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.