Downregulation of Erythrocyte miR-210 Induces Endothelial Dysfunction in Type 2 Diabetes

Red blood cells (RBC) act as mediators of vascular injury in type 2 diabetes mellitus (T2DM). miR-210 plays a protective role in cardiovascular homeostasis and is decreased in whole blood of T2DM mice. We hypothesized that downregulation of RBC miR-210 induces endothelial dysfunction in T2DM. RBC were coincubated with arteries and endothelial cells ex vivo and transfused in vivo to identify the role of miR-210 and its target protein tyrosine phosphatase 1B (PTP1B) in endothelial dysfunction. RBC from patients with T2DM and diabetic rodents induced endothelial dysfunction ex vivo and in vivo. miR-210 levels were lower in human RBC from patients with T2DM (T2DM RBC) than in RBC from healthy subjects. Transfection of miR-210 in human T2DM RBC rescued endothelial function, whereas miR-210 inhibition in healthy subjects RBC or RBC from miR-210 knockout mice impaired endothelial function. Human T2DM RBC decreased miR-210 expression in endothelial cells. miR-210 expression in carotid artery plaques was lower in T2DM patients than in patients without diabetes. Endothelial dysfunction induced by downregulated RBC miR-210 involved PTP1B and reactive oxygen species. miR-210 mimic attenuated endothelial dysfunction induced by RBC via downregulating vascular PTP1B and oxidative stress in diabetic mice in vivo. These data reveal that the downregulation of RBC miR-210 is a novel mechanism driving the development of endothelial dysfunction in T2DM.

Deletion of PTP1B From Brain Neurons Partly Protects Mice From Diet-Induced Obesity and Minimally Improves Fertility

Reproductive dysfunction in women has been linked to high caloric diet (HCD)-feeding and obesity. Central resistance to leptin and insulin have been shown to accompany diet-induced infertility in rodent studies, and we have previously shown that deleting suppressor of cytokine signaling 3, which is a negative regulator of leptin signaling, from all forebrain neurons partially protects mice from HCD-induced infertility. In this study, we were interested in exploring the role of protein tyrosine phosphatase 1B (PTP1B), which is a negative regulator of both leptin and insulin signaling, in the pathophysiology of HCD-induced obesity and infertility. To this end, we generated male and female neuron-specific PTP1B knockout mice and compared their body weight gain, food intake, glucose tolerance, and fertility relative to control littermates under both normal calorie diet and HCD feeding conditions. Both male and female mice with neuronal PTP1B deletion exhibited slower body weight gain in response to HCD feeding, yet only male knockout mice exhibited improved glucose tolerance compared with controls. Neuronal PTP1B deletion improved the time to first litter in HCD-fed mice but did not protect female mice from eventual HCD-induced infertility. While the mice fed a normal caloric diet remained fertile throughout the 150-day period of assessment, HCD-fed females became infertile after producing only a single litter, regardless of their genotype. These data show that neuronal PTP1B deletion is able to partially protect mice from HCD-induced obesity but is not a critical mediator of HCD-induced infertility.

Pharmacological Inhibition of miR-130 Family Suppresses Bladder Tumor Growth by Targeting Various Oncogenic Pathways via PTPN1

Previously, we have revealed that the miR-130 family (miR-130b, miR-301a, and miR-301b) functions as an oncomiR in bladder cancer. The pharmacological inhibition of the miR-130 family molecules by the seed-targeting strategy with an 8-mer tiny locked nucleic acid (LNA) inhibits the growth, migration, and invasion of bladder cancer cells by repressing stress fiber formation. Here, we searched for a functionally advanced target sequence with LNA for the miR-130 family with low cytotoxicity and found LNA #9 (A(L)^i^i^A(L)^T(L)^T(L)^G(L)^5(L)^A(L)^5(L)^T(L)^G) as a candidate LNA. LNA #9 inhibited cell growth in vitro and in an in vivo orthotopic bladder cancer model. Proteome-wide tyrosine phosphorylation analysis suggested that the miR-130 family upregulates a wide range of receptor tyrosine kinases (RTKs) signaling via the expression of phosphorylated Src (pSrc^Tyr416). SILAC-based proteome analysis and a luciferase assay identified protein tyrosine phosphatase non-receptor type 1 (PTPN1), which is implicated as a negative regulator of multiple signaling pathways downstream of RTKs as a target gene of the miR-130 family. The miR-130-targeted LNA increased and decreased PTPN1 and pSrc^Tyr416 expressions, respectively. PTPN1 knockdown led to increased tumor properties (cell growth, invasion, and migration) and increased pSrc^Tyr416 expression in bladder cancer cells, suggesting that the miR-130 family upregulates multiple RTK signaling by targeting PTPN1 and subsequent Src activation in bladder cancer. Thus, our newly designed miR-130 family targeting LNA could be a promising nucleic acid therapeutic agent for bladder cancer.

