Functional analysis of a newly identified TAAT-box of the rat insulin-II gene promoter

The activation of the rat insulin gene II by BETA2 and PDX-1 in rat insulinoma cells is repressed by Pax6

The transcriptional transactivator Pax6 binds the pancreatic islet cell-specific enhancer sequence (PISCES) of the rat insulin I gene. However the human, mouse, and rat insulin gene II promoters do not contain a PISCES element. To analyze the role of Pax6 in those PISCES-less promoters, we investigated its influence on rat insulin gene II expression and included in our studies the main activators: pancreatic and duodenal homeobox protein-1 (PDX-1) and BETA2/E47. Luciferase assays, Northern blots, and RIA were used to study effects of Pax6 overexpression, gel shift and chromatin precipitation assays to study its binding to the DNA, and yeast two-hybrid assays and glutathione S transferase capture assays to investigate its interactions with PDX-1 and BETA2. Finally, glucose-dependent intracellular transport of Pax6 was demonstrated by fluorescence microscopy. Overexpression of Pax6 prevents activation of the rat insulin II gene by BETA2 and PDX-1 and hence suppresses insulin synthesis and secretion. In vitro, Pax6 binds to the A-boxes, thereby blocking binding of PDX-1, and at the same time, its paired domain interacts with BETA2. Fluorescence microscopy demonstrated that the nuclear-cytoplasmic localization of Pax6 and PDX-1 are oppositely regulated by glucose. From the results, it is suggested that at low concentrations of glucose, Pax6 is localized in the nucleus and prevents the activation of the insulin gene by occupying the PDX-1 binding site and by interacting with BETA2.

A novel transgenic model to characterize the specific effects of follicle-stimulating hormone on gonadal physiology in the absence of luteinizing hormone actions

Gonadal function is wholly reliant on the two pituitary-derived gonadotropins, FSH and LH. Identifying the specific effects of FSH has been difficult because of the intimate relationship between LH and FSH action and inherent limitations of classic research paradigms. We describe a novel transgenic model to characterize the definitive actions of FSH alone, distinct from LH effects, created by combining transgenic FSH expression with the gonadotropin-deficient background of the hypogonadal (hpg) mouse. A tandem transgene construct encoding each alpha- and beta-subunit of human FSH, under the rat insulin II promoter, expressed biologically active heterodimers at serum levels, by immunoassay, equivalent to circulating FSH concentrations in fertile humans (0.1-25 IU/liter). Transgenic mice were crossed into the hpg mouse genotype to obtain LH-deficient animals secreting FSH alone. Testis weights of adult FSHxhpg mice were increased up to 5-fold, relative to nontransgenic hpg controls (P < 0.001). However, only transgenic males with serum FSH levels more than 1 IU/liter showed testis weights increased relative to hpg controls, indicating a physiological FSH threshold for the testicular response. Histology of enlarged FSHxhpg testes revealed round spermatids and sparse numbers of elongated spermatids, demonstrating that the testosterone-independent FSH response targeting the Sertoli cell can facilitate completion of meiosis and minimal initiation, but not completion, of spermiogenesis. Transgenic FSH also induced inhibin B secretion in FSHxhpg mice, but showed a distinct sexual dimorphism with only females exhibiting a strong FSH dose-dependent increase in serum inhibin B levels (r(2) = 0.84). In addition, ovaries of FSHxhpg females were enlarged up to 10-fold (P < 0.001), characterized by increased follicular recruitment and development to type 7 antral follicles. Thus, these findings show that the transgenic FSHxhpg mouse provides a unique model for detailed investigations of the definitive in vivo actions of FSH alone.

The nuclear localization signal (NLS) of PDX-1 is part of the homeodomain and represents a novel type of NLS

The beta-cell homeodomain transcription factor PDX-1 has vital functions both in controlling the expression of pancreatic polypeptide hormones and in the development of the pancreas. The transactivating and DNA-binding properties of PDX-1 have been well characterized, but nuclear transport is still undefined. Here we show that PDX-1 bears a nuclear localization signal (NLS) that is part of helix 3 of the homeodomain. PDX-1 deletion mutants were tagged with enhanced green fluorescent protein (EGFP) and expressed in COS-7 cells. Subcellular localization of the respective PDX-1-EGFP fusion proteins was analyzed by direct fluorescence microscopy and Western immunoblotting using an anti-(GFP). As a result we were able to demonstrate that the homeodomain or helix 3 alone was sufficient and necessary for transport into the nucleus. Point mutations of basic amino acid residues within helix 3 led to identification of an NLS with six amino acids being crucial for nuclear transport of PDX-1. Because this NLS does not match known examples of NLSs, the PDX-1 NLS may represent a novel class of NLS.