continuous pge2 leads to net bone loss

7/30/2019 Continuous PGE2 leads to net bone loss

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Continuous PGE2 leads to net bone loss while intermittent PGE2 leads to net

bone gain in lumbar vertebral bodies of adult female rats

X.Y. Tian a, Q. Zhang a, R. Zhao a, R.B. Setterberg a, Q.Q. Zeng b,S.J. Iturria b,c, Y.F. Ma b, W.S.S. Jee a,

aDivision of Radiobiology, D epartment of Radiology, University of Utah School of Medicine, 729 Arapeen Dr., Suite 2338, Salt Lake City, UT, 84108-121 8, USAb Department of Endocrinology, Musculoskeletal Research, Lilly Corporate Center, In dianapolis, IN, USA

c Global Statistical Sciences Discovery Statistics, Lilly Corporate Center, Indianapolis, IN, USA

Received 9 July 2007; revised 19 November 2007; accepted 12 December 2007

Available online 5 February 2008

Abstract

The present study examined the effects of continuous and intermittent PGE2 administration on the cancellous and cortical bone of lumbar

vertebral bodies (LVB) in female rats. Six-month-old SpragueDawley female rats were divided into 6 groups with 2 control groups and 1 or 3 mg

PGE2/kg given either continuously or intermittently for 21 days. Histomorphometric analyses were performed on the cancellous and cortical bone

of the fourth and fifth LVBs. Continuous PGE2 exposure led to bone catabolism while intermittent administration led to bone anabolism. Both

routes of administration stimulated bone remodeling, but the continuous PGE2 stimulated more than the intermittent route to expose more basic

multicellular units (BMUs) to the negative bone balance. The continuous PGE2 caused cancellous bone loss by stimulating bone resorption greater

than formation (i.e., negative bone balance) and shortening the formation period. It caused more cortical bone loss than gain, the magnitude of the

negative endocortical bone balance and increased intracortical porosity bone loss was greater than for periosteal bone gain. The anabolic effects of

intermittent PGE2 resulted from cancellous bone gain by positive bone balance from stimulated bone formation and shortened resorption period;

while cortical bone gain occurred from endocortical bone gain exceeding the decrease in periosteal bone and increased intracortical bone loss.

Lastly, a scheme to take advantage of the marked PGE2 stimulation of lumbar periosteal apposition in strengthening bone by converting it to an

anabolic agent was proposed.

2008 Elsevier Inc. All rights reserved.

Keywords: Continuous PGE2; Intermittent PGE2; Negative and positive; Endosteal bone balance; Periosteal expansion

Introduction

Numerous in vivo studies have identified prostaglandin ofthe E series (PGE) as a potent anabolic agent that stimulates

both modeling (i.e. formation drift on quiescent surface) and

remodeling-dependent (i.e. positive basic multicellular unit

[BMU] bone balance) bone gain when delivered intermittently

by daily subcutaneous injections [18]. However, there is

limited knowledge on the tissue and cellular level changes of

continuous PGE administration. There are several reports of

continuous infusion of PGE1 in human infants with congenital

heart disease in which PGE1 administration resulted in cortical

hyperostosis of the long bones due to periosteal bone formation

but lacking detailed histomorphometric analysis [9

11]. Onlyone pre-clinical study exists in which PGE2 was administered

by subcutaneous implantation of a controlled release pellet in

7-week-old female rats [12]. In this model, there was cancellous

bone loss but no other response. This route of administration

induced inflammation that led these investigators to conclude that

the pellet administration route does not provide a reliable method

of systemic PGE2 administration. This lack of literature indicates

a need for a pre-clinical study of the effect of continuous PGE2administration on lumbar vertebral bodies.

The current study was designed to compare the effects of

continuous PGE2 infusion in 6-month-old female rats to the

Bone 42 (2008) 914920www.elsevier.com/locate/bone

Corresponding author. Fax: +1 801 581 7008.

