Figure 3

Ribosomal and extraribosomal functions of the ribosomal proteins up-regulated in this study. Microarray results of polysomal fractions from 3T3-L1 cell lysate (6 h after hormonal induction) show that that ribosomal proteins (RP) are prominent among the up-regulated genes. For some RPs, extraribosomal functions have been demonstrated. In the nucleus, RPSa binds to DNA by histone binding, in the cytoplasm it is associated with the 40S small ribosomal subunit and at the cell surface it acts as a receptor for various components [34]. RPL6 over-expression promotes G1 to S phase transition of gastric cancer cells and promotes cell growth [28]. RPL7a interacts with the human thyroid hormone receptor and inhibits transactivation. Hyperthyroidism favours osteosarcoma cell growth and down-regulation of RPL7a might enhance sensitivity to TR and disrupt growth control [61]. RPL18 was shown to inhibit autophosphorylation of the double-stranded RNA-activated protein kinase (PKR) and PKR mediated phosphorylation of the translation initiation factor eIF2α. Over-expression of RPL18 reduced eIF2α phosphorylation and stimulated translation of a reporter gene in vivo [31]. A polymorphism in the promoter region of the RPL27a gene was associated with meat marbling in Japanese Black beef cattle [32]. These known extraribosomal functions might be important in early adipogenesis. Additionally an enhanced amount of RPs promotes an increase in translation process of adipocyte specific genes. In the beginning of translational process, the 43S ribosomal subunit scans mRNAs for start codons. Strong secondary structures inhibit processing of the complex on the mRNAs. eIF4B increases the helicases activity of the complex and allows translation mRNAs with strong secondary structures in the 5´UTR.