To differentiate the effects of inhibition of specific small interfering RNA (siRNA) of SPARC (secreted protein, acidic and rich in cysteine) and siRNA of connective tissue growth factor (CTGF) in cultured human fibroblasts, and to identify potential interrelationships between SPARC and CTGF.

Fibroblasts from skin biopsy specimens of 2 normal individuals were transfected with siRNA of SPARC and siRNA of CTGF. The fibroblasts were stimulated with or without transforming growth factor β1 (TGFβ1) and examined by real‐time quantitative reverse transcription–polymerase chain reaction to determine the transcription levels of several extracellular matrix genes.

After exogenous TGFβ1 stimulation, both SPARC siRNA and CTGF siRNA showed a protective role against overexpression of collagen genes. Following TGFβ1 stimulation, SPARC siRNA–transfected fibroblasts showed a greater reduction in expression of the collagen genes compared with CTGF siRNA–transfected fibroblasts, as well as a significantly decreased expression of CTGF (P < 0.05). Using linear structure equations to quantitatively model a genetic network based on expression levels of each gene, a positive regulatory role of SPARC on CTGF, COL1A2, COL3A1, COL11A1, and TIMP3 was observed. However, the regulatory role of CTGF on SPARC appeared to be negative and very small, while the positive regulatory effects of CTGF on COL1A2, COL3A1, COL11A1, and TIMP3 were less than those of SPARC.

The results of this quantitative comparison support the hypothesis that in these cultured fibroblasts, the regulatory effects of SPARC on some major extracellular matrix structural components are greater than those of CTGF. In addition, SPARC appears to regulate CTGF in a predominantly positive manner, while CTGF may act as a negative feedback control on SPARC following TGFβ stimulation.