Identification of Novel miR-21 Target Proteins in Multiple Myeloma Cells by Quantitative Proteomics

Substantial evidence indicates that microRNA-21 (miR-21)
is a key
oncomiR in carcinogenesis and is significantly elevated in multiple
myeloma (MM). In this study, we explored the role of miR-21 in human
MM cells and searched for miR-21 targets. By knocking down the expression
of endogenous miR-21 in U266 myeloma cells, we observed reduced growth,
an arrested cell cycle, and increased apoptosis. To further understand
its molecular mechanism in the pathogenesis of MM, we employed a SILAC
(stable isotope labeling by amino acids in cell culture)-based quantitative
proteomic strategy to systematically identify potential targets of
miR-21. In total, we found that the expression of 178 proteins was
up-regulated significantly by miR-21 inhibition, implying that they
could be potential targets of miR-21. Among these, the protein inhibitor
of activated STAT3 (PIAS3) was confirmed as a direct miR-21 target
by Western blotting and reporter gene assays. We further demonstrated
that miR-21 enhances the STAT3-dependent signal pathway by inhibiting
the function of PIAS3 and that down-regulation of PIAS3 contributes
to the oncogenic function of miR-21. This elucidation of the role
of PIAS3 in the miR-21-STAT3 positive regulatory loop not only may
shed light on the molecular basis of the biological effects of miR-21
observed in MM cells but also has direct implications for the development
of novel anti-MM therapeutic strategies.