Evidence that sequence homologous region in LRAT-like proteins possesses anti-proliferative activity and DNA binding properties: Translational implications and mechanism of action

LRAT (lecithin:retinol acyltransferase), an enzyme whose levels are modulated during malignant conversion, has been reported as the founder member of a new LRAT-like family that includes tumor suppressors TIG-3_1-164 and Ha-Rev107_1-162. The mechanisms that link these three proteins to carcinogenesis as well as the significance of a reported shared sequence homologous region remain unclear. This begs the question if the tumor suppressors possess enzyme properties and/or if the LRAT enzyme possesses tumor suppressor properties. We use the reported homologous region as a first approach to address the question from the perspective that all three proteins can possess tumor suppressor properties. We postulated that the homologous sequence harbors an anti-proliferation domain within the full-length proteins and that dodecapeptides of this sequence possess anti-proliferative activity. We report that H-TIG-3_111-123, H-Ha-Rev107-1_111-123 and H-LRAT_{160-171:C168L} exhibited in vitro growth inhibitory activity in a human cutaneous melanoma (HCM) model and affected tumor growth in a nude mouse model. Further, in peptide-sensitive HCM cells, these peptides crossed the plasma membrane and localized to the nucleus, where they could bind and activate promoters of transcription factors involved in G1→S transition. Moreover, peptide-induced abrogation of cyclin dependent kinase-2 expression was concomitant with sub-cellular re-distribution of cyclins E and A. Indeed, the sequence homologous region within each full-length wild-type protein as well as the growth inhibitory peptides can form alpha helices, a likely configuration for binding to DNA. This is the first report that this sequence homologous region (AA_111-123) within these LRAT-like proteins harbors an anti-proliferative domain with DNA binding properties. Sequences from this sequence homologous region can be used as templates for anti-tumor drug design and as probes to investigate disease-related mechanisms and structure-activity relationships of the full-length proteins, TIG-3_1-164, Ha-Rev107_1-162 and LRAT_160-171.