-molecule inhibitors that target TME are very promising.

•

The target efficiency can be improved by specific deliver and release systems.

Autophagy Modulators: Mechanistic Aspects and Drug Delivery Systems

Autophagy modulation is considered to be a promising programmed cell death mechanism to prevent and cure a great number of disorders and diseases. The crucial step in designing an effective therapeutic approach is to understand the correct and accurate causes of diseases and to understand whether autophagy plays a cytoprotective or cytotoxic/cytostatic role in the progression and prevention of disease. This knowledge will help scientists find approaches to manipulate tumor and pathologic cells in order to enhance cellular sensitivity to therapeutics and treat them. Although some conventional therapeutics suffer from poor solubility, bioavailability and controlled release mechanisms, it appears that novel nanoplatforms overcome these obstacles and have led to the design of a theranostic-controlled drug release system with high solubility and active targeting and stimuli-responsive potentials. In this review, we discuss autophagy modulators-related signaling pathways and some of the drug delivery strategies that have been applied to the field of therapeutic application of autophagy modulators. Moreover, we describe how therapeutics will target various steps of the autophagic machinery. Furthermore, nano drug delivery platforms for autophagy targeting and co-delivery of autophagy modulators with chemotherapeutics/siRNA, are also discussed.

The PI3K/AKT/mTOR and CDK4/6 Pathways in Endocrine Resistant HR+/HER2- Metastatic Breast Cancer: Biological Mechanisms and New Treatments

Endocrine-based treatments are the normal standard-of-care in women with hormone receptor-positive/Human Epidermal growth factor Receptor 2-negative metastatic breast cancer. Despite the well-known efficacy of these drugs as first-line therapies, about 50% of women develop endocrine resistance and disease progression. The treatment of these patients has represented one of the most important research fields in the last few years, with several multicenter phase II/III trials published or still ongoing. Novel therapies, such as cyclin-dependent kinase (CDK)4/6 and phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) inhibitors, have significantly changed the prognosis of patients progressing to a previous endocrine treatment, allowing a great benefit in terms of progression-free survival and, in some cases, of overall survival. However, identifying response predictors is essential for the rational use of these drugs to avoid unnecessary toxicity and costs, and to ensure the optimal therapeutic sequence is used. In this review, we analyze the PI3K/AKT/mTOR and CDK4/6 pathways and their roles in endocrine resistant metastatic breast cancer. We then focus on the new treatments developed and the roles of these drugs in overcoming endocrine resistance, describing the latest clinical trials that led to the approval of the drugs in clinical practice.

Combating pancreatic cancer with PI3K pathway inhibitors in the era of personalised medicine

Pancreatic ductal adenocarcinoma (PDAC) is among the most deadly solid tumours. This is due to a generally late-stage diagnosis of a primarily treatment-refractory disease. Several large-scale sequencing and mass spectrometry approaches have identified key drivers of this disease and in doing so highlighted the vast heterogeneity of lower frequency mutations that make clinical trials of targeted agents in unselected patients increasingly futile. There is a clear need for improved biomarkers to guide effective targeted therapies, with biomarker-driven clinical trials for personalised medicine becoming increasingly common in several cancers. Interestingly, many of the aberrant signalling pathways in PDAC rely on downstream signal transduction through the mitogen-activated protein kinase and phosphoinositide 3-kinase (PI3K) pathways, which has led to the development of several approaches to target these key regulators, primarily as combination therapies. The following review discusses the trend of PDAC therapy towards molecular subtyping for biomarker-driven personalised therapies, highlighting the key pathways under investigation and their relationship to the PI3K pathway.

Anti-inflammatory signaling by mammary tumor cells mediates prometastatic macrophage polarization in an innovative intraductal mouse model for triple-negative breast cancer

Background

Murine breast cancer models relying on intraductal tumor cell inoculations are attractive because they allow the study of breast cancer from early ductal carcinoma in situ to metastasis. Using a fully immunocompetent 4T1-based intraductal model for triple-negative breast cancer (TNBC) we aimed to investigate the immunological responses that guide such intraductal tumor progression, focusing on the prominent role of macrophages.

