Prostate-specific membrane antigen (PSMA) is one of the most specific cell surface markers for prostate cancer diagnosis and targeted treatment. However, achieving molecular imaging using non-invasive MRI with high resolution has yet to be achieved due to the lack of contrast agents with significantly improved relaxivity for sensitivity, targeting capabilities and metal selectivity. We have previously reported our creation of a novel class of protein Gd 3+ contrast agents, ProCA32, which displayed significantly improved relaxivity while exhibiting strong Gd 3+ binding selectivity over physiological metal ions. In this study, we report our effort in further developing biomarker-targeted protein MRI contrast agents for molecular imaging of PSMA. Among three PSMA targeted contrast agents engineered with addition of different molecular recognition sequences, ProCA32.PSMA exhibits a binding affinity of 1.1 ± 0.1 μM for PSMA while the metal binding affinity is maintained at 0.9 ± 0.1 × 10 −22 M. In addition, ProCA32.PSMA exhibits r 1 of 27.6 mM −1 s −1 and r 2 of 37.9 mM −1 s −1 per Gd (55.2 and 75.8 mM −1 s −1 per molecule r 1 and r 2 , respectively) at 1.4 T. At 7 T, ProCA32.PSMA also has r 2 of 94.0 mM −1 s −1 per Gd (188.0 mM −1 s −1 per molecule) and r 1 of 18.6 mM −1 s −1 per Gd (37.2 mM −1 s −1 per molecule). This contrast capability enables the first MRI enhancement dependent on PSMA expression levels in tumor bearing mice using both T 1 and T 2 -weighted MRI at 7 T. Further development of these PSMAtargeted contrast agents are expected to be used for the precision imaging of prostate cancer at an † Electronic supplementary information (ESI) available. See

c Unlike other Rho GTPases, RhoB is rapidly induced by DNA damage, and its expression level decreases during cancer progression. Because inefficient repair of DNA double-strand breaks (DSBs) can lead to cancer, we investigated whether camptothecin, an anticancer drug that produces DSBs, induces RhoB expression and examined its role in the camptothecin-induced DNA damage response. We show that in camptothecin-treated cells, DSBs induce RhoB expression by a mechanism that depends notably on Chk2 and its substrate HuR, which binds to RhoB mRNA and protects it against degradation. RhoB-deficient cells fail to dephosphorylate ␥H2AX following camptothecin removal and show reduced efficiency of DSB repair by homologous recombination. These cells also show decreased activity of protein phosphatase 2A (PP2A), a phosphatase for ␥H2AX and other DNA damage and repair proteins. Thus, we propose that DSBs activate a Chk2-HuR-RhoB pathway that promotes PP2A-mediated dephosphorylation of ␥H2AX and DSB repair. Finally, we show that RhoB-deficient cells accumulate endogenous ␥H2AX and chromosomal abnormalities, suggesting that RhoB loss increases DSB-mediated genomic instability and tumor progression.