ABSTRACT
The receptor tyrosine kinase KIT is an established oncogenic driver of tumor growth in certain tumor types, including gastrointestinal stromal tumors, in which constitutively active mutant forms of KIT represent an actionable target for small-molecule tyrosine kinase inhibitors. There is also considerable potential for KIT to influence tumor growth indirectly based on its expression and function in cell types of the innate immune system, most notably mast cells. We have evaluated syngeneic mouse tumor models for antitumor effects of an inhibitory KIT mAb, dosed either alone or in combination with immune checkpoint inhibitors. Anti-KIT mAb treatment enhanced the antitumor activity of anti-CTLA-4 and anti-PD-1 mAbs, and promoted immune responses by selectively reducing the immunosuppressive monocytic myeloid-derived suppressor cell population and by restoring CD8+ and CD4+ T-cell populations to levels observed in naïve mice. These data provide a rationale for clinical investigation of the human KIT-specific mAb KTN0158 in novel immuno-oncology combinations with immune checkpoint inhibitors and other immunotherapeutic agents across a range of tumor types. Mol Cancer Ther; 16(4); 671-80. ©2017 AACR.
Subject(s)
Antibodies, Monoclonal/administration & dosage , CTLA-4 Antigen/antagonists & inhibitors , Cell Cycle Checkpoints/drug effects , Colonic Neoplasms/drug therapy , Programmed Cell Death 1 Receptor/antagonists & inhibitors , Proto-Oncogene Proteins c-kit/antagonists & inhibitors , Animals , Antibodies, Monoclonal/pharmacology , CD4-Positive T-Lymphocytes/drug effects , CD8-Positive T-Lymphocytes/drug effects , Cell Line, Tumor , Colonic Neoplasms/immunology , Colonic Neoplasms/metabolism , Cytotoxicity, Immunologic/drug effects , Drug Synergism , Humans , Immunosuppression Therapy , Mice , Myeloid-Derived Suppressor Cells/drug effects , Phosphorylation/drug effects , Xenograft Model Antitumor AssaysABSTRACT
We have measured the effects of cofilin on the conformation and dynamics of actin filaments labeled at Cys374 with erythrosin-iodoacetemide (ErIA), using time-resolved phosphorescence anisotropy (TPA). Cofilin quenches the phosphorescence intensity of actin-bound ErIA, indicating that binding changes the local environment of the probe. The cofilin concentration-dependence of the phosphorescence intensity is sigmoidal, consistent with cooperative actin filament binding. Model-independent analysis of the anisotropies indicates that cofilin increases the rates of the microsecond rotational motions of actin. In contrast to the reduction in phosphorescence intensity, the changes in the rates of rotational motions display non-nearest-neighbor cooperative interactions and saturate at substoichiometric cofilin binding densities. Detailed analysis of the TPA decays indicates that cofilin decreases the torsional rigidity (C) of actin, increasing the thermally driven root-mean-square torsional angle between adjacent filament subunits from approximately 4 degrees (C = 2.30 x 10(-27) Nm2 radian(-1)) to approximately 17 degrees (C = 0.13 x 10(-27) Nm2 radian(-1)) at 25 degrees C. We favor a mechanism in which cofilin binding shifts the equilibrium between thermal ErIA-actin filament conformers, and facilitates two distinct structural changes in actin. One is local in nature, which affects the structure of actin's C terminus and is likely to mediate nearest-neighbor cooperative binding and filament severing. The second is a change in the internal dynamics of actin, which displays non-nearest-neighbor cooperativity and increases the torsional flexibility of filaments. The long-range effects of cofilin on the torsional dynamics of actin may accelerate P(i) release from filaments and modulate interactions with other regulatory actin filament binding proteins.