E-mail address: [email protected] (W.S.S. Jee).

8756-3282/$ - see front matter 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.bone.2007.12.228
mailto:[email protected]://dx.doi.org/10.1016/j.bone.2007.12.228http://dx.doi.org/10.1016/j.bone.2007.12.228mailto:[email protected]


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well-established effects of intermittent administration. Histo-morphometric analysis was performed on the lumbar vertebrae

to profile tissue and cellular changes of bone gain or loss. In

addition, we propose a scheme to take advantage of the fact that

continuous PGE2 stimulates periosteal apposition to increase

bone strength to convert continuous administration PGE2 to an

anabolic agent.

Materials and methods

Experimental protocol

Forty-eight, 6-month-old, female SpragueDawley rats (Harlan Sprague

Dawley Inc., Indianapolis, IN), weighing approximately 220 g, were maintainedon a 12-h light/12-h dark cycle at 22 C with ad libitum access to food (TD 5001

with 0.95% calcium and 0.67% phosphorus, vitamin D3 4500 IU/kg; Teklad,

Madison, WI) and water. Eight rats were sacrificed as the baseline+ 9 days and

aging controls, while the remaining rats were randomly divided into 4 groups

with eight rats in each group. Prostaglandin E2 (Upjohn Company, Kalamazoo,

MI) at 1 or 3 mg/kg/d was given to the rats by continuous infusion via Infu-

Diskpump (Med-E-Cell, San Diego, CA) or daily subcutaneous (sc) injection

on the back for 21 days. The Infu-Disks were connected to the jugular vein,

and placed on the back with a special jacket and changed every 7 days.

Prostaglandin E2 was first dissolved in ethanol then further diluted into the final

injection solution (10% ethanol with 1 ml/kg injection volume). All rats received

sc injections with Calcein 5 mg/kg (Sigma, St Louis, MO) on days 10, 9, 3 and 2

before sacrifice. The Institutional Animal Care and Use Committee of Lilly

Research Laboratories approved the animal protocol to ensure compliance with

the NIH animal care guidelines.

At necropsy, the fourth and fifth lumbar vertebrae were stained with

Villanueva osteochrome bone stain (Arizona Histology & Histomorphometry

Center, Phoenix, AZ, USA), and embedded in methyl methacrylate. Sawed and

ground 20-m longitudinal lumbar sections (LVB4), and 30-m cross lumbar

sections (LVB5) were prepared for analyses. Histomorphometric measurements

were acquired by using an Image Analysis System (Osteomeasure, Inc., Atlanta,

GA). Cancellous bone analysis was performed in the LVB4 longitudinal

sections, which included the spongiosa between 0.1 mm from the cranial and

caudal growth plates; while in the LVB5 cross sections analysis was limited tothe spongiosa of the body and the ventral cortices [7,1315].

The data gathered from the LVB4 and LVB5 sections were very similar so the

data were combined for analyses. All data were presented as a group mean

standard deviation (SD). The statistical analyses were performed using the

Ultimate Integrated Data Analysis and Presentation System (StatView 5.0.1,

SAS Institute Inc. Cary, NC, USA). Across group comparisons were made with

a parametric analysis of variance (ANOVA); pb0.05 was considered significant.

The one-sample KolmogorovSmirnov test for normality was applied to each

Table 1

Static and dynamic histomorphometric changes in the cancellous bone of LVB4 +LVB5

Groups %B.Ar (%) Tb.Wi (m) Tb.N (#/mm) Tb.Sp (m) %Er.Pm (%) %L.Pm (%) MAR (m/d) BFR/BS (m3/m2/d)

Baseline 33.27 5.74 73.44 7.51 4.53 0.69 151.66 33.56 1.14 0.39 32.92 5.13 1.00 0.08 32.98 6.46

