Cell stemness is maintained upon concurrent expression of RB and the mitochondrial ribosomal protein S18-2

Proc Natl Acad Sci U S A. 2020 Jul 7;117(27):15673-15683. doi: 10.1073/pnas.1922535117. Epub 2020 Jun 22.

Abstract

Stemness encompasses the capability of a cell for self-renewal and differentiation. The stem cell maintains a balance between proliferation, quiescence, and regeneration via interactions with the microenvironment. Previously, we showed that ectopic expression of the mitochondrial ribosomal protein S18-2 (MRPS18-2) led to immortalization of primary fibroblasts, accompanied by induction of an embryonic stem cell (ESC) phenotype. Moreover, we demonstrated interaction between S18-2 and the retinoblastoma-associated protein (RB) and hypothesized that the simultaneous expression of RB and S18-2 is essential for maintaining cell stemness. Here, we experimentally investigated the role of S18-2 in cell stemness and differentiation. Concurrent expression of RB and S18-2 resulted in immortalization of Rb1-/- primary mouse embryonic fibroblasts and in aggressive tumor growth in severe combined immunodeficiency mice. These cells, which express both RB and S18-2 at high levels, exhibited the potential to differentiate into various lineages in vitro, including osteogenic, chondrogenic, and adipogenic lineages. Mechanistically, S18-2 formed a multimeric protein complex with prohibitin and the ring finger protein 2 (RNF2). This molecular complex increased the monoubiquitination of histone H2ALys119, a characteristic trait of ESCs, by enhanced E3-ligase activity of RNF2. Furthermore, we found enrichment of KLF4 at the S18-2 promoter region and that the S18-2 expression is positively correlated with KLF4 levels. Importantly, knockdown of S18-2 in zebrafish larvae led to embryonic lethality. Collectively, our findings suggest an important role for S18-2 in cell stemness and differentiation and potentially also in cancerogenesis.

Keywords: cell immortalization; embryogenesis; stemness and differentiation; tumorigenesis.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Differentiation / genetics
  • Cell Proliferation / genetics
  • Cell Self Renewal / genetics
  • Fibroblasts / metabolism
  • Gene Expression Regulation, Developmental / genetics
  • Histones / genetics
  • Human Embryonic Stem Cells / metabolism
  • Humans
  • Kruppel-Like Factor 4
  • Kruppel-Like Transcription Factors / genetics
  • Mice
  • Mitochondria / genetics*
  • Mitochondria / metabolism
  • Mouse Embryonic Stem Cells / metabolism*
  • Polycomb Repressive Complex 1 / genetics
  • Retinoblastoma Binding Proteins / genetics*
  • Ribosomal Proteins / chemistry
  • Ribosomal Proteins / genetics*
  • Tumor Microenvironment / genetics
  • Ubiquitin-Protein Ligases / genetics

Substances

  • Histones
  • KLF4 protein, human
  • Klf4 protein, mouse
  • Kruppel-Like Factor 4
  • Kruppel-Like Transcription Factors
  • Rb1 protein, mouse
  • Retinoblastoma Binding Proteins
  • Ribosomal Proteins
  • ribosomal protein S18
  • Polycomb Repressive Complex 1
  • Rnf2 protein, mouse
  • Ubiquitin-Protein Ligases