Biphasic behavior of the kinetics of 31P-containing metabolites in ischemic porcine kidneys

Determination of phosphorylated proteins in tissue specimens requires high-quality samples collected under stringent conditions

AIMS

For selection of patients who will benefit from targeted therapies, identification of biomarkers predictive of treatment response is desirable. Activation of the targeted pathway becomes apparent by protein phosphorylation. Determination of this phenomenon is therefore considered a promising biomarker approach. To date, however, it is unclear whether routinely collected tissue specimens allow determination of in-vivo phosphorylation states.

METHODS AND RESULTS

To investigate whether routinely collected tissue specimens retain the true phosphorylation states of a tumour's proteins, we compared protein phosphorylation states between matched tumour samples that were subjected to different ischaemic times by immunohistochemistry. The influence of formalin fixation and paraffin-embedding on phosphorylation states was investigated by comparison of matched fresh frozen and formalin-fixed paraffin-embedded surgical specimens as well as small biopsies. We show that ischaemia influences protein phosphorylation in a tumour-specific, unpredictable manner. Formalin fixation and paraffin-embedding lead to a decrease in detectable protein phosphorylation in larger surgical specimens, but not in small biopsies.

CONCLUSIONS

Determination of protein phosphorylation using routinely collected surgical specimens results in artefacts which do not reflect a tumour's true states of pathway activation. Valid measurement of phosphorylated biomarkers requires that tissue collection procedures are tightly controlled, avoiding ischaemia and large-specimen fixation.

Kinetics of PME/Pi in pig kidneys during cold ischemia

Quality assessment of renal grafts via (31)P magnetic resonance spectroscopy (MRS) has been investigated since 1986. As ATP concentrations decay rapidly during cold ischemia, the ratio of phosphomonoesters (PME) to inorganic phosphate (Pi(O)) within the organ (PME/Pi(O)) is commonly used as a quality marker and is considered to be the most reliable parameter. MRS did not lead to any delay in the transplantation procedure since it was performed during the time necessary for immunological matching (cross-match). Differences in the time period until transplantation call for extrapolation of the measured ratio to the end of cold ischemia before correlating with graft performance after transplantation. Therefore, quantitative determination of PME/Pi(O) kinetics is essential. As a model for metabolite decay in human renal grafts, pig kidneys obtained from a slaughterhouse were monitored for up to 80 h via (31)P MRS at 2 T. By employing chemical shift imaging (CSI) with a spatial resolution of approximately 1 x 1 x 4 cm(3), it was possible to reduce partial volume effects significantly. The improved spectral resolution gained through CSI enabled reliable PME/Pi(O) ratios to be determined only from those voxels containing renal tissue. Spectra were fitted automatically using the magnetic resonance user interface (MRUI), with prior knowledge obtained from unlocalized spectra when necessary. A monoexponential time dependence of PME/Pi(O) for histidine-tryptophane-alpha-ketoglutarate (HTK)-perfused kidneys during cold ischemia was observed, and the determined value of the decay constant alpha was 0.0099 +/- 0.0012 h(-1). In University of Wisconsin solution (UW)-perfused kidneys, an alpha of 0.0183 +/- 0.0053 h(-1) was determined. Determination of the decay constant enables a usable extrapolation of PME/Pi(O) for quality assessment of UW perfusion and a reliable extrapolation for HTK-perfused human renal grafts.