Mer receptor tyrosine kinase signaling: Prevention of apoptosis and alteration of cytoskeletal architecture without stimulation or proliferation

Mer is a member of the Axl/Mer/Tyro3 receptor tyrosine kinase family, a family whose physiological function is not well defined. We constructed a Mer chimera using the epidermal growth factor receptor (EGFR) extracellular and transmembrane domains and the Mercytoplasmic domain. Stable transfection of the Mer chimera into interleukin 3 (IL-3)-dependent murine 32D cells resulted in ligand-activable surface receptor that tyrosine autophosphorylated, stimulated intracellular signaling, and dramatically reduced apoptosis initiated by IL-3 withdrawal. However, unlike multiple other ectopically expressed receptor tyrosine kinases including full-length EGFR or an EGFR/Axl chimera, the Mer chimera did not stimulate proliferation. Moreover, and in contrast to EGFR, Mer chimera activation induced adherence and cell flattening in the normally suspension-growing 32D cells. The Mer chimera signal also blocked IL-3-dependent proliferation leading to G₁/S arrest, dephosphorylation of retinoblastoma protein, and elongation of cellular processes. Unlike other agonists that lead to a slow (4-8 days) ligand-dependent differentiation of 32D cells, the combined Mer and IL-3 signal resulted in differentiated morphology and growth cessation in the first 24 h. Thus the Mer chimera blocks apoptosis without stimulating growth and produces cytoskeletal alterations; this outcome is clearly separable from the proliferative signal produced by most receptor tyrosine kinases.