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J. Cell Biol.,
Volume 141, Number 3, May 4, 1998 741-753



* Research School of Biological Sciences, The Australian National University, Canberra, A.C.T. 2601, Australia; and 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.
Japan
Immunoresearch Laboratories Company Ltd., Takasaki City, Gunma Prefecture 370, Japan
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