Protein kinase NII and the regulation of rDNA transcription in mammalian cells

Abstract

Transcription of ribosomal RNA genes is generally accepted to correlate with cell growth. Using primary cultures of adult bovine aortic endothelial (ABAE) cells, we have shown that transcription of rDNA in confluent cells falls to 5% of the transcription level in growing cells. Protein kinase NII appears to be a limiting factor to promote rDNA transcription in isolated nuclei of confluent cells. Protein kinase NII was detected by immunocytochemistry in the cytoplasm, nuclei and nucleoli of growing cells while it was no longer present in nucleoli of confluent cells. The kinase activity, in isolated nuclei, was estimated by endogenous phosphorylation of a specific substrate, nucleolin. A 10% residual activity was present in confluent cell nuclei compared to growing cell nuclei. Concomitantly, the transcription 'in vitro' of rDNA in the corresponding nuclei was also highly reduced (by 85%). Addition of exogenous protein kinase NII to confluent cell nuclei induced a strong increase in the phosphorylation of specific proteins including nucleolin. In parallel, the transcription of rDNA was increased by a factor of 5, to nearly the level observed in nuclei prepared from growing cells. These data suggest that, in confluent cells, factors necessary for rDNA transcription machinery are present but inactive in the nucleolus and that the phosphorylation of one or several of these factors (nucleolin, topoisomerase I,...) by protein kinase NII is a key event in the regulation of rDNA transcription.

Specific Interaction between Casein Kinase 2 and the Nucleolar Protein Nopp140

“…Its growth-related accumulation in the nucleolus has also been observed (32,33). The finding of the association of CK2 with Nopp140 suggests that CK2 may play an important role in the nucleolus.…”

Immunocytochemical localization of casein kinase II during interphase and mitosis.

“…Confluent bovine adrenocortical cells showed the presence of CKII in the cytoplasm and an absence of nuclear CKII, while actively growing cells showed the predominant CKII concentration in nuclei (Filhol et al ., 1990) . In confluent bovine aortic endothelial cells, CKII was barely detected in either the cytoplasm or the nucleus, while it was prominent in both the nucleus and the cytoplasm of exponentially growing cells (Belenguer et al, 1989) . These variabilities in nuclear staining using antibodies against holoenzyme observed by those authors may be due to the movement or disintegration of CKII a' from the nucleus in specific cell cycle stages.…”

“…Nucleolus-associated CKII has been identified, has been shown to phosphorylate nucleolin as a substrate, and may be involved in ribosome assembly (Caizergues-Ferrer et al ., 1987 ;Schneider and Issinger, 1988) and regulation of rDNA transcription (Belenguer et al, 1989).…”

“…Immunocytochemical localization studies using antibodies against holoenzyme have been published (Pfaff and Anderer, 1988 ;Serrano et al ., 1989 ;Belenguer et al ., 1989 ;Filhol et al ., 1990) . CKII was detected by inununocytochemistry in the cytoplasm, nuclei, and nucleoli of growing cells, while it was no longer present in nucleoli of confluent cells of adult aortic endothelial cells (Belenguer et al ., 1989).…”

“…CKII was detected by inununocytochemistry in the cytoplasm, nuclei, and nucleoli of growing cells, while it was no longer present in nucleoli of confluent cells of adult aortic endothelial cells (Belenguer et al ., 1989). In addition, the distribution of CKII exhibited a shift toward an increased nuclear concentration during active proliferation of bovine adrenocortical cells in primary culture (Filhol et al ., 1990) .…”