Phosphoinositide 3-Kinase Signalling Pathways in Tumor Progression, Invasion and Angiogenesis

Synthesis and Activity of C11-Modified Wortmannin Probes for PI3 Kinase

The key role played by PI3 kinase in cancer, hormone action, and a host of other biological functions suggests that specific inhibitors whose disposition could be ascertained in vivo would be useful in biological research or, potentially, for imaging PI3K in a clinical setting. Wortmannin (Wm, 1) is an inhibitor of PI3 kinase with high specificity for this enzyme. We synthesized three modified Wm probes, a biotinylated Wm (7a), a 4-hydroxy-3-iodophenylated Wm, which was obtained both unlabeled (7b) and labeled with (125)I (8), and a fluoresceinated Wm (7c), through modification at C-11, and evaluated their inhibitive activity as inhibitors of PI3 kinase. Biotinylated (7a) and 4-hydroxy-3-iodophenylated Wm's (7b) had IC(50)s for PI3K of 6.11 and 11.02 nM, respectively, compared to an IC(50) for Wm of 1.63 nM. Fluoresceinated Wm (7c) lost considerably more activity than the other derivatives, with an IC(50) of 64.9 nM. The (125)I labeled 4-hydroxy-3-iodophenylated Wm (8) could be detected after reaction with an immunoprecipitate of PI3 kinase. The activity of these reporter Wm's is discussed in relationship to earlier findings on the pharmacological activity of Wm derivatives and the ability of inhibitors to fit into the ATP pocket of PI3 kinase.

Inhibition of cyclooxygenase (COX)-2 expression by Tet-inducible COX-2 antisense cDNA in hormone-refractory prostate cancer significantly slows tumor growth and improves efficacy of chemotherapeutic drugs

PURPOSE

Overexpression of the proinflammatory enzyme cyclooxygenase (COX)-2 is associated with the progression of various malignancies; the role of COX-2 in prostate cancer is less clear. The significance of COX-2 in prostate cancer growth and response to chemotherapy was investigated in an androgen-refractory prostate cancer cell line using a Tet-inducible antisense COX-2 expression system.

EXPERIMENTAL DESIGN

An antisense COX-2 cDNA construct under the control of a doxycycline-inducible promoter was transfected into a prostate cancer cell line, PC-3ML. Modulations of cell growth, apoptosis, and chemosensitivity in the presence or absence of doxycycline were analyzed. Tumor incidence, growth rate, and response to two cytotoxic drugs, COL-3 [chemically modified tetracycline-3-(6-demethyl-6-deoxy-4-dedimethylamino-tetracycline)] and Taxotere (docetaxel), were investigated in tumor xenografts. Apoptotic incidences and tumor microvessel density in tumors were determined by immunohistochemistry.

RESULTS

Conditional suppression of COX-2 in PC-3ML caused reduced cell proliferation, decreased levels of phosphorylated AKT, G(0)-G(1) arrest, and increased apoptosis and caspase-3 activity. Suppression of COX-2 increased Bax protein and decreased Bcl-x(L) protein in vitro. COX-2 antisense-expressing PC-3ML tumors showed a 57% growth delay compared with nontransfected or vector controls. Oral administration of COL-3 (40 mg/kg, oral gavage) or Taxotere (2.3 mg/kg, intraperitoneally; 3x per week) in tumor-bearing mice further slowed tumor growth (65% and approximately 94%, respectively). Compared with the control group, the occurrence of apoptosis in antisense COX-2 tumors was eight times higher, and the tumor microvessel density was three times lower.

CONCLUSIONS

These results provide direct evidence that constitutive expression of COX-2 in prostate cancer has both angiogenic and cytoprotective functions. Suppression of tumor cell COX-2 is sufficient to enhance chemotherapy response in prostate cancer.

Targeting multiple signaling pathways as a strategy for managing prostate cancer: multifocal signal modulation therapy

The aberrant behavior of cancer reflects upregulation of certain oncogenic signaling pathways that promote proliferation, inhibit apoptosis, and enable the cancer to spread and evoke angiogenesis. Theoretically, it should be feasible to decrease the activity of these pathways-or increase the activity of pathways that oppose them-with noncytotoxic agents. Since multiple pathways are dysfunctional in most cancers, and cancers accumulate new oncogenic mutations as they progress, the greatest and most durable therapeutic benefit will likely be achieved with combination regimens that address several targets. Thus, a multifocal signal modulation therapy (MSMT) of cancer is proposed. This concept has already been documented by researchers who have shown that certain combinations of signal modulators-of limited utility when administered individually-can achieve dramatic suppression of tumor growth in rodent xenograft models. The present essay attempts to guide development of MSMTs for prostate cancer. Androgen ablation is a signal-modulating measure already in standard use in the management of delocalized prostate cancer. The additional molecular targets considered here include the type 1 insulin-like growth factor receptor, the epidermal growth factor receptor, mammalian target of rapamycin, NF-kappaB, hypoxia-inducible factor-1alpha, hsp90, cyclooxygenase-2, protein kinase A type I, vascular endothelial growth factor, 5-lipoxygenase, 12-lipoxygenase, angiotensin II receptor type 1, bradykinin receptor type 1, c-Src, interleukin-6, ras, MDM2, bcl-2/bclxL, vitamin D receptor, estrogen receptor-beta, and PPAR-. Various nutrients and phytochemicals suspected to have potential utility in prostate cancer prevention and therapy, but whose key molecular targets are still unknown, might reasonably be incorporated into MSMTs for prostate cancer; these include lycopene, selenium, green tea polyphenols, genistein, and silibinin. MSMTs can be developed systematically by testing various combinations of signal-modulating agents, in concentrations that can feasibly be achieved and maintained clinically, on human prostate cancer cell lines; combinations that appear promising can then be tested in xenograft models and, ultimately, in the clinic. Some signal modulators can increase response to cytotoxic drugs by upregulating effectors of apoptosis. When MSMTs fail to raise the spontaneous apoptosis rate sufficiently to achieve tumor stasis or regression, incorporation of appropriate cytotoxic agents into the regimen may improve the clinical outcome.

The integrity of cholesterol-enriched microdomains is essential for the constitutive high activity of protein kinase B in tumour cells

A deregulated activity of PKB/Akt (where PKB stands for protein kinase B) renders tumour cells resistant to a variety of apoptosis-inducing stimuli. Elucidation of the mechanisms responsible for this deregulation is of prime importance for the development of novel anti-cancer drugs. Results of the present study demonstrate that the constitutive activity of PKB/Akt in B16BL6 melanoma cells depends on the integrity of cholesterol-enriched membrane microdomains, since the exposure of cells to cholesterol-depleting agents decreases the phosphorylation of this enzyme, with no change in its total protein level. Inhibitors of Hsp90 (heat-shock protein 90) decreased phosphorylation of PKB/Akt with a similar pattern. Dephosphorylation of the enzyme, as a consequence of raft disintegration, could be precluded by inhibition of serine/threonine (but not tyrosine) phosphatases. Our results imply that destabilization of lipid rafts seemingly affects the association of Hsp90 with the respective serine/threonine phosphatases, thereby increasing the accessibility to PKB/Akt to deactivating phosphatases. We have found recently that reconstituted expression of H-2K class I glycoproteins in class I-deficient B16BL6 cells also decreases the phosphorylation of PKB/Akt. Therefore it is possible that raft-associated regulation of this important enzyme involves both H-2K glycoproteins and Hsp90.