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Regulation of cholesterol responsive genes in ovary cells: impact of cholesterol delivery systems
S Medicherla1, S Azhar, A Cooper
1Geriatric Research, Education and Clinical Center, VA Palo Alto Health Care System, California 94304, USA.
Abstract:
The "selective" cholesterol uptake pathway represents a bulk pathway by which many steroidogenic cells internalize lipoprotein-delivered cholesteryl esters. In the current report, we question whether cholesteryl esters entering cells via this pathway are capable of governing standard cholesterol end product feedback repression mechanisms. Cultured rat ovary granulosa cells which utilize both the "selective" and "endocytic" pathways to internalize lipoprotein-derived cholesteryl esters were used as a model system. ApoE-free hHDL3 was used to deliver cholesteryl esters to the cells exclusively by the selective pathway; hLDL was used as a control ligand which when internalized by the endocytic pathway releases cholesteryl esters which subsequently regulate the expression of the B/E (LDL)-receptor, HMG CoA reductase, and acyl-CoA:cholesterol acyltransferase (ACAT). Whereas trophic hormone (Bt2cAMP) stimulation by itself increased the activity, mRNA, and protein levels of both B/E-receptor and HMG CoA reductase, pretreatment with either lipoprotein (adjusted for equal cholesterol ester content) down-regulated this expression. Linked with these lipoprotein-related changes was an increase in activity (though not gene expression) of ACAT. The level of change in mRNA levels, protein content, and activity for the examined regulatory proteins was essentially equivalent whether the lipoprotein provided to the cells was hLDL or hHDL3. Thus, similar signals appear to have been received by the cells despite differences in the uptake and processing of the ligand-derived cholesteryl esters, and these signals resulted in identical homeostatic responses by the cells.
Insights
Cholesterol uptake pathways, selective and endocytic, equally regulate cellular cholesterol homeostasis. Both pathways, using HDL3 and LDL respectively, trigger similar feedback responses in rat ovary cells.
Area of Science:
- Biochemistry
- Cell Biology
- Endocrinology
Background:
- Steroidogenic cells utilize selective and endocytic pathways for cholesterol uptake.
- Cholesteryl esters from lipoproteins are critical for cellular cholesterol homeostasis.
- The regulation of cholesterol feedback mechanisms by different uptake pathways is not fully understood.
Purpose of the Study:
- To investigate if cholesteryl esters entering via the selective cholesterol uptake pathway can regulate cellular cholesterol feedback mechanisms.
- To compare the regulatory effects of the selective pathway (using HDL3) versus the endocytic pathway (using LDL) on cholesterol metabolism genes.
Main Methods:
- Primary cultured rat ovary granulosa cells were used as a model system.
- Cholesteryl esters were delivered via ApoE-free hHDL3 (selective pathway) and hLDL (endocytic pathway).
- Expression and activity of B/E receptor, HMG CoA reductase, and ACAT were measured following lipoprotein treatment and trophic hormone stimulation.
Main Results:
- Both hHDL3 and hLDL, despite different uptake mechanisms, similarly down-regulated B/E receptor and HMG CoA reductase expression and activity.
- Trophic hormone stimulation alone increased B/E receptor and HMG CoA reductase levels.
- Cholesterol ester delivery via both pathways led to increased ACAT activity, but not gene expression.
Conclusions:
- The selective cholesterol uptake pathway effectively governs cellular cholesterol feedback repression mechanisms, similar to the endocytic pathway.
- Cellular responses to cholesteryl esters are conserved regardless of the specific lipoprotein uptake and processing pathway.
- These findings highlight the integrated nature of cholesterol homeostasis regulation in steroidogenic cells.