A somatostatin analogue induces translocation of Ku 86 autoantigen from the cytosol to the nucleus in colon tumour cells.

Flow cytometric and electron microscopic immunocytochemical studies have been performed in HT-29 human colon tumour cells in vitro, to determine and localise p86 Ku protein, which is a regulatory subunit of DNA-dependent kinase and a specific binding site for somatostatin. We have demonstrated that HT-29 cells contain p86 Ku and that the distribution between the cytoplasm and the nucleus is even. After administration of the somatostatin analogues Sandostatin and TT-232 to HT-29 cells, the p86 Ku content of the cytoplasmic compartment decreased in the first 4 h. An increase in the content of this protein in the nuclear compartment was observed at hour 1 followed by a decrease at hour 4 after treatment. Quantitative differences between the two analogues have been observed in this respect. The practical significance of these findings is discussed.

The somatostatin analogue TT-232 induces apoptosis in A431 cells: sustained activation of stress-activated kinases and inhibition of signalling to extracellular signal-regulated kinases.

TT-232 is a somatostatin analogue containing a five-residue ring structure. The present report describes TT-232-induced signalling events in A431 cells, where a 4-h preincubation with the peptide irreversibly induced a cell death program, which involves DNA-laddering and the appearance of shrunken nuclei, but is unrelated to somatostatin signalling. Early intracellular signals of TT-232 include a transient two-fold activation of the extracellular signal-regulated kinase (ERK2) and a strong and sustained activation of the stress-activated protein kinases c-Jun NH(2)-terminal kinase (JNK)/SAPK and p38MAPK. Blocking the signalling to ERK or p38MAPK activation had no effect on the TT-232-induced cell killing. At the commitment time for inducing cell death, TT-232 decreased EGFR-tyrosine phosphorylation and prevented epidermial growth factor (EGF)-induced events like cRaf-1 and ERK2 activation. Signalling to ERK activation by FCS, phorbol 12-myristate 13-acetate (PMA) and platelet-derived growth factor (PDGF) was similarly blocked. Our data suggest that TT-232 triggers an apoptotic type of cell death, concomitant with a strong activation of JNK and a blockade of cellular ERK2 activation pathways.

Physical and functional interactions between protein tyrosine phosphatase alpha, PI 3-kinase, and PKCdelta.

The somatostatin analogue, TT-232 inhibits cell proliferation and induces apoptosis in a variety of tumor cells both in vivo and in vitro. While the early transient activation of Erk/MAPK was found to be important for the induction of cell cycle arrest, the signaling pathway leading to the activation of Erk/MAPK had not been fully established. Here we present evidence that activation of the Erk/MAPK pathway by TT-232 involves PI 3-kinase, PKCdelta and the protein tyrosine phosphatase alpha (PTPalpha). We show a physical interaction of PI 3-kinase and PKCdelta with PTPalpha and show that the tyrosine phosphatase plays a role in the activation of MAPK. In this process, PTPalpha Ser-180 and Ser-204 phosphorylation is critical for the induction of phosphatase activity, which is required for dephosphorylation of pp60(c-src). Taken together, we demonstrate the physical and functional association between PI 3-kinase, PKCdelta and PTPalpha in a signaling complex that mediates the antitumor activity of the somatostatin analogue TT-232.

The antitumor somatostatin analogue TT-232 induces cell cycle arrest through PKCdelta and c-Src.

The heptapeptide TT-232 is structurally related to the hypothalamic hormone somatostatin and shows promise as an anticancer drug because of its tumor-specific cytotoxic effects. Apart from the ability to induce apoptosis, the synthetic peptide can trigger an alternative pathway that leads to cell cycle arrest in certain tumor cell systems. We found that pulse treatment with TT-232 blocks the cell cycle G(1)/S transition irreversibly in A431 cells. Investigation of the TT-232 signaling pathway yielded results similar to those reported for somatostatin although its affinity to the somatostatin receptor 1 is significantly reduced. We show that functional protein kinase C (PKC) delta as well as c-Src are necessary mediators of the TT-232 cytostatic effect and we propose a signaling pathway that leads to cell cycle arrest.