Control 33.80 4.70 72.74 5.64 4.65 0.59 145.62 29.33 1.10 0.39 33.14 4.08 0.98 0.08 32.31 4.70

Continuous 1 mg 26.562.14a 60.274.62a 4.420.43 167.6819.09a 2.300.72a 31.21 3.19 0.98 0 .12 30.36 3.97

(

21) (

17) (

5) (15) (110) (

6) (0) (

6)Continuous 3 mg 25.963.19a 59.336.32a 4.390.43 170.5421.13a 2.860.75a 38.097.76ab 1.120.10ab 42.267.45ab

(23) (18) (6) (17) (161) (15) (15) (31)

Intermittent 1 mg 36.936.00bc 81.648.95abc 4.540.70 143.0431.45bc 1.000.25bc 45.128.37abc 1.150.11ab 51.6110.16abc

(9) (12) (2) (2) (9) (36) (17) (60)

Intermittent 3 mg 37.503.91abc 82.939.24abc 4.570.70 140.2326.49bc 1.200.36bc 50.092.75abcd 1.060.10ad 53.195.09abc

(11) (14) (2) (4) (9) (51) (9) (65)

MeanSD. % change from control in parentheses.

Note. pb0.05 vs. acontrol, bcontinuous 1 mg, ccontinuous 3 mg, dintermittent 1 mg. LVB4: Fourth lumbar vertebral body longitudinal section. LVB5: Fifth lumbar

vertebral body cross section. Continuous 1 mg and 3 mg: 1 and 3 mg PGE2/kg/d continuous infusion; intermittent 1 mg and 3 mg: 1 and 3 mg PGE2/kg/d daily sc

injection. %B.Ar: percent trabecular bone area, Tb.Wi: trabecular width, Tb.N: trabecular number, Tb.Sp: trabecular separation, %Er.Pm: percent eroded perimeter, %

L.Pm: percent mineralized perimeter, MAR: mineral apposition rate, BFR/BS: bone formation rate per unit of bone surface.

Table 2

Bone remodeling and bone balance changes in cancellous bone of LVB4+ LVB5

Groups BFR/BV (%/yr) W.Wi (m) FP (d) RP (d) Rm.P (d) Act.F (cycle/yr) %L.Pm BFR/BS

%Er.Pm %Er.Pm

Baseline 276.86 63.32 12.43 0.89 12.53 1.50 2.52 1.60 15.06 2.93 2.00 0.64 32.70 13.65 33.17 15.14Control 272.26 48.05 12.88 0.95 13.31 1.71 2.63 1.58 15.93 3.12 1.90 0.67 34.60 16.09 34.29 17.53

Continuous 1 mg 308.7748.42 12.580.91 13.132.39 3.341.69 16.473.96 2.820.79a 15.005.52a 14.735.84a

(13) (2) (1) (27) (3) (49) (57) (57)

Continuous 3 mg 433.17 53.84ab 12.761.22 11.511.83a 2. 64 0.74 14.15 2.29 4.00 0 .45ab 13.933.83a 15.514.00a

(59) (1) (14) (1) (11) (111) (60) (55)

Intermittent 1 mg 385.47 66.53abc 13.661.29 12.051.90 1.120.36abc 13.182.20ab 3.390.85ac 47.0911.50abc 54.2214.85abc

(42) (6) (9) (57) (17) (79) (36) (58)

Intermittent 3 mg 397.36 76.08ab 13.881.23abc 13.202.03c 1.690.67abcd 14.892.62 2.720.44acd 45.3013.26abc 48.9917.47abc

(46) (8) (1) (36) (7) (43) (31) (43)


Note. pb0.05 vs. acontrol, bcontinuous 1 mg, ccontinuous 3 mg, dintermittent 1 mg. LVB4: Fourth lumbar vertebral body longitudinal section. LVB5: Fifth lumbar

vertebral body cross section. Continuous 1 mg and 3 mg: 1 and 3 mg PGE2/kg/d continuous infusion; intermittent 1 mg and 3 mg: 1 and 3 mg PGE2/kg/d daily sc

injection. BFR/BV: bone formation rate per unit of bone volume; W.Wi: wall width. FP: formation period, RP: resorption period, Rm.P: remodeling period, Act.F:

activation frequency. %L.Pm/%Er.Pm: ratio of percent mineralized perimeter to percent eroded perimeter; BFR/BS/%Er.Pm: ratio of bone surface bone formation rateto percent eroded perimeter.

