Noninvasive Measurement of mTORC1 Signaling with 89Zr-Transferrin

Abstract

Purpose mTOR regulates many normal physiological processes and when hyperactive, can drive numerous cancers and human diseases. However, it is very challenging to detect and quantify mTOR signaling non-invasively in clinically relevant animal models of disease or man. We hypothesized that a nuclear imaging tool measuring intracellular mTOR activity could address this unmet need. Experimental Design Although the biochemical activity of mTOR is not directly amenable to nuclear imaging probe development, we show that the transferrin receptor can be used to indirectly measure intracellular changes in mTOR activity. Results After verifying that the uptake of radiolabeled transferrin (the soluble ligand of the transferrin receptor) is stimulated by active mTORC1 in vitro, we showed that 89Zr-labeled transferrin can measure mTORC1 signaling dynamics in normal and cancerous mouse tissues with positron emission tomography (PET). Lastly, we show that 89Zr-Tf can detect the upregulation of mTORC1 by tumor cells to escape the antitumor effects of a standard of care antiandrogen, which is to our knowledge the first example of applying PET to interrogate the biology of treatment resistant cancer. Conclusions In summary, we have developed the first quantitative assay to provide a comprehensive measurement of mTOR signaling dynamics in vivo, in specific normal tissues and during tumor development in genetically engineered animal models using a nuclear imaging tool that is readily translatable to man.

Targeting iron metabolism in high-grade glioma with 68Ga-citrate PET/MR

“…to high-grade glioma, most notably mTORC1 hyperactivity, can increase Fe(III) uptake into cancer cells by upregulating the activity of the transferrin (Tf) receptor (TFRC) in preclinical models (10)(11)(12)(13)(14). On this basis, we evaluated whether 68 Ga-citrate, a radiometal for PET that like Fe(III) binds to Tf in vivo (15,16), can detect high-grade glioma using combined PET and MRI modalities.…”

Real-Time Transferrin-Based PET Detects MYC-Positive Prostate Cancer

“…Because blood pool activity was approximately equivalent with tumor uptake of 68 Ga-citrate in both mouse cohorts, we next tested if tumor uptake of 68 Ga-citrate was equivalent with that of 89 Zr-Tf at time points we previously showed to reflect receptor mediated binding to tumors(15–17, 23, 24). We reasoned that equivalent levels of radiotracer uptake in tumor would be strong evidence of specific receptor binding by 68 Ga-citrate.…”

Therapeutic Potential of Everolimus on Core Autism Symptoms and Increasing Serum Ceruloplasmin and Transferrin Levels in a Pubescent Boy with Tuberous Sclerosis

“…With respect to increased serum Tf levels, the mTOR signaling pathway modulates Tf uptake [10], and mTOR regulates iron homeostasis through the Tf receptor [9]. Additionally, the Tf receptor may be useful for measuring intracellular changes in mTOR activity [46]. These findings suggest that everolimus may induce increases in serum Cp and Tf levels in relation to the improvement of social and behavioral symptoms.…”

“…Because mTOR is closely associated with antioxidant capacity [23,24], serum levels of antioxidant proteins such as Cp, and Tf were measured using a Bering BN Nephelometer (Siemens Healthcare Diagnostics K.K., USA) and a standard turbid metric assay and an automated biochemical analyzer (JCA-BM8000 series, JEOL Ltd., Tokyo, Japan). Interestingly, serum levels of Cp gradually increased from baseline to 16 weeks and then remained higher than the baseline levels ( Figure 2).…”