The proto-oncogene Ras undergoes a series of post-translational modifications at its carboxyl-terminal CAAX motif that are essential for its proper membrane localization and function. One step in this process is the cleavage of the CAAX motif by the enzyme Ras-converting enzyme 1 (RCE1). Here we show that the deubiquitinating enzyme USP17 negatively regulates the activity of RCE1. We demonstrate that USP17 expression blocks Ras membrane localization and activation, thereby inhibiting phosphorylation of the downstream kinases MEK and ERK. Furthermore, we show that this effect is caused by the loss of RCE1 catalytic activity as a result of its deubiquitination by USP17. We also show that USP17 and RCE1 co-localize at the endoplasmic reticulum and that USP17 cannot block proliferation or Ras membrane localization in RCE1 null cells. These studies demonstrate that USP17 modulates Ras processing and activation, at least in part, by regulating RCE1 activity.

The collagen-induced phosphorylation of discoidin domain receptor 1 (DDR1) in Wnt-5a-expressing HB2 mammary cells was effectively inhibited by pertussis toxin, but not by cholera toxin or antibodies blocking ␤ 1 integrins. Moreover, pertussis toxin reduced adhesion of the cells to collagen by approximately 50%, and antibodies against ␤ 1 integrins had a similar effect that was in fact additive to that of pertussis toxin. Cholera toxin had accordingly no such effect on adhesion. By comparison, pertussis toxin did not influence adhesion of Wnt-5a-antisense HB2 cells or MCF-7 mammary tumor cells, neither of which express Wnt-5a or exhibit activation of DDR1. In accordance with these results, direct mastoparan-induced activation of G-proteins in Wnt-5a-deficient MCF-7 cells enabled collagen-induced phosphorylation of DDR1 and enhanced their adhesion. The inactive analogue mastoparan-17 had no such effects on MCF-7 cells nor did active mastoparan affect adhesion of Wnt-5a-expressing HB2 cells. A possible explanation for how DDR1, a receptor tyrosine kinase (RTK), potentiates mammary cell adhesion comes from our observations that pertussis toxin also inhibited the recruitment of the cytoskeletal regulator phosphatidylinositol 3-kinase (PI3K) to DDR1 as well as its phosphorylation/activation. In accordance with that, the PI3K inhibitor wortmannin significantly impaired adhesion of normal Wnt5a-expressing HB2 cells but had little effect on adhesion of Wnt-5a-antisense HB2 cells. Thus, a G i/o -protein signaling pathway mediates the effect of Wnt-5a expression by enabling collagen-induced activation of DDR1, which, in parallel with ␤ 1 integrins, regulates adhesion of mammary cells.

Deubiquitinating enzymes are now emerging as potential therapeutic targets that control many cellular processes, but few have been demonstrated to control cell motility. Here, we show that ubiquitin-specific protease 17 (USP17) is rapidly and transiently induced in response to chemokines SDF-1/CXCL12 and IL-8/CXCL8 in both primary cells and cell lines, and that its depletion completely blocks chemokine-induced cell migration and cytoskeletal rearrangements. Using live cell imaging, we demonstrate that USP17 is required for both elongated and amoeboid motility, in addition to chemotaxis. USP17 has previously been reported to disrupt Ras localization and we now find that USP17 depletion blocks chemokine-induced subcellular relocalization of GTPases Cdc42, Rac and RhoA, which are GTPases essential for cell motility. Collectively, these results demonstrate that USP17 has a critical role in cell migration and may be a useful drug target for both inflammatory and metastatic disease.

Two naturally occurring mutant insulin receptors, Arg-1174 --> Gln and Leu-1178 --> Pro, found in patients with dominantly inherited Type A insulin resistance, showed unusual signaling properties when stably expressed in Chinese hamster ovary (CHO) cells. Both mutant receptors were expressed on the cell surface and bound insulin normally, but showed markedly impaired autophosphorylation in response to insulin. In addition, the in vitro tyrosine kinase activity of both mutant receptors toward an artificial substrate was also severely impaired. Despite these defects of kinase activity, anti-phosphotyrosine immunoblotting of whole cell lysates and anti-phosphotyrosine immunoprecipitation of 32P-labeled cells showed insulin-stimulated tyrosine phosphorylation of a protein of approximately 185 kDa to an extent comparable to that seen in CHO cells expressing wild-type human insulin receptors. Anti-insulin receptor substrate-1 (IRS-1) immunoprecipitation followed by anti-phosphotyrosine immunoblotting confirmed that this tyrosine-phosphorylated protein was IRS-1. In contrast, CHO cells expressing an insulin receptor mutated at the ATP binding site (Lys-1030 --> Arg) showed no insulin-stimulated autophosphorylation or phosphorylation of IRS-1. Despite exhibiting apparently normal insulin stimulation of IRS-1 tyrosine-phosphorylation, cells expressing the Arg-1174 --> Gln or Pro-1178 --> Leu receptors showed marked impairment in insulin stimulation of glycogen synthesis, thymidine incorporation, and activation of MAP kinase. The inability of these mutant receptors to signal normally to metabolic and mitogenic responses suggests that insulin-stimulated tyrosine phosphorylation of IRS-1 alone is insufficient to fully mediate insulin action.