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The bone-sparing effect of estrogen is primarily mediated via estrogen receptor-α (ERα), which stimulates target gene transcription through two activation functions (AFs), AF-1 in the N-terminal and AF-2 in the ligand binding domain. To evaluate the role of ERα AF-1 and ERα AF-2 for the effects of estrogen in bone in vivo, we analyzed mouse models lacking the entire ERα protein (ERα. Estradiol (E2) treatment increased the amount of both trabecular and cortical bone in ovariectomized (OVX) WT mice. Neither the trabecular nor the cortical bone responded to E2 treatment in OVX ERα −/− or OVX ERαAF-2 0 mice. OVX ERαAF-1 0 mice displayed a normal E2 response in cortical bone but no E2 response in trabecular bone. Although E2 treatment increased the uterine and liver weights and reduced the thymus weight in OVX WT mice, no effect was seen on these parameters in OVX ERα −/− or OVX ERαAF-2 0 mice. The effect of E2 in OVX ERαAF-1 0 mice was tissue-dependent, with no or weak E2 response on thymus and uterine weights but a normal response on liver weight. In conclusion, ERα AF-2 is required for the estrogenic effects on all parameters evaluated, whereas the role of ERα AF-1 is tissue-specific, with a crucial role in trabecular bone and uterus but not cortical bone. Selective ER modulators stimulating ERα with minimal activation of ERα AF-1 could retain beneficial actions in cortical bone, constituting 80% of the skeleton, while minimizing effects on reproductive organs.

Conspectus Interest in increasing drug delivery efficiency has risen over the past decade both as a means to improve efficacy of already clinically available drugs and due to the increased difficulties of approving new drugs. As a functional group for targeted drug delivery, boronic acids (BAs) have been incorporated in polymeric particles both as a stimuli-responsive functional group and as a targeting ligand. Here, BA chemistry presents a wealth of opportunities for biological applications. It not only reacts with several chemical markers of disease such as reactive oxygen species (ROS), adenosine triphosphate (ATP), glucose, and reduced pH, but it also acts as ligands for diols such as sialic acid. These stimuli-responsive drug delivery systems optimize delivery of therapeutics based on rational design and precise molecular engineering. When designing materials containing BA, the unique chemical properties are important to take into consideration such as its vacant p-orbital, its molecular geometry, and the designed acid’s pK a. Instead of behaving as most carboxylic acids that donate protons, BAs instead primarily act as Lewis acids that accept electrons. In aqueous solution, most polymers containing BA exist in an equilibrium between their triangular hydrophobic form and a tetrahedral hydrophilic form. The most common pK a’s are in the nonphysiological range of 8–10, and much ongoing research focuses on modifying BAs into materials sensitive to a more physiologically relevant pH range. So far, BA moieties have been incorporated into a stunning array of materials, ranging from small molecules that can self-assemble into higher order structures such as micelles and polymeric micelles, via larger polymeric assemblies, to large scale hydrogels. With the abundance of biological molecules containing diols and polyhydroxy motifs, BA-containing materials have proven valuable in several biomedical applications such as treatment of cancer, diabetes, obesity, and bacterial infections. Both materials functionalized with BA and boronic esters display good safety profiles in vitro and in vivo; thus, BA-containing materials represent promising carriers for responsive delivery systems with great potential for clinical translation. The intention of this Account is to showcase the versatility of BA for biomedical applications. We first discuss the chemistry of BA and what to consider when designing BA-containing materials. Further, we review how its chemistry recently has been applied to nanomaterials for enhanced delivery efficiency, both as a stimuli-responsive group and as a targeting ligand. Lastly, we discuss the current limitations and further perspectives of BA in biomaterials, based on the great benefits that can come from utilizing the unique BA chemistry to enhance drug delivery efficiency.

Stimuli-responsive nanoparticles (NPs) are especially interesting to enhance the drug delivery specificity for biomedical applications. With the aim to achieve a highly stable and inflammation-specific drug release, we designed a reactive oxygen species (ROS)-responsive dextran-drug conjugate (Nap-Dex). By blending Nap-Dex with the acid-sensitive acetalated dextran polymer, we achieved a dual-responsive NP with high specificity toward the inflammatory environment. The inflammatory environment not only has elevated ROS levels but also has a lower pH than healthy tissues, making pH and ROS highly suitable triggers to target inflammatory diseases. The anti-inflammatory cyclooxygenase inhibitor naproxen was modified with an ROS-responsive phenylboronic acid (PBA) and conjugated onto dextran. The dextran units were functionalized with up to 87% modified naproxen. This resulted in a complete drug release from the polymer within 20 min at 10 mM HO. The dual-responsive NPs reduced the levels of the proinflammatory cytokine IL-6 120 times more efficiently and TNFα 6 times more efficiently than free naproxen from lipopolysaccharide (LPS)-activated macrophages. These additional anti-inflammatory effects were found to be mainly attributed to ROS-scavenging effects. In addition, the model cargo fluorescein diacetate was released in an LPS-induced inflammatory response in vitro. We believe that drug conjugation using PBA can be applied to various drugs and dextran-based materials for enhanced drug efficacy, where this work demonstrates the significance of functionalized carbohydrates polymer-drug conjugates.