The CHO miRNA Transcriptome - MicroRNAs as Tools in Biopharmaceutical Production - page 50

4.1 MicroRNA Expression and Cellular Phenotypes

in Mammalian Cells

Most of our knowledge on miRNA expression and functionality derives from studies
performed in organisms ranging from C. elegans to human, with a notable lack of
CHO data. Only a handful of studies have been published describing the use of tran-
scriptomic tools for the analysis of microRNA (miRNA) expression in CHO cells
(Table 4.1 ). Most of these studies were designed to measure miRNA levels under
specific growth or culture conditions, in order to explore their functions and subse-
quently estimate their relevance for modulating CHO cell phenotypes. That changes
in miRNA transcript levels can indeed impact on cellular phenotype seems realistic
based on miRNA research in other mammalian organisms: in models of develop-
mental biology, controlled changes in miRNA expression are known to mediate cell
differentiation and organism development (Guo et al. 2011 ). In other studies co-
delivery of hsa-miR-93 with transcription factors Oct-4, Klf-4, Sox2 and cMyc can
increase the efficiency of re-differentiating somatic cells into pluripotent stem cells
(Li et al. 2011 ). In addition the development of certain diseases was shown to be
accompanied by changes in miRNA expression, for example as a consequence of
genetic mutations or rearrangements. In this respect, specific miRNA expression
signatures have been associated to various types of cancer (Lu 2005 ), since both
oncogenes and tumor suppressor proteins are known to be post-transcriptionally
controlled by miRNAs. Consequently, aberrant expression of miRNAs can allow
for rapid and uncontrolled proliferation or loss of cell contact inhibition, which
eventually will result in oncogenic cellular behavior (Iorio and Croce 2009 ).
Whether the biological relevance of miRNAs during these alterations in cell phe-
notypes lies in the induction or in the stabilization of the newly adopted cellular
state, currently is unclear (Kosik 2010 ;Wuetal. 2009 ); that the transcription of
miRNA genes and the subsequent processing of primary transcripts into mature
miRNAs need to be highly regulated processes, which result in defined levels of
mature miRNA transcripts depending on cell type or environmental condition is
undisputed (Krol et al. 2010 ). At transcriptional level, this control is mediated by
RNA polymerase II (Pol II) dependent transcription of most miRNA genes (Fig. 4.1 ),
allowing Pol II-associated transcription factors to bind and regulate the production
of primary miRNA transcripts (pri-miRs). In addition, the activity and levels of pro-
teins involved in processing of pri-miRs into mature miRNAs—especially Drosha
and Dgcr8 (Fig. 4.1 )—affects the accumulation of miRNAs on a post-transcriptional
level (Krol et al. 2010 ). Recently it was found that a number of well-known proteins,
such as p53 or members of the SMAD family, are able to interact with Drosha and
Dicer, thereby promoting the biogenesis of specific subsets of miRNAs (Suzuki et al.
2009 ; Davis et al. 2010 ). Overall these findings indicate that the composition of the
miRNA transcriptome at a given timepoint under given conditions is not random but
likely to significantly influence cellular state.
With respect to recombinant protein production cell lines, this means that mea-
suring changes in miRNA levels in response to specific culture conditions or
transitions in cellular state, is likely to increase our understanding of which genes and