The novel SPARC family member SMOC-2 potentiates angiogenic growth factor activity.

SMOC-2 is a novel member of the SPARC family of matricellular proteins. The purpose of this study was to determine whether SMOC-2 can modulate angiogenic growth factor activity and angiogenesis. SMOC-2 was localized in the extracellular periphery of cultured human umbilical vein endothelial cells (HUVECs). Ectopically expressed SMOC-2 was also secreted into the tissue culture medium. In microarray profiling experiments, a recombinant SMOC-2 adenovirus induced the expression of transcripts required for cell cycle progression in HUVECs. Consistent with a growth-stimulatory role for SMOC-2, its overexpression stimulated DNA synthesis in a dose-dependent manner. Overexpressed SMOC-2 also synergized with vascular endothelial growth factor or with basic fibroblast growth factor to stimulate DNA synthesis. Ectopically expressed SMOC-2 stimulated formation of network-like structures as determined by in vitro matrigel angiogenesis assays. Fetal calf serum enhanced the stimulatory effect of overexpressed SMOC-2 in this assay. Conversely, small interference RNA directed toward SMOC-2 inhibited network formation and proliferation. The angiogenic activity of SMOC-2 was also examined in experimental mice by subdermal implantation of Matrigel plugs containing SMOC-2 adenovirus. SMOC-2 adenovirus induced a 3-fold increase in the number of cells invading Matrigel plugs when compared with a control adenoviral vector. Basic fibroblast growth factor and SMOC-2 elicited a synergistic effect on cell invasion. Taken together, our results demonstrate that SMOC-2 is a novel angiogenic factor that potentiates angiogenic effects of growth factors.

Functional linkage between the endoplasmic reticulum protein Hsp47 and procollagen expression in human vascular smooth muscle cells.

Hsp47 is a heat stress protein that interacts with procollagen in the lumen of the endoplasmic reticulum, which is vital for collagen elaboration and embryonic viability. The precise actions of Hsp47 remain unclear, however. To evaluate the effects of Hsp47 on collagen production we infected human vascular smooth muscle cells (SMCs) with a retrovirus containing Hsp47 cDNA. SMCs overexpressing Hsp47 secreted type I procollagen faster than SMCs transduced with empty vector, yielding a greater accumulation of pro alpha1(I) collagen in the extracellular milieu. Interestingly, the amount of intracellular pro alpha1(I) collagen was also increased. This was associated with an unexpected increase in the rate of pro alpha1(I) collagen chain synthesis and 2.5-fold increase in pro alpha1(I) collagen mRNA expression, without a change in fibronectin expression. This amplification of procollagen expression, synthesis, and secretion by Hsp47 imparted SMCs with an enhanced capacity to elaborate a fibrillar collagen network. The effects of Hsp47 were qualitatively distinct from, and independent of, those of ascorbate and the combination of both factors yielded an even more intricate fibril network. Given the in vitro impact of altered Hsp47 expression on procollagen production, we sought evidence for interindividual variability in Hsp47 expression and identified a common, single nucleotide polymorphism in the Hsp47 gene promoter among African Americans that significantly reduced promoter activity. Together, these findings indicate a novel means by which type I collagen production is regulated by the endoplasmic reticulum constituent, Hsp47, and suggest a potential basis for inherent differences in collagen production within the population.