The HSV-1 mechanisms of cell-to-cell spread and fusion are critically dependent on host PTP1B

All herpesviruses have mechanisms for passing through cell junctions, which exclude neutralizing antibodies and offer a clear path to neighboring, uninfected cells. In the case of herpes simplex virus type 1 (HSV-1), direct cell-to-cell transmission takes place between epithelial cells and sensory neurons, where latency is established. The spreading mechanism is poorly understood, but mutations in four different HSV-1 genes can dysregulate it, causing neighboring cells to fuse to produce syncytia. Because the host proteins involved are largely unknown (other than the virus entry receptor), we were intrigued by an earlier discovery that cells infected with wild-type HSV-1 will form syncytia when treated with salubrinal. A biotinylated derivative of this drug was used to pull down cellular complexes, which were analyzed by mass spectrometry. One candidate was a protein tyrosine phosphatase (PTP1B), and although it ultimately proved not to be the target of salubrinal, it was found to be critical for the mechanism of cell-to-cell spread. In particular, a highly specific inhibitor of PTP1B (CAS 765317-72-4) blocked salubrinal-induced fusion, and by itself resulted in a dramatic reduction in the ability of HSV-1 to spread in the presence of neutralizing antibodies. The importance of this phosphatase was confirmed in the absence of drugs by using PTP1B^-/- cells. Importantly, replication assays showed that virus titers were unaffected when PTP1B was inhibited or absent. Only cell-to-cell spread was altered. We also examined the effects of salubrinal and the PTP1B inhibitor on the four Syn mutants of HSV-1, and strikingly different responses were found. That is, both drugs individually enhanced fusion for some mutants and reduced fusion for others. PTP1B is the first host factor identified to be specifically required for cell-to-cell spread, and it may be a therapeutic target for preventing HSV-1 reactivation disease.

It is estimated that 67% of the global population is infected with herpes simplex virus type 1 (HSV-1). This virus resides in sensory neurons in a quiescent state but periodically reactivates, producing virus particles that travel down the axon to infect epithelial cells of the skin, where it can be transmitted to additional people. To avoid neutralizing antibodies, herpesviruses have evolved mechanisms for moving directly from one cell to another through their sites of intimate contact; however, the mechanism of cell-to-cell spread is poorly understood. Studies of HSV-1 mutants have implicated numerous viral proteins, but the necessary cellular factors are unknown except for the one that the virus uses to enter cells. Our experiments have identified a cellular enzyme (PTP1B, a tyrosine phosphatase) that is dispensable for the production of infectious virions but is critically important for the cell-to-cell spreading mechanism. Promising drugs targeting PTP1B have already been tested in early clinical trials for possible treatment of obesity and type-2 diabetes, and thus, our study may have immediate utility for attenuating HSV-1 reactivation disease in immunocompromised patients.

Protein tyrosine phosphatase-1B contributes to LPS-induced leptin resistance in male rats

Leptin resistance is induced by the feedback inhibitors tyrosine phosphatase-1B (PTP1B) and decreased Src homology 2 domain-containing tyrosine phosphatase-2 (SHP-2) signaling. To investigate the participation of PTP1B and SHP-2 in LPS-induced leptin resistance, we injected repeated (6-LPS) intraperitoneal LPS doses (100 μg/kg ip) for comparison with a single (1-LPS) treatment and evaluated the expression of SHP-2, PTP1B, p-ERK1/2, and p-STAT3 in the hypothalamus of male Wistar rats. The single LPS treatment increased the expression of p-STAT3 and PTP1B but not SHP-2. The repeated LPS treatment reduced SHP-2, increased PTP1B, and did not change p-STAT3. We observed that the PTP1B expression induced by the endotoxin was highly colocalized with leptin receptor cells in the hypothalamus of LepRb-IRES-Cre-tdTomato reporter mice. The single, but not the repeated, LPS treatment decreased the food intake and body weight. Leptin had no stimulatory effect on the hypophagia, body weight loss, or pSTAT3 expression in 6-LPS rats, indicating leptin unresponsiveness. Notably, the PTP1B inhibitor (3.0 nmol/rat in 5 μl icv) restored the LPS-induced hypophagia in 6-LPS rats and restored the ability of leptin to reduce food intake and body weight as well as to phosphorylate STAT3 in the arcuate, paraventricular, and ventromedial nuclei of the hypothalamus. The present data suggest that an increased PTP1B expression in the hypothalamus underlies the development of leptin resistance during repeated exposure to LPS. Our findings contribute to understanding the mechanisms involved in leptin resistance during low-grade inflammation as seen in obesity.

