Effects of β-caryophyllene and oxygen availability on cholesterol and fatty acids in breast cancer cells

Hypoxia is a common feature of most solid tumors, one that favors tumor progression and limits treatment effectiveness. Targeting hypoxia has long been a goal in cancer therapy, by identifying factors that reverse or ameliorate the effects of hypoxia on cancer cells. We, and others, have shown that β-caryophyllene (BCP) exhibits anti-proliferative properties in cancer cells. We have further shown that non-cytotoxic concentrations of BCP affect cholesterol and lipid biosynthesis in hypoxic hBrC cells at both transcriptional and translational levels. This led us to hypothesize that BCP may reverse the hypoxic phenotype in hBrC cells. To test this, we determined the effect of BCP on hypoxic sensitive pathways, including oxygen consumption, glycolysis, oxidative stress, cholesterol and fatty acid biosynthesis, and ERK activation. While each of these studies revealed new information on the regulation by hypoxia and BCP, only the lipidomic studies showed reversal of hypoxic-dependent effects by BCP. These later studies showed that hypoxia-treated samples lowered monounsaturated fatty acid levels, shifting the saturation ratios of the fatty acid pools. This signature was ameliorated by sub-lethal concentrations of BCP, possibly through an effect on the C:16 fatty acid saturation ratios. This is consistent with BCP-induced upregulation of the stearoyl-CoA desaturase (SCD) gene, observed previously. This suggests that BCP may interfere with the lipid signature modulated by hypoxia which could have consequences for membrane biosynthesis or composition, both of which are important for cell replication.

Inhibition of Carbonic Anhydrase Using SLC-149: Support for a Noncatalytic Function of CAIX in Breast Cancer

Carbonic anhydrase IX (CAIX) is considered a target for therapeutic intervention in solid tumors. In this study, the efficacy of the inhibitor, 4-(3-(2,4-difluorophenyl)-oxoimidazolidin-1-yl)benzenesulfonamide (SLC-149), is evaluated on CAIX and a CAIX-mimic. We show that SLC-149 is a better inhibitor than acetazolamide against CAIX. Binding of SLC-149 thermally stabilizes CAIX-mimic at lower concentrations compared to that of CAII. Structural examinations of SLC-149 bound to CAIX-mimic and CAII explain binding preferences. In cell culture, SLC-149 is a more effective inhibitor of CAIX activity in a triple-negative breast cancer cell line than previously studied sulfonamide inhibitors. SLC-149 is also a better inhibitor of activity in cells expressing CAIX versus CAXII. However, SLC-149 has little effect on cytotoxicity, and high concentrations are required to inhibit cell growth, migration, and invasion. These data support the hypothesis that CAIX activity, shown to be important in regulating extracellular pH, does not underlie its ability to control cell growth.

β-caryophyllene enhances the transcriptional upregulation of cholesterol biosynthesis in breast cancer cells

β-caryophyllene (BCP) exhibits anti-proliferative properties in cancer cells. Here, we examine the hypothesis that BCP induces membrane remodeling. Our data show that high concentrations of BCP increase membrane permeability of human breast cells (hBrC) causing detachment and cell death. At a sub-lethal concentration of BCP, we show that BCP induces a striking upregulation of genes involved in cholesterol biosynthesis, including the gene that encodes for HMGCoA reductase (HMGCR), the rate-determining step in cholesterol biosynthesis. In addition, stearoyl-CoA desaturase (SCD) is also upregulated which would lead to the enhanced formation of monounsaturated fatty acids, specifically oleate and palmitoleate from stearoyl CoA and palmitoyl CoA, respectively. These fatty acids are major components of membrane phospholipids and cholesterol esters. Together, these data suggest that cells respond to BCP by increasing the synthesis of components found in membranes. These responses could be viewed as a repair mechanism and/or as a mechanism to mount resistance to the cytotoxic effect of BCP. Blocking HMGCR activity enhances the cytotoxicity of BCP, suggesting that BCP may provide an additional therapeutic tool in controlling breast cancer cell growth.

Selective inhibition of carbonic anhydrase IX over carbonic anhydrase XII in breast cancer cells using benzene sulfonamides: Disconnect between activity and growth inhibition

Carbonic anhydrases (CAs) have been linked to tumor progression, particularly membrane-bound CA isoform IX (CA IX). The role of CA IX in the context of breast cancer is to regulate the pH of the tumor microenvironment. In contrast to CA IX, expression of CA XII, specifically in breast cancer, is associated with better outcome despite performing the same catalytic function. In this study, we have structurally modeled the orientation of bound ureido-substituted benzene sulfonamides (USBs) within the active site of CA XII, in comparison to CA IX and cytosolic off-target CA II, to understand isoform specific inhibition. This has identified specific residues within the CA active site, which differ between isoforms that are important for inhibitor binding and isoform specificity. The ability of these sulfonamides to block CA IX activity in breast cancer cells is less effective than their ability to block activity of the recombinant protein (by one to two orders of magnitude depending on the inhibitor). The same is true for CA XII activity but now they are two to three orders of magnitude less effective. Thus, there is significantly greater specificity for CA IX activity over CA XII. While the inhibitors block cell growth, without inducing cell death, this again occurs at two orders of magnitude above the K_i values for inhibition of CA IX and CA XII activity in their respective cell types. Surprisingly, the USBs inhibited cell growth even in cells where CA IX and CA XII expression was ablated. Despite the potential for these sulfonamides as chemotherapeutic agents, these data suggest that we reconsider the role of CA activity on growth potentiation.

Differential expression and function of CAIX and CAXII in breast cancer: A comparison between tumorgraft models and cells

Carbonic anhydrase IX (CAIX) and XII (CAXII) are transmembrane proteins that are associated with cancer progression. We have previously described the catalytic properties of CAIX in MDA-MB-231 breast cancer cells, a line of cells that were derived from a patient with triple negative breast cancer. We chose this line because CAIX expression in breast cancer is a marker of hypoxia and a prognosticator for reduced survival. However, CAXII expression is associated with better survival statistics than those patients with low CAXII expression. Yet CAIX and CAXII have similar catalytic activities. Here we compare the potential roles of CAIX and CAXII in the context of TNBC and estrogen receptor (ER)-positive breast cancer. In tumor graft models, we show that CAIX and CAXII exhibit distinct expression patterns and non-overlapping. We find the same pattern across a panel of TNBC and luminal breast cancer cell lines. This affords an opportunity to compare directly CAIX and CAXII function. Our data suggest that CAIX expression is associated with growth potentiation in the tumor graft model and in a TNBC line using knockdown strategies and blocking activity with an impermeant sulfonamide inhibitor, N-3500. CAXII was not associated with growth potentiation. The catalytic activities of both CAIX and CAXII were sensitive to inhibition by N-3500 and activated at low pH. However, pH titration of activity in membrane ghosts revealed significant differences in the catalytic efficiency and pKa values. These features provide evidence that CAIX is a more efficient enzyme than CAXII at low pH and that CAIX shifts the equilibrium between CO₂ and bicarbonate in favor of CO₂ production by consuming protons. This suggests that in the acidic microenvironment of tumors, CAIX plays a role in stabilizing pH at a value that favors cancer cell survival.