915X.Y. Tian et al. / Bone 42 (2008) 914920


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combination of response variables and measurement sites. In no instance was

there a statistically significant departure from a normal distribution (all p

valuesN0.05) [16].

Results

Significant changes ( pb0.05) in lumbar vertebral cancellous

bone

Continuous 1 mg PGE2/kg for 21 days decreased cancellous

bone (%B.Ar 21%) and architecture (Tb.Wi 17%, Tb.Sp

+15%) compared to aging control (Table 1). Its histomorpho-

metric profile between 11 and 21 days exhibited significantly

increased bone resorption (%Er.Pm + 110%; Table 1), bone

remodeling (Act.F +49%) resulting in a negative cancellous

bone balance (%L.Pm/%Er.Pm and BFR/BS/%Er.Pm of57%)

(Table 2).

Continuous 3 mg PGE2/kg significantly reduced cancellousbone (%B.Ar23%) and architecture (Tb.Wi 18% and Tb.Sp

+17%) (Table 1; Figs. 1B and 2B). The histomorphometric profile

consisted of significantly elevated bone resorption (%Er.Pm

+161%) greater than formation (%L.Pm + 15%, MAR + 15% and

BFR/BS +31%) (Table 1) that increased bone remodeling (Act.F

+111%) and turnover (BFR/BV +59%) accompanied by a de-

creased formation period (FP -14%), which contributed to the

negative bone balance (%L.Pm/%Er.Pm60% and BFR/BS/%Er.

Pm 55%) (Table 2).

In contrast, intermittent 1 mg PGE2/kg significantly increased

trabecular thickness only (Tb.Wi +12%; Table 1). Its histomor-

phometric profile included stimulated bone formation (%L.Pm+ 36%, MAR + 17% and BFR/BS +60%) (Table 1) that increased

bone remodeling (Act.F + 79%), turnover (BFR/BV +42%), and

decreased resorption and remodeling periods (RP 57% and Rm.

P 17%), which contributed to a positive cancellous bone

balance (%L.Pm/%Er.Pm +36% and BFR/BS/%Er.Pm +43%)

(Table 2).

Intermittent 3 mg PGE2/kg increased cancellous bone mass

(%B.Ar +11%), trabecular thickness (Tb.Wi +14%) (Table 1),

and wall width (W.Wi +8%; Table 2). Its histomorphometric

profile was made up of stimulated bone formation (%L.Pm

+51%, MAR +9% and BFR/BS+ 65%; Table 1; Figs. 1C and 2C)

that activated bone remodeling (Act.F + 43%) and turnover (BFR/

BV +46%), while decreasing resorption period (RP

36%),which contributed toward a positive cancellous bone balance

(%L.Pm/%Er.Pm +31% and BFR/%Er.Pm +43%) (Table 2).

Significant changes (pb0.05) in lumbar vertebral cortical bone

Continuous 1 mg PGE2/kg decreased cortical bone mass

(%Ct.B.Ar 17%, %Ma.Ar +10%, Ct.Th 18% and Ic-%

Fig. 1. Von Kossa stained cross sections of fifth lumbar vertebral body from (A) control, (B) continuous treatment of 3 mg PGE2/kg/d and (C) intermittent treatment of

3 mg PGE2/kg/d. Continuous treatment (B) reduced trabecular bone mass, trabecular thickness and cortical thickness; intermittent treatment (C) increased trabecular

bone mass and trabecular thickness.

