Analysis of the 5'-upstream region of mouse P/Q-type Ca2+ channel alpha1A subunit gene for expression in pancreatic islet beta cells using transgenic mice and HIT-T15 cells

The role of voltage-gated calcium channels in pancreatic beta-cell physiology and pathophysiology

Voltage-gated calcium (CaV) channels are ubiquitously expressed in various cell types throughout the body. In principle, the molecular identity, biophysical profile, and pharmacological property of CaV channels are independent of the cell type where they reside, whereas these channels execute unique functions in different cell types, such as muscle contraction, neurotransmitter release, and hormone secretion. At least six CaValpha1 subunits, including CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1, have been identified in pancreatic beta-cells. These pore-forming subunits complex with certain auxiliary subunits to conduct L-, P/Q-, N-, R-, and T-type CaV currents, respectively. beta-Cell CaV channels take center stage in insulin secretion and play an important role in beta-cell physiology and pathophysiology. CaV3 channels become expressed in diabetes-prone mouse beta-cells. Point mutation in the human CaV1.2 gene results in excessive insulin secretion. Trinucleotide expansion in the human CaV1.3 and CaV2.1 gene is revealed in a subgroup of patients with type 2 diabetes. beta-Cell CaV channels are regulated by a wide range of mechanisms, either shared by other cell types or specific to beta-cells, to always guarantee a satisfactory concentration of Ca2+. Inappropriate regulation of beta-cell CaV channels causes beta-cell dysfunction and even death manifested in both type 1 and type 2 diabetes. This review summarizes current knowledge of CaV channels in beta-cell physiology and pathophysiology.

Beta-cell CaV channel regulation in physiology and pathophysiology

The beta-cell is equipped with at least six voltage-gated Ca2+ (CaV) channel alpha1-subunits designated CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1. These principal subunits, together with certain auxiliary subunits, assemble into different types of CaV channels conducting L-, P/Q-, N-, R-, and T-type Ca2+ currents, respectively. The beta-cell shares customary mechanisms of CaV channel regulation with other excitable cells, such as protein phosphorylation, Ca2+-dependent inactivation, and G protein modulation. However, the beta-cell displays some characteristic features to bring these mechanisms into play. In islet beta-cells, CaV channels can be highly phosphorylated under basal conditions and thus marginally respond to further phosphorylation. In beta-cell lines, CaV channels can be surrounded by tonically activated protein phosphatases dominating over protein kinases; thus their activity is dramatically enhanced by inhibition of protein phosphatases. During the last 10 years, we have revealed some novel mechanisms of beta-cell CaV channel regulation under physiological and pathophysiological conditions, including the involvement of exocytotic proteins, inositol hexakisphosphate, and type 1 diabetic serum. This minireview highlights characteristic features of customary mechanisms of CaV channel regulation in beta-cells and also reviews our studies on newly identified mechanisms of beta-cell CaV channel regulation.

Mouse transgenic for murine oviduct-specific glycoprotein promoter-driven simian virus 40 large T-antigen: tumor formation and its hormonal regulation

Transgenic mice were generated in which a 2.2-kb segment of the 5'-flanking sequence of the mouse oviduct-specific glycoprotein (OGP) gene was used to drive expression of the simian virus 40 large T antigen (Tag). These mice spontaneously developed tumors in the female reproductive tract. Analysis using reverse transcriptase-polymerase chain reaction showed that the 2.2-kb OGP 5'-flanking region drove Tag mRNA expression in the oviduct, uterus, vagina, and ovary, but not in other tissues. Histological and immunohistochemical analyses revealed that the tumor cells were distributed in the oviduct, endometrium, myometrium, and vagina; and had atypical features, abnormal mitosis, and Tag expression. Ovariectomy suppressed Tag expression, and thereby, blocked tumorigenesis in the transgenic mice. Estradiol administration to ovariectomized transgenic mice led to dramatic hyperplasia of the reproductive tract tissues in association with enhanced Tag expression, both in intensity and distribution. These results demonstrated that a 2.2-kb fragment of the 5'-flanking sequence of the mouse OGP gene was capable of directing the expression of Tag and inducing tumorigenesis in female reproductive tract tissues in an estrogen-dependent manner. Estrogen response elements present in the promoter region were functional in vivo. These transgenic mice represent a unique model, since they develop tumors in the oviducts as well as in other tissues derived from the Mullerian duct.