Biomedical Engineering Reference
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heatmap patterns, miRNA expression levels were generally observed to be similar be-
tween different CHO cell lines. However, explorative data analysis identified a clear
difference between serum-free CHO-K1 cells and serum-free DUKXB-11 (dhfr + / )
as well as in response to presence of serum in cultivation media (Hackl et al. 2011 ).
Indeed, differential expression analysis showed that the adaptation of three distinct
cell lines (CHO-K1, CHO-K1SV and DUKXB-11), both host and recombinant, to
serum-free growth was accompanied by a global down-regulation of miRNA ex-
pression as indicated in a cumulative fraction plot calculated from available read
count data (Hackl et al. 2011 ) (Fig. 4.3 b). Given the fact that serum withdrawal from
media commonly also decreases the specific growth rate, this observation would
attribute miRNAs an important role in mediating growth of CHO cells, which shall
be reviewed next.
4.4.2
Linking miRNA Expression and Cellular Proliferation
in CHO Cells
A great amount of time and work has been devoted by the research community to
elucidating the importance of miRNAs for cellular proliferation, mainly with the
goal of gaining a better understanding of the oncogenic phenotype of tumor cells.
Today, microarrays are extensively used to compare miRNA expression in various
tumor cells or tissues to the respective normal controls, and miRNAs reoccurring in
these screenings as up- or down-regulated in tumor cells are commonly designated as
either oncomiRs or tumor suppressor miRNAs. To the dismay of cancer researchers,
however, oncogenic or tumor suppressive functions of miRNAs turned out to be
specific for certain types of cells or tissues (Lee and Dutta 2009 ) since miRNA
function is likely to depend on the individual genetic background of cells (Brenner
et al. 2010 ). For miRNA research in CHO cells this means that despite the avail-
ability of a variety of miRNA expression data from highly proliferating tumor cells,
independent—CHO specific—data need to be generated, followed by functional
characterization of prioritized miRNAs based on their expression characteristics.
In the first study on miRNA expression in CHO cell factories, Gammell et al.
looked at changes in miRNA levels upon temperature shift from 37 Cto31 C
(Gammell et al. 2007 ). This shift to lower temperature is commonly used in biphasic
cultivation strategies to switch cells from a proliferative state to growth arrest, typ-
ically accompanied by prolonged viability as well as increased protein production
and secretion (Trummer et al. 2006a , 2006b ). While this initial study in 2007 was
mainly focused on establishing and validating the chosen cross-species approach to
miRNA profiling, in a subsequent study the cold-shock experiment was repeated, this
time looking at acute temperature-sensitive miRNAs after 24 h incubation at 31 C
(Barron et al. 2011 ). Ten miRNAs with significant differential expression were de-
tected, four down-regulated and six up-regulated, which by in silico prediction of
target interactions were associated with regulation of nucleic acid metabolism and
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