Methods

Intraductal inoculations were performed in lactating female mice with luciferase-expressing 4T1 mammary tumor cells either with or without additional RAW264.7 macrophages, mimicking basal versus increased macrophage-tumor cell interactions in the ductal environment. Imaging of 4T1-derived luminescence was used to monitor primary tumor growth and metastases. Tumor proliferation, hypoxia, disruption of the ductal architecture and tumor immune populations were determined immunohistochemically. M1- (pro-inflammatory) and M2-related (anti-inflammatory) cytokine levels were determined by Luminex assays and ELISA to investigate the activation state of the macrophage inoculum. Levels of the metastatic proteins matrix metalloproteinase 9 (MMP-9) and vascular endothelial growth factor (VEGF) as well as of the immune-related disease biomarkers chitinase 3-like 1 (CHI3L1) and lipocalin 2 (LCN2) were measured by ELISA to evaluate disease progression at the protein level.

Results

Mice intraductally co-injected with macrophages showed severe splenomegaly with faster ductal breakthrough of tumor cells and increased metastases in axillary lymph nodes and lungs. These mice showed higher M1-related cytokines in the early disease stages (at 1 to 3 weeks post-inoculation) due to the pro-inflammatory nature of RAW264.7 macrophages with increased Ly6G-positive neutrophils and decreased anti-inflammatory macrophages in the tumor microenvironment. However, upon metastasis (at 5 weeks post-inoculation), a prominent increase in M2-related cytokine levels was detected and established a tumor microenvironment with similar immune populations and cytokine responses as in mice which received only 4T1 tumor cells. The observed tumor-associated immune responses and the increased metastasis were associated with significantly induced local and systemic levels of MMP-9, VEGF, CHI3L1 and LCN2.

Conclusions

The current experimental study with an innovative immunocompetent intraductal model for TNBC pinpoints towards a metastasis-supporting M1 to M2 macrophage polarization in the mammary ducts mediated by 4T1-derived signaling. We propose to explore this process as immunotherapeutic target.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0860-x) contains supplementary material, which is available to authorized users.

Acetazolamide protects rat articular chondrocytes from IL-1β-induced apoptosis by inhibiting the activation of NF-κB signal pathway

Because the excessive apoptosis of articular chondrocytes contributes to extracellular matrix (ECM) loss and cartilage damage in rheumatoid arthritis (RA), inhibiting chondrocyte apoptosis might be a promising strategy for RA. Aquaporin1 (AQP1) is overexpressed in RA cartilage and synovial tissues, and play a vital pathogenic role in RA development. Particularly, we previously reported that acetazolamide (AZ) as an AQP1 inhibitor suppressed secondary inflammation and promoted ECM production in cartilage of adjuvant-induced arthritis rats. Here, we investigated the antiapoptotic effect of AZ on interleukin-1β (IL-1β)-induced apoptosis, a classic in vitro model of chondrocyte apoptosis. AZ treatment could inhibit IL-1β-induced apoptosis, evidenced by increasing cell viability, relieving apoptotic nuclear morphology, decreasing apoptosis rates, and restoring mitochondrial membrane potential. Additionally, AZ reversed IL-1β-induced decrease of Bcl-2 protein and reduced IL-1β-induced increases of Bax and caspase 3 protein, accompanied by inhibiting IκBα degradation and phosphorylation in cytoplasm, reducing NF-κB p65 protein level in nucleus and preventing NF-κB p65 translocation from cytoplasm to nucleus. In conclusion, our findings indicated that AZ could effectively attenuate IL-1β-induced chondrocyte apoptosis mediated by regulating the protein levels of apoptosis-related genes and inhibiting the activation of NF-κB signal pathway, suggesting that AZ might be of potential clinical interest in RA treatment.

