Molecular imaging of protein kinases

Quantitative and Dynamic Imaging of ATM Kinase Activity by Bioluminescence Imaging

Ataxia telangiectasia mutated (ATM) is a serine/threonine kinase critical to the cellular DNA damage response, including DNA double strand breaks (DSBs). ATM activation results in the initiation of a complex cascade of events facilitating DNA damage repair, cell cycle checkpoint control, and survival. Traditionally, protein kinases have been analyzed in vitro using biochemical methods (kinase assays using purified proteins or immunological assays) requiring a large number of cells and cell lysis. Genetically encoded biosensors based on optical molecular imaging such as fluorescence or bioluminescence have been developed to enable interrogation of kinase activities in live cells with a high signal to background. We have genetically engineered a hybrid protein whose bioluminescent activity is dependent on the ATM-mediated phosphorylation of a substrate. The engineered protein consists of the split luciferase-based protein complementation pair with a CHK2 (a substrate for ATM kinase activity) target sequence and a phospho-serine/threonine-binding domain, FHA2, derived from yeast Rad53. Phosphorylation of the serine residue within the target sequence by ATM would lead to its interaction with the phospho-serine-binding domain, thereby preventing complementation of the split luciferase pair and loss of reporter activity. Bioluminescence imaging of reporter-expressing cells in cultured plates or as mouse xenografts provides a quantitative surrogate for ATM kinase activity and therefore the cellular DNA damage response in a noninvasive, dynamic fashion.

Quantitative and Dynamic Imaging of ATM Kinase Activity

Ataxia telangiectasia mutated (ATM) is a serine/threonine kinase critical to the cellular DNA-damage response, including DNA double-strand breaks (DSBs). ATM activation results in the initiation of a complex cascade of events facilitating DNA damage repair, cell cycle checkpoint control, and survival. Traditionally, protein kinases have been analyzed in vitro using biochemical methods (kinase assays using purified proteins or immunological assays) requiring a large number of cells and cell lysis. Genetically encoded biosensors based on optical molecular imaging such as fluorescence or bioluminescence have been developed to enable interrogation of kinase activities in live cells with a high signal to background. We have genetically engineered a hybrid protein whose bioluminescent activity is dependent on the ATM-mediated phosphorylation of a substrate. The engineered protein consists of the split luciferase-based protein complementation pair with a CHK2 (a substrate for ATM kinase activity) target sequence and a phospho-serine/threonine-binding domain, FHA2, derived from yeast Rad53. Phosphorylation of the serine residue within the target sequence by ATM would lead to its interaction with the phospho-serine-binding domain, thereby preventing complementation of the split luciferase pair and loss of reporter activity. Bioluminescence imaging of reporter expressing cells in cultured plates or as mouse xenografts provides a quantitative surrogate for ATM kinase activity and therefore the cellular DNA damage response in a noninvasive, dynamic fashion.

Basic fibroblast growth factor upregulates survivin expression in hepatocellular carcinoma cells via a protein kinase B-dependent pathway

Basic fibroblast growth factor (bFGF) plays an important role in tumor angiogenesis. Several studies have reported that bFGF may influence cell apoptosis through different signaling pathways. The aim of the present investigation was to study the effect of bFGF on the activities of protein kinase B (PKB)/survivin and cell apoptosis in hepatocellular carcinoma cells (Bel-7402). We treated Bel-7402 cells with bFGF and wortmannin [phosphatidylinositol 3-kinase (PI3K)-specific inhibitor] separately to observe the expression of PKB and survivin detected with RT-PCR and western blotting. The cell cycle and apoptosis were assayed with flow cytometry. We found a significant increase in PKB expression in the group treated with 25 ng/ml bFGF for 10 min (P<0.05), and this effect was significantly inhibited by pretreatment with wortmannin (200 nM) for 1 h. After treatment with 10 ng/ml bFGF, the expression of survivin mRNA in Bel-7402 cells increased significantly, and reached the peak at 16 h (P<0.05); however, this effect could be significantly inhibited by pretreatment with wortmannin (200 mM) in a time-dependent manner. Following incubation with 25 ng/ml bFGF for 10 min, the apoptosis rate and M phase were significantly decreased and S phase cells increased compared with the wortmannin (200 nM)-treated group. When this group was pretreated with wortmannin (200 nM) for 1 h, the apoptosis rate and S phase were significantly increased, M phase cells decreased. The results revealed that wortmannin could induce high apoptosis rates in hepatocellular carcinoma cells, and bFGF could inhibit the cell apoptosis induced by wortmannin. These findings indicate that bFGF could rapidly activate the PKB activities, enhance the expression of survivin and the proliferation of hepatocellular carcinoma cells via the PI3K pathway, thus it may serve as a novel molecule for early targeting therapy of hepatocellular carcinoma.

Theranostic Imaging of the Kinases and Proteases that Modulate Cell Death and Survival

Several signaling cascades are involved in cell death, with a significant amount of crosstalk between them. Despite the complexity of these cascades several key pro-survival and pro-death players have been identified. These include PI3-kinase, AKT and caspase-3. Here we review the approaches used to date to perform molecular imaging of these important targets. We focus in particular on approaches that include the possibility of modulating the activity of these kinases and proteases in a theranostic approach.

