Development of tolerance to chemokine receptor antagonists: current paradigms and the need for further investigation.

Chemokine G-protein coupled receptors are validated drug targets for many diseases, including cancer, neurological, and inflammatory disorders. Despite much time and effort spent on therapeutic development, very few chemokine receptor antagonists are approved for clinical use. Among potential reasons for the slow progress in developing chemokine receptor inhibitors, antagonist tolerance, a progressive reduction in drug efficacy after repeated administration, is likely to play a key role. The mechanisms leading to antagonist tolerance remain poorly understood. In many cases, antagonist tolerance is accompanied by increased receptor concentration on the cell surface after prolonged exposure to chemokine receptor antagonists. This points to a possible role of altered receptor internalization and presentation on the cell surface, as has been shown for agonist (primarily opioid) tolerance. In addition, examples of antagonist tolerance in the context of other G-protein coupled receptors suggest the involvement of noncanonical signal transduction in opposing the effects of the antagonists. In this review, we summarize the available progress and challenges in therapeutic development of chemokine receptor antagonists, describe the available knowledge about antagonist tolerance, and propose new avenues for future investigation of this important phenomenon. Furthermore, we highlight the modern methodologies that have the potential to reveal novel mechanisms leading to antagonist tolerance and to propel the field forward by advancing the development of potent "tolerance-free" antagonists of chemokine receptors.

Conformational Dynamics Allows Sampling of an "Active-like" State by Oncogenic K-Ras-GDP.

Mutations in K-Ras GTPase replacing Gly12 with either Asp or Val are common in cancer. These mutations decelerate intrinsic and catalyzed GTP hydrolysis, leading to accumulation of K-Ras-GTP in cells. Signaling cascades initiated by K-Ras-GTP promote cell proliferation, survival, and invasion. Despite functional differences between the most frequent G12D mutation and the most aggressive and chemotherapy resistant G12V mutation, their long-suspected distinct structural features remain elusive. Using NMR, X-ray structures, and computational methods, we found that oncogenic mutants of K-Ras4B, the predominant splice variant of K-Ras, exhibit distinct conformational dynamics when GDP-bound, visiting the "active-like" conformational state similar to the one observed in GTP-bound K-Ras. This behavior distinguishes G12V from wild type and G12D K-Ras4B-GDP. The likely reason is interactions between the aliphatic sidechain of V12 and the Switch II region of K-Ras4BG12V-GDP, which are distinct in K-Ras4BG12D-GDP. In the X-ray structures, crystal contacts reduce the dynamics of the sidechain at position 12 by stabilizing the Switch I region of the protein. This explains why structural differences between G12V and G12D K-Ras have yet not been reported. Together, our results suggest a previously unknown mechanism of K-Ras activation. This mechanism relies on conformational dynamics caused by specific oncogenic mutations in the GDP-bound state. Our findings also imply that the therapeutic strategies decreasing the level of K-Ras-GTP by interfering with nucleotide exchange or by expediting GTP hydrolysis may work differently in different oncogenic mutants.

The Hypervariable Region of K-Ras4B Governs Molecular Recognition and Function.

The flexible C-terminal hypervariable region distinguishes K-Ras4B, an important proto-oncogenic GTPase, from other Ras GTPases. This unique lysine-rich portion of the protein harbors sites for post-translational modification, including cysteine prenylation, carboxymethylation, phosphorylation, and likely many others. The functions of the hypervariable region are diverse, ranging from anchoring K-Ras4B at the plasma membrane to sampling potentially auto-inhibitory binding sites in its GTPase domain and participating in isoform-specific protein-protein interactions and signaling. Despite much research, there are still many questions about the hypervariable region of K-Ras4B. For example, mechanistic details of its interaction with plasma membrane lipids and with the GTPase domain require further clarification. The roles of the hypervariable region in K-Ras4B-specific protein-protein interactions and signaling are incompletely defined. It is also unclear why post-translational modifications frequently found in protein polylysine domains, such as acetylation, glycation, and carbamoylation, have not been observed in K-Ras4B. Expanding knowledge of the hypervariable region will likely drive the development of novel highly-efficient and selective inhibitors of K-Ras4B that are urgently needed by cancer patients.