ИСТИНА

The level of many nucleolar proteins including major rRNA processing factors is known to vary in cell cycle progression being increased in actively proliferating cells. Quantitative analysis of these proteins is routinely used for evaluation of cell proliferative potencies in clinical oncology. In lymphoprolifertive disorders, the particular value has quantification of Ki-67-positive cells as far as the nucle(ol)ar Ki-67 protein is absent in resting lymphocytes. In order to examine whether other nucleolar proteins may also serve as a diagnostic oncomarkers, in the current study we examined the level of expression of an evolutionary conserved rRNA processing factor SURF6 in resting and PHA-activated lymphocytes obtained from a statistically significant number of healthy donors. Two specific anti-SURF6 antibodies were used to assess the SURF6 level by quantitative immunocytochemistry and on immunoblots. Activation of lymphocytes was monitored by FACS analysis after 16 h (G1 period), 24 h (the onset of the first S period), 48 h and 72 h, which correspond to the most high proliferation activity of lymphocytes. In parallel, the expression level of a multifunctional nucleolar protein NPM1/B23, the Ki-67 and PCNA (proliferating cell nuclear antigen) proteins were also analyzed. Our results show that similar to Ki-67 and PCNA but unlike NPM1, the SURF6 protein remains undetectable in resting (or G0) lymphocytes. Its expression commences around 16 h of PHA-activation when neither Ki-67 nor PCNA can be detected. Further activation of lymphocytes is accompanied by augmentation of the number of SURF6-positive cells as well as by an increase of specific SURF6 signals on immunoblots. However, even after 72 hours of PHA activation, the number of SURF6-positive lymphocytes exceeds that of Ki-67-posive cells. Based on these results we conclude that the nucleolar rRNA processing factor SURF6 is a novel and early marker of lymphocyte activation for proliferation. The study was supported by the Russian Foundation for Basic Research (grant 18-34-00767)