Abstract 1436: Beta-caryophyllene regulates lipid biosynthesis in breast cancer cells

The search for therapeutic anti-cancer drugs has spanned both synthetic and natural products approaches, and notable success has been achieved from unique chemistries produced by plants. Early studies in E.coli showed that cyclic hydrocarbons, including terpenes, interact directly with biological membranes. Accumulation of hydrocarbons results in membrane swelling and increased membrane fluidity, both signs of cell stress. At biological temperatures, membrane fluidity is controlled by the saturation state of the acyl chains of fatty acids (primarily in phospholipids) and cholesterol content. Changes in either of these parameters leads to membrane remodeling which can affect membrane function. Terpenes are, themselves, precursors to complex sterols across all kingdoms of life. Beta-caryophylene (BCP), a bicyclic sesquiterpene, induces cell death across a variety of cancer cell types, although the mechanism(s) by which this occurs is not completely known. Our data show that BCP induces membrane permeability in breast cancer lines representing both ER-positive and triple negative phenotypes (TNBC), as measured by lactate dehydrogenase release from cells. We sought to understand this phenomenon by looking at changes in the transcriptome of treated cells compared to controls. We chose to examine cells exposed to hypoxia, rationalizing that this condition, in vivo, creates an aggressive phenotype, is associated with TNBC and drug-resistant (recurrent) breast cancers, and is an independent prognosticator for poor patient outcome. We isolated high quality RNA (RIN>9) at the University of Florida. The Genomics Core at the University of Louisville prepared libraries and performed the sequencing run (Illumina NextSeq 500). This generated over 144 million 75bp reads that aligned to the human genome (96.3% alignment rate), or approximately 24 million reads per sample. As a first approach in data analysis, we selected differential expression based on the FDR adjusted p-values (q-values) <0.05 (as determined by CuffDiff), and analyzed this gene set for pathway involvement using the reactome data base. Reactome also assesses FDR values for pathway analysis, and we selected pathway FDR <0.05 providing the highest level of specificity. This search modality revealed an orchestrated up-regulation of lipid metabolism, including that of cholesterol biosynthesis. This is consistent with BCP-induced membrane remodeling. We have validated these results, not by PCR, but through protein expression and metabolic assays. Because these events occur at concentrations that are sub-cytotoxic, this potentially reveals a new mechanism underlying the development of drug resistance.

Citation Format: Mam Y. Mboge, Adam P. Bullock, Riley O'Dennell, John V. Matthias, Julie A. Davila, Christopher J. Frost, Susan C. Frost. Beta-caryophyllene regulates lipid biosynthesis in breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1436.

Cancer Drug Development of Carbonic Anhydrase Inhibitors beyond the Active Site

Carbonic anhydrases (CAs) catalyze the reversible hydration of carbon dioxide to produce bicarbonate and a proton. Multiple CA isoforms are implicated in a range of diseases, including cancer. In solid tumors, continuously dividing cells create hypoxic conditions that eventually lead to an acidic microenvironment. Hypoxic tumor cells have different mechanisms in place to regulate and adjust the surrounding microenvironment for survival. These mechanisms include expression of CA isoform IX (CA IX) and XII (CA XII). These enzymes help maintain a physiological intracellular pH while simultaneously contributing to an acidic extracellular pH, leading to tumor cell survival. Expression of CA IX and CA XII has also been shown to promote tumor cell invasion and metastasis. This review discusses the characteristics of CA IX and CA XII, their mechanism of action, and validates their prospective use as anticancer targets. We discuss the current status of small inhibitors that target these isoforms, both classical and non-classical, and their future design in order to obtain isoform-specificity for CA IX and CA XII. Biologics, such as monoclonal antibodies, monoclonal-radionuclide conjugated chimeric antibodies, and antibody-small molecule conjugates are also discussed.

UFH-001 cells: A novel triple negative, CAIX-positive, human breast cancer model system

Human cell lines are an important resource for research, and are often used as in vitro models of human diseases. In response to the mandate that all cells should be authenticated, we discovered that the MDA-MB-231 cells that were in use in our lab, did not validate based on the alleles of 9 different markers (STR Profile). We had been using this line as a model of triple negative breast cancer (TNBC) that has the ability to form tumors in immuno-compromised mice. Based on marker analysis, these cells most closely resembled the MCF10A line, which are a near diploid and normal mammary epithelial line. Yet, the original cells express carbonic anhydrase IX (CAIX) both constitutively and in response to hypoxia and are features that likely drive the aggressive nature of these cells. Thus, we sought to sub-purify CAIX-expressing cells using Fluorescence Activated Cell Sorting (FACS). These studies have revealed a new line of cells that we have name UFH-001, which have the TNBC phenotype, are positive for CAIX expression, both constitutively and in response to hypoxia, and behave aggressively in vivo. These cells may be useful for exploring mechanisms that underlie progression, migration, and metastasis of this phenotype. In addition, constitutive expression of CAIX allows its evaluation as a therapeutic target, both in vivo and in vitro.

Carbonic Anhydrases: Role in pH Control and Cancer

The pH of the tumor microenvironment drives the metastatic phenotype and chemotherapeutic resistance of tumors. Understanding the mechanisms underlying this pH-dependent phenomenon will lead to improved drug delivery and allow the identification of new therapeutic targets. This includes an understanding of the role pH plays in primary tumor cells, and the regulatory factors that permit cancer cells to thrive. Over the last decade, carbonic anhydrases (CAs) have been shown to be important mediators of tumor cell pH by modulating the bicarbonate and proton concentrations for cell survival and proliferation. This has prompted an effort to inhibit specific CA isoforms, as an anti-cancer therapeutic strategy. Of the 12 active CA isoforms, two, CA IX and XII, have been considered anti-cancer targets. However, other CA isoforms also show similar activity and tissue distribution in cancers and have not been considered as therapeutic targets for cancer treatment. In this review, we consider all the CA isoforms and their possible role in tumors and their potential as targets for cancer therapy.

Abstract 5936: Comparison of carbonic anhydrase & activity between triple-negative & luminal breast cancer cells

Tumor microenvironment substantially influences the process of tumorigenesis. Extracellular acidification within the tumor microenvironment is an indicator of an aggressive cancer and a marker for poor patient outcome. In solid tumors, hypoxia leads to extracellular acidosis. Carbonic anhydrases (CA) are thought to regulate intracellular and extracellular pH (pHi and pHe, respectively). To explore the effect of CAs in breast cancer, we compared the expression and activity of two membrane bound CAs, CAIX and CAXII, between triple negative breast cancer cells (TNBCs) and luminal breast cancer cells (LBCs).

We chose five different TNBC and LBC lines. Our data show that, among the TNBC lines, CAIX expression increased in three of the five lines: HBL100, SUM159, and the new UFH-001 cells under hypoxic condition. UFH-001 cells also showed strong constitutive expression. None of these TNBC lines expressed CAXII or estrogen receptor (ER). In LBC lines, four of the five lines constitutively expressed CAXII: T47D, MCF7, SKBR and SUM52 cells. CAXII expression was not hypoxia-dependent. Each of the five luminal lines expressed ER. We also examined CA expression in a tumor graft model. In tumors grown from cells derived from TNBC patients, we observed CAIX expression in four of six sample sets. In tumors derived from ER-positive LBC patients, all five expressed CAXII.

We also used the 18O exchange method to assess CA activity. Two TNBC lines: UFH-001 and HBL100 cells showed that CAIX activity increased in hypoxic conditions which was blocked by an impermeant sulfonamide CA inhibitor (N3500). In the luminal lines, we detected CAXII activity in T47D and MCF7 cells that was also inhibited by N3500. Like CAXII protein expression in these cells, CAXII activity was not affected by hypoxia. We also evaluated the effect of pH on CA activity in TNBC and LBC lines. Both CAIX and CAXII showed increased activity in response to reduced pH, which is expected in a bicarbonate-based system. However, UFH-001 cells also exhibited a hypoxic-dependent increase in CAIX activity which is associated with increased protein expression.

In conclusion, these observations demonstrate that CAIX expression is associated with the TNBC phenotype. Based on our activity data, we would predict that CA activity in TNBC tumors will be sensitive to both hypoxia (based on enhanced expression) and reduced pH. This change in activity may serve to regulate pH in the tumor microenvironment favoring an aggressive phenotype. On the other hand, LBC tumors, which are ER-positive, are only associated with CAXII expression. In luminal cells, we expect that only pH and not hypoxia will affect CAXII activity. This may, in part, explain the more positive prognosis in patients with CAXII expression.

Note: This abstract was not presented at the meeting.

