CAIR-1/BAG-3 abrogates heat shock protein-70 chaperone complex-mediated protein degradation: accumulation of poly-ubiquitinated Hsp90 client proteins.

BAG family proteins are regulatory co-chaperones for heat shock protein (Hsp) 70. Hsp70 facilitates the removal of injured proteins by ubiquitin-mediated proteasomal degradation. This process can be driven by geldanamycin, an irreversible blocker of Hsp90. We hypothesize that CAIR-1/BAG-3 inhibits Hsp-mediated proteasomal degradation. Human breast cancer cells were engineered to overexpress either full-length CAIR-1 (FL), which binds Hsp70, or a BAG domain-deletion mutant (dBAG) that cannot bind Hsp70. FL overexpression prevented geldanamycin-mediated loss of total and phospho-Akt and other Hsp client proteins. dBAG provided no protection, indicating a requirement for Hsp70 binding. Ubiquitinated Akt accumulated in FL-expressing cells, mimicking the effect of lactacystin proteasomal inhibition, indicating that CAIR-1 inhibits proteasomal degradation distal to protein ubiquitination in a BAG domain-dependent manner. Protein protection in FL cells was generalizable to downstream Akt targets, GSK3beta, P70S6 kinase, CREB, and other Hsp client proteins, including Raf-1, cyclin-dependent kinase 4, and epidermal growth factor receptor. These findings suggest that Hsp70 is a chaperone driving a multiprotein degradation complex and that the inhibitory co-chaperone CAIR-1 functions distal to client ubiquitination. Furthermore, poly-ubiquitination is not sufficient for efficient proteasomal targeting of Hsp client proteins.

Both protein activation and gene expression are involved in early vascular tube formation in vitro.

PURPOSE: Gene expression and protein translation regulate and direct endothelial cell proliferation and differentiation. We initiated an unbiased global search for transcriptional changes occurring during endothelial cell vascular differentiation in vitro, focusing on genes not previously implicated in vascularization and angiogenesis. EXPERIMENTAL DESIGN: cDNA and protein from human umbilical vein endothelial cells forming vascular tubes on the basement membrane surrogate, Matrigel, were collected and subjected to a global unbiased search for alterations in expression of genes not previously linked to angiogenesis. RESULTS: Transcriptional inhibitors blocked vascular tube formation only when present within the first hour of incubation (P < 0.05). cDNA array analysis yielded 31 differentially regulated transcripts (of 5100 queried; false positive rate, 0.4%) from gene classes representing transcription, translational regulation, cell structure, and cell adhesion. mRNA levels of caldesmon, a cytoskeleton-associated protein not previously linked to angiogenesis, were markedly reduced during early tube formation. Caldesmon protein quantity was also markedly decreased as demonstrated by laser capture microdissection of tubule cells followed by immunoblotting. Strikingly, no significant changes in transcription of genes previously demonstrated to contribute to angiogenesis, invasion, or signal transduction contained on the array were observed. To investigate the possibility that posttranslational rather than transcriptional changes were involved in facilitating tube formation, we evaluated the activation status of two dominant signal pathways, RAS/mitogen-activated protein kinase and phosphatidylinositol 3-kinase/AKT. A net 3-fold reduction in phospho-AKT and a 4-fold reduction in phospho-extracellular signal-regulated kinase-1/2 occurred in a transcription-independent fashion. CONCLUSIONS: These data suggest that both changes in gene expression and transcription-independent activation of signal transduction pathways may be involved in vascular tube formation. A combination of transcriptional and proteomic analysis has the potential to identify novel transcription-dependent and -independent molecular targets of angiogenesis.