Fig. 2. Villanueva bone stained cross sections of fifth lumbar vertebral body under fluorescent microscope from (A) control, (B) continuous 3 mg PGE2/kg/d and

(C) intermittent 3 mg PGE2/kg/d showing morphology and fluorochrome labeling. Continuous treatment (B) resulted in fewer and thinner trabeculae with increased

eroded perimeter and labeling and thinner cortical thickness but stimulated periosteal new bone formation (arrow) and increased intracortical porosity (asterisks).

Intermittent PGE2 treatment (C) caused increased and thicker trabeculation, abundant double labeled surfaces with formation greater than resorption, but withdecreased periosteal bone formation.

916 X.Y. Tian et al. / Bone 42 (2008) 914920


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Po.Ar +34%; Table 3). The endocortical bone loss fromincreased endocortical bone resorption and negative endo-

cortical bone balance (Ec-%Er.Pm + 111%, Ec-%L.Pm/%Er.

Pm 52%, and Ec-BFR/%Er.Pm 53%; Table 4) exceeded

the stimulated periosteal bone formation (Ps-MAR +54%

and Ps-BFR +95%; Table 3).

Continuous 3 mg PGE2/kg decreased cortical bone mass (%

Ct.B.Ar20%, %Ma.Ar +12%, Ct.Th 23%, and Ic-%Po.Ar

+97%; Table 3; Fig. 2B). Endocortical bone loss occurred by a

stimulated bone resorption (Ec-%Er.Pm +321%) and a negative

endocortical bone balance (Ec-%L.Pm/%Er.Pm 75% and Ec-

BFR/%Er.Pm 76%; Table 4) that exceeded the stimulated

periosteal bone formation (Ps-%L.Pm + 43%, MAR + 63% and

BFR +132%; Table 3).Intermittent 1 mg PGE2/kg lacked effect on mass but

increased intracortical porosity (Ic-%Po.Ar + 36%; Table 3).

The endocortical bone gain from stimulated endocortical bone

formation (Ec-%L.Pm +123%, MAR +37% and BFR +196%)

and positive endocortical bone balance (Ec-%L.Pm/%Er.Pm

+125% and Ec-BFR/%Er.Pm +200%) (Table 4) was offset by

increased intracortical porosity (Ic-%Po.Ar +36%) and

decreased periosteal bone formation (Ps-%L.Pm

36%, MAR52% and BFR68%; Table 3).

Intermittent 3 mg PGE2/kg also lacked effect on cortical

bone but did increase intracortical porosity (Ic-%Po.Ar + 63%;

Table 3; Fig. 2C). Again, the endocortical bone gain from

stimulated bone formation (Ec-%L.Pm +125%, MAR +36%

and BFR +200%; Table 4) was offset by the increased

intracortical porosity (Ic-%Po.Ar +63%) and by the decreased

periosteal bone formation (Ps-%L.Pm 25%, MAR58% and

BFR67%; Table 3).

Discussion

The net response of cancellous bone to continuous admin-istration of PGE2 was catabolic. In as little as 21 days, it induced

cancellous bone loss by stimulated bone turnover and shortened

formation and remodeling periods combined with negative

cancellous bone balance to yield an imbalance of resorption and

formation in favor of resorption. In cortical bone, the bone loss

was due to an imbalance of bone loss over gain. The loss from

negative endocortical bone balance and increased intracortical

Table 3

Static and dynamic histomorphometric and periosteal bone formation changes in cortical bone of LVB5

Groups T.Ar (mm2) %Ma.Ar (%) %Ct.B.Ar (%) Ct.Th (m) Ic-%Po.Ar (%) Ps-%L.Pm (%) Ps-MAR (m/d) Ps-BFR (m/d100)

Baseline 3.78 0.37 6 0.36 4.58 39.32 4.52 315.81 35.65 0.81 0.22 59.48 8.63 0.99 0.01 58.92 8.94

