Fig. 3 Target threshold in relief from miRNA mediated
repression using molecular titration model . The model assumes that the
target mRNA can get produced and degraded intrinsically. After
interacting with miRNA, mRNA gets degraded or dissociated from the
complex, while the miRNA is recycled back into the system. The model
also assumes that the protein translation can occur only from the free
mRNA and the mRNA bound to the miRNA cannot undergo translation. Plots
of target gene expression by varying, (a) the number of miRNA
binding sites on the target, and (b) the concentration of the
miRNA mimics for the same target with ‘n’ number of binding sites.
Violet dotted lines represent the target expression in the absence of a
miRNA binding site. Adapted from Mukherji et al53.
Gam et al54 investigated the fold repression using a
similar dual fluorescence reporter but with a miRNA target site
consisting of four perfectly complementary repeat sequences for the
miRNA in the 3’UTR of the fluorescent gene.54 They
created a library of miRNA sensors for various miRNAs and observed the
threshold kind of behavior similar to that of Mukherji et
al53 for a single miRNA binding site (four repeat
sequences) with the perfect match. They found that there are three
output regimes for miRNA mediated gene expression: (i) mRNA repressed
regime (ii) mRNA derepressed regime and (iii) an in-between threshold
regime (where switching from mRNA repression to de-repression takes
place). They have demonstrated that the number of binding sites and
miRNA levels can shift the threshold for miRNA-mediated mRNA
repression.54 High throughput assays performed to
measure miRNA activity for a large number of miRNAs in other cell lines
further suggest that the factors such as miRNA abundance and no. of
binding sites highly influence the miRNA repression
activity.52
Such sharp threshold response is observed for Notch (a signaling
molecule) expression for color cancer stem cells (CCSCs), where mir-34a
creates a bimodality in Notch expression by sequestering Notch
mRNA.63 The Notch pathway plays an important role in
regulating asymmetric division in both normal and cancer stem cells,
thus creating a CCSC daughter cell with self-renewal potential and a
differentiated non-CCSC daughter cell. It has been observed that
injecting these cells into the mouse xenograft models, low miR-34a
expressing CCSCs promoted tumor growth due to symmetric division of
CCSCs to give two CCSC daughter cells. On the other hand, high mir-34
expressing cells promoted differentiation with reduced tumorigenicity.
Hence, the ability of microRNAs to fine-tune the symmetric and
asymmetric division of cancer stem cells can be used as a potential
strategy for cancer treatment.63