Inhibition of nucleolar protein nucleolin by electroporation with anti-nucleolin antibodies results in an increase of the nucleolar size.

Electroporation of exponentially growing human larynx epidermoid carcinoma cells (HEp-2) with a serum against nucleolin, one of the most abundant non-histone nuclear proteins, has shown, 24 h after electroporation, a significant increase in the size of the nucleolus of these cells compared with normal HEp-2 cells (non-electroporated) and electroporated HEp-2 cells in the absence of anti-nucleolin serum (P < 0.01). Image analysis evaluation of the different nucleolar components proved a major contribution of the dense fibrillar component to the total nucleolar size in cells electroporated with anti-nucleolin antibodies, more than that corresponding to the dense fibrillar component in cells from any of the control groups (P < 0.01), indicating that the reported increase in nucleolar size was due to a marked enlargement of the dense fibrillar regions. These results, in agreement with previous biochemical and molecular biology studies, suggest a pivotal role for nucleolin in pre-rRNA processing and constitute morphological evidence supporting this role. Following nucleolin inhibition, impaired pre-rRNA processing might result in an accumulation of this molecular species in the dense fibrillar component of the nucleolus, where pre-rRNA is first present.