Citation Format: Zhijuan Chen, Mam Y. Mboge, Chingkuang Tu, Lingbao Ai, Coy Heldermon, Susan C. Frost. Comparison of carbonic anhydrase & activity between triple-negative & luminal breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5936. doi:10.1158/1538-7445.AM2017-5936

Abstract 2931: Targeting membrane-bound carbonic anhydrases in breast cancer to intervene in the metastatic phenotype

Breast cancer is the second leading cause of cancer related deaths among women in the United States. Despite the tremendous progress that has been made towards treating localized tumors, nearly 40,000 women die each year, predominantly from metastatic drug resistance. The tumor microenvironment plays a pivotal role in determining tumor growth, invasion, metastasis, and therapeutic success or failure. Therefore, a therapy targeting the tumor microenvironment is needed to sufficiently preserve the quality of life of cancer patients, by inhibiting metastasis. Elevated levels of Carbonic anhydrase IX (CAIX) expression in primary breast cancers is a marker for highly aggressive and metastatic tumors, especially of the triple negative subtype (TNBC). It is also associated with hypoxia, extracellular acidification, and poor prognosis. Low pH (values of ~6.5-6.8) is toxic to normal cells in the tumor microenvironment while enhancing cancer cell proliferation and tumor growth. Our goal was to compare the structure of a CAIX-mimic bound to ureidosulfonamide inhibitors with the biological activity of these inhibitors in triple negative and estrogen receptor positive (ER+) breast cancer cell lines. CAIX is a reversible enzyme and at low pH (high proton concentration), the enzyme will consume protons, raising pH. Our hypothesis is that CAIX inhibition, in the context of an acidic microenvironment, will dysregulate its ability to maintain the acidic pH preferred by cancer cells which favors their growth and migration. In this study, we have shown the interaction of sulfonamide-based inhibitors with a CAIX-mimic using X-ray crystallography. These structures show that the inhibitors make multiple contacts within the active site cavity. This is consistent with the inhibitor-induced decrease in CAIX activity and to some extent expression. We have also investigated the effect of CA inhibition on breast cancer cell growth, proliferation, activation of cell death pathways and migration. This reveals that, although CA inhibition with the sulfonamide-based compounds inhibits cell growth and migration, it does not activate apoptotic pathways. In total, these observations indicate that CAIX is a viable small molecular drug target for the treatment of metastatic breast cancer.

Citation Format: Mam Y. Mboge, Zhijuan Chen, Brian P. Mahon, Chingkkuang Tu, Alyssa S. Wolff, John V. Mathias, Fabrizio Carta, Claudiu T. Supuran, Rob McKenna, Susan C. Frost. Targeting membrane-bound carbonic anhydrases in breast cancer to intervene in the metastatic phenotype [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2931. doi:10.1158/1538-7445.AM2017-2931

Structure activity study of carbonic anhydrase IX: Selective inhibition with ureido-substituted benzenesulfonamides

Ureido-substituted benzenesulfonamides (USBs) show great promise as selective and potent inhibitors for human carbonic anhydrase hCA IX and XII, with one such compound (SLC-0111/U-F) currently in clinical trials (clinical trials.gov, NCT02215850). In this study, the crystal structures of both hCA II (off-target) and an hCA IX-mimic (target) in complex with selected USBs (U-CH3, U-F, and U-NO2), at resolutions of 1.9 Å or better, are presented, and demonstrate differences in the binding modes within the two isoforms. The presence of residue Phe 131 in hCA II causes steric hindrance (U-CH3, 1765 nM; U-F, 960 nM; U-NO2, 15 nM) whereas in hCA IX (U-CH3, 7 nM; U-F, 45 nM; U-NO2, 1 nM) and hCA XII (U-CH3, 6 nM; U-F, 4 nM; U-NO2, 6 nM), 131 is a Val and Ala, respectively, allows for more favorable binding. Our results provide insight into the mechanism of USB selective inhibition and useful information for structural design and drug development, including synthesis of hybrid USB compounds with improved physiochemical properties.

The Structure of Carbonic Anhydrase IX Is Adapted for Low-pH Catalysis

Human carbonic anhydrase IX (hCA IX) expression in many cancers is associated with hypoxic tumors and poor patient outcome. Inhibitors of hCA IX have been used as anticancer agents with some entering Phase I clinical trials. hCA IX is transmembrane protein whose catalytic domain faces the extracellular tumor milieu, which is typically associated with an acidic microenvironment. Here, we show that the catalytic domain of hCA IX (hCA IX-c) exhibits the necessary biochemical and biophysical properties that allow for low pH stability and activity. Furthermore, the unfolding process of hCA IX-c appears to be reversible, and its catalytic efficiency is thought to be correlated directly with its stability between pH 3.0 and 8.0 but not above pH 8.0. To rationalize this, we determined the X-ray crystal structure of hCA IX-c to 1.6 Å resolution. Insights from this study suggest an understanding of hCA IX-c stability and activity in low-pH tumor microenvironments and may be applicable to determining pH-related effects on enzymes.

Abstract B25: Characterization, targeting, and modulation of carbonic anhydrase IX activity for the development of small-molecule inhibitors to treat triple-negative breast cancer

The microenvironment within a solid tumor is usually heterogeneous with certain regions being acidic and hypoxic. These acidic and hypoxic regions arise from rapidly proliferating cells combined with poor tumor perfusion. Cancer cells cope with these hostile changes in their microenvironment by expressing genes that are essential for survival. One of the coping mechanisms is an upregulation of pH regulatory factors, including carbonic anhydrase IX (CAIX). The action of CAIX helps to maintain physiological pH inside the cell (pHi) while regulating extracellular acidification (pHe). Extracellular acidification of the tumor microenvironment promotes local invasion, metastasis and decreases the effectiveness of adjuvant therapies, thus contributing to poor clinical outcome. Our goal was to compare the structure of a CAIX-mimic bound to ureidosulfonamide inhibitors with the biological activity of these inhibitors in a triple negative breast cancer cell line. CAIX is a reversible enzyme and at low pH (high proton concentration), the enzyme will consume protons, raising pH. Our hypothesis is that CAIX inhibition, in the context of an acidic microenvironment, will dysregulate its ability to maintain the acidic pH preferred by cancer cells which favors their growth and migration. In this study, we have shown the interaction of sulfonamide-based inhibitors using X-ray crystallography methods. These structures show the inhibitors make multiple contacts within the active site cavity. This is consistent with the inhibitor-induced decrease in CAIX activity measured as 18O exchange between H2O16 and H13C18O3 . We have also investigated the effect CAIX inhibition on cancer cell metabolism and extracellular acidification using Seahorse technology. This reveals that CAIX may contribute to the “non-glycolytic” acidification process. In total, these observations indicate that CAIX is a viable small molecular drug target and contribute to our understanding of the function of CAIX in modulating pH in cancer cells.

Citation Format: Mam Y. Mboge, Zhijuan Chen, Brian P. Mahon, Nicole Lamas, Shingkuang Tu, Fabrizio Carta, Claudiu T. Superan, Robert McKenna, Susan C. Frost. Characterization, targeting, and modulation of carbonic anhydrase IX activity for the development of small-molecule inhibitors to treat triple-negative breast cancer. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr B25.

Abstract C37: Differential expression of carbonic anhydrases and proton transporters in co-cultured models of the breast cancer microenvironment

Extracellular acidification (pHe) within the tumor microenvironment is a well-accepted hallmark for aggressive tumor behavior. Hypoxia is thought to play a key role in the acidic tumor microenvironment. Hypoxia triggers a shift to glycolytic metabolism and leads to an excess of acidic products, including lactic acid, protons and carbon dioxide. To counteract the acidosis, cells activate the pH regulatory machinery which has two major transport arms: the lactate and proton export arm and bicarbonate import arm. These two arms operate through several types of transmembrane proton transporters (TPTs). These transporters include the monocarboxylate transporters (MCTs), the vacuolar ATPase (v-ATPase), the sodium/proton exchangers (NHE-1), and the sodium/bicarbonate transporters (NBC). In addition, it is hypothesized that membrane-bound isoforms of the carbonic anhydrase family (CAIX and CAXII) may balance pHe to the advantage of cancer cells by improved buffering utilizing the CO2/bicarbonate system.

