UVRAG mutations associated with microsatellite unstable colon cancer do not affect autophagy

Abstract

Reduced levels of autophagy correlate with tumorigenesis, and several inducers of autophagy have been found to be tumor suppressors. One such autophagic inducer is the Beclin 1 binding protein UVRAG, a positive regulator of the class III PI3K/Vps34 complex. UVRAG has been implicated in the formation and maturation of autophagosomes, as well as in endocytic trafficking and suppression of proliferation and in vivo tumorigenicity. In this study we show that approximately one-third of a large series of colon carcinomas with microsatellite instability (MSI) (n = 102) carry a monoallelic UVRAG mutation, leading to expression of a truncated protein, indicating that this event is involved in tumorigenesis. In order to investigate whether the high incidence of UVRAG mutation in MSI colorectal carcinomas is associated with dysfunctional autophagy we analyzed autophagy levels in several colon cancer cell lines that express wild-type or mutant UVRAG protein. No reduction in autophagy was detected in cell lines expressing mutant UVRAG. Consistent with this, depletion of UVRAG in HEK cells stably expressing GFP-LC3 did not inhibit autophagy, but did decrease epidermal growth factor receptor (EGFR) degradation. Overall our results show that there is no correlation between the presence of the monoallelic UVRAG mutation and inhibition of autophagy. Thus, our data indicate that mechanisms other than autophagy contribute to the tumorigenicity of microsatellite unstable colon carcinomas with monoallelic UVRAG mutation.