Assessment of Aspartate and Bicarbonate Produced From Hyperpolarized [1-13C]Pyruvate as Markers of Renal Gluconeogenesis

Oxidation of hyperpolarized [1-13 C]pyruvate in isolated rat kidneys

Kidneys play a central role in numerous disorders but current imaging methods have limited utility to probe renal metabolism. Hyperpolarized (HP) 13 C magnetic resonance imaging is uniquely suited to provide metabolite-specific information about key biochemical pathways and it offers the further advantage that renal imaging is practical in humans. This study evaluated the feasibility of hyperpolarization examinations in a widely used model for analysis of renal physiology, the isolated kidney, which enables isolation of renal metabolism from the effects of other organs and validation of HP results by independent measurements. Isolated rat kidneys were supplied with either HP [1-13 C]pyruvate only or HP [1-13 C]pyruvate plus octanoate. Metabolic activity in both groups was confirmed by stable renal oxygen consumption. HP [1-13 C]pyruvate was readily metabolized to [13 C]bicarbonate, [1-13 C]lactate, and [1-13 C]alanine, detectable seconds after HP [1-13 C]pyruvate was injected. Octanoate suppressed but did not eliminate the production of HP [13 C]bicarbonate from [1-13 C]pyruvate. Steady-state flux analyses using non-HP 13 C substrates validated the utilization of HP [1-13 C]pyruvate, as observed by HP 13 C NMR. In the presence of octanoate, lactate is generated from a tricarboxylic acid cycle intermediate, oxaloacetate. The isolated rat kidney may serve as an excellent model for investigating and establishing new HP 13 C metabolic probes for future kidney imaging applications.

Hyperpolarized (1-13C)Alaninamide Is a Multifunctional In Vivo Sensor of Aminopeptidase N Activity, pH, and CO2

Spin hyperpolarization enables real-time metabolic imaging of carbon-13-labeled substrates. While hyperpolarized l-(1-¹³C)alaninamide is a probe of the cell-surface tumor marker aminopeptidase-N (APN, CD13), its activity in vivo has not been described. Scanning the kidneys of rats infused with hyperpolarized alaninamide shows both conversion to [1-¹³C]alanine and several additional spectral peaks with distinct temporal dynamics. The (1-¹³C)alaninamide chemical shift is pH-sensitive, with a pK_a of 7.9 at 37 °C, and the peaks correspond to at least three different compartments of pH 7.46 ± 0.02 (1), 7.21 ± 0.02 (2), and 6.58 ± 0.05 (3). An additional peak was assigned to the carboxyamino adduct formed by reaction with dissolved CO₂. Spectroscopic imaging showed nonuniform distribution, with the low-pH signal more concentrated in the inner medulla. Treatment with the diuretic acetazolamide resulted in significant pH shifts in compartment 1 to 7.38 ± 0.03 (p = 0.0057) and compartment 3 to 6.80 ± 0.05 (p = 0.0019). While the pH of compartment 1 correlates with blood pH, the pH of compartment 3 did not correspond to the pH of urine. In vitro experiments show that alaninamide readily enters blood cells and can detect intracellular pH. While carbamate formation depends on pH and pCO₂, the carbamate-to-alaninamide ratio did not correlate with either arterial blood pH or pCO₂, suggesting that it may reflect variations in tissue pH and pCO₂. This study demonstrates the feasibility of using hyperpolarized sensors to simultaneously image enzyme activity, pCO₂, and pH in vivo.

Developing a Method to Estimate the Downstream Metabolite Signals from Hyperpolarized [1-13C]Pyruvate

Hyperpolarized carbon-13 MRI has the advantage of allowing the study of glycolytic flow in vivo or in vitro dynamically in real-time. The apparent exchange rate constant of a metabolite dynamic signal reflects the metabolite changes of a disease. Downstream metabolites can have a low signal-to-noise ratio (SNR), causing apparent exchange rate constant inconsistencies. Thus, we developed a method that estimates a more accurate metabolite signal. This method utilizes a kinetic model and background noise to estimate metabolite signals. Simulations and in vitro studies with photon-irradiated and control groups were used to evaluate the procedure. Simulated and in vitro exchange rate constants estimated using our method were compared with the raw signal values. In vitro data were also compared to the Area-Under-Curve (AUC) of the cell medium in ¹³C Nuclear Magnetic Resonance (NMR). In the simulations and in vitro experiments, our technique minimized metabolite signal fluctuations and maintained reliable apparent exchange rate constants. In addition, the apparent exchange rate constants of the metabolites showed differences between the irradiation and control groups after using our method. Comparing the in vitro results obtained using our method and NMR, both solutions showed consistency when uncertainty was considered, demonstrating that our method can accurately measure metabolite signals and show how glycolytic flow changes. The method enhanced the signals of the metabolites and clarified the metabolic phenotyping of tumor cells, which could benefit personalized health care and patient stratification in the future.

Overcoming chemoresistance to b-raf inhibitor in melanoma via targeted inhibition of phosphoenolpyruvate carboxykinase1 using 3-mercaptopropionic acid

The resistance of melanoma to BRAF inhibitors remains a tough clinical challenge. In order to explore the underlying mechanism of drug resistance in melanoma, we established the resistant cell line to vemurafenib, and assessed the changes of drug-resistant cells on proliferation, apoptosis, oxidative stress and tumor stemness. Our results suggest that phosphoenolpyruvate carboxykinase1 (PCK1) is activated and inhibits the oxidative stress caused by vemurafenib in drug-resistant cells. Long term treatment of vemurafenib increases the expression of PCK1 which reduces the production of reactive oxygen species (ROS) by activating PI3K/Akt pathway. After the inhibition of PCK1 by 3-mercaptopropionic acid (3-MPA), the therapeutic sensitivity of vemurafenib is restored. In conclusion, this study disclosed that drug-resistant cells appeared to regulate their own proliferation, oxidative stress and tumor dryness by activating Akt/PCK1/ROS pathway, and shed new insights into acquiring drug resistance in melanoma.