Altered Cellular Responses by Varying Expression of a Ribosomal Protein Gene: Sequential Coordination of Enhancement and Suppression of Ribosomal Protein S3a Gene Expression Induces Apoptosis

Abstract

A growing body of evidence indicates that individual ribosomal proteins and changes in their expression, participate in, and modulate, a variety of cellular activities. Our earlier studies have found that apoptosis could be induced by inhibiting expression of ribosomal protein S3a (RPS3a) in many tumor cells which constitutively express RPS3a at levels much higher than in normal cells. This study aimed to investigate cellular responses to enhancement of RPS3a expression, and whether apoptosis could be induced by sequential alterations in RPS3a expression involving enhancement from an initially low constitutive level, followed by suppression. Stably transfected NIH 3T3– derived cell lines were established in which exogenous RPS3a expression could be readily manipulated. Enhancement of RPS3a expression appeared to induce transformation as assessed by well-established criteria such as foci formation and anchorage-independent growth in vitro, and formation of tumors in nude mice. These properties were compared with those observed in ras-transformed NIH 3T3 cells. Apparent transformation occurred only when enhanced RPS3a-expressing cells were in close cell–cell contact. Suppression of enhanced RPS3a expression was observed to induce apoptosis as assessed by various morphological and biochemical characteristics including cell shrinkage, membrane blebbing, chromatin condensation, nuclear and cell fragmentation, phosphatidylserine externalization, and internucleosomal DNA fragmentation. This induction of apoptosis was not specific to apparently transformed cells, as cells at low confluence, which likewise expressed RPS3a at enhanced levels but exhibited no morphological transformation, underwent apoptosis when RPS3a expression was inhibited. These results support a role for RPS3a in the apoptotic process, but not as an oncoprotein per se.

In silico whole-genome scanning of cancer-associated nonsynonymous SNPs and molecular characterization of a dynein light chain tumour variant

“…In this regard, we found a variant causing a Gly to Val change at residue 165 in ribosomal protein S19, a protein that has been associated to cancer predisposition in Diamond-Balckfan anaemia (Draptchinskaia et al, 1999). Another ribosomal protein, S3a, which is able to induce transformation (Naora et al, 1998) also displayed genetic variations associated with tumoral context in our analysis. The ability of the MYC oncogene to regulate genes involved in ribosome biogenesis and translation control is now well documented (Coller et al, 2000;Boon et al, 2001;Menssen and Hermeking, 2002).…”

Ribosomal Proteins and Colorectal Cancer

“…Although, RPS3 was identified as an endonuclease participating into DNA repair and may be involved in carcinogenesis, but unluckily no direct evidence supported this assumption. Because of no experiment ascertaining the relation between the overexpression of RPs and high proliferating speed, it will be a great challenge to reveal the real roles of RPs in diseases especially when multiple RPs were abnormal in one disease [54]. Moreover, the tissue and disease-specific expression patterns of RPs also prevent us to answer the question effortlessly.…”

“…For example, the enhanced RPL5, RPL11 and RPL23 proteins may form a complex with MDM2 and consequently inhibited the combination MDM2 with p53, resulting in reduced MDM2-mediated p53 ubiquitination and also induced p53 activity and G1 arrest [50][51][52][53]. The overexpression of RPS3a gene caused cell malignant transformation and tumorigenesis of nude mouse and was assumed to promote cell transformation by suppressing cell apoptosis since it induced synthesis of anti-apoptosis proteins [54]. Lastly, RPs may play critical roles in DNA repair and cell apoptosis leading to tumourgenesis, such as RPS3.…”

“…Thirdly, some RPs only increased in senescent cells, [38] some in quiescent cells and some in tumor cells well differentiated [31] but some decreased in metastatic CRC [29]. Fourthly, p53 mutation, a common event in CRC, which is considered the key factor in colon carcinogenesis can upregulate only several RPs but not all of them; [39] Fifthly, overexpression of specific RPs did not increase the synthesis of proteins [40]. RPs could still been produced when rRNA synthesis was blocked which means RPs synthesis can not be consistent with rRNA synthesis.…”

Ribosome biogenesis and cell growth: mTOR coordinates transcription by all three classes of nuclear RNA polymerases

“…In accordance with this, overexpression of RPs seems to be frequently associated with tumorigenesis (Ferrari et al, 1990). Deregulated expression of the rpS3a, an endonuclease that cleaves DNA in response to ultraviolet irradiation, induced transformation of NIH3T3 cells (Naora et al, 1998). Studies elucidating novel functions of RPs apart from their role as ribosome constituents and their control by mTOR signaling may provide insights into the role of ribosomal components in cancer.…”