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Published on: March 8, 2012
Molecular pathways executing the "trophic sentinel" response in HPV-16 E7-expressing normal human diploid fibroblasts
Alexandra Eichten1, Debrah S Rud, Miranda Grace
1Department of Pathology, Harvard Medical School, Boston, MA 02115, USA.
Abstract:
In response to oncogenic insults, normal human cells execute a defense response that culminates in cellular suicide, apoptosis. Normal human diploid fibroblasts expressing the human papillomavirus type 16 (HPV-16) E7 oncoprotein are predisposed to apoptosis when they are deprived of growth factors. Even though a dominant negative p53 mutant abrogates the cell death response, it is not accompanied by p53 phosphorylation, the DNA binding capacity of p53 remains unaltered, and no activation of common p53-dependent transcriptional targets is observed. Expression of two insulin-like growth factor-1 binding proteins, IGFBP-2 and -5, is increased presumably in response to enhanced NF-kappaB activity in HPV-16 E7-expressing serum-starved cells. Phosphorylation of AKT, an important modulator of IGF-1 survival signaling, is lower in serum-starved E7-expressing cells, and exogenously added IGF-1 can partially inhibit the cell death response. This suggests that IGFBP-2 and -5 may limit IGF-1 availability thus decreasing survival signaling. Caspase 3 but not caspase 8 is activated in serum-starved HPV-16 E7-expressing cells. Caspase inhibition affects nuclear DNA fragmentation, but cell death is not inhibited. Although mitochondria play important roles in caspase-dependent as well as -independent forms of cell death, there is no evidence for cytochrome c release and thus for mitochondrial permeabilization in growth factor deprived HPV-16 E7-expressing cells.
Insights
Human papillomavirus type 16 (HPV-16) E7 oncoprotein-expressing cells undergo apoptosis when growth factors are removed. This cell death involves specific insulin-like growth factor-binding proteins and caspase 3 activation, but not mitochondrial pathways.
Area of Science:
- Cell Biology
- Molecular Biology
- Oncology
Background:
- Normal human cells undergo apoptosis in response to oncogenic insults.
- Human papillomavirus type 16 (HPV-16) E7 oncoprotein expression can alter cellular responses to stress.
Purpose of the Study:
- To investigate the mechanisms of apoptosis in human diploid fibroblasts expressing the HPV-16 E7 oncoprotein when deprived of growth factors.
Main Methods:
- Utilized dominant-negative p53 mutants, assessed p53 phosphorylation and DNA binding, analyzed NF-kappaB activity, measured insulin-like growth factor-1 (IGF-1) signaling components (IGFBP-2, IGFBP-5, AKT), and examined caspase activation (caspase 3, caspase 8) and mitochondrial integrity.
Main Results:
- HPV-16 E7-expressing cells showed increased apoptosis upon serum starvation.
- A dominant-negative p53 mutant abrogated cell death without altering p53 phosphorylation or DNA binding.
- Expression of IGFBP-2 and -5 increased, potentially limiting IGF-1 availability and survival signaling.
- AKT phosphorylation was reduced, and exogenous IGF-1 partially rescued cell death.
- Caspase 3 was activated, but caspase 8 and cytochrome c release were not observed.
Conclusions:
- Growth factor deprivation induces apoptosis in HPV-16 E7-expressing cells via a p53-independent pathway.
- Increased IGFBP-2 and -5, reduced AKT phosphorylation, and caspase 3 activation contribute to this cell death.
- The apoptotic process does not appear to involve mitochondrial permeabilization in this context.
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