Inhibition of activated Ras-induced neuronal differentiation of PC12 cells by the LIM domain of LIM-kinase 1

LIM-kinase 1 and 2 (LIMK1 and LIMK2) are members of a novel class of protein kinases with structures composed of two LIM motifs at the N-terminus and an unusual protein kinase domain at the C-terminus. The cellular functions of the LIMK family proteins have remained unknown. In the present study, we examined effects of LIMKs on neuronal differentiation of PC12 pheochromocytoma cells. Transient expression analyses revealed that LIMK1, in itself, had no apparent effect on PC12 cells, but the oncogenic Pas-induced differentiation of PC12 cells was notably inhibited by co-expression with LIMK1 or LIMK2. A mutant of LIMK1 lacking a protein kinase domain (ΔK) similarly inhibited Ras-induced differentiation of PC12 cells, but a mutant lacking a LIM domain (ΔLIM) failed to do so, indicating that a LIM domain but not a protein kinase domain is required for the inhibitory activity. This notion was further supported by the finding that mutation, changing conserved cysteines involved in zinc coordination to glycines in both of two LIM motifs, abolished the inhibitory activity of ΔK. Additionally, we also found that the constitutively activated MAP kinase kinase (MAPKK)-induced differentiation of PC12 cells was inhibited by co-expression with ΔK. Furthermore, ΔK did not inhibit the kinase activity of MAP kinase (MAPK) stimulated by MAPKK, when co-expressed in COS7 cells. These findings suggest that LIMK1 inhibits neuronal differentiation of PC12 cells, through its LIM domain and by interfering with events downstream of MAPK activation.