BERKELEY, CA (UroToday.com) -
In this report, we demonstrated that atorvastatin induces autophagy through activation of transcription of LC3, which is mediated by Erk and JNK signaling pathways and leads to elevation of LC3-II level. Based on the data, we propose a mechanism by which atorvastatin induces LC3-II expression and activates autophagy. Atorvastatin impairs protein geranylgeranylation, which activates Erk and JNK signaling to promote the transcription of LC3 and leads to elevation of LC3-I protein. LC3-I protein is instantly converted to LC3-II, which results in the activation of the autophagic process in cells. This mechanism differs from the one underlying nutritional stress-induced autophagy. In nutritional stress-induced autophagy, mTOR inactivation is the key event and no translational and transcriptional processes are needed.[1] The differences in the mechanism may be associated with the function: nutritional stress-induced autophagy is for generating nutrients to sustain cell survival, while atorvastatin-induced autophagy may be for initiating the cell death process.
Atorvastatin-induced autophagy is dependent on geranylgeranylation. Our study has shown that the geranylgeranyltransferase (GGTase) I inhibitor GGTI-298, similar to atorvastatin, induces expression of LC3-II in PC3 cells, suggesting that GGTase I-catalyzed geranylgeranylation is involved in atorvastatin-induced autophagy. It has been observed that inactivation of Rho GTPase by lovastatin causes apoptosis in prostate cancer cells derived from transgenic mice with adenocarcinoma of the prostate (TRAMP cells).[2] However, treatment of PC3 cells with the Rho inhibitor C3 ADP-ribosyltransferase or ROCK inhibitor Y27632 did not cause elevation of LC3-II expression, implicating that inactivation of Rho is not the cause for atorvastatin-induced autophagy. Currently, the geranylgeranylated protein(s) mediating the effect of atorvastatin on activation of LC3-II expression has not been identified. It should be noticed that atorvastatin does not elevate expression of LC3-II in androgen receptor-positive prostate cancer LNCaP cells,[3] suggesting that the susceptibility to atorvastatin-induced autophagy is dependent on geranylgeranylation-associated signaling context.
We observed no activation of autophagy induced by nutritional starvation or by inhibition of mTOR in PC3 cells. Although the basal level of LC3, including both LC3-I and LC3-II, in PC3 cells is very low and difficult to detect by immunoblotting, treatment with the lysosomal inhibitor chloroquine rapidly accumulates LC3-II [3], suggesting that the turnover of autophagosomes is very fast in PC3 cells. This may imply that the nutritional stress-associated autophagic process is constitutively active in PC3 cells, thus is no longer sensitive to the treatment of nutritional starvation. Another interesting observation is that LC3-I level in PC3 cells is very low or undetectable with immunoblotting of the lysates even when LC3-II is significantly elevated by treatment with atorvastatin or chloroquine.[3] This implicates that the conversion of LC3-I to LC3-II is instant in PC3 cells, i.e. the Atg4/Atg7/Atg3-mediated PE ligation process of LC3-I is very active in PC3 cells. The conversion of LC3-I to LC3-II may determine the sensitivity to nutritional stress-induced autophagy. When the conversion of LC3-I to LC3-II is unlimited, the activation of LC3 expression (transcription and translation) may become critical for activation of autophagy.
References:
- Kabeya Y, Mizushima N, Ueno T, et al., LC3, a mammalian homologue of yeast Apg8p, is localized in autophagosome membranes after processing. EMBO J. 2000; 19(21):5720-5728.
- Cheng Y, Qiu F, Tashiro S, Onodera S, Ikejima T. (2008) ERK and JNK mediate TNFalpha-induced p53 activation in apoptotic and autophagic L929 cell death. Biochem Biophys Res Commun. 2008; 376(3):483-8. [
- Parikh A, Childress C, Deitrick K, Lin Q, Rukstalis D, Yang W. Statin-induced autophagy by inhibition of geranylgeranyl biosynthesis in prostate cancer PC3 cells. Prostate. 2010; 70(9):971-81.
Written by:
Wannian Yang,1 Nicolas Toepfer,2 Chandra Childress,1 Ankur Parikh,2 and Daniel Rukstalis2 as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
1 Weis Center for Research, and 2Department of Urology, Geisinger Clinic, 100 N. Academy Ave., Danville, PA 17822
Atorvastatin induces autophagy in prostate cancer PC3 cells through activation of LC3 transcription - Abstract
UroToday.com Prostate Cancer Section