Within the tumor microenvironment, there are both malignant cells and adjacent stromal cells, including fibroblasts, endothelial cells, adipocytes, pericytes, and inflammatory macrophages. Communication and interactions between tumor cells and stromal cells may contribute to the pH regulation of the tumor microenvironment. A recent study by Fiaschi et al. (2013) showed that cultured, healthy human prostate fibroblasts could be converted to the phenotype of cancer-associated fibroblasts (CAFs) by incubation with conditioned medium from cultured prostate cancer cells. This conversion induced the upregulation of CAIX. This might suggest that CAFs play a role in pH regulation in the tumor microenvironment.

To determine if breast cancer fibroblasts respond in like, we co-cultured MDA-MB-231 (triple-negative, CAIX-positive) and T47D (ER-positive, CAXII-positive) breast cancer cells with human, mammary-specific, cancer-associated fibroblasts using the transwell system. In this setting, we were unable to demonstrate that either CAIX or CAXII expression changed in the CAFs. We also explored the expression of the TPTs. Interestingly, triple negative and ER-positive breast cancer cells showed differential expression of these family members. In the co-culture system, vATPase expression increased in CAFs co-cultured with the MDA cells where reduced expression was observed. Knockdown of CAIX in the MDA cells did not alter this difference. Yet loss of CAIX in the MDA cells reduced their expression of NHE-1. We observed no expression difference in CAFs co-cultured with T47D cells. Knockdown of CAXII in T47D cells enhanced NHE-1 expression but reduced MCT4 expression. These preliminary data suggest that there are complicated interactions between CAFs and breast cancer cells not related to CAIX or CAXII expression. Yet, loss of expression of CAIX or CAXII did change the expression of TPTs in the cancer cells, suggesting the existence of compensatory mechanisms.

Citation Format: Zhijuan Chen, Susan C. Frost. Differential expression of carbonic anhydrases and proton transporters in co-cultured models of the breast cancer microenvironment. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr C37.

Effects of acid-base variables and the role of carbonic anhydrase on oxalate secretion by the mouse intestine in vitro

Hyperoxaluria is a major risk factor for calcium oxalate kidney stones and the intestine is recognized as an important extra-renal pathway for eliminating oxalate. The membrane-bound chloride/bicarbonate (Cl(-)/) exchangers are involved in the transcellular movement of oxalate, but little is understood about how they might be regulated. , CO2, and pH are established modulators of intestinal NaCl cotransport, involving Na(+)/H(+) and Cl(-)/ exchange, but their influence on oxalate transport is unknown. Measuring (14)C-oxalate and (36)Cl fluxes across isolated, short-circuited segments of the mouse distal ileum and distal colon we examined the role of these acid-base variables and carbonic anhydrase (CA) in oxalate and Cl(-) transport. In standard buffer both segments performed net oxalate secretion (and Cl(-) absorption), but only the colon, and the secretory pathway were responsive to and CO2. Ethoxzolamide abolished net oxalate secretion by the distal colon, and when used in tandem with an impermeant CA inhibitor, signaled an intracellular CA isozyme was required for secretion. There was a clear dependence on as their removal eliminated secretion, while at 42 mmol/L was also decreased and eradicated. Independent of pH, raising Pco2 from 28 to 64 mmHg acutely stimulated net oxalate secretion 41%. In summary, oxalate secretion by the distal colon was dependent on , CA and specifically modulated by CO2, whereas the ileum was remarkably unresponsive. These findings highlight the distinct segmental heterogeneity along the intestine, providing new insights into the oxalate transport mechanism and how it might be regulated.

Advances in Anti-Cancer Drug Development Targeting Carbonic Anhydrase IX and XII

The microenvironment within a solid tumor is heterogeneous with regions being both acidic and hypoxic. As a result of this, cancer cells upregulate genes that allow survival in such environments. Some of these genes are pH regulatory factors, including carbonic anhydrase IX (CA IX) and in some cases XII (CA XII). CA IX helps to maintain normal cytoplasmic pH (pHi) while simultaneously contributing to the extracellular pH (pHe). CA XII is also thought to be responsible for stabilizing pHe at physiological conditions. Extracellular acidification of the tumor microenvironment promotes local invasion and metastasis while decreasing the effectiveness of adjuvant therapies, thus contributing to poor cancer clinical outcomes. In this review, we describe the properties of CA IX and CA XII that substantiate their potential use as anticancer targets. We also discuss the current status of CA isoform-selective inhibitor development and patents of CA IX/XII targeted inhibitors that show potential for treating aggressive tumors. Some of the recently published patents discussed include sulfonamide-based small molecule inhibitors including derivatives of boron cluster compounds; metal complexes of poly(carboxyl)amine-containing ligands; nitroi-midazole-, ureidosulfonamide-, and coumarin-based compounds; as well as G250 and A610 monoclonal antibodies for cancer treatment.

Physiological functions of the alpha class of carbonic anhydrases

Carbonic anhydrases are ubiquitous enzymes that catalyze the reversible hydration of carbon dioxide. These enzymes are of ancient origin as they are found in the deepest of branches of the evolutionary tree. Of the five different classes of carbonic anhydrases, the alpha class has perhaps received the most attention because of its role in human pathology. This review focuses on the physiological function of this class of carbonic anhydrases organized by their cellular location.

Overview of the carbonic anhydrase family

The purpose of this collection of chapters is to provide a glimpse of where the carbonic anhydrase (CA) field is. This book is by no means fully inclusive, as only a few of the lead researchers around the world contributed; it serves only to show that the CA field is still pushing the boundaries of research as it has done since its discovery, and will do for a long time to come.

Role of zinc in catalytic activity of carbonic anhydrase IX

The carbonic anhydrases (CAs) in the α class are zinc-dependent metalloenzymes. Previous studies have reported that recombinant forms of carbonic anhydrase IX (CAIX), a membrane-bound form of CA expressed in solid tumors, appear to be activated by low levels of zinc independent of its well-studied role at the catalytic site. In this study, we sought to determine if CAIX is stimulated by zinc in its native environment. MDA-MB-231 breast cancer cells express CAIX in response to hypoxia. We compared CAIX activity associated with membrane ghosts isolated from hypoxic cells with that in intact hypoxic cells. We measured CA activity directly using (18)O exchange from (13)CO(2) into water determined by membrane inlet mass spectrometry. In membrane ghosts, there was little effect of zinc at low concentrations on CAIX activity, although at high concentration zinc was inhibitory. In intact cells, zinc had no significant effect on CAIX activity. This suggests that there is an appreciable decrease in sensitivity to zinc when CAIX is in its natural membrane milieu compared to the purified forms.

Abstract 2056: Evidence against hypoxic-dependent activation of carbonic anhydrase IX in MDA-MB-231 breast cancer cells

Carbonic anhydrase IX (CAIX) is a membrane-bound, tumor-related enzyme the expression of which is often considered a marker for hypoxia, an indicator of poor prognosis, and associated with acidification of the tumor microenvironment. Many studies have shown that CAIX expression is induced by hypoxia exposing its catalytic domain to the interstitial milieu. Several recent studies have suggested that hypoxic conditions may also permit activation of CAIX, perhaps by causing a conformational change which exposes the catalytic pocket. Our goal was to assess the effect of anoxic conditions on CAIX activity in MDA-MB-231 cells, previously exposed to hypoxia which increases CAIX expression in the absence of other membrane-bound carbonic anhydrase (CA) family members.