Control 3.45 0.14 60.82 1.30 38.86 1.26 305.95 7.21 0.84 0.19 54.21 11.61 0.90 0.12 49.47 14.31

Continuous 1 mg 3.640.39 67.19 4.84a 32.444.80a 252.2026.14a 1.130.16a 69.3313.07 1.390.08a 96.4921.05a

(6) (10) (

17) (

18) (34) (28) (54) (95)Continuous 3 mg 3.440.32 68.22 3.75a 31.253.65a 235.6235.99a 1.660.56ab 77.3810.08a 1.470.25a 114.9330.02a

(0) (12) (20) (23) (97) (43) (63) (132)

Intermittent 1 mg 3.620.76 58.43 2.74bc 41.102.75bc 285.6742.42c 1.140.18a 34.756.04abc 0.430.25abc 15.9510.13abc

(5) (4) (6) (7) (36) (36) (52) (68)

Intermittent 3 mg 3.650.44 59.44 0.86bc 39.731.19bc 295.7321.92bc 1.370.34a 42.982.62abcd 0.380.21abc 16.559.59abc

(6) (2) (2) (3) (63) (21) (58) (67)


Note. pb0.05 vs. acontrol, bcontinuous 1 mg, ccontinuous 3 mg, dintermittent 1 mg. LVB5: Fifth lumbar vertebral body cross section. Continuous 1 mg and 3 mg: 1

and 3 mg PGE2/kg/d continuous infusion; intermittent 1 mg and 3 mg: 1 and 3 mg PGE 2/kg/d daily sc injection. T.Ar: total tissue area, %Ma.Ar: percent marrow area,

Ct.Th: cortical thickness. Ic: Intracortical, Ic-%Po.Ar: percent intracortical porosity area, Ps-: Periosteal surface. Ps-%L.Pm: percent periosteal mineralized perimeter,

Ps-MAR: periosteal mineral apposition rate, Ps-BFR: bone formation rate per unit of periosteal surface.

Table 4

Endocortical surface histomorphometric changes in cortical bone of LVB5

Groups Ec-%Er.Pm (%) Ec-%L.Pm (mm) Ec-MAR (m/d) Ec-BFR (m/d100) Ec-%L.Pm/%Er.Pm Ec-BFR/%Er.Pm

Baseline 3.17 0.38 24.29 4.83 0.79 0.14 19.17 5.69 7.87 2.57 6.32 2.91Control 3.09 0.28 22.49 7.25 0.78 0.12 18.02 8.20 7.33 2.39 5.86 2.67

Continuous 1 mg 6.502.15a 20.85 3.78 0.79 0.05 16.57 3.30 3.53 1.36a 2.781.00a

(111) (7) (2) (8) (52) (53)

Continuous 3 mg 12.985.20ab 17.20 4.36 0.82 0.17 13.81 2.96 1.82 1.68a 1.401.06ab

(321) (24) (6) (23) (75) (76)

Intermittent 1 mg 3.060.35bc 50.074.43abc 1.060.05abc 53.295.31abc 16.521.95abc 17.592.20abc

(1) (123) (37) (196) (125) (200)

Intermittent 3 mg 3.410.35bc 50.696.23abc 1.060.11abc 54.1311.25abc 15.052.78abc 16.154.35abc

(11) (125) (36) (200) (105) (176)


Note. pb0.05 vs. acontrol, bcontinuous 1 mg, ccontinuous 3 mg, dintermittent 1 mg. LVB5: Fifth lumbar vertebral body cross section. Continuous 1 mg and 3 mg: 1

and 3 mg PGE2/kg/d continuous infusion; intermittent 1 mg and 3 mg: 1 and 3 mg PGE 2/kg/d daily sc injection. Ec-%Er.Pm: percent endocortical eroded perimeter,

Ec-%L.Pm: percent endocortical mineralized perimeter, Ec-MAR: endocortical mineral apposition rate, Ec-BFR: bone formation rate per unit of endocortical surface.

