Studies with the mouse mammary cell line HC11 showed that...

Studies with the mouse mammary cell line HC11 showed that CDH1 engagement triggers a dramatic surge in total RAC1 and CDC42 protein levels and activity through inhibition of proteolytic degradation by the 26S proteasome (Arulanandam et al. 2009, Arulanandam et al. 2010). In fact, polyubiquitination of RAC1 is strong at low cell densities, and this correlates with low total RAC protein and RAC:GTP levels (Arulanandam et al. 2010).

As constitutively active RAC1 mutants show the same pattern of expression and polyubiquitination as the wild-type RAC1, it is plausible that the GTP-bound RAC1 must also be ubiquitinated and targeted for degradation in subconfluent cells (Arulanandam et al. 2010).

In the Madin Darby canine kidney II (MDCKII) cell line, the dissolution of CDH1 adhesions triggers cell scattering which is accompanied by RAC1 ubiquitination (Lynch et al. 2006). CDC42 and its constituively active mutants show a similar pattern of expression and activation as RAC1 in subconfluent vs. confluent cells, but ubiquitination of CDC42 was not explicitly examined (Arulanandam et al. 2010).

There are several known E3 ubiquitin ligases that target these RAC1 and CDC42 for proteasome-mediated degradation. The action of these ligases may be directly or (likely) indirectly inhibited by homophilic cadherin engagement, and they are therefore annotated as candidate catalysts for the RAC1/CDC42 ubiquitination in this context. HACE1, cloned as a gene significantly downregulated in Wilm's tumour compared to normal kidney and located in the vicinity of the 6q21 breakpoint in the recurring Wilm's tumour t(6;15)(q21;q21) translocation (Anglesio et al. 2004), is the first discovered and the best characterized RAC1 E3 ubiquitin ligase, with a preference for GTP-bound RAC1 (Torrino et al. 2011: human HACE1 overexpressed in human embryonic kidney cell line HEK293 was used; Catillo-Lluva et al. 2013: an overexpressed recombinant human HACE1 and endogenous HEK293 RAC1 or recombinant mouse RAC1 were used). HACE1 expression is frequently lost in breast cancer, resulting in the accumulation of GTP-bound RAC1 (Goka and Lippman 2015). Cancer-associated missense mutations in HACE1 that impair its ability to polyubiquitinate RAC1 have been reported (Andrio et al. 2017). Other ubiquitin E3 ubiquitin ligases that polyubiquitinate RAC1 and target it for proteasome-mediated degradation are XIAP (Oberoi et al. 2012: recombinant proteins from different organisms, including humans, were used to demonstrate evolutionary conservation; Oberoi-Khanuja and Rajalingam 2014: recombinant human BIRC2 and RAC1 were used; Fujita et al. 2015: induction of XIAP in human pancreatic cancer cell lines coincides with binding of XIAP to GTP-bound RAC1 and RAC1 polyubiquitination) and BIRC2 (also known as c-IAP1) (Oberoi et al. 2012: recombinant proteins from different organisms, including humans, were used to demonstrate evolutionary conservation; Oberoi-Khanuja and Rajalingam 2014: recombinant human BIRC2 and RAC1 were used). XIAP and BIRC2 polyubiquitinate RAC1 irrespective of its activation status (Oberoi et al. 2012). Contrary to its effect on RAC1, BIRC2 stabilizes CDC42 (Marivin et al. 2014) and was confirmed to not polyubiquitinated CDC42 in another study (Murali et al. 2017). XIAP, on the other hand, does polyubiquitinate CDC42, targeting it for degradation (Murali et al. 2017: recombinant proteins of unspecified species origin were used, as well as endogenous proteins in human cervical carcinoma cell line HeLa). A Skp1-Cul1-F-box (SCF) E3 ubiquitin ligase complex containing FBXL19 as the substrate recognizing F-box protein (SCF-FBXL19) polyubiquitinates RAC1 and targets it for degradation only if RAC1 is previously phosphorylated by AKT, so it has not been annotated as a candidate RAC1 E3 ubiquitin ligase in this context (Zhao et al. 2013). TRAF7, an E3 ubiquitin ligase with a frequent loss of function in meningioma, binds both RAC1 and CDC42 (Najm et al. 2021). TRAF7 was shown to polyubiquitinate CDC42 and target it for degradation (Najm et al. 2021: recombinant TRAF7 and CDC42 of unspecified species origin, as well as endogenous TRAF7 and CDC42 in HEK293 cells were used). The effect of TRAF7 on RAC1 has not been examined (Najm et al. 2021). TRIM72 (MG53) E3 ubiquitin ligase polyubiquitinates RAC1 and targets it for degradation (Ma et al. 2022: recombinant TRIM72 and RAC1 of unspecified species origin and endogenous TRIM72 and RAC1 in human hepatocellular carcinoma cell line HUH7 were used). RNF19B E3 ubiquitin ligase polyubiquitinates RAC1 and targets it for degradation, which is positively regulated by DIRAS3, which stimulates the binding of RNF19B to RAC1 (Wang et al. 2023: human recombinant proteins and non-small-cell lung cancer cell lines were used).