HOXB7 overexpression promotes differentiation of C3H10T1/2 cells to smoothmuscle cells

The presence or Immature smooth muscle cells and ectopic tissues such as fu lly-formed bone in artherosclerotic lesions, mat result from recapitulation or embryonic mechanisms In the artery wail. We hypothesized that expressio n of homeobox genes Is triggered in atherogenesis and that these regulate p roliferation and differentiation iii multipotential progenitor cells along one or more specific lineages. We Identified expression of the homeobox gen e HOXB7 in clones or bovine aortic medial cells previously shown to be mult ipotent. HOXB7; was subsequently detected In human atherosclerotic plaques by RT-PCR and in situ hybridization. Expression was localized to areas adja cent to calcification and scattered in media and neointima, which may be re flective or a role in either osteoblastic or smooth muscle cell differentia tion. To differentiate between these possibilities, we overexpressed HOXB7 in C3H10T1/2 cells, a multipotent cell line able to differentiate into vasc ular smooth muscle cells (SMC), as well as osteogenic and chondrogenic line ages. Results showed that overexpression or HOXB7 increased proliferation 3 .5-fold, and induced an SMC-like cell morphology, in addition, expression o f the early SMC markers calponin and SM22 alpha increased 4-fold and;3-fold respectively by semi-quantitative KP-PCR. Expression of the intermediate S MC marker smooth muscle myosin heavy chain (SM-MHC) did not change. No Incr ease in osteogenic or chondrogenic differentiation was detected, neither in the C3H10T1/2 cells nor in ML cells, a bone marrow stromal cell line used to confirm this result. These findings suggest that HOXB7; plays a role in expansion of Immature cell populations or dedifferentiation of mature cells .