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Immunological demonstration of the accumulation of insulin, but not insulin receptors, in nuclei of insulin-treated cells

Authors: Soler, A.P. Thompson, K.A. Smith, R.M. Jarett, L. (Univ. of Pennsylvania School of Medicine, Philadelphia (USA))

Abstract: Although insulin is known to regulate nuclear-related processes, such as cell growth and gene transcription, the mechanisms involved are poorly understood.^Previous studies suggested that translocation of insulin or its receptor to cell nuclei might be involved in some of these processes.^The present investigation demonstrated that intact insulin, but not the insulin receptor, accumulated in nuclei of insulin-treated cells.^Cell fractionation studies demonstrated that the nuclear accumulation of ¹²⁵I-labeled insulin was time-, temperature-, and insulin-concentration-dependent.^Electron microscopic immunocytochemistry demonstrated that the insulin that accumulated in the nucleus was immunologically intact and associated with the heterochromatin.^Only 1% of the ¹²⁵I-labeled insulin extracted from isolated nuclei was eluted from a Sephadex G-50 column as ¹²⁵I-labeled tyrosine.^Plasma membrane insulin receptors were not detected in the nucleus by immuno electron microscopy or when wheat germ agglutinin-purified extracts of the nuclei were subjected to PAGE, electrotransfer, and immunoblotting with anti-insulin receptor antibodies.^These results suggested that internalized insulin dissociated from its receptor and accumulated in the nucleus without its membrane receptor.^The authors propose that some of insulin`s effects on nuclear function may be caused by the translocation of the intact and biologically active hormone to the nucleus and its binding to nuclear components in the heterochromatin.