Lovastatin-mediated G ₁ arrest is through inhibition of the proteasome, independent of hydroxymethyl glutaryl-CoA reductase

Abstract

In this paper we present the finding that lovastatin arrests cells by inhibiting the proteasome, which results in the accumulation of p21 and p27, leading to G 1 arrest. Lovastatin is an inhibitor of hydroxymethyl glutaryl (HMG)-CoA reductase, the rate-limiting enzyme in cholesterol synthesis. Previously, we reported that lovastatin can be used to arrest cultured cells in the G 1 phase of the cell cycle, resulting in the stabilization of the cyclin-dependent kinase inhibitors (CKIs) p21 and p27. In this report we show that this stabilization of p21 and p27 may be the result of a previously unknown function of the pro-drug, ␤-lactone ring form of lovastatin to inhibit the proteasome degradation of these CKIs. The lovastatin mixture used in this study is 80% open-ring form and 20% pro-drug, ␤-lactone form. We show that while the lovastatin open-ring form and pravastatin (a lovastatin analogue, 100% open ring) inhibit the HMG-CoA reductase enzyme, lovastatin pro-drug inhibits the proteasome but does not inhibit HMG-CoA reductase. In addition, many of the properties of proteasome inhibition by the prodrug are the same as the specific proteasome inhibitor lactacystin. Lastly, mevalonate (used to rescue cells from lovastatin arrest) unexpectedly abrogates the lactacystin and lovastatin pro-drug inhibition of the proteasome. Mevalonate increases the activity of the proteasome, which results in degradation of the CKIs, allowing lovastatin-and lactacystinarrested cells to resume cell division. The lovastatin-mediated inhibition of the proteasome suggests a unique mechanism for the chemopreventative effects of this agent seen in human cancer.

Independent Inhibition of Alzheimer Disease β- and γ-Secretase Cleavage by Lowered Cholesterol Levels

“…A recent study has shown that statins cause intracellular accumulation of A␤ via a non-steroidal isoprenoid-dependent mechanism (67). In our study, however, we added mevalonate to avoid toxicity due to inhibition of these non-steroidal pathways (51,52,54).…”

“…De novo cholesterol synthesis was inhibited by lovastatin, an inhibitor of HMG-CoA reductase. A basic level of mevalonate was maintained by adding mevalonate to the cell culture media to avoid toxicity due to inhibition of non-steroidal pathways (51,52). Plasma membrane cholesterol was extracted by methyl-␤-cyclodextrin.…”

Simvastatin strongly reduces levels of Alzheimer's disease β-amyloid peptides Aβ42 and Aβ40 in vitro and in vivo

“…First, de novo synthesis of cholesterol was inhibited by lovastatin or simvastatin. Biochemical pathways for nonsteroidal products were allowed to proceed by supplementing cells with mevalonate in the culture medium (33,34). Second, plasma membrane cholesterol was extracted by CDX, which is very efficient in selectively and rapidly extracting cholesterol from the plasma membrane (35).…”

Cyclin E deregulation alters the biologic properties of ovarian cancer cells

“…We have previously shown that lovastatin leads to G1 arrest by inhibition of the proteosome and subsequent increase in p21 and p27 levels (Gray-Bablin et al, 1997; Rao et al, 1998Rao et al, , 1999. Cells were treated with escalating doses of lovastatin and cell cycle distribution assessed by flow cytometry (Figure 4a).…”