New pROSpects for PTP1B: micro-managing oncogene-induced senescence

Oncogene-induced senescence (OIS) provides an important, but incompletely understood, barrier to tumorigenesis. In this issue, Yang et al. (2014) surprisingly report that inactivation of PTP1B by reactive oxygen species is essential for OIS, via effects on AGO2 and microRNA maturation.

Dephosphorylation of tyrosine 393 in argonaute 2 by protein tyrosine phosphatase 1B regulates gene silencing in oncogenic RAS-induced senescence

Oncogenic RAS (H-RAS(V12)) induces premature senescence in primary cells by triggering production of reactive oxygen species (ROS), but the molecular role of ROS in senescence remains elusive. We investigated whether inhibition of protein tyrosine phosphatases by ROS contributed to H-RAS(V12)-induced senescence. We identified protein tyrosine phosphatase 1B (PTP1B) as a major target of H-RAS(V12)-induced ROS. Inactivation of PTP1B was necessary and sufficient to induce premature senescence in H-RAS(V12)-expressing IMR90 fibroblasts. We identified phospho-Tyr 393 of argonaute 2 (AGO2) as a direct substrate of PTP1B. Phosphorylation of AGO2 at Tyr 393 inhibited loading with microRNAs (miRNAs) and thus miRNA-mediated gene silencing, which counteracted the function of H-RAS(V12)-induced oncogenic miRNAs. Overall, our data illustrate that premature senescence in H-RAS(V12)-transformed primary cells is a consequence of oxidative inactivation of PTP1B and inhibition of miRNA-mediated gene silencing.

Ventromedial hypothalamus-specific Ptpn1 deletion exacerbates diet-induced obesity in female mice

Protein-tyrosine phosphatase 1B (PTP1B) regulates food intake (FI) and energy expenditure (EE) by inhibiting leptin signaling in the hypothalamus. In peripheral tissues, PTP1B regulates insulin signaling, but its effects on CNS insulin action are largely unknown. Mice harboring a whole-brain deletion of the gene encoding PTP1B (Ptpn1) are lean, leptin-hypersensitive, and resistant to high fat diet-induced (HFD-induced) obesity. Arcuate proopiomelanocortin (POMC) neuron-specific deletion of Ptpn1 causes a similar, but much milder, phenotype, suggesting that PTP1B also acts in other neurons to regulate metabolism. Steroidogenic factor-1-expressing (SF-1-expressing) neurons in the ventromedial hypothalamus (VMH) play an important role in regulating body weight, FI, and EE. Surprisingly, Ptpn1 deletion in SF-1 neurons caused an age-dependent increase in adiposity in HFD-fed female mice. Although leptin sensitivity was increased and FI was reduced in these mice, they had impaired sympathetic output and decreased EE. Immunohistochemical analysis showed enhanced leptin and insulin signaling in VMH neurons from mice lacking PTP1B in SF-1 neurons. Thus, in the VMH, leptin negatively regulates FI, promoting weight loss, whereas insulin suppresses EE, leading to weight gain. Our results establish a novel role for PTP1B in regulating insulin action in the VMH and suggest that increased insulin responsiveness in SF-1 neurons can overcome leptin hypersensitivity and enhance adiposity.

