Developmental function of Nm23/awd: a mediator of endocytosis

Abstract

The metastasis suppressor gene Nm23 is highly conserved from yeast to human, implicating a critical developmental function. Studies in cultured mammalian cells have identified several potential functions, but many have not been directly verified in vivo. Here we summarize the studies on the Drosophila homologue of the Nm23 gene, named abnormal wing discs (awd), which shares 78% amino acid identity with the human Nm23-H1 and H2 isoforms. These studies confirmed that awd gene encodes a nucleoside diphosphate kinase, and provided strong evidence of a role for awd in regulating cell differentiation and motility via regulation of growth factor receptor signaling. The latter function is mainly mediated by control of endocytosis. This review provides a historical account of the discovery and subsequent analyses of the awd gene. We will also discuss the possible molecular function of the Awd protein that underlies the endocytic function. KeywordsNm23; awd; prune; Killer-of-prune; Drosophila Nm23 was first isolated as a cDNA clone that was down-regulated in highly metastatic derivatives of murine melanoma cell line K-1735 (1). The gene is highly conserved in the eukaryotes from yeast to mammals. In vertebrates it consists of a gene family of eight.Nm23 has been shown to inhibit metastasis, but not primary tumor growth, in xenografts of cells from human breast cancer, murine melanoma, rat colon cancer and human oral squamous cancer (2). However, in clinical cancer samples, this correlation is much weaker. While in initial breast cancer studies reduced Nm23 expression levels were consistent with increased metastasis (3), in later studies no clear correlation could be discerned when comparing benign tumors and metastatic cancers (4-8). It has become clear that Nm23 plays a critical role in normal tissue functions, and both up-and down-regulation of this gene can disrupt growth and differentiation. The function of Nm23Further complicating the issue of Nm23's role in tumor progression is the multiple enzymatic functions assigned to Nm23: (a) Nucleoside diphosphate kinase (NDPK) that transfers the terminal phosphate group from ATP to a non-ATP nucleoside diphosphate (such as GDP), through the formation of an intermediate histidine-phosphate linkage at histidine 118 (residue 119 in Drosophila) (2). (b) DNA binding and DNA nuclease that is involved in transcriptional regulation (9,10). (c) DNase activity that is activated by granzyme A in caspase-independent apoptosis (11). (d) Histidine-dependent protein kinase (12)(13)(14). This enzymatic activity is similar to that of NDPK but toward protein substrates (15). Some tantalizing kinase targets have been identified such as aldolase C and kinase suppressor of Ras (Ksr), but the in vivo relevance of which requires further investigation (16). It is therefore critical that the apparently complex Nm23 functions be examined in physiologically relevant model systems. The Drosophila model to date has provided the most extensive set of studies, in part because Drosophila genome enc...

Complex cellular functions of the von Hippel–Lindau tumor suppressor gene: insights from model organisms

“…Indeed, FGFR-ERK-Ets1 signaling pathway is over-activated in dVHL mutant tracheal cells, which is the result of accumulated FGFR on the mutant tracheal cell surface (Hsouna et al , 2010). Genetic epistasis analysis indicates that the surface accumulation of FGFR is the result of defective endocytic pathway and that dVHL functionally interacts with and stabilizes Abnormal Wing Discs (AWD), the homolog of human anti-metastasis factor Nm23, which has been shown to regulate endocytosis (Dammai et al , 2003; Hsu et al , 2006; Nallamothu et al , 2008, 2009; Woolworth et al , 2009) in part by stabilizing Rab5 (Woolworth et al , 2009). The detailed phenotypic analysis also revealed that the endocytic function of dVHL not only regulates cell motility but also controls the size and length of the tubule lumen (Hsouna et al , 2010).…”

Progress on Nme (NDP kinase/Nm23/Awd) gene family-related functions derived from animal model systems: studies on development, cardiovascular disease, and cancer metastasis exemplified

“…The only ortholog of the Nme genes in Drosophila, abnormal wing discs (awd), was discovered soon after the human Nme1 (also known as Nm23H1 or NDPK A) metastasis suppressor activity was identified (Dearolf et al 1988;Nallamothu et al 2009;Steeg et al 1988). Early studies in Drosophila showed that awd mutant larval brain exhibited mitotic defects correlated with defective microtubule polymerization (Biggs et al 1990).…”

NME genes in epithelial morphogenesis

“…To date, the most physiologically relevant function of NME was revealed by genetic studies in Drosophila [reviewed in (Nallamothu et al 2009)]. Initially, awd mutations were shown to cause imaginal disc defects, hence its name a bnormal w ing d iscs (Dearolf et al 1988a, b).…”

“…In an attempt to further characterize the function of VHL in Drosophila , a yeast two-hybrid screen was conducted to isolate VHL-interacting proteins, and Awd was isolated as such (Hsu, personal communication). It was subsequently shown that the NME protein is stabilized by VHL in Drosophila and in human kidney and renal carcinoma cells (Hsouna et al 2010; Hsu et al 2006; Nallamothu et al 2009; Urakami et al 1997). Whether NME is involved in renal carcinoma progression requires further investigation, although a few clinical studies have suggested a link between loss of NME and renal cell carcinoma (Ayhan et al 1998; Hiasa et al 1996; Nakagawa et al 1998).…”