Insulin receptor signaling in the beta-cell influences insulin gene expression and insulin content: evidence for autocrine beta-cell regulation.
The insulin receptor (IR) is expressed by insulin-secreting beta-cells, but its cellular function is unknown. We transfected betaTC6-F7 beta-cells with cDNAs encoding either wild-type or mutant kinase-inactive (A/K1018) IRs, and by fluorescence-activated cell sorting generated polyclonal beta-cell lines that overexpressed each receptor type at levels two- to fourfold higher than the parental cells. Beta-cells overexpressing wild-type IRs had a proportional increase in insulin-stimulated tyrosine kinase activity; no change occurred in beta-cells expressing the mutant IR. We observed a threefold increase in cellular insulin content in beta-cells that overexpressed the wild-type IR, as determined by radioimmunoassay. This increase occurred despite a fivefold elevated rate of both basal and 10 mmol/l glucose-induced insulin secretion. Fractional insulin secretion (percentage of total cell insulin releasable at 10 mmol/l glucose) was unchanged in beta-cells overexpressing the wild-type IR compared with the parental beta-cell line. Insulin content and insulin secretion were unaffected by overexpression of kinase-inactive IRs. Steady-state insulin mRNA levels were elevated twofold in the beta-cells overexpressing the wild-type IR and unchanged in the beta-cells expressing the kinase-inactive receptor, as determined by Northern blot analysis. The rate of insulin mRNA degradation measured in the presence of 5 microg/ml actinomycin D was not significantly affected in either cell line. In the absence of glucose, the basal level of (pro)insulin biosynthesis in the beta-cells overexpressing the wild-type IR increased significantly (61%) compared with the beta-cells transfected with the kinase-inactive IR. These data indicate that IR signaling can regulate insulin gene transcription and can modulate the steady-state insulin content of beta-cells.
Department of Pathology and Laboratory Medicine, University of Pennsylvania School of Medicine, Philadelphia, USA.
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D000818:Animals; D019898:Autocrine Communication / Q000502:physiology*; D002460:Cell Line; D015870:Gene Expression / Q000502:physiology*; D007328:Insulin / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D007515:Islets of Langerhans / Q000378:metabolism / Q000502:physiology*; D051379:Mice; D012333:RNA, Messenger / Q000378:metabolism; D011972:Receptor, Insulin / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D014162:Transfection / Q000502:physiology
D007328:Insulin; D012333:RNA, Messenger; D011972:Receptor, Insulin