Hypoglycemic effect of triterpenoid-rich extracts from Euryale ferox shell on normal and streptozotocin-diabetic mice

The antioxidant effects of the triterpenoid-rich extracts from Euryale ferox shell (ES) have been confirmed in vitro. This study examined whether the triterpenoid-rich extract from ES eases human hyperglycemia and diabetes caused by metabolic disorders. Normal and streptozocin (STZ)-induced diabetic mice were used as controls for the four groups that received the triterpenoid-rich extracts of ES suspended in distilled water orally at doses of 200, 300, 400, 500±2 mg/L. Body weight, blood glucose and pancreatic tissue morphology were observed after 4 weeks. The expression of protein tyrosine phosphatase-1B (PTP1B) and insulin receptor substrate (IRS-1) proteins, which are related to the regulation of glucose metabolism in vivo, were also investigated. Compared with the model group (LD50 900±2 mg/L), it was found that the triterpenoid-rich extracts of ES could regulate glucose metabolism (P<0.01) and cause body weight to return to normal levels (P<0.05). Islet morphology recovered well, the expression of the negative regulation protein PTP1B gene was reduced and insulin receptor IRS-1 protein expression was increased. These data prove that the triterpenoid-rich extracts from ES have a therapeutic effect on diabetes by insulin resistance.

Transmembrane protein ESDN promotes endothelial VEGF signaling and regulates angiogenesis

Aberrant blood vessel formation contributes to a wide variety of pathologies, and factors that regulate angiogenesis are attractive therapeutic targets. Endothelial and smooth muscle cell-derived neuropilin-like protein (ESDN) is a neuropilin-related transmembrane protein expressed in ECs; however, its potential effect on VEGF responses remains undefined. Here, we generated global and EC-specific Esdn knockout mice and demonstrated that ESDN promotes VEGF-induced human and murine EC proliferation and migration. Deletion of Esdn in the mouse interfered with adult and developmental angiogenesis, and knockdown of the Esdn homolog (dcbld2) in zebrafish impaired normal vascular development. Loss of ESDN in ECs blunted VEGF responses in vivo and attenuated VEGF-induced VEGFR-2 signaling without altering VEGF receptor or neuropilin expression. Finally, we found that ESDN associates with VEGFR-2 and regulates its complex formation with negative regulators of VEGF signaling, protein tyrosine phosphatases PTP1B and TC-PTP, and VE-cadherin. These findings establish ESDN as a regulator of VEGF responses in ECs that acts through a mechanism distinct from neuropilins. As such, ESDN may serve as a therapeutic target for angiogenesis regulation.

Redox signaling in cardiovascular health and disease

Spatiotemporal regulation of the activity of a vast array of intracellular proteins and signaling pathways by reactive oxygen species (ROS) governs normal cardiovascular function. However, data from experimental and animal studies strongly support that dysregulated redox signaling, resulting from hyperactivation of various cellular oxidases or mitochondrial dysfunction, is integral to the pathogenesis and progression of cardiovascular disease (CVD). In this review, we address how redox signaling modulates the protein function, the various sources of increased oxidative stress in CVD, and the labyrinth of redox-sensitive molecular mechanisms involved in the development of atherosclerosis, hypertension, cardiac hypertrophy and heart failure, and ischemia-reperfusion injury. Advances in redox biology and pharmacology for inhibiting ROS production in specific cell types and subcellular organelles combined with the development of nanotechnology-based new in vivo imaging systems and targeted drug delivery mechanisms may enable fine-tuning of redox signaling for the treatment and prevention of CVD.

Tub has a key role in insulin and leptin signaling and action in vivo in hypothalamic nuclei

Mutation of tub gene in mice induces obesity, suggesting that tub could be an important regulator of energy balance. In the current study, we investigated whether insulin, leptin, and obesity can modulate Tub in vivo in hypothalamic nuclei, and we investigated possible consequences on energy balance, neuropeptide expression, and hepatic glucose metabolism. Food intake, metabolic characteristics, signaling proteins, and neuropeptide expression were measured in response to fasting and refeeding, intracerebroventricular insulin and leptin, and Tub antisense oligonucleotide (ASO). Tub tyrosine phosphorylation (Tub-p-tyr) is modulated by nutritional status. Tub is a substrate of insulin receptor tyrosine kinase (IRTK) and leptin receptor (LEPR)-Janus kinase 2 (JAK2) in hypothalamic nuclei. After leptin or insulin stimulation, Tub translocates to the nucleus. Inhibition of Tub expression in hypothalamus by ASO increased food intake, fasting blood glucose, and hepatic glucose output, decreased O(2) consumption, and blunted the effect of insulin or leptin on proopiomelanocortin, thyroid-releasing hormone, melanin-concentrating hormone, and orexin expression. In hypothalamus of mice administered a high-fat diet, there is a reduction in leptin and insulin-induced Tub-p-tyr and nuclear translocation, which is reversed by reducing protein tyrosine phosphatase 1B expression. These results indicate that Tub has a key role in the control of insulin and leptin effects on food intake, and the modulation of Tub may contribute to insulin and leptin resistance in DIO mice.

