Regulation of nov by WT1: a potential role for nov in nephrogenesis

The nov gene encodes a putative Insulin-like-Growth Factor-Binding-Protein (IGFBP) of a novel type which is structurally related to a family of growth-factors likely to play a role in the control of cell proliferation. In the kidney, nov is expressed essentially at the embryonic stage and alterations of nov expression, relative to the normal kidney, have been detected in both avian nephroblastomas and human Wilms' tumors. The levels of human nov (novH) and WT1 mRNA in individual Wilms' tumors have been shown to be inversely correlated, suggesting that the expression of novH could be under the negative control of WT1. We have now established the nucleotide sequence of the 5' flanking region and identified two transcription start sites by RNase protection assays and primer extension. We report that in transient cotransfection experiments the transcription activity of novH promoter constructs was repressed by two isoforms of WT1 proteins (WT1 and WT1+KTS). Repression of the novH promoter required both intact zinc finger regions and the NH2 transcription repression domain of WT1. Inasmuch as the minimal region of novH promoter required to mediate WT1 repression in vivo failed to bine recombinant WT1 protein in in vitro footprinting assays this repression may be mediated by either (i) low affinity sites cooperative interactions or (ii) indirectly via protein-protein interactions with another factor(s). Furthermore, constitutive expression of wild type WT1 into 293 cells resulted in a decrease of endogenous NOVH protein levels, suggesting that novH may be a physiological target for WT1. The downregulation of novH expression by WT1 might represent a key element in normal and tumoral nephrogenesis.