Na-Pi cotransporter expressed during the differentiation of pancreatic AR42J cells.

Abstract

Pancreatic AR42J cells express both exocrine and neuroendocrine properties (1). When exposed to activin A and betacellulin, these cells convert to insulin-secreting cells (2). Hence, these cells provide a model system to study the mechanism of differentiation of pancreatic -cells. Using AR42J-B20 cells, a subclone of AR42J, we studied the genes that had increased expression during the course of differentiation (3). In that study, we identified 20 genes upregulated by the treatment with a combination of activin A and betacellulin. Among those 20 genes, 5 were novel. One of those, AB20, was interesting in that its expression was inhibited by PD098059, an inhibitor of mitogen-activated protein kinase, which blocked the differentiation of AR42J cells. It appeared that expression of AB20 was tightly related with differentiation of AR42J cells into insulinsecreting cells. In the present study, we cloned and sequenced the cDNA for AB20. Sequence analysis revealed an open reading frame of 1,746 bp. The cDNA encoded a protein containing 582 amino acids with an apparent molecular weight of 64.5 kDa. The protein was related to the brain-specific Na-Pi cotransporter (BNPI), with an identity of 76.2%. This protein was therefore designated as the differentiation-associated Na-Pi cotransporter (DNPI). Structurally, DNPI belongs to the type 1 Na-Pi cotransporters, which include Napi-1, NPT-1, Npt1, BNPI, and RNaPi. To determine the functional property of DNPI, we injected DNPI mRNA into Xenopus oocytes and measured sodium-dependent P uptake. The results showed that DNPI functioned as a Na-Pi cotransporter and that the activity of DNPI was slightly lower than that of BNPI. Northern blot analysis showed that DNPI was expressed in the brain but not in other tissues, including the pancreas. However, when analyzed by reverse transcriptase–polymerase chain reaction, mRNA for DNPI was detected in pancreatic islets. These results indicate that DNPI is a Na-Pi cotransporter expressed in pancreatic islets.