Bioactives of Artemisia dracunculus L. mitigate the role of ceramides in attenuating insulin signaling in rat skeletal muscle cells

Ectopic lipids in peripheral tissues have been implicated in attenuating insulin action in vivo. The botanical extract of Artemisia dracunculus L. (PMI 5011) improves insulin action, yet the precise mechanism is not known. We sought to determine whether the mechanism by which PMI 5011 improves insulin signaling is through regulation of lipid metabolism. After differentiation, cells were separately preincubated with free fatty acids (FFAs) and ceramide C2, and the effects on glycogen content, insulin signaling, and ceramide profiles were determined. The effect of PMI 5011 on ceramide accumulation and ceramide-induced inhibition of insulin signaling was evaluated. FFAs resulted in increased levels of total ceramides and ceramide species in L6 myotubes. Saturated FFAs and ceramide C2 inhibited insulin-stimulated phosphorylation of protein kinase B/Akt and reduced glycogen content. PMI 5011 had no effect on ceramide formation or accumulation but increased insulin sensitivity via restoration of Akt phosphorylation. PMI 5011 also attenuated the FFA-induced upregulation of a negative inhibitor of insulin signaling, i.e., protein tyrosine phosphatase 1B (PTP1B), and increased phosphorylation of PTP1B. PMI 5011 attenuates the reduction in insulin signaling induced by ceramide accumulation, but the mechanism of improved insulin signaling is independent of ceramide formation.

Deficiency of PTP1B in POMC neurons leads to alterations in energy balance and homeostatic response to cold exposure

The adipose tissue-derived hormone leptin regulates energy balance through catabolic effects on central circuits, including proopiomelanocortin (POMC) neurons. Leptin activation of POMC neurons increases thermogenesis and locomotor activity. Protein tyrosine phosphatase 1B (PTP1B) is an important negative regulator of leptin signaling. POMC neuron-specific deletion of PTP1B in mice results in reduced high-fat diet-induced body weight and adiposity gain due to increased energy expenditure and greater leptin sensitivity. Mice lacking the leptin gene (ob/ob mice) are hypothermic and cold intolerant, whereas leptin delivery to ob/ob mice induces thermogenesis via increased sympathetic activity to brown adipose tissue (BAT). Here, we examined whether POMC PTP1B mediates the thermoregulatory response of CNS leptin signaling by evaluating food intake, body weight, core temperature (T(C)), and spontaneous physical activity (SPA) in response to either exogenous leptin or 4-day cold exposure (4°C) in male POMC-Ptp1b-deficient mice compared with wild-type controls. POMC-Ptp1b(-/-) mice were hypersensitive to leptin-induced food intake and body weight suppression compared with wild types, yet they displayed similar leptin-induced increases in T(C). Interestingly, POMC-Ptp1b(-/-) mice had increased BAT weight and elevated plasma triiodothyronine (T(3)) levels in response to a 4-day cold challenge, as well as reduced SPA 24 h after cold exposure, relative to controls. These data show that PTP1B in POMC neurons plays a role in short-term cold-induced reduction of SPA and may influence cold-induced thermogenesis via enhanced activation of the thyroid axis.

Regulation of brown fat adipogenesis by protein tyrosine phosphatase 1B

BACKGROUND

Protein-tyrosine phosphatase 1B (PTP1B) is a physiological regulator of insulin signaling and energy balance, but its role in brown fat adipogenesis requires additional investigation.

