Gene ecology: a cis-acting gene-to-gene interaction due to the spatial arrangement of genes in chromosomes affects neighbouring transfected c-H-ras expression

Involvement of ribosomal proteins in regulating cell growth and apoptosis: translational modulation or recruitment for extraribosomal activity?

Gene recruitment is a mechanism of molecular evolution whereby a gene product can function in more than one distinct capacity. The 'one gene-dual function' phenomenon is well illustrated by crystallins, structural proteins that play both specialized roles in the eye lens and also 'housekeeping' enzyme roles. Ribosomal proteins are integral components of the basal cellular machinery involved in protein synthesis, whose roles have been regarded collectively as important, but individually somewhat mundane. However, various individual ribosomal proteins and also translation initiation and elongation factors have been found to play roles in regulating cell growth, transformation and death, giving rise to increasing speculation that components of the translational apparatus can act as multifunctional proteins. Recently, we have shown that ribosomal protein S3a (RPS3a) plays important roles in cell transformation and death, whereby constitutively or transiently enhanced RPS3a expression can be regarded as 'priming' a cell for apoptosis and suppression of such enhanced expression as 'execution'. While it is unclear whether RPS3a acts in a capacity mechanistically distinct from that in translation, such a possibility is discussed in this article in the light of recent, although not exhaustively reviewed, findings implicating the involvement of other individual ribosomal proteins in modulating and/or effecting changes in cellular responses and growth patterns in an extraribosomal capacity independent of their conventional role in translation.

Regulation of tumor growth as a 'total mass' in mice: apoptosis as a major mechanism in altering growth rates of single and multiple coexisting tumor nodules

Earlier studies have suggested that a solid tumor behaves, in its general pattern of growth, like a normal integrated organ. In this study, the growth patterns of spherically shaped tumor nodules are re-examined using an accurate tumor volume-measuring procedure, with the aim of investigating the possible role of apoptosis in regulating tumor growth. Observations revealed at least three distinct phases of growth: rapid growth phase I, slower growth phase II and 'stationary' phase III. Transition from one phase to the next was primarily due to an increase in the level of apoptosis and not to a decrease in the cell proliferation rate. The level of apoptosis, at a given phase, was similar in a single nodule and each of the multiple coexisting nodules of the same tumor line. However, temporal shifts in apoptosis levels caused early phase transition in coexisting nodules, such that their total volume was the same as that of a single nodule. It can be concluded that apoptosis appears to be a primary mechanism regulating tumor growth as a 'total mass', irrespective of whether the tumor exists in one or multiple nodules, if derived from the same tumor line.

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

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.