Both molecular and clinical evidence support a link between HCV infection and insulin resistance. We examined the in vitro interaction between the HCV core protein of genotypes 3a and 1b with the insulin-signaling pathway. We measured the expression levels of insulin receptor substrate 1 (IRS-1), IRS-2, and other factors involved in the insulin signal pathway in a human hepatoma cell line (Huh-7) transiently expressing the HCV core protein of genotypes 3a or 1b by molecular biology and biochemical techniques. The IRS-1 (but not IRS-2) protein level was significantly reduced in Huh-7 expressing the core protein of both genotypes 3a and 1b, as compared to cells transfected with the empty vector. However, while the core protein of genotype 3a promoted IRS-1 degradation through the downregulation of peroxisome proliferator-activated receptor γ (PPARγ) and by upregulating the suppressor of cytokine signal 7 (SOCS-7), the core protein of genotype 1b activated the mammalian target of rapamycin (mTOR). We confirmed these findings by using agonists for PPARγ (rosiglitazone) or short interfering RNAs for SOCS-7. Conclusion: Despite the small sequence divergence of the HCV core proteins of genotypes 3a and 1b, the 2 proteins appear to interfere with the insulin signaling pathway using genotype-specific mechanisms. (HEPATOLOGY 2007;45:1164–1171.)