Mishra Prajna, Paramasivam Suresh K, Thylur Ramesh P, Rana Ajay, Rana Basabi
Department of Medicine, Division of Gastroenterology, Hepatology and Nutrition, Loyola University, Chicago, 2160 South First Avenue, Maywood, IL 60153, USA.
J Mol Signal. 2010 Dec 13;5:20. doi: 10.1186/1750-2187-5-20.
Ligands of Peroxisome proliferator-activated receptor gamma (PPARγ) can inhibit growth and promote apoptosis in various cancer cells, and thus have the potential to be utilized as anticancer drugs. This potential however, has been seriously challenged by observations that they can lead to tumor promotion in some cancer models, possibly due to activation of different signaling mechanisms in various tumor environments. Elucidation of the specific signaling events that modulate PPARγ ligand-mediated events is thus critical to increase their efficacy. The studies described here were designed to elucidate the signaling pathway(s) that modulate the apoptotic potential of Troglitazone (TRG), an artificial PPARγ ligand in hepatocellular carcinoma (HCC) cells.
Our results indicate that the apoptotic potential of TRG was regulated by the presence or absence of serum in the media. When added in serum-containing media, TRG inhibited proliferation and cyclin D1 expression, but was unable to induce any apoptosis. However, TRG's apoptotic potential was induced significantly when added in serum deficient media, as indicated by increased PARP and Caspase-3 cleavage and results from apoptosis assay. Furthermore, TRG-induced apoptosis in serum deficient media was associated with a dramatic reduction in PI3Kinase downstream target AktSer473 and FoxO1Thr24/FoxO3aThr32 phosphorylation. On the contrary, there was an increase of PI3K-induced AktSer473 and FoxO1Thr24/FoxO3aThr32 phosphorylation involving Pak, when TRG was added in serum-containing media. Pharmacological inhibition of PI3Kinase pathway with LY294002 inhibited Aktser473 phosphorylation and sensitized cells towards apoptosis in the presence of serum, indicating the involvement of PI3K in apoptosis resistance. Interestingly, pharmacological inhibition or siRNA-mediated knockdown of Akt or inhibition of Pak was unable to sensitize cells towards TRG-induced apoptosis in the presence of serum. Similarly, TRG was unable to induce apoptosis in the Akt1-KO, Akt1&2-KO MEFs in serum-containing media.
These studies indicate that TRG-induced apoptosis is modulated by PI3K pathway in a novel Akt-independent manner, which might contribute to its tumor promoting effects. Since PI3K activation is linked with various cancers, combination therapy utilizing TRG and PI3K inhibitors has the potential to not only increase the efficacy of TRG as a chemotherapeutic agent but also reduce its off target effects.
过氧化物酶体增殖物激活受体γ（PPARγ）的配体可抑制多种癌细胞的生长并促进其凋亡，因此有潜力用作抗癌药物。然而，这一潜力受到了一些观察结果的严重挑战，即它们在某些癌症模型中可能导致肿瘤进展，这可能是由于在不同肿瘤环境中激活了不同的信号传导机制。因此，阐明调节PPARγ配体介导事件的特定信号转导事件对于提高其疗效至关重要。此处描述的研究旨在阐明调节曲格列酮（TRG，一种人工合成的PPARγ配体）在肝癌（HCC）细胞中凋亡潜力的信号通路。
我们的结果表明，TRG的凋亡潜力受培养基中血清的存在与否调节。当添加到含血清的培养基中时，TRG抑制增殖和细胞周期蛋白D1表达，但不能诱导任何凋亡。然而，当添加到无血清培养基中时，TRG的凋亡潜力被显著诱导，这通过PARP和Caspase-3切割增加以及凋亡检测结果得以表明。此外，TRG在无血清培养基中诱导的凋亡与PI3激酶下游靶点AktSer473和FoxO1Thr24/FoxO3aThr32磷酸化的显著降低相关。相反，当TRG添加到含血清的培养基中时，涉及Pak的PI3K诱导的AktSer473和FoxO1Thr24/FoxO3aThr32磷酸化增加。用LY294002对PI3激酶途径进行药理学抑制可抑制Aktser473磷酸化，并使细胞在有血清存在时对凋亡敏感，表明PI3K参与了凋亡抵抗。有趣的是，在有血清存在时，对Akt进行药理学抑制或siRNA介导的敲低或对Pak的抑制均不能使细胞对TRG诱导的凋亡敏感。同样，在含血清的培养基中，TRG不能在Akt1基因敲除、Akt1和2基因双敲除小鼠胚胎成纤维细胞（MEFs）中诱导凋亡。
这些研究表明，TRG诱导的凋亡以一种新的不依赖Akt的方式受PI3K途径调节，这可能有助于其肿瘤促进作用。由于PI3K激活与多种癌症相关，联合使用TRG和PI3K抑制剂进行治疗不仅有潜力提高TRG作为化疗药物的疗效，还能降低其脱靶效应。
