Stimulating cell proliferation through the pharmacologic activation of c-kit.

Previous studies have shown that expression of a membrane targeted chimeric protein containing the erythropoietin receptor (EpoR) cytoplasmic domain fused to the FK506-binding peptide FKBP12 allowed Ba/F3 cells to be rescued from interleukin-3 (IL-3) deprivation using a dimeric form of FK506, called FK1012. In this report, a similar approach is applied to the c-kit receptor. Expression of a membrane targeted fusion protein containing the c-kit receptor linked to one or more copies of FKBP12 allowed Ba/F3 cells to be switched from IL-3 dependence to FK1012-dependence. Similar results were obtained using an alternative dimerizer of FKBP12 domains called AP1510. Pharmacologic dimerization of chimeric proteins containing only a single FKBP12 domain confirmed that receptor dimerization is sufficient for proliferative signaling. Interestingly, while the proliferative effects of both FK1012 and AP1510 were reversible, FK1012-driven proliferation persisted for several days after drug withdrawal. Furthermore, much higher concentrations of FK506 were required to inhibit FK1012-mediated proliferation than were required to inhibit AP1510-mediated proliferation. The persistence of FK1012's effect appeared to be specific to clones expressing c-kit-containing fusion proteins. These results suggest that pharmacologically-responsive fusion proteins containing c-kit may be useful for specifically and reversibly expanding genetically modified hematopoietic cell populations.