On the lysosomal degradation of neurofibromin and its phosphorylation in cultured melanocytes

Neurofibromatosis type 1 (NF1) is one of the most common inherited disorders in humans. Most of the NF1 gene mutations result in a reduction of the amount of neurofibromin to about 50%. Recently, we found that the level of neurofibromin can be regulated post-translationally through the alteration of its half-life. Here, we investigated whether lysosomes are involved in this post-translational regulation in cultured melanocytes of NF1 patients and controls. When the lysosomal degradation was inhibited by chloroquine, an increase of neurofibromin by a factor of 2 to 3, correlating with an increased half-life, was measured. Incubation with phosphoprotein-phosphatase inhibitors also increased the neurofibromin content in melanocytes. Investigations on phosphorylation of neurofibromin revealed a basal phosphorylation in melanocytes cultured with growth factor-deprived medium that increased upon incubation with the growth stimulators PMA or bFGF. Because both factors are also able to increase the half-life of neurofibromin, we suggest its phosphorylation to be an important step in protecting neurofibromin against specific lysosomal degradation.