Intravital Imaging to Monitor Therapeutic Response in Moving Hypoxic Regions Resistant to PI3K Pathway Targeting in Pancreatic Cancer

Application of advanced intravital imaging facilitates dynamic monitoring of pathway activity upon therapeutic inhibition. Here, we assess resistance to therapeutic inhibition of the PI3K pathway within the hypoxic microenvironment of pancreatic ductal adenocarcinoma (PDAC) and identify a phenomenon whereby pronounced hypoxia-induced resistance is observed for three clinically relevant inhibitors. To address this clinical problem, we have mapped tumor hypoxia by both immunofluorescence and phosphorescence lifetime imaging of oxygen-sensitive nanoparticles and demonstrate that these hypoxic regions move transiently around the tumor. To overlay this microenvironmental information with drug response, we applied a FRET biosensor for Akt activity, which is a key effector of the PI3K pathway. Performing dual intravital imaging of drug response in different tumor compartments, we demonstrate an improved drug response to a combination therapy using the dual mTORC1/2 inhibitor AZD2014 with the hypoxia-activated pro-drug TH-302.

Fine-Tuning Tumor Endothelial Cells to Selectively Kill Cancer

Tumor endothelial cells regulate several aspects of tumor biology, from delivering oxygen and nutrients to shaping the immune response against a tumor and providing a barrier against tumor cell dissemination. Accordingly, targeting tumor endothelial cells represents an important modality in cancer therapy. Whereas initial anti-angiogenic treatments focused mainly on blocking the formation of new blood vessels in cancer, emerging strategies are specifically influencing certain aspects of tumor endothelial cells. For instance, efforts are generated to normalize tumor blood vessels in order to improve tumor perfusion and ameliorate the outcome of chemo-, radio-, and immunotherapy. In addition, treatment options that enhance the properties of tumor blood vessels that support a host’s anti-tumor immune response are being explored. Hence, upcoming anti-angiogenic strategies will shape some specific aspects of the tumor blood vessels that are no longer limited to abrogating angiogenesis. In this review, we enumerate approaches that target tumor endothelial cells to provide anti-cancer benefits and discuss their therapeutic potential.

Resistance to mTORC1 Inhibitors in Cancer Therapy: From Kinase Mutations to Intratumoral Heterogeneity of Kinase Activity

Targeting mTORC1 has been thoroughly explored in cancer therapy. Following encouraging preclinical studies, mTORC1 inhibitors however failed to provide substantial benefits in cancer patients. Several resistance mechanisms have been identified including mutations of mTOR and activation of alternate proliferation pathways. Moreover, emerging evidence discloses intratumoral heterogeneity of mTORC1 activity that further contributes to a reduced anticancer efficacy of mTORC1 inhibitors. Genetic heterogeneity as well as heterogeneous conditions of the tumor environment such as hypoxia profoundly modifies mTORC1 activity in tumors and hence influences the response of tumors to mTORC1 inhibitors. Intriguingly, the heterogeneity of mTORC1 activity also occurs towards its substrates at the single cell level, as mutually exclusive pattern of activation of mTORC1 downstream effectors has been reported in tumors. After briefly describing mTORC1 biology and the use of mTORC1 inhibitors in patients, this review will give an overview on concepts of resistance to mTORC1 inhibition in cancer with a particular focus on intratumoral heterogeneity of mTORC1 activity.

Targeting pH regulating proteins for cancer therapy-Progress and limitations

Tumour acidity induced by metabolic alterations and incomplete vascularisation sets cancer cells apart from normal cellular physiology. This distinguishing tumour characteristic has been an area of intense study, as cellular pH (pH_i) disturbances disrupt protein function and therefore multiple cellular processes. Tumour cells effectively utilise pH_i regulating machinery present in normal cells with enhancements provided by additional oncogenic or hypoxia induced protein modifications. This overall improvement of pH regulation enables maintenance of an alkaline pH_i in the continued presence of external acidification (pH_e). Considerable experimentation has revealed targets that successfully disrupt tumour pH_i regulation in efforts to develop novel means to weaken or kill tumour cells. However, redundancy in these pH-regulating proteins, which include Na⁺/H⁺ exchangers (NHEs), carbonic anhydrases (CAs), Na⁺/HCO₃^- co-transporters (NBCs) and monocarboxylate transporters (MCTs) has prevented effective disruption of tumour pH_i when individual protein targeting is performed. Here we synthesise recent advances in understanding both normoxic and hypoxic pH regulating mechanisms in tumour cells with an ultimate focus on the disruption of tumour growth, survival and metastasis. Interactions between tumour acidity and other cell types are also proving to be important in understanding therapeutic applications such as immune therapy. Promising therapeutic developments regarding pH manipulation along with current limitations are highlighted to provide a framework for future research directives.

