The expression and activity of D-type cyclins in F9 embryonal carcinoma cells: Modulation of growth by RXR-selective retinoids

The growth rate of malignant F9 embryonal carcinoma cells slows considerabl y following all-trans-retinoic acid-induced differentiation into benign par ietal endoderm. To determine the mechanism of this process, we examined the expression of cyclins D1, D2, and D3 and the activity of their associated kinases. Cyclin D1 and D3 mRNA levels decreased during complete differentia tion induced by all-trans-retinoic acid and dibutyryl cAMP, while the level s of cyclin D2 and the cyclin-dependent kinase (Cdk) inhibitor p27 mRNAs in creased. Ultimately, terminally differentiated cells possessed 50% of the C dk4-associated kinase activity observed in undifferentiated cells. Since nu merous genes are differentially regulated during parietal endoderm differen tiation, it is difficult to determine whether retinoic acid affects cell cy cle gene expression directly or if these changes are caused by differentiat ion. We found that the retinoid X receptor (RXR)-selective agonists LG10015 3 and LG100268 significantly inhibited F9 cell growth without causing overt terminal differentiation as assessed by anchorage-independent growth and d ifferentiation-associated gene expression, As seen in cells induced to diff erentiate by the RAR agonist all-trans-retinoic acid, RXR activation led to an increase in the number of cells in G1 phase. RXR agonists also sharply induced the levels of the Cdk, regulatory subunits, cyclin D2 and D3. Howev er, Cdk4-dependent kinase activity was reduced by RXR-selective retinoid tr eatment. These observations suggest that some retinoids can directly inhibi t proliferation and regulate Cdk4-dependent kinase activity without inducin g terminal differentiation. (C) 1999 Academic Press.