Molecular imaging of akt enables early prediction of response to molecular targeted therapy

Development of noninvasive, real-time molecular imaging tools to assess responsiveness of a given therapy may be a critical component of the success of individualized therapy approach for patients. Toward this, we have previously developed and validated molecular sensors for Akt and caspase-3 activity, and in this report, we have explored the utility of these reporters in assessing the responsiveness of tumors to a combination of gemcitabine (Gem) and cetuximab (Cet) delivered in two opposite schedules. We found that human head and neck cancer (UMSCC1) xenografts responded significantly better in a schedule where cetuximab was administered after gemcitabine when compared with the schedule of cetuximab followed by gemcitabine. Wilcoxon two-sample tests suggested that the difference in tumor volumes in two schedules became significant on day 7 (P > .05 on day 4, and P < .05 on days 7 and 10), and the difference in activity of Akt in two schedules became significant on day 4 (P < .05 on days 4, 6, and 10). Using Akt reporter activity and cubic spline interpolation, the distinction between the two schedules could be detected 2 days before using the tumor volume, suggesting that molecular imaging of Akt may allow early prediction of therapy responsiveness. We did not observe a significant difference between the two schedules in the caspase-3 activity. In summary, this proof-of-concept study provides a basis for using molecular imaging of Akt as an early indicator of therapeutic efficacy.

Molecular imaging of c-Met tyrosine kinase activity

The receptor tyrosine kinase c-Met and its ligand, hepatocyte growth factor/scatter factor (HGF/SF), modulate signaling cascades implicated in cellular proliferation, survival, migration, invasion, and angiogenesis. Therefore, dysregulation of HGF/c-Met signaling can compromise the cellular capacity to moderate these activities and can lead to tumorigenesis, metastasis, and therapeutic resistance in various human malignancies. To facilitate studies investigating HGF/c-Met receptor coupling or c-Met signaling events in real time and in living cells and animals, here we describe a genetically engineered reporter where bioluminescence can be used as a surrogate for c-Met tyrosine kinase activity. c-Met kinase activity in cultured cells and tumor xenografts was monitored quantitatively and dynamically in response to the activation or inhibition of the HGF/c-Met signaling pathway. Treatment of tumor-bearing animals with a c-Met inhibitor and the HGF neutralizing antibody stimulated the reporter's bioluminescence activity in a dose-dependent manner and led to a regression of U-87 MG tumor xenografts. Results obtained from these studies provide unique insights into the pharmacokinetics and pharmacodynamics of agents that modulate c-Met activity and validate c-Met as a target for human glioblastoma therapy.

Novel molecular imaging platform for monitoring oncological kinases

Recent advances in oncology have lead to identification of a plethora of alterations in signaling pathways that are critical to oncogenesis and propagation of malignancy. Among the biomarkers identified, dysregulated kinases and associated changes in signaling cascade received the lion's share of scientific attention and have been under extensive investigations with goal of targeting them for anti-cancer therapy. Discovery of new drugs is immensely facilitated by molecular imaging technology which enables non-invasive, real time, dynamic imaging and quantification of kinase activity. Here, we review recent development of novel kinase reporters based on conformation dependent complementation of firefly luciferase to monitor kinase activity. Such reporter system provides unique insights into the pharmacokinetics and pharmacodynamics of drugs that modulate kinase signaling and have a huge potential in drug discovery, validation, and drug-target interactions.

Lung vascular targeting using antibody to aminopeptidase P: CT-SPECT imaging, biodistribution and pharmacokinetic analysis

BACKGROUND/AIMS

Aminopeptidase P (APP) is specifically enriched in caveolae on the luminal surface of pulmonary vascular endothelium. APP antibodies bind lung endothelium in vivo and are rapidly and actively pumped across the endothelium into lung tissue. Here we characterize the immunotargeting properties and pharmacokinetics of the APP-specific recombinant antibody 833c.

METHODS

We used in situ binding, biodistribution analysis and in vivo imaging to assess the lung targeting of 833c.

RESULTS

More than 80% of 833c bound during the first pass through isolated perfused lungs. Dynamic SPECT acquisition showed that 833c rapidly and specifically targeted the lungs in vivo, reaching maximum levels within 2 min after intravenous injection. CT-SPECT imaging revealed specific targeting of 833c to the thoracic cavity and co-localization with a lung perfusion marker, Tc99m-labeled macroaggregated albumin. Biodistribution analysis confirmed lung-specific uptake of 833c which declined by first-order kinetics (t(½) = 110 h) with significant levels of 833c still present 30 days after injection.

CONCLUSION

These data show that APP expressed in endothelial caveolae appears to be readily accessible to circulating antibody rather specifically in lung. Targeting lung-specific caveolar APP provides an extraordinarily rapid and specific means to target pulmonary vasculature and potentially deliver therapeutic agents into the lung tissue.

Enhancing Akt imaging through targeted reporter expression

The serine/threonine kinase PKB/Akt is a key mediator of survival and resistance to cancer therapy. Pharmacologic inhibition of Akt and its biologic sequelae may significantly impact the treatment of cancer. The use of molecular imaging technologies has contributed significantly to drug discovery research with an emphasis on drug efficacy, the mechanism of action, and target validation studies. We constructed a genetically engineered hybrid bioluminescent Akt reporter (BAR) molecule that reports on Akt serine/threonine kinase activity. Based on the fact that Akt is recruited to the plasma membrane on activation, we here describe a modified version of this reporter molecule (myristoylated and palmitoylated bioluminescent Akt reporter [MyrPalm-BAR]), which is membrane bound and whose bioluminescence activity can be used to monitor Akt activity at the cell membrane. Using changes in Akt activation status with small molecule inhibitors of Akt, we demonstrated that the membrane-targeted Akt reporter was more sensitive and quantitative. In addition, inhibition of upstream signaling kinases such as epidermal growth factor receptor and phosphatidylinositol 3-kinase activity resulted in changes in Akt activity that were quantitatively monitored by bioluminescence imaging. Based on these results, we propose that the membrane-associated Akt reporter may be better suited for identification of novel compounds that modulate the Akt pathway by high-throughput screening.