LAMININ VARIANTS AND INTEGRIN LAMININ RECEPTORS IN DEVELOPING AND ADULT HUMAN SMOOTH-MUSCLE

Distribution of laminin variant chains and of laminin-binding β1- associated α integrin subunits was studied in human arterial and visceral smooth muscle during development using the immunofluorescence method. In aortic media of 10-week-old fetuses (early fetal stage of development), B1, B2, and A chains were found, and at 27 weeks of gestation (late fetal step), in addition, the S chain was reviewed. In the major part of adult aortic media, staining for the β1 chain was substituted by staining for the S chain. A reasonable explanation for this is that the basement membrane of arterial SMCs contains the A-B1-B2 laminin trimer at the early fetal stage of development, a mixture of A-B1-B2 and A-S-B2 variants during the late fetal period, and the A-S-B2 form in adult media. In the intimal thickening of adult arteries, staining for A and S chains was decreased, and the B1 chain was detected, thus suggesting coexistence of A-B1-B2 and A-S-B2 variants similar to the fetal aorta. These data led us to propose that maturation of arterial SMCs is accompanied by a switch from expression of A-B1-B2 to A-S- B2 laminin, the only protein variant associated with the differentiated SMC phenotype. The M chain was not detected in fetal and adult arterial smooth muscle. In differentiated visceral SMCs from adult colon, in contrast to arterial medial SMCs, B1, S, B2, and A chains, i.e., both B1 chain- and S chain-containing laminin molecules, were present. Similar to vascular smooth muscle, the expression of A and S laminin chains in SMCs of the intestine was developmentally regulated. of the four α1-associated α integrin subunits that have been shown to recognize laminin (α1, α2, α3, and α6), only α1 was revealed during the early fetal stage of development in smooth muscle of the arteries and digestive tract. α3 appeared in aortic media only during the late fetal period. In adult arterial media and smooth muscle of the colon, α1 and α3 were the major potential laminin-binding integrins. The spatial-temporal distribution of laminin variants in developing and adult smooth muscle suggests that a variant laminin (possibly A-S-B2) interacting with its receptor (α1α1 or α3α1) may be important for maintenance of the differentiated SMC phenotype in vivo. © 1993 by Academic Press, Inc.