| Summary: | The mature FMS receptor has been demonstrated here as being subject to constitutive loss as shown by S35 methionine labelling, live antibody labelling surface loss, surface biotinylation and FACS analysis with a half-life estimated at between 1 and 2 hours. This receptor turnover has been shown to be independent of kinase activity by the use of kinase impaired mutants and also a highly specific FMS kinase inhibitor, GW2580 that is able to inhibit the ligandinduced downregulation of FMS. The intracellular domain of the FMS receptor is believed not to be involved in the proteins constitutive downregulation from experiments performed here using domain deletion mutations and broad spectrum inhibition of other intracellular kinases. All experiments demonstrated FMS constitutive loss at the same rate as the wild-type and untreated receptor. The constitutive turnover has been further demonstrated to be distinct from FMS downregulation occurring via shedding and RIP mediated by ADAM17 and the γ-secretase complex respectively. We have demonstrated that the mechanism of receptor downregulation is independent of clathrin, caveolin and dynamin. FMS has been demonstrated here to pass through EEA1 and Rab7 positive compartments before its delivery to the lysosome for degradation. Our preliminary data suggests that the mechanism of FMS downregulation by the constitutive pathway is equivalent to that induced by M-CSF, but dissimilar from that of shedding and RIP degradation. These results also suggest that the c-KIT and VEGFR2 receptors are also subject to a similar mechanism of constitutive downregulation and both are also committed to degraded via the lysosome. To gain further insight into a possible raison d'être for the seemingly wasteful process of constitutively degrading the FMS receptor and also possibly the c-KIT and VEGFR2 receptors the 3 synthesis rates of FMS was analysed. The rates of FMS synthesis were shown to be constant irrespective of the receptors mature levels. Inhibition of the receptors degradation by lysosomal inhibition resulted in an increase in the levels of the mature receptor within cells. This was also demonstrated for the c-KIT and VEGFR2 receptors. As the synthesis rate of FMS was shown to be indepedent of its mature levels, we suggest that the constitutive degradation pathway is part of a homoeostatic mechanism whose function is to buffer receptor levels, preventing their overexpression in a simplistic system where they are synthesised at a constant rate.
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