A peptide inhibitor derived from p55PIK phosphatidylinositol 3-kinase regulatory subunit: a novel cancer therapy

Abstract

Abstractp55PIK, a regulatory subunit of phosphatidylinositol 3-kinase (PI3K), specifically interacts with retinoblastoma protein (Rb) through the unique NH 2 terminus of p55PIK, N24. This interaction is critical for cell proliferation and cell cycle progression. To examine p55PIK as a potential target for cancer therapy, we generated an adenovirus expressing N24 (Ad-N24-GFP) and studied its effects on the proliferation of cultured cancer cells, including human colon (HT29) and thyroid (FTC236) cancer cells. Ad-N24-GFP blocked cell proliferation and induced cell cycle arrest in all cancer cell lines tested. N24 induced cell cycle arrest at G 0 -G 1 phase in cell lines that expressed Rb. Interestingly, N24 inhibited cell proliferation by blocking cell cycle transition at both S and G 2 -M phases in FTC236 cells, which did not express Rb. When Rb was knocked down by short hairpin RNA in HT29 cells, N24 also inhibited cell cycle progression at S and G 2 -M phases, suggesting that p55PIK regulates cell cycle progression by Rb-dependent and Rbindependent mechanisms. Finally, Ad-N24-GFP markedly decreased the growth of xenograft tumors derived from HT29 and FTC236 cancer cells in athymic nude mice. Our data strongly suggest that N24 peptide is an effective anticancer therapy, which specifically inhibits PI3K signaling pathways mediated by p55PIK. Moreover, they show that the regulatory subunit of an enzyme, in addition to its catalytic subunit, can be an important target for drug development.

Blocking p55PIK signaling inhibits proliferation and induces differentiation of leukemia cells

“…Cells were collected 48 h after incubation with TAT-N24 or TAT-N24M (120 mg/ml), PBS solvent was added to the control cells and cell cycle analyses were performed as described. 7 Shown are representative histograms from an individual experiment. Similar results were obtained in three independent experiments.…”

“…Incorporation of BrdU into DNA and the DNA content in nuclei was determined by flow cytometry analysis. 7 Shown here are the representative data from three independent experiments. Note that % cells in DNA synthesis is defined as BrdU-positive cells (R2)/total cells.…”

“…Cells were incubated with TAT-N24 (120 mg/ml), TAT-N24M (120 mg/ml), PMA (20 ng/ml) or PBS (Control) for 48 h. Incorporation of BrdU into DNA and the DNA content in nuclei was determined. 7 Shown here are the representative data from three independent experiments. Note that % cells in DNA synthesis is defined as BrdU-positive cells/total cells.…”

“…26 BrdU incorporation into DNA (DNA synthesis) in tumor samples was determined as described. 7 Shown here are representative images of slides. Expression of Cd11b in tumor samples was determined with PE-labeled anti-Cd11b antibodies (red), and nuclear DNA was stained with DAPI (green) as described.…”

“…Moreover, Ad-N24 also inhibited the growth of cancer cells, which do not express Rb, indicating that N24 inhibits tumor growth by Rbindependent mechanisms as well. 7 While Ad-N24 is effective when injected locally into tumors, inefficient systemic delivery and safety concerns limit Ad-N24's clinical utility. Also, due to lack of adenovirus receptors, most leukemia cells are not permissive to Ad-N24.…”