MicroRNAs as Engineering Targets: Pathway Manipulation to Impact Bioprocess Phenotypes - MicroRNAs as Tools in Biopharmaceutical Production - page 69

genes encoding the p21 CIP1 and p27 KIP1 cyclin E-dependant kinase cdk2 inhibitors
which cause arrest of the cell cycle at G1 phase (Fussenegger and Bailey 1998 ).
This strategy of cell cycle arrest may also have its own limitations since it is difficult
to engineer higher cell growth rates and growth arrest simultaneously. Besides, the
improvement of cell specific productivity resulting from this approach may also be
cell and product-specific (Dinnis and James 2005 ).
A microRNA-based strategy is a possible alternative to the existing methods of
manipulating cell growth and arrest. There are reports on miRNA involvement in
cell proliferation and growth by targeting essential genes of cell cycle regulation .
The function of several members of miRNA 17-92 polycistronic cluster was inves-
tigated. Inhibition of miR-18a with antagomirs decreased cell proliferation (Scherr
et al. 2007 ) and miR-17-5p and miR-20a negatively regulated the expression of E2F1
protein (which, when up-regulated promotes cell cycle progression and overrides the
growth arrest), (O'Donnell et al. 2005 ). Another study showed that the inhibition of
miR-21 and miR-24 resulted in a significant increase in mammalian cell growth
(Cheng et al. 2005 ). The growth-inhibitory properties of miR-21 and miR-24 were
also confirmed by their up-regulation during the stationary phase and after a tem-
perature shift when compared to their levels during the exponential growth phase
(Gammel 2007 ). It was suggested that miR-24 inhibits the progression thorough G1
phase and transition to S phase of the cell cycle by targeting E2F2 and MYC, and
may also regulate DNA repair (Lal et al. 2009 ).
Several miRNAs have been reported to act as regulators of cell growth cycle.
(1) miR-125b was reported to support cell growth in liver cells, and its inhibition
in human cancer cell lines caused decreased growth (Gammell 2007 ; Lee et al.
2005 ); (2) over expression of Let-7 suppressed growth in colon cancer and lung
adenocarcinoma by targeting MYC and RAS gene expression, thus affecting the
pathways controlled by these oncogenes (Akao et al. 2006 ; Johnson et al. 2005 );
(3) expression of miR-133 was found to be correlated with a higher skeletal muscle
growth rate by repression of serum response factor (SRF) in cultured myoblasts (Chen
et al. 2006 ); (4) the miR-34 family (miR-34 a, b, c) was found to target growth and
growth-arrest related genes including CDK4,and CDK6, c-MYC, CREB, Notch1,
E2F3 (Hermeking 2010 ).
Other miRNAs affect proliferation of mammalian cell lines: (1) miR-143 and
miR-145 negatively affect cell proliferation by regulating genes of the mitogen-
activated protein kinase (MAPK) family, MAPK7 and ERK5 (Esau et al. 2004 );
(2) miR-221 and miR-222 inhibit proliferation in hematopoietic progenitor cells
lines by targeting the Kit gene, which encodes receptor protein with tyrosine-kinase
activity (Felli et al. 2005 ); (3) miR-372 and miR-373 control proliferation by direct
interference with cell cycle regulation, regardless of increased levels of the inhibitor
p21 CIPI and inhibition of CDK by p53; (4) over expression of miR-31 exhibits anti-
proliferative effects in multiple cancer types by associating with the p53-mediated
growth inhibitory pathway, and may trigger caspase activation and apoptosis by sup-
pressing E2F2 gene (Creighton et al. 2010 ); (5) over expression of miR-7 inhibited
growth by targeting epidermal growth factor receptor (EGFR) and Akt pathway in
glioblastomas. Exogenous increase of miR-7 levels blocked CHO-K1 proliferation