Clinical Endocrinology (1997) 47, 65–70 Expression of the putative inhibitor of the insulin receptor tyrosine kinase PC-1 in dermal fibroblasts from patients with syndromes of severe insulin resistance Jonathan P. Whitehead*, Phillippa J. Humphreys*, Karim Dib*, James W. Goding†, Stephen O’Rahilly* *Department of Medicine and Clinical Biochemistry, University of Cambridge, Addenbrooke’s Hospital, Cambridge, UK, †Department of Pathology and Immunology, Monash Medical School, Melbourne, Australia (Received 19 September 1996; returned for revision 11 November 1996; finally revised 23 December 1996; accepted 10 April 1997) Summary OBJECTIVE To date, mutations in the insulin receptor gene are the only clearly defined cause of extreme insulin resistance in man. Recently, however, some patients with severe insulin resistance have been reported to have marked over-expression of the trans- membrane glycoprotein PC-1 in their cultured fibro- blasts. This protein appears to act as an endogenous inhibitor of the insulin receptor tyrosine kinase which suggests that primary over-expression of PC-1 may play an aetiological role in some forms of insulin resistance. One sub-type of extreme insulin resis- tance in which the insulin receptor gene has been reported to be normal is pseudo-acromegalic insulin resistance. The main aim of this study was to deter- mine whether overexpression of PC-1 might contri- bute to the severe insulin resistance exhibited by some patients with pseudo-acromegaly. DESIGN AND PATIENTS PC-1 phosphodiesterase activity and PC-1 protein and mRNA content were measured in cultured dermal fibroblasts from three severely insulin resistant pseudo-acromegalic patients. These were compared with fibroblasts from normoinsulinaemic normoglycaemic controls and from subjects with known genetic defects in the insulin receptor or IRS-1. RESULTS In the fibroblasts from pseudo-acromegalic insulin resistant subjects PC-1 activity and PC-1 pro- tein and mRNA levels were indistinguishable from the normoinsulinaemic controls. Consistent with this observation, insulin receptor tyrosine kinase activity was similar in extracts from fibroblasts of pseudo- acromegalic subjects and normal controls. Surpris- ingly, subjects with insulin receptor or IRS-1 mutations had a profound reduction in PC-1 activity (p  0 . 005), protein (p  0 . 05) and mRNA levels (P  0 . 005). CONCLUSIONS The results indicate that PC-1 over- expression does not appear to contribute to the insu- lin resistant state of pseudo-acromegalic patients. The finding of normal insulin receptor tyrosine kinase activity in these subjects suggests that the site of defective insulin signalling is likely to be distal to the receptor. The unexpected finding that PC-1 activity, protein and mRNA were all dramatically reduced in patients with lesions early in the insulin signalling cascade provides further evidence for a link, albeit as yet poorly understood, between cellular insulin action and the expression of PC-1. Extreme insulin resistance occurs in a number of rare genetic disorders (Garvey & Birnbaum, 1993). To date, mutations in the gene encoding the insulin receptor remain the only well- defined cause of insulin resistance in man (Garvey & Birnbaum, 1993). Recently, an inhibitor of insulin receptor tyrosine kinase activity was isolated from the cultured dermal fibroblasts of a patient with severe insulin resistance who had normal insulin receptor structure (Sbraccia et al., 1991; Maddux et al., 1993). This inhibitor proved to be identical to a previously cloned Type II transmembrane protein of unknown function, called PC-1 (Maddux et al., 1995), a molecule which was originally discovered as an antigen on myeloma cells having multiple biochemical activities but no proven physiological function (Takahashi et al., 1970; Uriarte et al., 1995). Maddux et al. (1995) detected increased PC-1 activity in fibroblasts from seven of nine typical NIDDM subjects and overexpression of PC-1 in MCF-7 cells resulted in a reduction of insulin receptor tyrosine kinase activity in these cells. Thus, it was concluded 65  1997 Blackwell Science Ltd Correspondence: Professor Stephen O’Rahilly, University of Cambridge, Department of Medicine and Clinical Biochemistry, Addenbrooke’s Hospital, Cambridge, CB2 2QR, UK, Fax: 01223 330598