MTOR and hepatocellular carcinoma: In this study, our finding that rapamycin significantly up-regulated the expression and phosphorylation of PDGFRβ in HCC cells was partly consistent with these prior studies, whereas the expression of PDGFRα was not affected by rapamycin, suggesting that only PDGFRβ, instead of PDGFRα, was the major negative-regulation target of mTOR and paradoxical up-regulation of PDGFRβ may contribute to rapamycin resistance in HCC.