Steroid Probes Conjugated with Protein-Protected Gold Nanocluster: Specific and Rapid Fluorescence Imaging of Steroid Receptors in Target Cells

Abstract

Steroid ligands can easily diffuse through the cell membrane and this property makes it feasible to be used for in-situ staining of the nuclear receptors. However, nonspecific binding of the internalized ligand probe with the cellular components has caused serious interferences for the detection of receptor-expressing cells. We report a novel gold nanocluster (AuNC)-conjugated estrogen probe that can eliminate nonspecific internalization and accelerate nuclear localization to achieve selective and rapid detection of estrogen receptors (ERs) in live cells. The AuNC, protected by bovine serum albumin (BSA), BSA-AuNCs, was prepared by the synthesis and confirmed to be 1.9 nm in core size and 18 nm in diameter. Ethinyl estradiol was used as the precursor of 17β-estradial (E2) to conjugate with BSA-protected AuNCs via polyethylene glycol linker (E2-PEG/BSA-AuNCs) or to conjugate with Cy3 dyes (E2-Cy3). The conjugated probe was determined to contain five E2 molecules per BSA-AuNC by mass spectrometry and exhibit an emission maximum of around 640 nm, which was not altered by E2 conjugation indicating that the structural integrity of BSA-AuNCs was conserved. E2-PEG/BSA-AuNCs probes were quickly internalized by MCF-7 (ER+) cells and localized to the nuclei in 2 h. Such internalization was sensitive to competition by free E2 and was rarely detected in the controls using either non-conjugated BSA-AuNCs in MCF-7 (ER+) cells or E2-PEG/BSA-AuNCs in MDA-MB-231 (ER-) cells. In contrast to the high specificity of E2-PEG/BSA-AuNCs probe, the uptake of E2-Cy3 probe could not differentiate between MCF-7(ER+) and MDA-MB-231(ER-) cells during the early phases of the treatment. Moreover, nuclear targeting by E2-Cy3 was three times slower than that by the E2-PEG/BSA-AuNC probe. Such accelerated nuclei targeting was consistent with the enhanced cell viability by conjugating E2 with BSA-AuNC. In conclusion, the E2-PEG/BSA-AuNC probes are promising candidates that can be used for the detection of ER+ tumor tissues and the same strategy can be applied to fabricate other steroid probes.

Atomically Precise Clusters of Noble Metals: Emerging Link between Atoms and Nanoparticles

“…Several clusters have been tabulated in Table S1 with a brief description. Along with thiol protection, protein protected nanoclusters ,,− such as Au@BSA, Au@lysozyme, etc., are also known and most of them are highly luminescent in nature. For these systems, the number of metal atoms has been found from MALDI MS data after subtracting the protein mass (more details about protein protected cluster can found in section ).…”

“…Size evolution of protein protected clusters has also been investigated by small-angle X-ray scattering (SAXS) . Because of their biocompatibility, they have been used in several applications and many of them have been discussed in the application section. ,,,,,− Many review articles ,,,, and book chapters , including some from the present authors have covered this topic in detail.…”

Identifying Reducing and Capping Sites of Protein-Encapsulated Gold Nanoclusters

“…BSA-AuNCs were synthesized at pH 12 and 37 °C (experimental details available in supporting information) following the method published previously [6,46]. A volume of 500 μL of aqueous HAuCl 4 solution (10 mM) was added to 500 μL of BSA solution (50 mg/mL) under vigorous stirring.…”