Authors: Indu Jose, Faisal Ahmed Moinuddin, Vishnu Priya
Ribosomal proteins (RPs) have traditionally been associated with ribosomal biogenesis, assembly and translation. Recent evidence though, has thrust the spotlight on the extra-ribosomal functions of these proteins that include tumor progression or suppression as well as immune surveillance in various cellular contexts. Here, we highlight a few ribosomal protein antibodies that could assist the emerging research on non-ribosomal functions of RPs in cancer.
Old targets, new roles- RPs and Cancer
The synthesis of the ribosome, called ribosome biogenesis, is a highly coordinated cellular process and any dysregulation may result in aberrant cell proliferation and clinical manifestations such as cancer and metabolic disorders. The earliest links between ribosomes and cancer were established while studying ‘ribosomopathies’ – congenital disorders of ribosomal malfunction that elevated the risk of developing cancer (1). The best studied ribosomopathy is Diamond-Blackfan Anemia (DBA) that is characterized by mutations or deletions in RPS19, RPS28, RPL5, RPL11 and RPL36A among other RPs (2, 3). Apart from its role in DBA, RPL5 is also extensively mutated in T-cell acute lymphoblastic anemia (4). RPL5 mutations or deletions have been reported in a variety of cancers such as glioblastomas, breast cancers and melanomas (5, 6). Somatic mutations in RPL11 are rarer, but heterozygous Rpl11 deletion has shown susceptibility to radiation-induced lymphomagenesis in mice (7). Invitrogen offers specific antibodies against some of these key targets (Table 1). RPS28 (Figure 1A) and RPL5 (Figure 1B) antibodies have been validated by genetic modification strategies where reduction or abrogation of target expression was used to demonstrate antibody specificity.
Differential expression of RPs are also observed in a wide spectrum of cancers. RPL29 is upregulated in colon cancer cells and its depletion leads to cellular differentiation via modulation of p21 and p53 pathways (8). It has also been found to regulate tumor angiogenesis (9). RPL3 is overexpressed in colorectal carcinomas and several adenomas (10) whereas RPLP0 is upregulated in both colorectal and hepatocellular carcinomas (11). RPS3 is reported to have a role in DNA repair and apoptosis regulation in addition to being overexpressed in several adenomas (12). The specificity of RPL29, RPLP0 and RPS3 antibodies have been demonstrated through siRNA mediated knockdown experiments (Figures 1C-E). Learn more about advanced verification testing methods.
In contrast to the oncogenic properties of RPs described above, some of them can also activate tumor suppressors/ inactivate oncoproteins. During ribosome biogenesis, the nascent RPs synthesized within the cytoplasm are imported into nucleolus and assembled into pre-ribosomal particles along with rRNA. Perturbations in the ribosome biogenesis via specific extracellular/intracellular stimulations can result in ‘ribosomal/nucleolar stress’ leading to the accumulation of ribosome-free RPs. Upon nucleolar stress induction, RPL5, RPL11, RPS3 along with an array of various of RPs can bind to and inhibit MDM2 mediated ubiquitination and degradation of p53 (13). The resulting stabilization of p53 can result in induction of cellular senescence or apoptosis. RPS3 can also collaborate with E2F1 and induce apoptosis (14). RPL5 and RPL11 bind to specific regions in the central acidic domain of MDM2. It is quite possible that multiple stimulatory signals are required for the binding of various RPs to MDM2 and their synergy could augment the inhibition of MDM2 and activation of p53 during nucleolar stress conditions (15, 16). RPs could bind to nucleoplasmic MDM2 while they are being transported into the nucleolus from cytoplasm. Whenever there is an increase in global translational levels, the increased RP traffic and consequent binding to MDM2 also acts as a ‘sensing mechanism’ (16).
RPL11 has been reported to specifically bind to the oncoprotein, c-Myc and prevent the recruitment of its transcriptional co-activator TRRAP, thus inhibiting c-Myc mediated transcription of downstream targets (17, 18). RPS14 and RPL5 also inhibit c-Myc via a similar mechanism along with regulation of c-Myc mRNA turnover. The tumor suppressive action of RPs can also be attributed to the inhibition of angiogenesis. Knockdown of RPL17 was found to promote VSMC proliferation in this regard (19). The reduction of expression of RPL17 demonstrated by knockdown experiment underlines the specificity of our antibody against RPL17 (Figures 1F).
RPs and Inflammation
Beyond their role in oncogenesis, RPs are also implicated in the regulation of inflammatory responses. RPL13A has been reported to engage the interferon-gamma mediated inflammatory response (20). Interferon-gamma activates DAPK1-ZIPK cascade that triggers phosphorylation and release of ribosome bound RPL13A which subsequently associates with various proteins to form a translational inhibitory complex. Similarly, RPS3 also plays a role in modulation of NF-kB target gene expression either by TNF-alpha induced sulfhydration of RelA subunit or IKK-beta induced RPS3 phosphorylation (21, 22).
Ribosome biogenesis is an emerging target in the treatment of cancer. Many of the existing chemotherapeutic drugs interfere with RNA pol I mediated transcription or ribosomal RNA metabolism to target proliferating cancer cells (23). In addition to this, targeting of ribosomal proteins could pave way to new cancer treatment modalities. Explore our ribosomal protein antibodies at thermofisher.com/antibodies and select the best candidate for your research.
Table 1: Ribosomal protein antibodies for cancer research
Knockdown of RPS28 (panel A), RPL5 (panel B), RPL29 (panel C), RPLP0 (panel D), RPS3 (panel E) and RPL17 (panel F) was achieved by transfecting HeLa cells with respective ribosomal protein specific siRNAs. Western blot analysis was performed using whole cell extracts from the knockdown cells (lane 3), non-specific scrambled siRNA transfected cells (lane 2) and untransfected cells (lane 1). RPS28 Polyclonal Antibody (Product # PA5-45721) for RPS28 in panel A; RPL5 Polyclonal Antibody (Product # PA5-27539) for RPL5 in panel B; RPL29 Polyclonal Antibody (Product # PA5-27545) for RPL29 in panel C; RPLP0 Polyclonal Antibody (Product # PA5-30158) for RPLP0 in panel D; RPS3 Polyclonal Antibody (Product # PA5-27974) for RPS3 in panel E; and RPL17 Polyclonal Antibody (Product # PA5-29397) for RPL17 in panel F were used to probe the blots.