SGLT2 Inhibitors as Potential Anticancer Agents.

Sodium-glucose cotransporter 2 (SGLT2) serves as a critical glucose transporter that has been reported to be overexpressed in cancer models, followed by increased glucose uptake in both mice and humans. Inhibition of its expression can robustly thwart tumor development in vitro and in vivo. SGLT2 inhibitors are a comparatively new class of antidiabetic drugs that have demonstrated anticancer effects in several malignancies, including breast, liver, pancreatic, thyroid, prostate, and lung cancers. This review aims to assess the extent of SGLT involvement in different cancer cell lines and discuss the pharmacology, mechanisms of action, and potential applications of SGLT2 inhibitors to reduce tumorigenesis and its progression. Although these agents display a common mechanism of action, they exhibit distinct affinity towards the SGLT type 2 transporter compared to the SGLT type 1 transporter and varying extents of bioavailability and half-lives. While suppression of glucose uptake has been attributed to their primary mode of antidiabetic action, SGLT2 inhibitors have demonstrated several mechanistic ways to combat cancer, including mitochondrial membrane instability, suppression of ß-catenin, and PI3K-Akt pathways, increase in cell cycle arrest and apoptosis, and downregulation of oxidative phosphorylation. Growing evidence and ongoing clinical trials suggest a potential benefit of combination therapy using an SGLT2 inhibitor with the standard chemotherapeutic regimen. Nevertheless, further experimental and clinical evidence is required to characterize the expression and role of SGLTs in different cancer types, the activity of different SGLT subtypes, and their role in tumor development and progression.

Preclinical and Clinical Research Models of Prostate Cancer: A Brief Overview.

The incidence and mortality from prostate cancer (PCa) are on the rise which poses a major public health concern worldwide. In this narrative review, we have summarized the characteristics of major in vitro and in vivo PCa models including their utility in developing treatment strategies. Androgens, particularly, testosterone and dihydrotestosterone (DHT) activate the androgen receptor (AR) signaling pathway that facilitates the development and progression of castration resistant PCa. Several enzymes namely, CYP17A1, HSD17B, and SRD5A are essential to furnishing DHT from dehydroepiandrosterone in the classical pathway while DHT is formed from androstanediol in the backdoor pathway. The advancement in delineating the molecular heterogeneity of PCa has been possible through the development of several in vitro and in vivo research models. Generally, tissue culture models are advantageous to understand PCa biology and investigate the efficacy and toxicity of novel agents; nevertheless, animal models are indispensable to studying the PCa etiology and treatment since they can simulate the tumor microenvironment that plays a central role in initiation and progression of the disease. Moreover, the availability of several genetically engineered mouse models has made it possible to study the metastasis process. However, the conventional models are not devoid of limitations. For example, the lack of heterogeneity in tissue culture models and the variation of metastatic characteristics in xenograft models are obviously challenging. Additionally, due to the racial and ethnic disparities in PCa pathophysiology, a new model that can represent PCa encompassing different ethnicities is urgently needed. New models should continue to evolve to address the genetic and molecular complexities as well as to further elucidate the finer details of the steroidogenic pathway associated with PCa.

Comparison of Anticancer Drug Toxicities: Paradigm Shift in Adverse Effect Profile.

The inception of cancer treatment with chemotherapeutics began in the 1940s with nitrogen mustards that were initially employed as weapons in World War II. Since then, treatment options for different malignancies have evolved over the period of last seventy years. Until the late 1990s, all the chemotherapeutic agents were small molecule chemicals with a highly nonspecific and severe toxicity spectrum. With the landmark approval of rituximab in 1997, a new horizon has opened up for numerous therapeutic antibodies in solid and hematological cancers. Although this transition to large molecules improved the survival and quality of life of cancer patients, this has also coincided with the change in adverse effect patterns. Typically, the anticancer agents are fraught with multifarious adverse effects that negatively impact different organs of cancer patients, which ultimately aggravate their sufferings. In contrast to the small molecules, anticancer antibodies are more targeted toward cancer signaling pathways and exhibit fewer side effects than traditional small molecule chemotherapy treatments. Nevertheless, the interference with the immune system triggers serious inflammation- and infection-related adverse effects. The differences in drug disposition and interaction with human basal pathways contribute to this paradigm shift in adverse effect profile. It is critical that healthcare team members gain a thorough insight of the adverse effect differences between the agents discovered during the last twenty-five years and before. In this review, we summarized the general mechanisms and adverse effects of small and large molecule anticancer drugs that would further our understanding on the toxicity patterns of chemotherapeutic regimens.

