Homocysteine Induces Smooth Muscle Cell Proliferation Through Differential Regulation of Cyclins A and D1 Expression

The mechanism of homocysteine-induced cell proliferation in human vascular smooth muscle cells (SMCs) remains unclear. We investigated the molecular mechanisms by which homocysteine affects the expression of cyclins A and D1 in human umbilical artery SMCs (HUASMCs). Homocysteine treatment induced proliferation of HUASMCs and increased the expression levels of cyclins A and D1. Knocking down either cyclin A or cyclin D1 by small interfering RNA (siRNA) inhibited homocysteine-induced cell proliferation. Furthermore, treatment with extracellular signal-related kinase (ERK) inhibitor (PD98059) and dominant negative Ras (RasN17) abolished homocysteine-induced cyclin A expression; and treatment with phosphatidylinositol 3-kinase (PI3K) inhibitor (LY294002) and mammalian target of rapamycin (mTOR) inhibitor (rapamycin) attenuated the homocysteine-induced cyclin D1 expression. Homocysteine also induced transient phosphorylation of ERK, Akt, and p70 ribosomal S6 kinase (p70S6K). Neutralizing antibody and siRNA for beta 1 integrin blocked cell proliferation, expression of cyclins A and D1, and phosphorylation of ERK and Akt. In conclusion, homocysteine-induced differential activation of Ras/ERK and PI3K/Akt/p70S6K signaling pathways and consequent expression of cyclins A and D1 are dependent on beta 1 integrin. Homocysteine may accelerate progression of atherosclerotic lesions by promoting SMC proliferation. J. Cell. Physiol. 226: 1017-1026, 2011. (C) 2010 Wiley-Liss, Inc.