We have taken advantage of membrane inlet mass spectrometry (MIMS) to directly analyze CA activity in intact cells by measuring the 18O exchange between CO2 and H2O. This method distinguishes between intracellular and extracellular CA activity. MDA-MB-231 cells were exposed to 1% oxygen for 16 hours after which they were isolated under normoxic or anoxic conditions. CA activity was then measured, again under normoxic or anoxic conditions. These data show biphasic depletion of 18O from CO2 under both normoxic and anoxic assay conditions. The first phase (which occurs over the first 20-40 seconds) represents a rapid diffusion of CO2 into cells where it is exposed to intracellular CAII. A catalytic cycle depletes 18O followed by efflux of CO2 from the cell. There was no difference in this phase between cells prepared and assayed under normoxic or anoxic conditions. The second phase (from 200-500 sec) is dominated by the hydration-dehydration reaction of CO2/HCO3− catalyzed by exofacial CA activity (CAIX). In cells exposed to anoxia, the slope of the second phase was greater than that observed with cells exposed to normoxic conditions indicating elevated CAIX activity. While this provided evidence that oxygen limitation might influence CAIX activity, the Ki values for two impermeant CA inhibitors, Cpd 5C and a polymeric sulfonamide, did not differ between anoxic and normoxic cells. We conclude from these data that the catalytic site of CAIX is exposed and functional under both anoxic or normoxic conditions.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2056. doi:10.1158/1538-7445.AM2011-2056

Catalysis and pH control by membrane-associated carbonic anhydrase IX in MDA-MB-231 breast cancer cells

Carbonic anhydrase IX (CAIX) is a membrane-bound, tumor-related enzyme whose expression is often considered a marker for hypoxia, an indicator of poor prognosis in the majority of cancer patients, and is associated with acidification of the tumor microenvironment. Here, we describe for the first time the catalytic properties of native CAIX in MDA-MB-231 breast cancer cells that exhibit hypoxia-inducible CAIX expression. Using (18)O exchange measured by membrane inlet mass spectrometry, we determined catalytic activity in membrane ghosts and intact cells. Exofacial carbonic anhydrase activity increases with exposure to hypoxia, an activity which is suppressed by impermeant sulfonamide CA inhibitors. Inhibition by sulfonamide inhibitors is not sensitive to reoxygenation. CAIX activity in intact cells increases in response to reduced pH. Data from membrane ghosts show that the increase in activity at reduced pH is largely due to an increase in the dehydration reaction. In addition, the kinetic constants of CAIX in membrane ghosts are very similar to our previous measurements for purified, recombinant, truncated forms. Hence, the activity of CAIX is not affected by the proteoglycan extension or membrane environment. These activities were measured at a total concentration for all CO(2) species at 25 mm and close to chemical equilibrium, conditions which approximate the physiological extracellular environment. Our data suggest that CAIX is particularly well suited to maintain the extracellular pH at a value that favors the survival fitness of tumor cells.

Role of hypoxia and EGF on expression, activity, localization and phosphorylation of carbonic anhydrase IX in MDA-MB-231 breast cancer cells

Carbonic anhydrase IX (CAIX) is a zinc metalloenzyme that catalyzes the reversible hydration of CO(2). CAIX is overexpressed in many types of cancer, including breast cancer, but is most frequently absent in corresponding normal tissues. CAIX expression is strongly induced by hypoxia and is significantly associated with tumor grade and poor survival. Herein, we show that hypoxia induces a significant increase in CAIX protein in MDA-MB-231 breast cancer cells. Using a unique mass spectrophotometric assay, we demonstrate that CAIX activity in plasma membranes isolated from MDA-MB-231 is correlated with CAIX content. We also show that CAIX exists predominantly as a dimeric, high-mannose N-linked glycoprotein. While there is some evidence that the dimeric form resides specifically in lipid rafts, our data do not support this hypothesis. EGF, alone, did not affect the distribution of CAIX into lipid rafts. However, acute EGF treatment in the context of hypoxia increased the amount of CAIX in lipid rafts by about 5-fold. EGF did not stimulate tyrosine phosphorylation of CAIX, although EGFR and down-stream signaling pathways were activated by EGF. Interestingly, hypoxia activated Akt independent of EGF action. Together, these data demonstrate that the active form of CAIX in the MDA-MB-231 breast cancer cell line is dimeric but that neither lipid raft localization nor phosphorylation are likely required for its dimerization or activity.

Expression and activity of carbonic anhydrase IX is associated with metabolic dysfunction in MDA-MB-231 breast cancer cells

The expression of carbonic anhydrase IX (CAIX), a marker for hypoxic tumors, is correlated with poor prognosis in breast cancer patients. We show herein that the MDA-MB-231 cells, a "triple-negative," basal B line, express exclusively CAIX, while a luminal cell line (T47D) expresses carbonic anhydrase XII (CAXII). CAIX expression in the basal B cells is both density- and hypoxia-dependent and is correlated with carbonic anhydrase activity. Evidence is provided that CAIX contributes to extracellular acidification through studies on pH, lactic acid production, and CAIX inhibition. Together, these studies suggest that CAIX expression and activity is associated with metabolic dysfunction in MDA-MB-231 cells.

Soluble fibroin enhances insulin sensitivity and glucose metabolism in 3T3-L1 adipocytes

Type 2 diabetes is characterized by hyperglycemia and hyperinsulinemia, features of insulin resistance. In vivo treatment of ob/ob mice with hydrolyzed fibroin reverses these pathological attributes. To explore the mechanism underlying this effect, we used the murine, 3T3-L1 adipocyte cell line, which has been used extensively to model adipocyte function. Chronic exposure of 3T3-L1 adipocytes to insulin leads to a 50% loss of insulin-stimulated glucose uptake. Chronic exposure to different preparations of fibroin partially blocked the response to insulin but also increased the sensitivity of control cells to the acute action of insulin. The latter effect was most robust at physiologic concentrations of insulin. Fibroin did not prevent the insulin-induced downregulation of the insulin receptor or the tyrosine kinase activity associated with the receptor. Further, fibroin had no effect on the activity of the insulin-sensitive downstream kinase, Akt. Interestingly, fibroin accelerated glucose metabolism and glycogen turnover independent of insulin action. In addition, fibroin upregulated glucose transporter (GLUT)1, which increased its expression at the cell surface and enhanced GLUT4 translocation. Together, these phenomena may underlie the improvement in diabetic hyperglycemia noted in vivo in response to fibroin.

Glucose deprivation enhances targeting of GLUT1 to lipid rafts in 3T3-L1 adipocytes

Glucose deprivation dramatically increases glucose transport activity in 3T3-L1 adipocytes without changing the concentration of GLUT1 in the plasma membrane (PM). Recent data suggest that subcompartments within the PM, specifically lipid rafts, may sequester selected proteins and alter their activity. To evaluate this possibility, we examined the distribution of GLUT1 in Triton X-100-soluble and -insoluble fractions. Our data show that 77% of the GLUT1 pool in PMs isolated from control 3T3-L1 adipocytes was extracted by 0.2% Triton X-100. After glucose deprivation for 12 h, only 56% of GLUT1 was extracted by detergent. In contrast, there was a twofold increase in the GLUT1 content of the detergent-resistant fraction. To evaluate whether GLUT1 interacts with a specific protein within lipid rafts, we focused on stomatin, recently shown to interact with and inhibit GLUT1 activity. Stomatin is distributed about equally between the PM and the biosynthetic compartments, and its expression is not affected by glucose deprivation. Nearly 90% of the PM pool of stomatin is in detergent-resistant lipid rafts. In normal 3T3-L1 adipocytes, we were unable to demonstrate an interaction between GLUT1 and stomatin in coimmunoprecipitation experiments. However, in stomatin-overexpressing cells, there was clear coprecipitation of stomatin with GLUT1 antibodies. Glucose deprivation increased this interaction threefold, which may reflect the increase of GLUT1 in lipid rafts. Despite this, there was little change in transport activity in glucose-deprived, stomatin-overexpressing cells vs. that in control cells. Thus GLUT1 interacts with stomatin in lipid rafts, but this interaction per se does not alter transport activity. Rather, stomatin may serve as an anchor for GLUT1 in lipid rafts, the environment of which favors activation.

