Expression of ribosomal protein S5 cloned gene during differentiation and apoptosis in murine erythroleukemia (MEL) cells.

Murine erythroleukemia (MEL) cells have been used as a suitable model system for studying cellular and molecular mechanisms of erythroid differentiation. In an effort to isolate and characterize genes whose expression change during differentiation, we cloned and sequenced a cDNA of 715 bp (rpS5) from a MEL cDNA library. The cloned mouse cDNA showed significant degree of structural homology in both DNA and protein level to known human and rat genes that encode the S5 proteins of 40S ribosomal subunit. The use of 715-bp cDNA as probe revealed the presence of an RNA transcript in the cytoplasm of MEL and human neuroectodermal RD/TE-671 cells. The steady-state accumulation level of this RNA transcript decreased upon induction of differentiation of both cell lines by treatment with DMSO and UDP-4, two structurally different inducers. Blockade of MEL cell differentiation by the inhibitor N6-methyladenosine preserved the constitutive expression of the rpS5 gene. DNA methylation analysis at CCGG sites located at the rpS5 gene locus in undifferentiated and differentiated MEL cells revealed that the suppression of the rpS5 gene during MEL cell differentiation is not related to any change in methylation at these sites. Moreover, the rpS5 gene continued to be expressed in cells undergoing serum-deprived apoptosis, like in control untreated cells. Therefore, we conclude that there may be a disparate pattern of expression of the rpS5 gene in differentiating and apoptotic cells. These data can be valuable in understanding the role of ribosomal proteins during differentiation and cell death (apoptosis) of neoplastic cells, although there is no experimental evidence that the suppression of the rpS5 gene is related mechanistically to the induction of differentiation. It may well be considered as part of the differentiation process.