Blockade of Protein Geranylgeranylation Inhibits Cdk2-Dependent p27^Kip1 Phosphorylation on Thr187 and Accumulates p27^Kip1 in the Nucleus: Implications for Breast Cancer Therapy

Abstract

We describe the design of a potent and selective peptidomimetic inhibitor of geranylgeranyltransferase I (GGTI), GGTI-2418, and its methyl ester GGTI-2417, which increases the levels of the cyclin-dependent kinase (Cdk) inhibitor p27Kip1 and induces breast tumor regression in vivo. Experiments with p27Kip1 small interfering RNA in breast cancer cells and p27Kip1 null murine embryonic fibroblasts demonstrate that the ability of GGTI-2417 to induce cell death requires p27Kip1. GGTI-2417 inhibits the Cdk2-mediated phosphorylation of p27Kip1 at Thr187 and accumulates p27Kip1 in the nucleus. In nude mouse xenografts, GGTI-2418 suppresses the growth of human breast tumors. Furthermore, in ErbB2 transgenic mice, GGTI-2418 increases p27Kip1 and induces significant regression of breast tumors. We conclude that GGTIs' antitumor activity is, at least in part, due to inhibiting Cdk2-dependent p27Kip1 phosphorylation at Thr187 and accumulating nuclear p27Kip1. Thus, GGTI treatment might improve the poor prognosis of breast cancer patients with low nuclear p27Kip1 levels.

The Deubiquitinating Enzyme USP17 Is Highly Expressed in Tumor Biopsies, Is Cell Cycle Regulated, and Is Required for G1-S Progression

“…Attenuation of GTPase processing using these inhibitors causes cells to arrest in G 1 (47,48). In agreement with our observations in USP17-depleted cells, this was thought to occur as a result of improper GTPase activation that leads to elevated p21 cip1 and p27 kip1 levels (49,50). In summary, our findings clearly show that, like other cell cycle-regulated proteins, the expression of USP17 is tightly controlled during cell cycle progression.…”

Mevalonate Metabolism Regulates Basal Breast Cancer Stem Cells and Is a Potential Therapeutic Target

“…Conditional knockout mice for Pggt1b, the gene encoding GGTase-I, develop significantly less KRAS-induced lung tumors and myeloproliferative neoplasms [40]. Furthermore, GGTI treatment inhibits the growth of human breast cancer xenografts and induces regression of mammary tumors in transgenic mice [25,41]. Here, we have shown that MVA metabolism blockade through GG inhibition targets breast CSCs in vivo.…”

“…RHOA regulates P27 kip1 by mediating its phosphorylation on Thr187 by CDK2, subsequent translocation of P27 from the nucleus to the cytosol, and enhancing its degradation in the cytoplasm [23,24]. In the absence of GG, RHOA is unable to carry out these functions and P27 kip1 accumulates in the nucleus [25]. Because P27 kip1 is known to regulate stem cell self-renewal [26,27], we explored the role of RHOA/P27 kip1 signaling in mediating the effect of GGTI treatment on the CSC population.…”

The perplexing case of the geranylgeranyl transferase–deficient mouse

“…FTase inhibitors failed in the clinic because oncogenic K-Ras and N-Ras can escape the blockade by serving as substrates for GGTase-I (23). This has led to an effort to develop GGTase-I inhibitors as anticancer drugs (24,25). If pharmacological inhibition of GGTase-I has the same effects as genetic disruption of Pggt1b, as suggested by some of the data presented by Khan et al (3), then one might expect such drugs to exhibit significant toxicity in the form of autoimmune disease.…”