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Failure of senescent human fibroblasts to express the insulin-like growth factor-1 gene
1Jefferson Cancer Institute, Thomas Jefferson University, Philadelphia, Pennsylvania 19107.
Abstract:
Senescent human diploid fibroblasts express several growth-regulated genes but fail to express others. In this paper we show, by a very sensitive technique (reverse transcriptase-polymerase chain reaction), that senescent cells fail to express insulin-like growth factor-1 (IGF-1) mRNA, which is expressed in moderate amounts by young cells. Human fibroblasts immortalized by transfection with a temperature-sensitive SV40 T antigen gene regain the ability to express IGF-1 mRNA, but only at the permissive temperature of 34 degrees C. Under these conditions, the immortalized human fibroblasts grow even in 1% serum. At the restrictive temperature of 39 degrees C, the temperature-sensitive T antigen is nonfunctional, IGF-1 RNA is not detectable, and the cells fail to grow even in 10% serum. The failure to express IGF-1 mRNA in postsenescent cells can be ascribed, at least in part, to a transcriptional mechanism. Despite the correlation among immortalization by SV40 T antigen, expression of IGF-1, and growth, it seems unlikely that the failure to express IGF-1 is the sole cause of cellular senescence; other requirements must be postulated.
Insights
Senescent cells lose expression of insulin-like growth factor-1 (IGF-1) mRNA, impacting cellular growth. Immortalized cells regain IGF-1 expression and growth under specific conditions, suggesting complex senescence mechanisms.
Area of Science:
- Cell Biology
- Molecular Biology
- Genetics
Background:
- Cellular senescence is a state of irreversible growth arrest.
- Senescent cells exhibit altered gene expression profiles.
- Insulin-like growth factor-1 (IGF-1) plays a role in cell growth and proliferation.
Purpose of the Study:
- To investigate the expression of IGF-1 mRNA in senescent human fibroblasts.
- To determine the role of SV40 T antigen in IGF-1 expression and cellular growth.
- To elucidate the mechanisms underlying IGF-1 downregulation in senescence.
Main Methods:
- Reverse transcriptase-polymerase chain reaction (RT-PCR) for sensitive mRNA detection.
- Culturing of senescent and SV40-immortalized human fibroblasts.
- Manipulation of temperature to control SV40 T antigen function.
Main Results:
- Senescent human diploid fibroblasts fail to express IGF-1 mRNA.
- SV40-immortalized fibroblasts express IGF-1 mRNA and grow at the permissive temperature (34°C).
- IGF-1 mRNA expression and cell growth are lost at the restrictive temperature (39°C) in immortalized cells.
- Failure to express IGF-1 mRNA in senescent cells is partly due to transcriptional regulation.
Conclusions:
- Downregulation of IGF-1 mRNA is a characteristic of cellular senescence.
- SV40 T antigen can restore IGF-1 expression and promote growth in senescent cells.
- While linked to senescence, IGF-1 deficiency is likely not the sole cause of this complex cellular state.