Treatment of ovarian
carcinomas with the
antimitotic antitumor drug paclitaxel is highly efficacious. However, development of drug resistance presents a major obstacle. The common cellular phenotypes associated with
paclitaxel resistance are an increased expression of the
drug transport protein P-glycoprotein (P-gp), an alteration in the levels of
beta-tubulin isotypes, and/or changes in the
drug binding affinity of the microtubules. We established two
paclitaxel-resistant human ovarian
carcinoma cell lines. The 2008/17/4 cells exhibited a "classic" multidrug-resistant phenotype (overexpression of P-gp associated with cross-resistance to
natural product drugs), whereas the 2008/13/4 cells were an atypical multidrug-resistant subline (no overexpression of P-gp). In addition to being
paclitaxel resistant (250-fold), the 2008/13/4 cells were also cross-resistant to
etoposide (39-fold) and
vincristine (460-fold). To identify the alterations in the gene expression profile associated with the development of atypical
paclitaxel resistance, we used the Clontech Atlas Human
Cancer cDNA Microarray (spotted with 588 genes). The expression of
retinoic acid receptor (
RAR)-gamma was significantly higher in the
paclitaxel-resistant (2008/13/4 and 2008/17/4) cells than in the parental (2008) cells. Northern blotting analysis demonstrated that the expression of
RAR-gamma was 7-fold higher in the 2008/13/4 and 2008/17/4 cells than in the 2008 cells, whereas the expression of RAR-alpha and
RAR-beta was not observed in any cell line. Whereas the 2008, 2008/13/4, and 2008/17/4 cells were found to resist the antiproliferative effects of
all-trans-retinoic acid, the
paclitaxel-resistant cells were 6- to 7-fold cross-resistant to the antiproliferative effects of CD437 (a synthetic
RAR-gamma-selective agonist; 6-[-(1-admantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic
acid) compared with the sensitivity of the parental cells. To further understand the association of
paclitaxel and CD437 resistance with the observed
RAR-gamma overexpression, we transfected the 2008 cells with a full-length
RAR-gamma cDNA construct. Two transfectants with increased expression of the
RAR-gamma mRNA and
protein were isolated and subjected to growth inhibition assays in the presence of various concentrations of
paclitaxel,
etoposide,
vincristine, and CD437. The sensitivity of the 2008 transfected clones (displaying increased expression of
RAR-gamma) to the cytotoxic effects of
paclitaxel,
etoposide,
vincristine, and CD437 was similar to that observed in the parental 2008 cells. These results suggest that the overexpression of
RAR-gamma (observed in the 2008/13/4 and 2008/17/4 cells) by itself is not capable of inducing
paclitaxel and CD437 resistance (or resistance to
etoposide and
vincristine).