Isolation and characterization of heparin-binding growth factors in human leiomyomas and normal myometrium

Uterine leiomyomas (fibroids) are benign, smooth muscle cell (SMC) tumors of the myometrium containing abundant extracellular matrix (ECM). Heparin-binding growth factors present in leiomyoma and normal myometrial fresh tissue were isolated using heparin-affinity fast protein liquid chromatography. Purification of these growth factors was monitored by the stimulation of [3H]thymidine incorporation into BALBc-3T3 cells and myometrial SMC. Western blot analysis confirmed that two consistent peaks of growth factor activity (eluting at 0.5 M NaCl and 1.7 M NaCl) were platelet-derived growth factor (PDGF), 31 kDa, and basic fibroblast growth factor (bFGF), 18 kDa, respectively. Northern blot analysis of leiomyoma and myometrial tissue revealed three RNA transcripts (2.8, 2.3, and 1.9 kb) for PDGF-A chain, one RNA transcript (4.0 kb) for PDGF-B chain, and two RNA transcripts (3.7 and 3.5 kb) for bFGF. RNase protection assay showed elevated expression of the bFGF mRNA transcript in leiomyomas in 3 out of 5 patients. Immunoperoxidase staining of paraffin-embedded tissue showed that PDGF was predominantly intracellular in both vascular and myometrial SMC. Basic FGF, by contrast, was found primarily bound to the ECM of myometrium and fibroids. Leiomyomas showed much stronger staining for bFGF due to the large areas of ECM in these tumors. A third mitogenic peak eluting at 1.1 M NaCl was also seen in both myometrial and leiomyoma tissue. This peak was not definitively identified by Western blotting. However, Northern analysis for heparin binding-epidermal growth factor (HBEGF), which also elutes at 1.1 M NaCl, detected one RNA transcript for HBEGF (2.5 kb) in normal myometrium but little or no expression in the corresponding leiomyoma tissue. Immunoperoxidase staining showed that HBEGF was a cell-membrane-associated protein in both normal myometrial and leiomyoma SMC with more intense staining in normal myometrium. These results show that both leiomyomas and myometrium synthesize a number of heparin-binding growth factors. The enhanced growth of leiomyomas may be due, in part, to the presence of large quantities of bFGF that are stored in the ECM of these tumors. In addition, the level of HBEGF mRNA declines during the transformation of myometrial SMC into leiomyomas.