A specific anti-cyclin D1 intrabody represses breast cancer cell proliferation by interrupting the cyclin D1-CDK4 interaction.

Background: Cyclin D1 overexpression may contribute to development of various cancers, including breast cancer, and thus may serve as a key cancer diagnostic marker and therapeutic target. In our previous study, we generated a cyclin D1-specific single-chain variable fragment antibody (ADκ) from a human semi-synthetic single-chain variable fragment library. ADκ specifically interacted with recombinant and endogenous cyclin D1 proteins through an unknown molecular basis to inhibit HepG2 cell growth and proliferation.

Results: Here, using phage display and in silico protein structure modeling methods combined with cyclin D1 mutational analysis, key residues that bind to ADκ were identified. Notably, residue K112 within the cyclin box was required for cyclin D1-ADκ binding. In order to elucidate the molecular mechanism underlying ADκ anti-tumor effects, a cyclin D1-specific nuclear localization signal-containing intrabody (NLS-ADκ) was constructed. When expressed within cells, NLS-ADκ interacted specifically with cyclin D1 to significantly inhibit cell proliferation, induce G1-phase arrest, and trigger apoptosis of MCF-7 and MDA-MB-231 breast cancer cells. Moreover, the NLS-ADκ-cyclin D1 interaction blocked binding of cyclin D1 to CDK4 and inhibited RB protein phosphorylation, resulting in altered expression of downstream cell proliferation-related target genes.

Conclusion: We identified amino acid residues in cyclin D1 that may play key roles in the ADκ-cyclin D1 interaction. A nuclear localization antibody against cyclin D1 (NLS-ADκ) was constructed and successfully expressed in breast cancer cells. NLS-ADκ exerted tumor suppressor effects via blocking the binding of CDK4 to cyclin D1 and inhibiting phosphorylation of RB. The results presented here demonstrate anti-tumor potential of intrabody-based cyclin D1-targeted breast cancer therapy.