Evading apoptosis is a cancer hallmark that remains a serious obstacle in 
current treatment approaches. Although proteasome inhibitors (PIs) have 
transformed management of multiple myeloma (MM), drug resistance emerges through 
induction of the aggresome+autophagy pathway as a compensatory protein clearance 
mechanism. Genome-wide profiling identified microRNAs (miRs) differentially 
expressed in bortezomib-resistant myeloma cells compared with drug-naive cells. 
The effect of individual miRs on proteasomal degradation of short-lived 
fluorescent reporter proteins was then determined in live cells. MiR-29b was 
significantly reduced in bortezomib-resistant cells as well as in cells 
resistant to second-generation PIs carfilzomib and ixazomib. Luciferase reporter 
assays demonstrated that miR-29b targeted PSME4 that encodes the proteasome 
activator PA200. Synthetically engineered miR-29b replacements impaired the 
growth of myeloma cells, patient tumor cells and xenotransplants. MiR-29b 
replacements also decreased PA200 association with proteasomes, reduced the 
proteasome's peptidase activity and inhibited ornithine decarboxylase turnover, 
a proteasome substrate degraded through ubiquitin-independent mechanisms. 
Immunofluorescence studies revealed that miR-29b replacements enhanced the 
bortezomib-induced accumulation of ubiquitinated proteins but did not reveal 
aggresome or autophagosome formation. Taken together, our study identifies 
miR-29b replacements as the first-in-class miR-based PIs that also disrupt the 
autophagy pathway and highlight their potential to synergistically enhance the 
antimyeloma effect of bortezomib.