Ripmeester et al rDNA.pzfx

BMP7 is a
morphogen capable of counteracting the OA chondrocyte hypertrophic phenotype
via NKX3-2. NKX3-2 represses expression of RUNX2, an important transcription
factor for chondrocyte hypertrophy. Since RUNX2 has previously been described
as an inhibitor for 47S pre-rRNA transcription, we hypothesized that BMP7
positively influences 47S pre-rRNA transcription through NKX3-2, resulting in
increased protein translational capacity. Therefor SW1353 cells and human
primary chondrocytes were exposed to BMP7 and rRNA (18S, 5.8S, 28S) expression
was determined by RT-qPCR. NKX3-2 knockdown was achieved via transfection of a
NKX3-2-specific siRNA duplex. Translational capacity was assessed by the SUNsET
assay, and 47S pre-rRNA transcription was determined by transfection of a 47S
gene promoter-reporter plasmid. BMP7 treatment increased protein translational
capacity. This was associated by increased 18S and 5.8S rRNA and NKX3-2 mRNA
expression, as well as increased 47S gene promotor activity. Knockdown of
NKX3-2 led to increased expression of RUNX2, accompanied by decreased 47S gene
promotor activity and rRNA expression, an effect BMP7 was unable to restore.
Our data demonstrate that BMP7 positively influences protein translation
capacity of SW1353 cells and chondrocytes. This is likely caused by an
NKX3-2-dependent activation of 47S gene promotor activity. This finding
connects morphogen-mediated changes in cellular differentiation to an aspect of
ribosome biogenesis via key transcription factors central to determining the
chondrocyte phenotype.