Wound healing involves reepithelialization, granulation tissue formation and contraction (reviewed in refs. [1
]). Reepithelialization and granulation tissue formation in turn involve proliferation and migration of cells from the wound edge to fill the wound site. In higher vertebrate adult animals, reepithelialization of a wound begins within hours after injury while granulation tissue begins to form approximately 4 days after injury. Besides fibroblasts, granulation tissue consists of macrophages, blood vessels, and extracellular matrix (ECM) produced by fibroblasts. Once the wound is filled with granulation tissue and covered with a neoepidermis, a portion of the wound fibroblasts transforms into myofibroblasts, which contract the wound to reduce and strengthen the defect. Myofibroblasts are rich in F-actin bundles, which establish cell-cell and cell-matrix linkages and thus generate the force for wound contraction.
SPARC (secreted protein acidic and rich in cysteine) is an extracellular glycoprotein expressed in elevated levels in actively proliferating cells and organs such as developing embryos and adult tissues associated with remodeling (bone and gut), wound healing, angiogenesis (reviewed in refs. [3
]), and tumorigenesis [5
]. SPARC belongs to a class of recently denoted matricellular proteins that are involved in cell-ECM interactions. It binds to ECM proteins and interacts with cells. SPARC has been postulated to modulate cell shape [3
] and cell adhesions [4
], to regulate cell proliferation [3
] and migration [9
], and to influence matrix contraction [11
]. In addition, SPARC is known to bind to certain growth factors [12
] and to modulate expression of ECM components [13
]. Many of these functions are requisite to wound repair.
To further investigate the role of SPARC in wound healing, we used cells from our recently generated SPARC-null mice [14
]. As we reported previously SPARC plays a role in regulating expression of growth factors, growth factor receptors, and ECM proteins and in modulating cell proliferation. In the absence of SPARC, expression of transforming growth factor-β1 and type I collagen is down-regulated in kidney mesangial cells [15
]. The expression of cell surface receptor for insulin-like growth factor I in SPARC-null embryonic fibroblasts is also diminished [16
]. Studies with SPARC-null cells have confirmed that SPARC functions in cell proliferation. Mesangial cells, adult skin fibroblasts and smooth muscle cells from SPARC-null mice proliferate faster than their respective wild-type counterparts in media containing fetal bovine serum (FBS) [17
]. Although embryonic fibroblasts from the SPARC-null mice grow at the same rate as the wild-type cells when they are cultured in FBS, the SPARC-null embryonic fibroblasts exhibit down-regulated proliferation as compared to the wild-type cells when the fibroblasts are cultured in serum-free medium containing insulin as the sole mitogenic factor [16
]. Thus, it appears that the role of SPARC in cell proliferation is cell type and environment dependent.
Although SPARC is expressed in fibroblasts and macrophages at wound sites [18
] and functions in many processes important to wound repair, the role of SPARC in wound healing in a whole animal has not yet been studied. Previously, studies of phenotypes in this laboratory have shown that SPARC-null mice develop osteopenia around 2.5 months of age [19
] in bones where SPARC is normally the most abundant non-collagenous glycoprotein [3
]. The second phenotype thus far uncovered is in the lens where SPARC is normally produced by lens epithelial cells [20
] and is a component of the lens capsule [20
]. SPARC-null mice show progressive cataract formation beginning approximately 1.5 months after birth [14
]. Electron microscopy of the SPARC-null lens revealed abnormality at the lens cell-ECM (capsule) interface with an intrusion of cellular processes into the basement membrane of the lens capsule, whereas wild-type lens exhibited a precise border at the cell-matrix interface [23
]. In the present study, we show for the first time that SPARC plays a role in wound repair in vivo and in vitro. Wound healing in SPARC-null mice is retarded due to delayed granulation tissue formation. Repair of a wound in vitro by dermal fibroblasts is impaired due to defect in cell migration.