Crystal structure of active CDK4-cyclin D and mechanistic basis for abemaciclib efficacy.

Despite the biological and therapeutic relevance of CDK4/6 for the treatment of HR+, HER2- advanced breast cancer, the detailed mode of action of CDK4/6 inhibitors is not completely understood. Of particular interest, phosphorylation of CDK4 at T172 (pT172) is critical for generating the active conformation, yet no such crystal structure has been reported to date. We describe here the x-ray structure of active CDK4-cyclin D3 bound to the CDK4/6 inhibitor abemaciclib and discuss the key aspects of the catalytically-competent complex. Furthermore, the effect of CDK4/6 inhibitors on CDK4 T172 phosphorylation has not been explored, despite its role as a potential biomarker of CDK4/6 inhibitor response. We show mechanistically that CDK4/6i stabilize primed (pT172) CDK4-cyclin D complex and selectively displace p21 in responsive tumor cells. Stabilization of active CDK4-cyclin D1 complex can lead to pathway reactivation following alternate dosing regimen. Consequently, sustained binding of abemaciclib to CDK4 leads to potent cell cycle inhibition in breast cancer cell lines and prevents rebound activation of downstream signaling. Overall, our study provides key insights demonstrating that prolonged treatment with CDK4/6 inhibitors and composition of the CDK4/6-cyclin D complex are both critical determinants of abemaciclib efficacy, with implications for this class of anticancer therapy.

Structure-based, multi-targeted drug discovery approach to eicosanoid inhibition: Dual inhibitors of mPGES-1 and 5-lipoxygenase activating protein (FLAP).

BACKGROUND: Due to the importance of both prostaglandins (PGs) and leukotrienes (LTs) as pro-inflammatory mediators, and the potential for eicosanoid shunting in the presence of pathway target inhibitors, we have investigated an approach to inhibiting the formation of both PGs and LTs as part of a multi-targeted drug discovery effort. METHODS: We generated ligand-protein X-ray crystal structures of known inhibitors of microsomal prostaglandin E2 synthase-1 (mPGES-1) and the 5-Lipoxygenase Activating Protein (FLAP), with their respective proteins, to understand the overlapping pharmacophores. We subsequently used molecular modeling and structure-based drug design (SBDD) to identify hybrid structures intended to inhibit both targets. RESULTS: This work enabled the preparation of compounds 4 and 5, which showed potent in vitro inhibition of both targets. SIGNIFICANCE: Our findings enhance the structural understanding of mPGES-1 and FLAP's unique ligand binding pockets and should accelerate the discovery of additional dual inhibitors for these two important integral membrane protein drug targets.

Nectin-like protein 2 defines a subset of T-cell zone dendritic cells and is a ligand for class-I-restricted T-cell-associated molecule.

Dendritic cells (DCs) are a phenotypically and functionally heterogenous population of leukocytes with distinct subsets serving a different set of specialized immune functions. Here we applied an in vitro whole cell panning approach using antibody phage display technology to identify cell-surface epitopes specifically expressed on human blood BDCA3(+) DCs. A single-chain antibody fragment (anti-1F12 scFv) was isolated that recognizes a conserved surface antigen expressed on both human BDCA3(+) DCs and mouse CD8alpha(+) DCs. We demonstrate that anti-1F12 scFv binds Nectin-like protein 2 (Necl2, Tslc1, SynCaM, SgIGSF, or Igsf4), an adhesion molecule involved in tumor suppression, synapse formation, and spermatogenesis. Thus, Necl2 defines a specialized subset of DCs in both mouse and human. We further show that Necl2 binds Class-I-restricted T-cell-associated molecule (CRTAM), a receptor primarily expressed on activated cytotoxic lymphocytes. When present on antigen presenting cells, Necl2 regulates IL-22 expression by activated CD8(+) T-cells. We propose that Necl2/CRTAM molecular pair could regulate a large panel of cell/cell interactions both within and outside of the immune system.

Purification, cloning, and characterization of Nek8, a novel NIMA-related kinase, and its candidate substrate Bicd2.

We describe the isolation, cloning, and characterization of human Nek8, a new mammalian NIMA-related kinase, and its candidate substrate Bicd2. Nek8 was isolated as a beta-casein kinase activity in rabbit lung and has an N-terminal catalytic domain homologous to the Nek family of protein kinases. Nek8 also contains a central domain with homology to RCC1, a guanine nucleotide exchange factor for the GTPase Ran, and a C-terminal coiled-coil domain. Like Nek2, Nek8 prefers beta-casein over other exogenous substrates, has shared biochemical requirements for kinase activity, and is capable of autophosphorylation and oligomerization. Nek8 activity is not cell cycle regulated, but like Nek3, levels are consistently higher in G(0)-arrested cells. During the purification of Nek8 a second protein co-chromatographed with Nek8 activity. This protein, Bicd2, is a human homolog of the Drosophila protein Bicaudal D, a coiled-coil protein. Bicd2 is phosphorylated by Nek8 in vitro, and the endogenous proteins associate in vivo. Bicd2 localizes to cytoskeletal structures, and its subcellular localization is dependent on microtubule morphology. Treatment of cells with nocodazole leads to dramatic reorganization of Bicd2, and correlates with Nek8 phosphorylation. This may be indicative of a role for Nek8 and Bicd2 associated with cell cycle independent microtubule dynamics.