Ec-%L.Pm/%Er.Pm: ratio of percent mineralized perimeter to percent eroded perimeter of endocortical surface; Ec-BFR/%Er.Pm: ratio of bone surface bone formationrate to percent eroded perimeter of endocortical surface.

917X.Y. Tian et al. / Bone 42 (2008) 914920


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porosity exceeded the bone gain from stimulated periosteal

bone formation.

Alternatively, the influence of intermittent PGE2 for 21 days

on cancellous bone was mostly anabolic. Bone turnover and

remodeling were stimulated along with positive cancellous bone

balance and a shorter resorption period (the magnitude of the

formation phase is greater than that of the resorption phase). Inthe cortical bone, the intermittent treatment did not increase bone

mass because of insufficient treatment time to generate mea-

surable bone gain. However, it stimulated positive endocortical

bone balance-induced bone gain that was offset by increased

intracortical porosity and decreased periosteal apposition.

Both routes of administration increased bone turnover (BFR/

BV) and remodeling (activation frequency). Continuous PGE2administration was more efficacious than intermittent adminis-

tration with a continuous to intermittent ratio for bone turnover

of 1.3 and activation frequency of 2.6 (Table 2). This suggests

more BMUs were available to be subjected to the negative

endosteal bone balance to generate the reduced cancellous bonemass in 2 remodeling cycles. The lower relative activation fre-

quency with intermittent treatment exposed BMUs to positive

endosteal bone balance only was able to generate a non-sig-

nificant 9% increase with 1 mg and a significant 11% increase

with the 3 mg dose.

Factors contributing to the negative cancellous bone balance

induced by continuous PGE2 administration partially involved

the imbalance caused by stimulation of bone resorption (+161%)

greater than bone formation (+31%) coupled with a shorter

formation period (14%). The former was possibly due to the

imbalance in RANKL to osteoprotegerin ratios in favor of

RANKL [17] and the latter was due to increased osteoblastic

apoptosis [18]. In contrast, the intermittent PGE2-induced posi-tive cancellous bone balance was partly due to stimulated

cancellous bone formation (+65%) coupled with a shorter

resorption period (36%). The latter response may be due to

accelerated osteoclast apoptosis [18].

Factors contributing to the cortical bone loss induced by

continuous PGE2 was attributed mainly to stimulated endocor-

tical bone resorption causing a negative bone balance that offset

the stimulated bone formation the magnitude of endocortical

bone resorption (+ 321%) was greater than the periosteal bone

formation (+132%). In contrast, the lack of effect of intermittent

PGE2 was due to the depression of periosteal apposition and

increased intracortical bone porosity that offset the stimulatedendocortical bone formation (+200%), with its positive bone

balance of + 105% and + 176%.

Another effect of continuous PGE2 administration was the

robust stimulation of periosteal bone formation opposed to the

decrease with intermittent treatment. The latter was a surprise

since previous studies have shown intermittent PGE2 markedly

increased periosteal bone formation in the tibial shaft of male,

intact and ovariectomized female rats [2,3,6,1925]. One

possible explanation is that this is a site-specific response of

lumbar vertebral cortices since this is the only report to date for

this site. More studies are needed to confirm this. Regardless,

the powerful ability of continuous PGE2 to stimulate periosteal

bone formation should improve bone strength. It is generally

accepted that periosteal expansion is an effective mechanism to

dramatically improve vertebral bone strength and reduce

fracture risk [2630].

Both continuous and intermittent PGE2 stimulated intracor-

tical bone remodeling to increase intracortical porosity. The

continuous route was found to be 1.5 times more efficacious

than by intermittent PGE2 (Table 3). The intracortical porosityactivated by both routes of administration was localized near the

endocortical surface adjacent to the marrow [19,3133] with

little influence on bone strength. Pores known to be close to

endocortical surfaces have less effect on mechanical properties

than pores near the periosteal surfaces [34]. In addition, these

were transiently increased intracortical pores refilled on

cessation of treatment [21,35].

