HUMAN SMOOTH-MUSCLE MYOSIN HEAVY-CHAIN ISOFORMS AS MOLECULAR MARKERS FOR VASCULAR DEVELOPMENT AND ATHEROSCLEROSIS

Smooth muscle myosin heavy chains (MHCs) exist in multiple isoforms. R abbit smooth muscles contain at least three types of MHC isoforms: SM1 (204 kD), SM2 (200 kD), and SMemb (200 kD). SMI and SM2 are specific to smooth muscles, but SMemb is a nonmuscle-type MHC abundantly expres sed in the embryonic aorta. We recently reported that these three MHC isoforms are differentially expressed in rabbit during normal vascular development and in experimental arteriosclerosis and atherosclerosis. The purpose of this study was to clarify whether expression of human smooth muscle MHC isoforms is regulated in developing arteries and in atherosclerotic lesions. To accomplish this, we have isolated and char acterized three cDNA clones from human smooth muscle: SMHC94 (SM1), SM HC93 (SM2), and HSME6 (SMemb). The expression of SM2 mRNA in the fetal aorta was significantly lower as compared with SM1 mRNA, but the rati o of SM2 to SM1 mRNA was increased after birth. SMemb mRNA in the aort a was decreased after birth but appeared to be increased in the aged. To further examine the MHC expression at the histological level, we ha ve developed three antibodies against human SM1, SM2, and SMemb using the isoform-specific sequences of the carboxyl terminal end. Immunohis tologically, SM1 was constitutively positive from the fetal stage to a dulthood in the apparently normal media of the aorta and coronary arte ries, whereas SM2 was negative in fetal arteries of the early gestatio nal stage. In human, unlike rabbit, aorta or coronary arteries, SMemb was detected even in the adult. However, smaller-sized arteries, like the vasa vasorum of the aorta or intramyocardial coronary arterioles, were negative for SMemb. Diffuse intimal thickening in the major coron ary arteries was found to be composed of smooth muscles, reacting equa lly to three antibodies for MHC isoforms, but reactivities with anti-S M2 antibody were reduced with aging. With progression of atheroscleros is, intimal smooth muscles diminished the expression of not only SM2 b ut also SMI, whereas alpha-smooth muscle actin was well preserved. We conclude from these results that smooth muscle MHC isoforms are import ant molecular markers for studying human vascular smooth muscle cell d ifferentiation as well as the cellular mechanisms of atherosclerosis.