p73 isoforms affect VEGF, VEGF165b and PEDF expression in human colorectal tumors: VEGF165b downregulation as a marker of poor prognosis

Abstract

The secreted mitogen vascular endothelial growth factor, VEGF, plays a pivotal role in angiogenesis. Hypoxia, inactivation of p53 and oncogenic K‐Ras induce VEGF expression. Other factors such as p73 may also affect VEGF levels. Curiously, p73 may also regulate angiogenesis by affecting the expression of the pigment epithelium‐derived factor, PEDF. Additionally, VEGF might harbor additional functions through the activation of E2F transcription factors. Recently, a new VEGF variant formed by alternative splicing, VEGF₁₆₅b, has been described as exerting anti‐angiogenic activity. We study here whether p73 isoforms levels ‐TAp73 and ΔTAp73‐ and p53 and K‐Ras status affect the expression of the above‐mentioned angiogenesis‐related genes (through the correlation between their expressions), the prognostic value of VEGF₁₆₅b and PEDF and the correlation between VEGF and E2F‐1 levels. Tumor and normal tissue of 112 colorectal cancer patients was analyzed to evaluate: (i) levels of TAp73, ΔTAp73, VEGF, VEGF₁₆₅b, PEDF and E2F‐1 by quantitative real‐time RT‐PCR, (ii) p53 status by immunohistochemistry and (iii) mutations in the first exon of K‐Ras by PCR‐SSCP. Tumor characteristics were examined in each patient. Associations were observed between: (i) specific p73 isoforms and VEGF and VEGF₁₆₅b expression; (ii) ΔEx2p73 variant and downregulation of PEDF; (iii) VEGF and PEDF expression; (iv) inactive p53 and VEGF₁₆₅b levels; (v) oncogenic K‐Ras and PEDF downregulation; (vi) E2F‐1 and VEGF expressions; (vii) VEGF₁₆₅b downregulation and poor prognosis parameters of tumors. We conclude that the levels of p73 isoforms could affect the expression of VEGF, VEGF₁₆₅b and PEDF. This scenario becomes complicated because a feedback between VEGF and PEDF may exist. VEGF may activate the E2F‐1 factor. Mutations in K‐Ras could negatively regulate PEDF expression. p53 inactivation might result in compensatory mechanisms such as over‐expression of VEGF₁₆₅b. Our data support the role of VEGF₁₆₅b as a tumor suppressor factor in colorectal carcinogenesis and its possible prognosis value.