In our previous paper on the effects of continuous versus

intermittent PGE2 treatment on the proximal tibial metaphysis

and tibial shaft, we proposed a treatment pattern in which

continuous PGE2 treatment could become anabolic by co-

treating with an anti-catabolic agent to suppress continuousPGE2-induced bone resorption [25]. Briefly, the plan empha-

sizes the beneficial effect of periosteal expansion and conversion

of negative to positive endosteal bone balance (the magnitude of

bone formation phase greater than that of resorption phase), an

approach to add cortical bone to the lumbar vertebral body,

which intermittent PGE2 did not. In this scheme, we assumed

that combining continuous PGE2 with an anti-catabolic agent

would have a better effect than intermittent PGE2. The effects of

co-treatment with intermittent PGE2 and an anti-catabolic agent

have been shown to be equal to or better than the intermittent

PGE2 alone [2224,36,37]. Thus, we postulated that co-

treatment of continuous PGE2 and an anti-catabolic agent

would continue to increase periosteal modeling, refill endostealremodeling spaces and intracortical porosity, and initiate

positive endosteal bone balance. This would lead to an increase

in cancellous and cortical bone mass, architecture and bone

geometry to yield increased bone strength. At cessation of

treatment, there would be refilling of intracortical porosity and

remodeling spaces with maintenance of the increased periosteal

and cancellous bone mass furthering increase of bone strength

[21,35]. Whether this strategy employing continuous PTH to the

clinical relevant site, the lumbar vertebra, would increase bone

strength is worth pursuing.

There are two obvious limitations to the current study. One is

the lack of pre-treatment baseline groups. The animals labeledas baseline controls were sacrificed at 9 days into the study.

Without a true baseline group, we avoided comparing the

terminal changes with the baseline group to avoid making any

misinterpretation. Secondly, we did not attempt to quantify the

PGE2 activation of modeling and remodeling. Previously we

reported that much of the intermittent PGE2-induced bone

formation was driven more by modeling than by remodeling

bone gain [7,8,37]. Thus, it would be informative in the current

study if we were able to determine to what degree continuous

PGE2 differs from intermittent PGE2 in influencing modeling

and remodeling bone formation.

In summary, continuous PGE2 exposure led to bone catab-

olism while intermittent administration led to bone anabolism.

918 X.Y. Tian et al. / Bone 42 (2008) 914920


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Both routes of administration stimulated bone turnover and

remodeling to expose more BMUs to either negative or positive

endosteal bone balance. The net catabolic effect of continuous

administration was the result of the imbalance in bone gain and

loss in favor of bone loss. The magnitude of the increased

periosteal bone formation-induced bone gain was less than the

magnitude of the increased negative endosteal bone balance andintracortical porosity-induced bone loss. The anabolic effect of

intermittent administration was the result of imbalance in bone

gain over loss. The magnitude of the increased positive endos-

teal bone balance-induced gain was more than the magnitude of

the decreased periosteal bone expansion and increased intracor-

tical porosity. The principal effects of continuous administration

were the robust stimulation of bone resorption and periosteal

bone formation while that for intermittent administration was the

stimulation of endosteal bone formation. In addition, this is the

first study to show the lack of intermittent PGE2 stimulation of

periosteal apposition in lumbar vertebral bodies, while inter-

mittent PGE2 shortened the resorption period and continuousPGE2 shortened the formation period. Lastly, a plan to convert

continuous PGE2 into an anabolic agent with co-treatment with

an anti-catabolic agent to generate lumbar vertebral cortical bone

with increased bone strength was proposed, a much needed

approach to increase vertebral cortical bone mass and strength.

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