METHODOLOGY/PRINCIPAL FINDINGS

To precisely determine the role of PTP1B in adipogenesis, we established preadipocyte cell lines from wild type and PTP1B knockout (KO) mice. In addition, we reconstituted KO cells with wild type, substrate-trapping (D/A) and sumoylation-resistant (K/R) PTP1B mutants, then characterized differentiation and signaling in these cells. KO, D/A- and WT-reconstituted cells fully differentiated into mature adipocytes with KO and D/A cells exhibiting a trend for enhanced differentiation. In contrast, K/R cells exhibited marked attenuation in differentiation and lipid accumulation compared with WT cells. Expression of adipogenic markers PPARγ, C/EBPα, C/EBPδ, and PGC1α mirrored the differentiation pattern. In addition, the differentiation deficit in K/R cells could be reversed completely by the PPARγ activator troglitazone. PTP1B deficiency enhanced insulin receptor (IR) and insulin receptor substrate 1 (IRS1) tyrosyl phosphorylation, while K/R cells exhibited attenuated insulin-induced IR and IRS1 phosphorylation and glucose uptake compared with WT cells. In addition, substrate-trapping studies revealed that IRS1 is a substrate for PTP1B in brown adipocytes. Moreover, KO, D/A and K/R cells exhibited elevated AMPK and ACC phosphorylation compared with WT cells.

CONCLUSIONS

These data indicate that PTP1B is a modulator of brown fat adipogenesis and suggest that adipocyte differentiation requires regulated expression of PTP1B.

PTP1B suppresses prolactin activation of Stat5 in breast cancer cells

Basal levels of nuclear localized, tyrosine phosphorylated Stat5 are present in healthy human breast epithelia. In contrast, Stat5 phosphorylation is frequently lost during breast cancer progression, a finding that correlates with loss of histological differentiation and poor patient prognosis. Identifying the mechanisms underlying loss of Stat5 phosphorylation could provide novel targets for breast cancer therapy. Pervanadate, a general tyrosine phosphatase inhibitor, revealed marked phosphatase regulation of Stat5 activity in breast cancer cells. Lentiviral-mediated shRNA allowed specific examination of the regulatory role of five tyrosine phosphatases (PTP1B, TC-PTP, SHP1, SHP2, and VHR), previously implicated in Stat5 regulation in various systems. Enhanced and sustained prolactin-induced Stat5 tyrosine phosphorylation was observed in T47D and MCF7 breast cancer cells selectively in response to PTP1B depletion. Conversely, PTP1B overexpression suppressed prolactin-induced Stat5 tyrosine phosphorylation. Furthermore, PTP1B knockdown increased Stat5 reporter gene activity. Mechanistically, PTP1B suppression of Stat5 phosphorylation was mediated, at least in part, through inhibitory dephosphorylation of the Stat5 tyrosine kinase, Jak2. PTP1B knockdown enhanced sensitivity of T47D cells to prolactin phosphorylation of Stat5 by reducing the EC(50) from 7.2 nmol/L to 2.5 nmol/L. Immunohistochemical analyses of two independent clinical breast cancer materials revealed significant negative correlations between levels of active Stat5 and PTP1B, but not TC-PTP. Collectively, our data implicate PTP1B as an important negative regulator of Stat5 phosphorylation in invasive breast cancer.

Enzyme occupancy measurement of intracellular protein tyrosine phosphatase 1B using photoaffinity probes

Protein tyrosine phosphatase 1B (PTP1B) is believed to be one of the enzymes involved in down-regulating the insulin receptor and is a drug target for the treatment of type II diabetes. To better understand the in vitro and in vivo behavior of PTP1B inhibitors, a cell-based assay to directly measure enzyme occupancy of PTP1B by inhibitors using photoaffinity labeling was developed. Two photoaffinity probes were synthesized containing the photolabile diazirine moiety. These photoprobes were specific for PTP1B and T-cell protein tyrosine phosphatase over CD45, with the most potent photoprobe having an IC(50) value of 0.2nM for PTP1B. Activation of the photoprobes with a 40-W UV lamp in the presence of purified AspTyrLysAspAspAspAspLys (Flag)-PTP1B formed a 1:1 irreversible adduct with the enzyme. The photolabeling was competed by known PTP1B inhibitors, vanadate, and the peptide inhibitor N-benzoyl-l-glutamyl-[4-phosphono(difluoromethyl)]-l-phenylalanyl-[4-phosphono(difluoromethyl)]l-phenylalanineamide (BzN-EJJ-amide). In HepG2 (human hepatoma cell line) cells, endogenous PTP1B was labeled by the UV-activated photoprobes in both lysed and intact cells. Enzyme occupancy measurements were conducted with a series of PTP1B inhibitors using the photoprobe affinity assay. Several compounds were shown to bind to endogenous PTP1B in the HepG2 intact cells.