DEVELOPMENTAL REGULATION OF PROENKEPHALIN GENE-EXPRESSION IN OSTEOBLASTS

Proenkephalin (PENK), a classically defined opioid gene, was originall y thought to be expressed almost exclusively in the mature nervous and neuroendocrine systems. In the last few years, it was demonstrated, h owever, that high levels of PENK messenger RNA and PENK-derived peptid es are expressed in embryonic mesenchymal tissues during differentiati on into mature tissues and organs. Shortly after birth, as development progresses, PENK expression drops in those tissues to undetectable le vels. Very little is known about the molecular mechanisms regulating t his transient expression. To investigate those mechanisms, we used pri mary cell cultures of calvaria-derived osteoblasts. These cultures exp ress PENK and exhibit a normal pattern of osteoblastic differentiation . In the present study we demonstrate that 1) a reciprocal interrelati onship exists between PENK expression and osteoblastic differentiation in vivo, ex vivo, and in vitro; namely, PENK expression is down-regul ated upon cellular differentiation; 2) PENK promoter usage and messeng er RNA splicing function similarly in osteoblasts and in neural cells; 3) osteoblastic PENK expression is modulated by bone-targeting hormon es; and 4) this down-regulation is inhibited by the serine/threonine k inase inhibitor H-8. The link between osteoblastic differentiation and down-regulation of PENK expression together with our preliminary find ings indicating the existence of an osteoblastic opioid receptor sugge st that opioids act in an autocrine/paracrine mechanism on undifferent iated osteoblasts and play a significant role in bone development.