Glycogen phosphorylase is activated in response to glucose deprivation but is not responsible for enhanced glucose transport activity in 3T3-L1 adipocytes

We have previously shown that glucose deprivation activates glucose transport in a time- and protein synthesis-dependent fashion in 3T3-L1 adipocytes, a mouse cell line. Coincident with this is loss of glycogen. Because glycogen phosphorylase (GP) is responsible for glycogen degradation, we have examined its regulation to determine the relationship between transport activation and glycogen turnover. We first cloned the adipose GP cDNA and found sequence similarity to rat and human liver GP. Because the mouse liver GP cDNA sequence was unavailable, we cloned this cDNA as well and showed 100% identity between mouse adipose and liver sequences. A 3.1 kb transcript was readily observed in total RNA isolated from mouse liver or adipose by Northern blot analysis but, surprisingly, not in either total or poly(A) selected RNA from 3T3-L1 adipocytes. To evaluate regulation in 3T3-L1 adipocytes, we amplified GP mRNA from total RNA using multiplex, semi-quantitative PCR but found that expression did not change in response to deprivation. GP protein levels did not change either. However, endogenous GP activity from glucose-deprived cells was significantly elevated relative to controls, due to an increase in the phosphorylated form of GP (GPa). Finally, we overexpressed GP to determine its direct influence on the glucose transport system. These results were negative, which suggests that the nutrient control of glucose transport and GP occurs independently despite kinetic similarities in transport activation and glycogen turnover.

Alternative glycosylation of the insulin receptor prevents oligomerization and acquisition of insulin-dependent tyrosine kinase activity

Glucose deprivation leads to the synthesis of an aberrantly glycosylated ('alternative') and inefficiently processed form of the insulin proreceptor in 3T3-L1 adipocytes. To further explore the effect of aberrant (rather than absent) N-linked glycosylation of the insulin receptor, we examined the relationship of processing to function. Our studies show that the alternative form of the proreceptor does not oligomerize nor does it acquire the ability to undergo insulin-sensitive autophosphorylation. This along with an interaction with the glucose-regulated stress protein GRP78/BiP implies inappropriate folding/dimerization and retention in the ER. Glucose refeeding causes the post-translational modification of the alternative form of the proreceptor to a novel 'intermediate' form which is independent of new protein synthesis. As little as 100 microM glucose (or mannose) can induce this modification. In vitro digestion of the alternative and intermediate proreceptors with SPC1/furin shows that both the alpha- and beta-subunit domains are glycosylated, albeit aberrantly. This implies that the aberrantly glycosylated proreceptor could serve as a substrate for SPC1 in a physiological setting if the receptor was able to interact with the enzyme in the appropriate compartment (i.e., the trans-Golgi network). Based on inhibitor studies, however, both the alternative and intermediate forms of the proreceptor appear to be primarily targeted to the proteasome for degradation.

Glucose deprivation does not affect GLUT1 targeting in 3T3-L1 adipocytes

We have previously demonstrated that glucose deprivation alters the glycosylation of the GLUT1 glucose transporter in 3T3-L1 adipocytes. Many aberrantly glycosylated proteins are retained in the endoplasmic reticulum by interaction with chaperones. Herein, we use three independent procedures to show that GLUT1 is targeted to the plasma membrane, despite alterations in glycosylation. While earlier experiments revealed that plasma membrane targeting of aglyco GLUT 1 transporter was significantly reduced, our data show for the first time that altered glycosylation provides sufficient information to drive appropriate trafficking.

Effect of alternative glycosylation on insulin receptor processing

The mature insulin receptor is a cell surface heterotetrameric glycoprotein composed of two alpha- and two beta-subunits. In 3T3-L1 adipocytes as in other cell types, the receptor is synthesized as a single polypeptide consisting of uncleaved alpha- and beta-subunits, migrating as a 190-kDa glycoprotein. To examine the importance of N-linked glycosylation on insulin receptor processing, we have used glucose deprivation as a tool to alter protein glycosylation. Western blot analysis shows that glucose deprivation led to a time-dependent accumulation of an alternative proreceptor of 170 kDa in a subcellular fraction consistent with endoplasmic reticulum localization. Co-precipitation assays provide evidence that the alternative proreceptor bound GRP78, an endoplasmic reticulum molecular chaperone. N-Glycosidase F treatment shows that the alternative proreceptor contained N-linked oligosaccharides. Yet, endoglycosidase H insensitivity indicates an aberrant oligosaccharide structure. Using pulse-chase methodology, we show that the synthetic rate was similar between the normal and alternative proreceptor. However, the normal proreceptor was processed into alpha- and beta-subunits (t((1)/(2)) = 1.3 +/- 0.6 h), while the alternative proreceptor was degraded (t((1)/(2)) = 5.1 +/- 0.6 h). Upon refeeding cells that were initially deprived of glucose, the alternative proreceptor was processed to a higher molecular weight form and gained sensitivity to endoglycosidase H. This "intermediate" form of the proreceptor was also degraded, although a small fraction escaped degradation, resulting in cleavage to the alpha- and beta-subunits. These data provide evidence for the first time that glucose deprivation leads to the accumulation of an alternative proreceptor, which can be post-translationally glycosylated with the readdition of glucose inducing both accelerated degradation and maturation.

Is stress radiography necessary in the diagnosis of acute or chronic ankle instability?

BACKGROUND

Clinicians often use the talar tilt (TT) and anterior drawer (AD) stress x-rays to diagnose acute or chronic mechanical ankle instability. However, the wide range of TT and AD values in normal and injured ankles makes interpretation of the test results difficult.

OBJECTIVE

To critically review the literature and determine the accuracy of stress radiography in the diagnosis of mechanical ankle instability.

DATA SOURCES

MEDLINE was searched for relevant articles published since 1966 using MEDLINE subject headings (MeSH) and textwords for English articles related to ankle injuries and radiography. Additional references were reviewed from the bibliographies of the retrieved articles. The total number of articles reviewed was 67. Of these, 8 studies met criteria for inclusion and were analyzed.

STUDY SELECTION

Only clinical studies that used surgical exploration as the gold standard for diagnosing lateral ligament rupture were evaluated for this study. Cadaver or laboratory studies were excluded.

DATA EXTRACTION AND SYNTHESIS

In reviewing the literature, pertinent strengths of the different study designs were emphasized. From these data, particular attention was paid to the diagnostic accuracy of each study in comparing TT and AD stress x-rays to surgical confirmation of lateral ligament rupture.

MAIN RESULTS

A total of eight prospective clinical series satisfied the inclusion criteria. Seven of the eight assessed acute ankle instability as the outcome and one assessed chronic ankle instability. Of the seven studies that focused on acute ankle injuries, only one concluded significant benefit in using stress views to diagnose lateral ligament rupture. Three of the seven reported a positive relationship between stress radiography and surgical findings, although all six studies concluded that TT and AD stress x-rays are not reliable enough to make the diagnosis. The authors who assessed chronic ankle instability stated that TT and AD stress views combined were not useful in defining ankle instability.

CONCLUSION

The published data regarding TT and AD stress x-rays are too variable to determine accepted normal values compared with injured values. There are insufficient data for comparison of the use of mechanical versus manual techniques, or use of local anesthetic to facilitate the stress test. Because the treatment evolution of all acute ankle sprains is toward functional nonoperative treatment and because treatment does not depend on the degree of ankle instability on stress views, the TT and AD stress x-rays have no clinical relevance in the acute situation. In cases of chronic instability, the large variability in TT and AD values in both injured and noninjured ankles precludes their routine use.