The Role of Oxidative Stress and Its Counteractive Utility in Colorectal Cancer (CRC).

An altered redox status accompanied by an elevated generation of reactive oxygen/nitrogen species (ROS/RNS) has been implicated in a number of diseases including colorectal cancer (CRC). CRC, being one of the most common cancers worldwide, has been reported to be associated with multiple environmental and lifestyle factors (e.g., dietary habits, obesity, and physical inactivity) and harboring heightened oxidative stress that results in genomic instability. Although under normal condition ROS regulate many signal transduction pathways including cell proliferation and survival, overwhelming of the antioxidant capacity due to metabolic abnormalities and oncogenic signaling leads to a redox adaptation response that imparts drug resistance. Nevertheless, excessive reliance on elevated production of ROS makes the tumor cells increasingly vulnerable to further ROS insults, and the abolition of such drug resistance through redox perturbation could be instrumental to preferentially eliminate them. The goal of this review is to demonstrate the evidence that links redox stress to the development of CRC and assimilate the most up-to-date information that would facilitate future investigation on CRC-associated redox biology. Concomitantly, we argue that the exploitation of this distinct biochemical property of CRC cells might offer a fresh avenue to effectively eradicate these cells.

Thymoquinone as a Potential Adjuvant Therapy for Cancer Treatment: Evidence from Preclinical Studies.

Thymoquinone (TQ), the main bioactive component of Nigella sativa, has been found to exhibit anticancer effects in numerous preclinical studies. Due to its multitargeting nature, TQ interferes in a wide range of tumorigenic processes and counteracts carcinogenesis, malignant growth, invasion, migration, and angiogenesis. Moreover, TQ can specifically sensitize tumor cells toward conventional cancer treatments (e.g., radiotherapy, chemotherapy, and immunotherapy) and simultaneously minimize therapy-associated toxic effects in normal cells. In this review, we summarized the adjuvant potential of TQ as observed in various in vitro and in vivo animal models and discussed the pharmacological properties of TQ to rationalize its supplementary role in potentiating the efficacy of standard therapeutic modalities namely surgery, radiotherapy, chemotherapy, and immunotherapy. Altogether, we suggest further comprehensive evaluation of TQ in preclinical and clinical levels to delineate its implied utility as a novel complementary adjuvant therapy for cancer treatment.

New insights into estrogenic regulation of O6-methylguanine DNA-methyltransferase (MGMT) in human breast cancer cells: Co-degradation of ER-α and MGMT proteins by fulvestrant or O6-benzylguanine indicates fresh avenues for therapy.

Endocrine therapy using estrogen receptor-α (ER-α) antagonists for attenuating horm2one-driven cell proliferation is a major treatment modality for breast cancers. To exploit any DNA repair deficiencies associated with endocrine therapy, we investigated the functional and physical interactions of ER-α with O6-methylguanine DNA methyltransferase (MGMT), a unique DNA repair protein that confers tumor resistance to various anticancer alkylating agents. The ER-α -positive breast cancer cell lines (MCF-7, T47D) and ER- negative cell lines (MDAMB-468, MDAMB-231), and established inhibitors of ER-α and MGMT, namely, ICI-182,780 (Faslodex) and O6-benzylguanine, respectively, were used to study MGMT- ER interactions. The MGMT gene promoter was found to harbor one full and two half estrogen-responsive elements (EREs) and two antioxidant-responsive elements (AREs). MGMT expression was upregulated by estrogen, downregulated by tamoxifen in Western blot and promoter-linked reporter assays. Similarly, both transient and stable transfections of Nrf-2 (nuclear factor-erythroid 2-related factor-2) increased the levels of MGMT protein and activity 3 to 4-fold reflecting novel regulatory nodes for this drug-resistance determinant. Of the different ER-α antagonists tested, the pure anti-estrogen fulvestrant was most potent in inhibiting the MGMT activity in a dose, time and ER-α dependent manner, similar to O6-benzylguanine. Interestingly, fulvestrant exposure led to a degradation of both ER-α and MGMT proteins and O6-benzylguanine also induced a specific loss of ER-α and MGMT proteins in MCF-7 and T47D breast cancer cells with similar kinetics. Immunoprecipitation revealed a specific association of ER-α and MGMT proteins in breast cancer cells. Furthermore, silencing of MGMT gene expression triggered a decrease in the levels of both MGMT and ER-α proteins. The involvement of proteasome in the drug-induced degradation of both proteins was also demonstrated. Fulvestrant enhanced the cytotoxicity of MGMT-targeted alkylating agents, namely, temozolomide and BCNU by 3 to 4-fold in ER-α positive cells, but not in ER-negative cells. We conclude that MGMT and ER-α proteins exist as a complex and are co-targeted for ubiquitin-conjugation and subsequent proteasomal degradation. The findings offer a clear rationale for combining alkylating agents with endocrine therapy.

