Involvement of integrins α3β1 and α5β1 and glycoprotein IIb in megakaryocyte‐induced osteoblast proliferation

Skeletal fragility has emerged as a major limitation to quality of life as we age. Osteoporosis and the ensuing hip, wrist, and vertebral fractures are significant sources of morbidity and pain among the elderly: such a fracture can be the sentinel event that transforms a relatively healthy, independent senior citizen into a person requiring significant assistance for daily living. This downward spiral is evidenced by a one-year post-hip fracture mortality of 24 percent (National Osteoporosis Foundation). As the prevalence of osteoporosis is expected to increase over the next few decades, the development of novel therapeutic strategies to combat this disorder becomes clinically imperative. These efforts draw extensively from an expanding body of knowledge pertaining to the physiologic mechanisms of skeletal homeostasis. To this body of knowledge, we contribute that cells of hematopoietic lineage may play a crucial role in balancing osteoblastic bone formation against osteoclastic resorption. Over the past decade, a new paradigm has emerged wherein MKs have been found to play an important role in skeletal homeostasis. In brief, data demonstrate that MKs may act to stimulate bone formation by expressing/secreting bone-related proteins, and by directly enhancing OB proliferation and differentiation (Thiede et al., 1994; Kelm et al., 1992; Breton-Gorius et al., 1992; Chenu and Delmas, 1992; Frank et al., 1993; Sipe et al., 2004; Kacena et al., 2004; Ciovacco et al., 2009; Miao et al., 2004; Bord et al., 2005; Ciovacco et al., in press). Simultaneously, MKs may regulate bone resorption by expressing/secreting several factors known to be involved in osteoclastogenesis, and recent studies demonstrate that MKs can inhibit osteoclast (OC) formation in vitro (Ciovacco et al., in press; Bord et al., 2003; Bord et al., 2004; Beeton et al., 2006; Pearse et al., 2001; Chagraoui et al., 2003; Kartsogiannis et al., 1999; Jiang et al., 1994; Soslau et al., 1997; Wickenhauser et al., 1995a; Wickenhauser et al., 1995b; Kacena et al., 2006). Specifically, our laboratory has demonstrated that MKs induce OB activation in vitro via a mechanism(s) requiring direct physical contact between the two cell types (Kacena et al., 2004), whereas MKs inhibit OC development in vitro via the elaboration of an as-yet unidentified soluble factor(s) (Kacena et al., 2006). The net result, as demonstrated in vivo, is that increases in MK number can lead to concomitant increases in bone mass (Kacena et al., 2004; Kacena et al., 2005; Suva et al., 2008; Frey et al., 1998a; Frey et al., 1998b; Yan et al., 1996; Yan et al., 1995; Villeval et al., 1997). In the present study, we have focused our efforts on characterization of the contact-dependent mechanism(s) by which MKs induce OB proliferation/differentiation. To this end, we have effectively neutralized several adhesion molecules known to function in the analogous interaction of MKs with another cell-type of mesenchymal origin - the FB (Wickenhauser et al., 2000; Schmitz et al., 1998). Furthermore, we have explored the effect of IL-3 on our MK-OB model system, as this cytokine has been shown to enhance MK-induced FB activation in vitro (Schmitz et al., 1999; Schmitz et al., 1995). Here we examine these new data which may offer insight as to the mechanism(s) of this interaction.