Development of insulin resistance in 3T3-L1 adipocytes

Insulin resistance is a manifestation of both diabetes mellitus and obesity. However, the mechanism is still not clearly identified. Herein, we describe a procedure that allows us to evaluate the development of insulin resistance in 3T3-L1 adipocytes. Under these conditions, we show that the concentration of insulin required for 50% desensitization of glucose transport activity is 100 pM; maximal desensitization could be achieved with 1 nM. This demonstrates for the first time that 3T3-L1 adipocytes develop insulin resistance in response to physiologically relevant concentrations of insulin. Glucose (or glucosamine), in addition to insulin, was required to establish desensitization. The expression of GLUT4 protein decreased by 50% with exposure to 10 nM insulin. The dose-dependent loss of GLUT4 was similar to the dose dependence for insulin-resistant transport activity. Translocation in the presence of acute insulin was apparent, but the extent of recruitment directly reflected the decrease in GLUT4 protein. GLUT4 mRNA also declined, but the ED50 was approximately 5 nM. Together, these data suggest that the loss of GLUT4 protein likely underlies the cause of desensitization. However, the loss of GLUT4 protein did not correlate with the loss in GLUT4 mRNA suggesting post-translational control of GLUT4 expression.

Insulin-stimulated glucose disposal: does adipose play a role?

Although muscle is thought to be a primary assimilator of glucose, adipose may provide a substantial amount of substrate for gluconeogenesis, even in the fed state.

Differential regulation of GRP78 and GLUT1 expression in 3T3-L1 adipocytes

We tested the hypothesis that the constitutive glucose transporter (GLUT1) in 3T3-L1 adipocytes belongs to the family of glucose-regulated proteins which are transcriptionally regulated by glucose deprivation. Using cDNA probes for both GRP78 (BiP) and GLUT1, we show that the level of GRP78 mRNA increased by 15-fold within 24 h of glucose deprivation with little change in GLUT1 mRNA. The elevated GRP78 mRNA in turn led to a time-dependent increase in GRP78 protein. While glucose deprivation did not alter the expression of the normal glycoform of GLUT1, a lower molecular weight glycoform accumulated with extended deprivation. Mannose and fructose, but not galactose, prevented the induction of GRP78 and accumulation of the abnormal GLUT1. Because GRP78 acts as a chaperone in other cell systems, we also sought evidence to support this activity in 3T3-L1 adipocytes. Using the technique of co-immunoprecipitation, we demonstrate that GRP78 bound several proteins unique to the glucose-deprived state. No deprivation-specific proteins could be detected in association with GLUT1. These data lead us to conclude that GLUT1 does not display characteristics of the glucose-regulated proteins, at least in 3T3-L1 adipocytes, a widely used model for differentiation, hormone action, and nutrient control. However, the mechanisms for activating traditional members of this family appear intact.

Translocation of GLUT1 does not account for elevated glucose transport in glucose-deprived 3T3-L1 adipocytes

Glucose deprivation increases the rate of glucose transport in 3T3-L1 adipocytes in a protein synthesis-dependent fashion. To determine if translocation of either GLUT1 or GLUT4 is responsible for this phenomenon, we adapted existing fractionation procedures toward isolating 3T3-L1 adipocyte membranes. By Western blot analysis of equal protein, GLUT1 was distributed between plasma membranes, high density "microsomal" membranes, and low density "microsomal" membranes isolated from control cells. GLUT4 comigrated with high density and low density membranes. Glucose deprivation for 12 h did not alter the distribution of either GLUT1 or GLUT4, despite an 8-10-fold increase in glucose transport activity in intact cells. Importantly, increased transport activity was retained in plasma membrane vesicles isolated from glucose-deprived cells. These data show for the first time that the increase in transport activity associated with glucose deprivation does not result from the translocation of either of the glucose transporters known to exist in 3T3-L1 adipocytes. As GLUT4 is excluded from the plasma membrane, these data provide evidence for activation of GLUT1.

Glycogen: a carbohydrate source for GLUT-1 glycosylation during glucose deprivation of 3T3-L1 adipocytes

In 3T3-L1 adipocytes, the glycosylation of the GLUT-1 transporter is altered beyond 12 h of glucose deprivation. To determine whether glycogen degradation provides substrate for normal protein glycosylation during this delay, we measured the glycogen content of 3T3-L1 adipocytes. From an initial value of 0.537 +/- 0.097 mumol glucose/10(6) cells, glycogen was depleted in a time-dependent manner in response to glucose deprivation, exhibiting a half-time of 6 h. Surprisingly, fructose did not prevent glycogen depletion. However, in such glycogen-depleted adipocytes, the alteration of GLUT-1 glycosylation in response to glucose deprivation was more rapid than in normal adipocytes. Chinese hamster ovary (CHO) cells, which synthesize abbreviated dolichol-linked oligosaccharides within minutes of glucose deprivation (J. I. Rearick, A. Chapman, and S. Kornfeld. J. Biol. Chem. 256: 6255-6261, 1981), contained only 1% of the level of glycogen found in 3T3-L1 adipocytes. Glycosylation of GLUT-1 was altered in CHO cells within 3 h of glucose deprivation. These data demonstrate that, during glucose stress, glycogen may serve as a buffer for oligosaccharide biosynthesis and provide a potential explanation for varying sensitivities of different cell types to glucose deprivation.

Nutrient control of GLUT1 processing and turnover in 3T3-L1 adipocytes

Metabolic labeling and immunoprecipitation were used to analyze the glucose-dependent regulation of GLUT1 synthesis, processing, and turnover in a murine adipocyte cell line. Metabolically labeled GLUT1 from control cells migrated as a 46-kDa protein, while GLUT1 from cells deprived of glucose for more than 12 h migrated as a 37-kDa protein. On the basis of tunicamycin sensitivity, both GLUT1 species arose from a common protein migrating at 36 kDa. In addition, the rate of synthesis of GLUT1 in control and glucose-deprived cells was similar. In short pulse-chase experiments, we distinguished two species arising from the core GLUT1 protein in control cells; an intermediate and the mature 46-kDa species. In contrast, only one glycoform, the 37-kDa species, arose from the core protein in glucose-deprived cells, which was not further processed in either the presence or absence of glucose. Although 12-18 h of glucose deprivation were required to affect GLUT1 glycosylation, glucose-deprived cells quickly recovered the ability to correctly glycosylate GLUT1 upon the readdition of glucose (t1/2 < 1 h). GLUT1 in control adipocytes exhibited a half-life of approximately 14 h, while that in glucose-deprived adipocytes was greater than 50 h. This effect was readily reversed upon the readdition of glucose. In total, these data show that glucose deprivation alters both the processing (glycosylation) and turnover (degradation) of GLUT1. These results are discussed in light of transport function.

Effect of glucose deprivation of GLUT 1 expression in 3T3-L1 adipocytes

Elevated glucose transport rates during glucose deprivation are phenomena that have been observed in several different types of cells in culture. We show here that glucose transport rates in 3T3-L1 adipocytes increased by 10-fold within 18 h in response to glucose deprivation, confirming earlier work by Van Putten and Krans (Van Putten, J. P. M., and Krans, H. M. J. (1985) J. Biol. Chem. 260, 7996-8001). Mannose and 3-O-methylglucose (a nonmetabolizable glucose analog), but not fructose or galactose, blocked the increase in transport activity. Although the increase in transport was dependent on new protein synthesis, only a small and transient increase in GLUT 1 mRNA (less than 2-fold) was observed. In addition, the level of the normal isoform of GLUT 1 (46 kDa) did not increase. A lower molecular mass isoform (37 kDa) was observed but not until 15 h after glucose removal, the appearance of which was clearly not correlated with the increase in activity. Further, the extracellular glucose concentration required to elicit accumulation of this form (p37) was 2 orders of magnitude less than that required for transport stimulation (5 microM versus 500 microM glucose; p37 accumulation and transport activation, respectively). Interestingly, p37 was seen in the presence of galactose, but not fructose, despite elevated transport activity with either sugar. The p37 isoform was slightly larger than N-glycosidase F-treated GLUT 1 (36 kDa), implying that this form is still glycosylated, albeit incompletely. It is not known if p37 is functional, but the time- and sugar-dependent appearance of the lower isoform suggests that p37 is not responsible for starvation-induced transport but potentially represents an underglycosylated precursor of the normal, 46-kDa isoform of GLUT 1.