Potent anti-proliferative actions of a non-diuretic glucosamine derivative of ethacrynic acid.

Ethacrynic acid (EA), a known inhibitor of the neoplastic marker glutathione S-transferase P1 and other GSTs, exerts a weak antiproliferative activity against human cancer cells. The clinical use of EA (Edecrin) as an anticancer drug is limited by its potent loop diuretic activity. In this study, we developed a non-diuretic 2-amino-2-deoxy-d-glucose conjugated EA (EAG) to target tumors cells via the highly expressed glucose transporter 1 (GLUT1). Cell survival assays revealed that EAG had little effect on normal cells, but was cytotoxic 3 to 4.5-fold greater than EA. Mechanistically, the EAG induced selective cell death in cancer cells by inhibiting GSTP1 and generating abundant reactive oxygen species. Furthermore, EAG induced p21(cip1) expression and a G2/M cell cycle block irrespective of the p53 gene status in tumor cells. These data encourage the development of new EA analogs.

Piperlongumine exerts cytotoxic effects against cancer cells with mutant p53 proteins at least in part by restoring the biological functions of the tumor suppressor.

Piperlongumine (PL), a small molecule alkaloid present in black pepper (Piper longum), has been reported to kill tumor cells irrespective of their p53 gene status, however, the mechanisms involved are unknown. Since p53 is a redox-sensitive protein, we hypothesized that the redox imbalance induced by PL may affect the structure and/or function of the mutant p53 protein and promote cell death. We used two human colon cancer cell lines, the HT29 and SW620 which harbor the R273H DNA contact abrogatory mutation in p53. PL treatment induced significant ROS production and protein glutathionylation with a concomitant increase in Nrf-2 expression in both cell lines. Surprisingly, immunoprecipitation with wt-p53 specific antibodies (PAb1620) or direct western blotting showed a progressive generation of wild-type-like p53 protein along with a loss of its mutant counterpart in PL-treated HT29 and SW620 cells. Moreover, the EMSA and DNA-affinity blotting revealed a time-dependent restoration of DNA-binding for the mutant p53, which was accompanied by the induction of p53 target genes, MDM2 and Bax. PL, while cytotoxic by itself, also increased the cell killing by many anticancer drugs. In nude mice bearing the HT29 tumors, PL alone (7.5 mg/kg daily) produced a 40% decrease in tumor volume, which was accompanied by diminished intratumoral mutant p53 protein levels. The antitumor efficacy of BCNU or doxorubicin in HT29 xenografts was highly potentiated by PL, followed by expression of apoptotic proteins. These clinically-relevant findings suggest that PL-induced oxidative milieu facilitates a weak functional restoration of mutant p53 through protein glutathionylation and contributes to the increased drug sensitivity.

Posttranslational Regulation of O(6)-Methylguanine-DNA Methyltransferase (MGMT) and New Opportunities for Treatment of Brain Cancers.

O(6)-Methylguanine-DNA-methyltransferase (MGMT) is an antimutagenic DNA repair protein highly expressed in human brain tumors. Because MGMT repairs the mutagenic, carcinogenic and cytotoxic O(6)-alkylguanine adducts, including those generated by the clinically used anticancer alkylating agents, it has emerged as a central and rational target for overcoming tumor resistance to alkylating agents. Although the pseudosubstrates for MGMT [O(6)-benzylguanine, O(6)-(4- bromothenyl)guanine] have gained attention as powerful and clinically-relevant inhibitors, bone marrow suppression due to excessive alkylation damage has diminished this strategy. Our laboratory has been working on various posttranslational modifications of MGMT that affect its protein stability, DNA repair activity and response to oxidative stress. While these modifications greatly impact the physiological regulation of MGMT, they also highlight the opportunities for inactivating DNA repair and new drug discovery in this specific area. This review briefly describes the newer aspects of MGMT posttranslational regulation by ubiquitination, sumoylation and glutathionylation and reveals how the reactivity of the active site Cys145 can be exploited for potent inhibition and depletion of MGMT by thiol-reacting drugs such as the disulfiram and various dithiocarbamate derivatives. The possible repurposing of these nontoxic and safe drugs for improved therapy of pediatric and adult brain tumors is discussed.