Targeting vasculature in urologic tumors: Mechanistic and therapeutic significance

Abstract

Recent advances toward understanding the molecular mechanisms regulating cancer initiation and progression provide new insights into the therapeutic value of targeting tumor vascularity by interfering with angiogenic signaling pathways. The functional contribution of key angiogenic factors toward increased vascularity characterizing metastatic tumors and their therapeutic exploitation is considered in three major urologic malignancies, renal, bladder, and prostate cancer. With the realization that the success of the therapeutic efficacy of the various anti-angiogenic approaches for the treatment of urologic tumors has yet to be proven clinically, the challenge remains to select critical angiogenesis pathways that can be targeted for an individual tumor. Here we discuss the major mechanisms that support formation of vasculature in renal, bladder, and prostate tumors and the current results of targeting of specific molecules/regulators for therapeutic intervention against metastastic disease. Keywordsvascularity; tumor growth; apoptosis; VEGF; bladder cancer; renal cancer; prostate cancer In 2007, there will be an estimated 346,440 new cases diagnosed with urologic cancer in the United States and 54,360 Americans will die from a urologic malignancy (SEER Cancer Statistics Review, http://cancernet.nci.nih.gov/statistics). This mortality rate is alarmingly high as it translates to one individual dying every 9 min in the US due to a urologic tumor and thus a significant health issue.Angiogenesis is an essential process in normal physiological functions such as ovarian cycle in female reproductive system [Kaczmarek et al., 2005] and a contributing factor in disease states such as chronic inflammation, arthritis, cancer, and macular degeneration . During the development of the embryo, mesoderm differentiates into angioblasts; these endothelial cells, not yet organized into a lumen, form primitive vessels toward development of blood vessel network, via vasculogenesis. In the adult, new blood vessels form from preexisting vasculature, via angiogenesis [Risau, 1997], while malignant conditions induce a hypercoagulable state in their hosts [Nash et al., 2001]. By early 1960s it was evident that tumors could elaborate diffusible substances that induce angiogenesis from the host vasculature [Algira et al., 1945;Greenblatt and Shubick, 1968]. The increased tumor vascularity was originally believed to be vasodilation of the host endothelium in response to metabolic waste products from within the tumor . A decade later Dr. Folkman's pioneering work identified angiogenesis as a required phenomenon for tumor growth and metastasis, first defining the potential therapeutic value of agents targeting this process , 1971]. Tumor blood vessels exhibit characteristic markers which are not present in normal angiogenic tissues . After enduring the circulation "journey," metastatic cancer cells can escape out of the endothelial vasculature and in the target tissue via extravasation. How do the metastastic cells signal activa...

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