Acidic tumor microenvironment abrogates the efficacy of mTORC1 inhibitors

Background

Blocking the mechanistic target of rapamycin complex-1 (mTORC1) with chemical inhibitors such as rapamycin has shown limited clinical efficacy in cancer. The tumor microenvironment is characterized by an acidic pH which interferes with cancer therapies. The consequences of acidity on the anti-cancer efficacy of mTORC1 inhibitors have not been characterized and are thus the focus of our study.

Methods

Cancer cell lines were treated with rapamycin in acidic or physiological conditions and cell proliferation was investigated. The effect of acidity on mTORC1 activity was determined by Western blot. The anticancer efficacy of rapamycin in combination with sodium bicarbonate to increase the intratumoral pH was tested in two different mouse models and compared to rapamycin treatment alone. Histological analysis was performed on tumor samples to evaluate proliferation, apoptosis and necrosis.

Results

Exposing cancer cells to acidic pH in vitro significantly reduced the anti-proliferative effect of rapamycin. At the molecular level, acidity significantly decreased mTORC1 activity, suggesting that cancer cell proliferation is independent of mTORC1 in acidic conditions. In contrast, the activation of mitogen-activated protein kinase (MAPK) or AKT were not affected by acidity, and blocking MAPK or AKT with a chemical inhibitor maintained an anti-proliferative effect at low pH. In tumor mouse models, the use of sodium bicarbonate increased mTORC1 activity in cancer cells and potentiated the anti-cancer efficacy of rapamycin. Combining sodium bicarbonate with rapamycin resulted in increased tumor necrosis, increased cancer cell apoptosis and decreased cancer cell proliferation as compared to single treatment.

Conclusions

Taken together, these results emphasize the inefficacy of mTORC1 inhibitors in acidic conditions. They further highlight the potential of combining sodium bicarbonate with mTORC1 inhibitors to improve their anti-tumoral efficacy.

Cancer: fundamentals behind pH targeting and the double-edged approach

The highly regulated pH of cells and the less-regulated pH of the surrounding extracellular matrix (ECM) is the result of a delicate balance between metabolic processes and proton production, proton transportation, chemical buffering, and vascular removal of waste products. Malignant cells show a pronounced increase in metabolic processes where the 10- to 15-fold rise in glucose consumption is only the tip of the iceberg. Aerobic glycolysis (Warburg effect) is one of the hallmarks of cancer metabolism that implies excessive production of protons, which if stayed inside the cells would result in fatal intracellular acidosis (maintaining a strict acid-base balance is essential for the survival of eukaryotic cells). Malignant cells solve this problem by increasing mechanisms of proton transportation which expel the excess acidity. This allows cancer cells to keep a normal intracellular pH, or even overshooting this mechanism permits a slightly alkaline intracellular tendency. The proton excess expelled from malignant cells accumulates in the ECM, where chronic hypoxia and relative lack of enough blood vessels impede adequate proton clearance, thus creating an acidic microenvironment. This microenvironment is quite heterogeneous due to the tumor's metabolic heterogeneity and variable degrees of hypoxia inside the tumor mass. The acidic environment (plus other necessary cellular modifications) stimulates migration and invasion and finally intravasation of malignant cells which eventually may result in metastasis. Targeting tumor pH may go in two directions: 1) increasing extracellular pH which should result in less migration, invasion, and metastasis; and 2) decreasing intracellular pH which may result in acidic stress and apoptosis. Both objectives seem achievable at the present state of the art with repurposed drugs. This hypothesis analyzes the altered pH of tumors and its implications for progression and metastasis and also possible repurposed drug combinations targeting this vulnerable side of cancer development. It also analyzes the double-edged approach, which consists in pharmacologically increasing intracellular proton production and simultaneously decreasing proton extrusion creating intracellular acidity, acid stress, and eventual apoptosis.