No abstract

The clinical efficacy of many chemotherapeutic agents has been reduced due to the development of drug resistance. In this article, we aimed to validate gossypol, a natural BH3 mimetic found in cottonseeds, as a potential therapeutic to overcome multidrug resistance (MDR). Gossypol was found to retain its efficacy in v-Ha-ras-transformed NIH 3T3 cells that overexpressed P-glycoprotein (Ras-NIH 3T3/Mdr), which was similar to the efficacy observed in their parental counterparts (Ras-NIH 3T3). A rhodamine assay revealed that the alteration of MDR activity did not contribute to the cytotoxic effect of gossypol. Gossypol caused a G2 /M arrest by the induction of p21(Cip1) and the down-regulation of p27(Kip1) expression in Ras-NIH 3T3 cells, whereas no significant G2 /M arrest was exhibited in Ras-NIH 3T3/Mdr cells. Surprisingly, a 48-h treatment with gossypol induced apoptotic cell death in Ras-NIH 3T3 cells; however, gossypol induced both apoptosis and necrosis in Ras-NIH 3T3/Mdr cells, as determined with flow cytometry analysis. More notably, gossypol preferentially induced autophagy in Ras-NIH 3T3 cells but not in Ras-NIH 3T3/Mdr cells. Coimmunoprecipitation and flow cytometric analysis revealed that gossypol-induced autophagy is independent of the dissociation of Beclin 1 from Bcl-2 in Ras-NIH 3T3 cells. Taken together, these results suggest that the antiproliferative activity of gossypol appears to be due to cell-cycle arrest at the G2 /M phase, with the induction of apoptosis in Ras-NIH 3T3 cells. In addition, defective autophagy might contribute to apoptotic and necrotic cell death in response to gossypol in Ras-NIH 3T3/Mdr cells.

Kaposi's sarcoma-associated herpesvirus (KSHV) is the infectious cause of the angioproliferative neoplasm Kaposi's sarcoma (KS). We first confirmed the susceptibility of NIH 3T3 fibroblasts to KSHV by infecting them with BCP-1-derived KSHV. Lytic replication of KSHV was confirmed by PCR amplification of viral DNA isolated from culture supernatants of KSHV-infected cells. The template from KSHV-infected NIH 3T3 cells resulted in an intense viral DNA PCR product. A time course experiment revealed the disappearance of KSHV-specific DNA in culture supernatant of NIH 3T3 cells during a period between 48 h and 72 h postinfection. Furthermore, 3 days postinfection, infected NIH 3T3 cells showed no evidence of latent or lytic transcripts, including LANA, vFLIP, vCyclin, and vIL-6. These results imply that KSHV infection in NIH 3T3 cells is unstable and is rapidly lost on subsequent culturing. Additionally, we detected an enhancement of autophagy early in infection with KSHV. More interestingly, inhibition of autophagy by Beclin 1 siRNA or 3-methyladenine significantly increased the amount of KSHV-specific DNA in the culture supernatant of NIH 3T3 cells when compared to the group treated with KSHV infection alone, implying that autophagy prevents lytic replication of KSHV. Taken together, our data suggest that autophagy could be one of the cellular mechanisms utilized by host cells to promote viral clearance.

The therapeutic efficacy of oncogenic BRAF inhibitor is limited by the onset of acquired resistance. In this study, we investigated the potential therapeutic effects of the mitotic inhibitor paclitaxel on three melanoma cell lines with differing sensitivity to the BRAF inhibitor. Of the two BRAF inhibitor-resistant cell lines, A375P/Mdr cells harboring the BRAF V600E mutant were resistant and the wild-type BRAF SK-MEL-2 cells were sensitive to paclitaxel. In particular, paclitaxel caused the growth inhibition of SK-MEL-2 cells to a much greater extent than it caused growth inhibition of A375P cells. Paclitaxel exhibited no significant effect on the phosphorylation of MEK-ERK in any cell lines tested, regardless of both the BRAF mutation and the drug resistance, implying that paclitaxel activity is independent of MEK-ERK inhibition. In A375P cells, paclitaxel treatment resulted in a marked emergence of apoptotic cells after mitotic arrest, concomitant with a remarkable induction of p21(Cip1). However, paclitaxel only moderately increased the levels of p21(Cip1) in A375P/Mdr cells, which exhibited a strong resistance to paclitaxel. The p21(Cip1) overexpression partially conferred paclitaxel sensitivity to A375P/Mdr cells. Interestingly, we found an extremely low background expression level of p21(Cip1) in SK-MEL-2 cells lacking normal p53 function, which caused much greater G2/M arrest than that seen in A375P cells. Taken together, these results suggest that paclitaxel may be an effective anticancer agent through regulating the expression of p21(Cip1) for the treatment of BRAF mutant melanoma cells resistant to BRAF inhibitors.

Developing potential therapeutic strategies to overcome multidrug resistance (MDR) is a major challenge in cancer research. Here, we explored the use of gossypol as a chemotherapeutic agent that is effective against both v‐Ha‐ras‐transformed NIH 3T3 cells (Ras‐NIH 3T3) and their mdr equivalents (Ras‐NIH 3T3/Mdr). We found that Ras‐NIH 3T3/Mdr cells, which exhibit a strong cross‐resistance to many chemotherapeutic agents, exhibited essentially no resistance to gossypol. This study also revealed that gossypol significantly decreased mdr‐1 and mdr‐3 mRNA expression in Ras‐NIH 3T3/Mdr cells. However, the rhodamine assay indicated that P‐glycoprotein is not likely to be a major mechanism of the chemotherapy sensitization by gossypol. While both cell lines can be growth arrested, cell death is apoptotic in the Ras‐NIH 3T3 cells but mixed necrotic and apoptotic in the Ras‐NIH 3T3/Mdr cells. More notably, gossypol preferentially induced autophagy in Ras‐NIH 3T3 cells but not in Ras‐NIH 3T3/Mdr cells, suggesting that defective autophagy makes MDR cells vulnerable to gossypol despite being resistant to G2/M block. Taken together, these results suggest that gossypol could be an effective anticancer agent to treat malignancies that are resistant to conventional therapies, either used alone or in association with other autophagy‐blocking treatments.