Cellular Mechanism Through Which Parathyroid Hormone–Related Protein Induces Proliferation in Arterial Smooth Muscle Cells

Abstract

Abstract-Parathyroid hormone-related protein (PTHrP) is present in vascular smooth muscle (VSM), is markedly upregulated in response to arterial injury, is essential for normal VSM proliferation, and also markedly accentuates neointima formation following rat carotid angioplasty. PTHrP contains a nuclear localization signal (NLS) through which it enters the nucleus and leads to marked increases in retinoblastoma protein (pRb) phosphorylation and cell cycle progression. Our goal was to define key cell cycle molecules upstream of pRb that mediate cell cycle acceleration induced by PTHrP. The cyclin D/cdk-4,-6 system and its upstream regulators, the inhibitory kinases (INKs), are not appreciably influenced by PTHrP. In striking contrast, cyclin E/cdk-2 kinase activity is markedly increased by PTHrP, and this is a result of a specific, marked, PTHrP-induced proteasomal degradation of p27 kip1 . Adenoviral restoration of p27 kip1 fully reverses PTHrP-induced cell cycle progression, indicating that PTHrP mediates its cell cycle acceleration in VSM via p27 kip1 . In confirmation, adenoviral delivery of PTHrP to murine primary vascular smooth muscle cells (VSMCs) significantly decreases p27 kip1 expression and accelerates cell cycle progression. p27 kip1 is well known to be a central cell cycle regulatory molecule involved in both normal and pathological VSM proliferation and is a target of widely used drug-eluting stents. The current observations define a novel "PTHrP/p27 kip1 pathway" in the arterial wall and suggest that this pathway is important in normal arterial biology and a potential target for therapeutic manipulation of the arterial response to injury. Key Words: cell cycle progression Ⅲ p27Kip1/pRb Ⅲ proliferation Ⅲ PTHrP Ⅲ vascular smooth muscle cell proliferation P arathyroid hormone-related protein (PTHrP) was originally identified through the search for the humoral factor responsible for humoral hypercalcemia of malignancy.

The p27 Pathway Modulates the Regulation of Skeletal Growth and Osteoblastic Bone Formation by Parathyroid Hormone–Related Peptide

“…(18) Consequently, PTHrP inhibition of the expression of p27 can promote the proliferation of vascular smooth muscle. (17) Our studies show that a similar mechanism may occur in vivo with respect to skeletal growth and development and osteoblastic bone formation. Nevertheless, a limitation of our study is that the use of a global knockout of p27 and of a model of global ablation of the NLS and C-terminus of PTHrP does not preclude the fact that indirect regulation of PTHrP action may still occur in the p27-/-Pthrp KI mouse model we have used.…”

“…(17) In view of our finding of upregulated p27 in skeletal tissue of Pthrp KI mice, we next determined whether the cell cycle inhibitor p27…”

“…(16) PTHrP induces proliferation in arterial smooth muscle cells via a PTHrP/p27 KIP1 pathway in which PTHrPinduces proteasomal degradation of p27. (17) In contrast, mutant PTHrP devoid of the NLS markedly inhibits arterial smooth muscle cell cycle and neointima formation by upregulation of p27 KIP1 . (18) However, it is unclear whether the NLS and C terminus of PTHrP regulate bone formation via a p27 KIP1 pathway.…”

PTHrP promotes malignancy of human oral cancer cell downstream of the EGFR signaling

“…3G-J). Recently, it is reported that PTHrP translocates into the nucleus and functions in an intracrine manner, which is associated with altered cell-cycle progression and protection from apoptosis [15][16][17]. We noted nuclear localization of PTHrP in addition to a cytoplasmic signal; however, PTHrP may not participate in apoptotic pathways in OSCC, since we found no cleaved Caspase-3, -9, or PARP in cells deficient in PTHrP (data not shown).…”

Severe growth retardation and early lethality in mice lacking the nuclear localization sequence and C-terminus of PTH-related protein

“…The nuclear presence of PTHrP appears to trigger pRb phosphorylation and release of G 1 /S arrest and thereby cell cycle progression (21). In this setting, cyclin E/Cdk-2 kinase activity is markedly increased by PTHrP as a result of marked PTHrP-induced proteasomal degradation of p27 kip1 (47). In contrast, we show here that in Pthrp KI cells, one consequence of the absence of nuclear PTHrP action in vivo is increased p21 levels, which in turn would lead to inhibition of cyclin E/Cdk2 and cyclin D1/Cdk4/Cdk6 activities (shown to be reduced in our studies) and to cell-cycle arrest in G 1 phase.…”