A General Introduction to MicroRNAs,Their Investigation and Exploitation in CHO Cell Lines - MicroRNAs as Tools in Biopharmaceutical Production - page 6

1.5 Cellular Effects of MicroRNA Manipulation

1.5.1 Signalling Pathways

Proteins involved in signalling cascades are often more likely to influence the cell
phenotype than other effectors. Consequently, even a subtle fluctuation in their levels
via miRNA binding could have drastic consequence. Saj and Lai examined the re-
lationship between microRNA-mediated regulation and known signalling pathways
and reported evidence of the role of TGF-

/BMP and Ras/MAPK in modulating mi-
croRNA biogenesis (Saj and Lai 2011 ). The p53 pathway is also known to module
miRNA biogenesis or expression upon DNA damage (for review see (Suzuki and
Miyazono 2010 )). On the other hand, some miRNAs have been shown to regulate
p53 (Park et al. 2009 ) and thus the relationship between regulation of microRNAs
and the regulation of targets by microRNAs is still a relatively unexplored area of
microRNA biology.

β

1.5.2 MicroRNA and Stress

The principles that govern target capture such as levels of mRNAs and the number
of mRNA targets remain true under conditions of stress (Stern-Ginossar et al. 2008 ;
Kroll et al. 2010). As exemplified in the above section, miRNA- mediated stress re-
sponses, for example via the p53 pathway, can involve intricate levels of regulation.
In the context of recombinant protein production, an abnormal stress is placed on the
host cells, often activating stress pathways such as those involved in protein folding
and the folding machinery (UPR), secretion, energy metabolism, lipid metabolism
and membrane biogenesis in protein secreting cells. Yang et al ( 2011 ) showed that
miR-122 negatively regulates UPR chaperones via the CDK4-PSMD10 pathway.
A similar line of evidence, showed that miR-30c-2* was up-regulated during UPR
activation, concomitant with Xbp1 and could bind the 3 UTR of Xbp1 (Byrd and
Brewer 2011). Another hallmark of cell stress is the re-localization of protein effec-
tors in specialized cell compartments such as Russell bodies (aggregated proteins),
and stress granules (RNAs). Following cellular insults, stress-sensing kinases induce
the formation of stress granules-via eIF2alpha phosphorylation and miRNAs which
normally reside in the cytoplasm can be shuttled to this self-organized compartment.

1.5.3 MicroRNA and Culture Conditions

In biotechnology, the growth rate, the lifespan and the productivity of the host cells
are major determinants of recombinant protein production. As a result, cell engineer-
ing strategies such as the manipulation of cell cycle progression and apoptosis have