The product of the retinoblastoma (Rb) susceptibility gene ( RB-1 ) regulates expression of a variety of growth control genes via discrete promoter elements termed retinoblastoma control elements (RCEs). We have previously shown that RCEs are bound and regulated by a common set of ubiquitously expressed nuclear proteins of 115, 95 and 80 kDa, termed retinoblastoma control proteins (RCPs). We have also previously determined that Sp3 and Sp1, two members of the Sp family of transcription factors, encode the 115 and 95 kDa RCPs respectively and that Rb stimulates Sp1/Sp3-mediated transcription in vivo. In this report we have extended these results by determining that the 80 kDa RCP arises from Sp3 mRNA via translational initiation at two internal sites located within the Sp3 trans -activation domain. Internally initiated Sp3 proteins readily bind to Sp1 binding sites in vitro yet have little or no capacity to stimulate transcription of Sp-regulated genes in vivo. Instead, these Sp3-derived proteins function as potent inhibitors of Sp1/Sp3- mediated transcription. Since cell cycle- or signal- induced expression of a variety of genes, including p21 waf1/cip1, p15 INK4B, CYP11A, mdr1 and acetyl-CoA carboxylase, have been mapped to GC-rich promoter elements that bind Sp family members, we speculate that alterations of the protein and/or DNA binding activities of internally initiated Sp3 isoforms may account in part for the regulation of such differentially expressed genes.

The transient expression of the retinoblastoma protein (Rb) regulates the transcription of a variety of growth-control genes, including c-fos, c-myc, and the gene for transforming growth factor j31 via discrete promoter sequences termed retinoblastoma control elements (RCE) The human retinoblastoma susceptibility gene RB1 is believed to participate in the orchestration of orderly cell growth and/or differentiation (1). Deletion or mutational inactivation of RBJ is correlated with the genesis of a variety of human cancers including retinoblastoma, osteosarcoma, and carcinomas of the breast, bladder, and lung (2-6). The retinoblastoma protein (Rb) may also be sequestered by viral oncoproteins, such as simian virus 40 large tumor antigen, via conserved regions of the oncoproteins that are essential for viral-induced transformation (7-11).The Rb gene encodes a set of ubiquitously expressed nuclear phosphoproteins that are distinguished by their extent of posttranslational modification (3,(12)(13)(14). Given its nuclear localization and an associated nonspecific affinity for DNA, a role for Rb in the regulation of gene expression has been suggested (3, 12). This supposition was supported by experiments demonstrating that a 30-bp sequence within the c-fos promoter is a target of Rb function (15). This sequence, termed "retinoblastoma control element" (RCE), was shown to be necessary and sufficient for Rb-mediated transcription control. Subsequently, similar elements have been described within the promoters of a variety of genes that themselves encode growth regulatory molecules (16)(17)(18)(19)(20)(21)(22). Interestingly,The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.Kim et at (18) showed that the transcriptional response of a given RCE to Rb coexpression is cell-type dependent and may be stimulatory or inhibitory. While a mechanism for Rbmediated transcriptional regulation was not indicated by these experiments, recent evidence strongly suggests that Rb functions to regulate transcription via its physical interaction with sequence-specific DNA-binding proteins (23)(24)(25)(26)(27)(28)(29).The binding of viral oncoproteins to Rb occurs within a discrete portion of the Rb carboxyl terminus that is a "hotspot" for mutation in human cancers (2,3,(30)(31)(32)(33)(34)(35)(36). This region, often referred to as the Rb "pocket," is also the site of interaction of Rb with a variety of proteins that are believed to be targets of Rb function, including transcription factors E2F-1, ATF-2, and myoD (23, 25, 27, 28). The functional consequence of the formation of such complexes can be quite distinct. Rb forms cell-cycle-regulated complexes with E2F-1 and down-regulates E2F-dependent transcription in vivo (for a review, see ref. 23). In contrast, the formation of complexes between Rb and ATF-2 or myoD leads to an increase in ATF-2 or myoDmediated transcription (25,...

The glaucomas comprise a genetically complex group of retinal neuropathies that typically occur late in life and are characterized by progressive pathology of the optic nerve head and degeneration of retinal ganglion cells. In addition to age and family history, other significant risk factors for glaucoma include elevated intraocular pressure (IOP) and myopia. The complexity of glaucoma has made it difficult to model in animals, but also challenging to identify responsible genes. We have used zebrafish to identify a genetically complex, recessive mutant that shows risk factors for glaucoma including adult onset severe myopia, elevated IOP, and progressive retinal ganglion cell pathology. Positional cloning and analysis of a non-complementing allele indicated that non-sense mutations in low density lipoprotein receptor-related protein 2 (lrp2) underlie the mutant phenotype. Lrp2, previously named Megalin, functions as an endocytic receptor for a wide-variety of bioactive molecules including Sonic hedgehog, Bone morphogenic protein 4, retinol-binding protein, vitamin D-binding protein, and apolipoprotein E, among others. Detailed phenotype analyses indicated that as lrp2 mutant fish age, many individuals—but not all—develop high IOP and severe myopia with obviously enlarged eye globes. This results in retinal stretch and prolonged stress to retinal ganglion cells, which ultimately show signs of pathogenesis. Our studies implicate altered Lrp2-mediated homeostasis as important for myopia and other risk factors for glaucoma in humans and establish a new genetic model for further study of phenotypes associated with this disease.