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Rapamycin response in tumorigenic and non-tumorigenic hepatic cell lines
Rosa H Jimenez1, Joan M Boylan, Ju-Seog Lee
1Department of Pediatrics, Rhode Island Hospital and Brown University, Providence, Rhode Island, USA.
Background:
The mTOR inhibitor rapamycin has anti-tumor activity across a variety of human cancers, including hepatocellular carcinoma. However, resistance to its growth inhibitory effects is common. We hypothesized that hepatic cell lines with varying rapamycin responsiveness would show common characteristics accounting for resistance to the drug.
Methodology/Principal Findings:
We profiled a total of 13 cell lines for rapamycin-induced growth inhibition. The non-tumorigenic rat liver epithelial cell line WB-F344 was highly sensitive while the tumorigenic WB311 cell line, originally derived from the WB-F344 line, was highly resistant. The other 11 cell lines showed a wide range of sensitivities. Rapamycin induced inhibition of cyclin E-dependent kinase activity in some cell lines, but the ability to do so did not correlate with sensitivity. Inhibition of cyclin E-dependent kinase activity was related to incorporation of p27(Kip1) into cyclin E-containing complexes in some but not all cell lines. Similarly, sensitivity of global protein synthesis to rapamycin did not correlate with its anti-proliferative effect. However, rapamycin potently inhibited phosphorylation of two key substrates, ribosomal protein S6 and 4E-BP1, in all cases, indicating that the locus of rapamycin resistance was downstream from inhibition of mTOR Complex 1. Microarray analysis did not disclose a unifying mechanism for rapamycin resistance, although the glycolytic pathway was downregulated in all four cell lines studied.
Conclusions/Significance:
We conclude that the mechanisms of rapamycin resistance in hepatic cells involve alterations of signaling downstream from mTOR and that the mechanisms are highly heterogeneous, thus predicting that maintaining or promoting sensitivity will be highly challenging.
Insights
Rapamycin resistance in liver cancer cells is common and mechanisms vary downstream of mTOR inhibition. Overcoming this resistance is challenging due to heterogeneous signaling alterations.
Area of Science:
- Oncology
- Molecular Biology
- Cancer Research
Background:
- Rapamycin, an mTOR inhibitor, shows anti-tumor effects in various cancers, including hepatocellular carcinoma.
- Resistance to rapamycin's growth inhibitory effects is frequently observed, necessitating investigation into its underlying mechanisms.
Purpose of the Study:
- To identify common characteristics in hepatic cell lines with varying rapamycin responsiveness.
- To elucidate the mechanisms of rapamycin resistance in liver cancer cells.
Main Methods:
- Profiling 13 cell lines for rapamycin-induced growth inhibition.
- Assessing inhibition of cyclin E-dependent kinase activity and p27(Kip1) incorporation.
- Evaluating sensitivity of global protein synthesis and phosphorylation of key mTORC1 substrates (S6, 4E-BP1).
- Utilizing microarray analysis to explore resistance mechanisms.
Main Results:
- Significant variation in rapamycin sensitivity was observed across cell lines, with a non-tumorigenic line being sensitive and a derived tumorigenic line being resistant.
- Rapamycin inhibited mTORC1 substrates (S6, 4E-BP1) in all cell lines, indicating resistance occurs downstream of mTOR Complex 1.
- No single mechanism, including cell cycle inhibition or protein synthesis changes, consistently explained resistance; however, the glycolytic pathway was downregulated in resistant lines.
Conclusions:
- Mechanisms of rapamycin resistance in hepatic cells involve heterogeneous alterations in signaling pathways downstream from mTOR.
- The diverse nature of these resistance mechanisms presents a significant challenge for maintaining or enhancing therapeutic sensitivity.