Regulation of annexin I in adipogenesis: cAMP-independent action of methylisobutylxanthine

The differentiation of 3T3-L1 fibroblasts to adipocytes can be accelerated by the addition of 1-methyl-3-isobutylxanthine (MIX), insulin, and dexamethasone to the culture medium. During differentiation, we have demonstrated that the level of both annexin I mRNA and protein decreases. The half-times for this reduction were 2 h and 10 h for annexin I mRNA and protein, respectively. Of the added agents in the differentiation medium, only MIX caused a decline in annexin I expression in 3T3-L1 fibroblasts. The MIX effect in fibroblasts was reversible and required de novo transcription but not protein synthesis. Although MIX could be replaced by high levels of theophylline, neither agonists of the beta-adrenergic receptor nor intracellular second messengers, cAMP and cGMP, were able to reduce annexin I. The potential role of annexin I in cellular differentiation is discussed.

Protein-synthesis-dependent induction of annexin I by glucocorticoid

We demonstrate that annexin I/lipocortin I (lipo I) gene expression is regulated by dexamethasone (DEX) in mouse 3T3-L1 fibroblasts and LA-4 lung epithelial cells. We have characterized this induction further in 3T3-L1 fibroblasts. At 24 h after addition of DEX, the levels of lipo I mRNA and protein increased 5-fold and 1.5-fold respectively. Time-course experiments revealed that the induction was delayed by 2-4 h after DEX addition. Half-maximal induction of both lipo I mRNA and protein was achieved with 10 nM-DEX. Both actinomycin D and cycloheximide blocked the DEX effect on lipo I mRNA expression. These results indicate that the induction of lipo I by DEX has a transcriptional component and requires protein synthesis de novo.

Annexin-I regulation in response to suckling and rat mammary cell differentiation

Expression of annexin-I was investigated in the rat mammary gland during pregnancy, lactation, and involution. Both the mRNA and protein were very abundant in the mature virgin gland, but declined significantly by midpregnancy. In the lactating gland, little or no annexin-I was detected. After weaning, mRNA and protein levels increased dramatically, corresponding to glandular involution. We also show that premature removal of the suckling stimulus caused a rapid increase in mRNA expression, but that translation of message was delayed, possibly until the gland was irreversibly committed to involution. Since high levels of annexin-I are associated with the quiescent epithelial cell, annexin-I may play an important role in blocking differentiation of the mammary gland.

Uptake and binding of radiolabelled phenylarsine oxide in 3T3-L1 adipocytes

Phenylarsine oxide (PAO), a trivalent arsenical, has been shown to inhibit insulin-stimulated glucose transport in 3T3-L1 adipocytes, implicating vicinal dithiols in signal transmission [Frost & Lane (1985) J. Biol. Chem. 260, 2646-2652]. To assist in the direct identification of a PAO-binding protein which might be involved in this process, we have synthesized [3H]acetylaminophenylarsine oxide [( 3H]APAO) from the amino derivative of phenylarsine oxide (NPAO). To assess the inhibitory effect of the product, a dual-labelling experiment was performed which showed that [3H]APAO inhibited insulin-stimulated 2-deoxy[1-14C]glucose transport in 3T3-L1 adipocytes with a Ki of 21 microM, identical with that of the parent compound, NPAO. Further characterization revealed that over a wide concentration range, uptake of the labelled arsine oxide was linear. Although the dithiol reagent 2,3-dimercaptopropanol (DMP) reversed PAO-induced inhibition of transport, it had no effect on the uptake of [3H]APAO. In a simple fractionation experiment approx. 50% of the radioactivity was associated with the cytosolic fraction and 50% with the total membrane fraction. Identification of radiolabelled proteins by non-reducing SDS/PAGE revealed fraction-specific binding, although many proteins were observed. Covalent modification was time-dependent and could be reversed by addition of DMP. These data further support a role for vicinal dithiols in insulin-stimulated glucose transport. Additionally, the probe described may offer a new means with which to identify the inhibitory protein or, more globally, to investigate mechanisms of action of vicinal dithiol-containing proteins.

Human relaxin inhibits division but not differentiation of 3T3-L1 cells

For the first time, we demonstrate here the ability of human relaxin to block cell division. During the induction of differentiation of 3T3-L1 fibroblasts to adipocytes, the cells typically undergo two rounds of cell division followed by accumulation of lipid droplets and expression of insulin-stimulated glucose transport as the cells attain the adipocyte phenotype. Human relaxin added during induction had no effect on the development of the adipocyte phenotype or insulin-stimulated glucose transport. However, it blocked cell division at a half-maximal concentration of 1.25 nM, well within physiological range. This could be reversed by the addition of antibodies specific for human relaxin. Thus relaxin joins a select number of hormones with growth inhibitory properties such as transforming growth factor-beta (TGF beta) and mammastatin. Potentially, this is an important but until now unidentified function of relaxin. Unlike other inhibitory polypeptides, like TGF beta, relaxin does not prevent differentiation but rather uncouples it from cell division.

Effect of phenylarsine oxide on fluid phase endocytosis: further evidence for activation of the glucose transporter

We have shown previously that insulin stimulates fluid phase endocytosis in 3T3-L1 adipocytes (Gibbs et al., 1986). Using [14C]sucrose as an endocytotic marker, we show here that phenylarsine oxide, a trivalent arsenical which binds neighboring dithiols, blocked not only insulin-stimulated fluid phase endocytosis, but basal endocytosis as well. The Ki for this process was 6 microM in the presence or absence of insulin and the time required for inhibition was less than 2.5 min, the limit of detection in our assay system. These results can be compared with the inhibitory effect of phenylarsine oxide on insulin-stimulated glucose transport. Although the Ki for insulin-stimulated transport (7 microM) was similar to that for inhibition of endocytosis, basal glucose transport was not affected by the inhibitor. Further, when cells were prestimulated with insulin causing maximal stimulation of the glucose transport rate, phenylarsine oxide induced a time-dependent reduction to the basal rate (t 1/2 of 10 min), despite the fact that endocytosis was blocked immediately. This observation suggests that if the transporter is recycled by an exocytotic/endocytotic mechanism, it is distinct from fluid-phase endocytosis/exocytosis, which is a vesicle-mediated process, and provides further evidence that the transporter may undergo intrinsic activation/inactivation which does not require vesicle movement.

Effect of relaxin on differentiation of 3T3-L1 preadipocytes

Relaxin is a member of the insulin-like growth factor family that functions primarily during pregnancy and parturition. Because previous studies have shown that rat adipocytes respond to relaxin, we undertook this study to investigate the effect of relaxin on the differentiation of 3T3-L1 fibroblasts in cell culture. First, relaxin prevented the normal course of cell division during the induction phase, but did not interfere with expression of the adipocyte phenotype, as indicated by lipid accumulation and insulin-sensitive glucose transport. Relaxin-treated cells were 2-3 times larger than cells induced by the normal procedure, as determined by both computer-assisted size analysis and intracellular water volume. These effects on cell division and cell size could be reversed by removal of relaxin during early induction. The concentration of relaxin required to elicit the half-maximal effect was 75 ng/ml (12.5 nM). These studies demonstrate that relaxin affects the proliferative response during the inductive phase of differentiation of the 3T3-L1 cell line without affecting differentiation directly. This effect of relaxin on proliferation is unique and is discussed in relation to the cell cycle. It is our hope that these cells will serve as a model system for investigating the mechanisms by which relaxin functions and allow further studies on receptor structure and function.

Effect of phenylarsine oxide on protein synthesis in 3T3-L1 adipocytes

In this report, we show that insulin stimulated the incorporation of tracer [3H]leucine into protein of 3T3-L1 adipocytes within 2 min of insulin addition. The concentration of insulin required to elicit 50% activation was 4nM. Phenylarsine oxide, an inhibitor of insulin-stimulated glucose transport, blocked not only insulin-stimulated protein synthesis but constitutive protein synthesis as well (Ki, 3 microM). Importantly, protein synthesis was not required for insulin-activated glucose transport since cycloheximide added either before or after insulin had no effect on the stimulated rates of glucose transport.