Assistant Professor of Biochemistry & Molecular Biology
PO Box 980694
Richmond, VA 23298-0694
Email: mjbeckma@hsc.vcu.edu
Telephone: 804-628-0225
Education
- B.S., 1986, University of Steubenville
- M.S., 1990, Iowa State University
- Ph.D., 1993, Iowa State University
Post-Doc
- Dept. of Biochemistry, University of Wisconsin, '93-'97
- Dept. of Pathobiological Sciences, Wisconsin School of Veterinary Medicine, '97-'99
- Dept. of Surgical Sciences, Wisconsin School of Veterinary Medicine, '99-'00
Research
The focus of my program is two-fold, (1) renal vitamin D metabolism and (2) bone
resorptive mechanisms in periprosthetic osteolysis. In the first project, it is
known that vitamin D requires two in vivo hydroxylations to become the potent
hormone that regulates intestinal Ca absorption. After hydroxylation in the
liver at position 25 of the molecule, renal proximal tubular cells may add a
second hydroxyl at the 1- or 24-position of the molecule. PTH, in response to
low blood Ca, is the major stimulus for 1α-hydroxylase induction in the renal
proximal tubule. A diverse array of factors influence vitamin D metabolism by a
coordinately regulated fashion in the proximal kidney. These factors can be
broadly divided into two groups based upon whether they are associated with
nutritional and physiological changes, or are the result of disease or drug
treatments that affect the renal nephron. This distinction is significant
because the two groups are globally and inversely regulated, with the induction
of 1a-hydroxylation occurring under conditions of mineral deficiencies of Ca and
P, and by physiological stress (pregnancy, lactation, hypoglycemia). On the
other hand, damage to the renal nephron and most disease conditions that affect
vitamin D metabolism result in a decrease in 1a-hydroxylation. The hallmark of
renal vitamin D metabolism is reciprocal regulation of 1α-hydroxylase with
24-hydroxylase. Specifically, it has been hypothesized that regulated blood
levels of 24,25(OH)2D3 are important for additional hormonal function within the
vitamin D system. Other studies, however, demonstrate that the major function of
24-hydroxylation is to initiate the catabolic pathway for vitamin D inactivation
and excretion. PTH potently represses renal 24-hydroxylation by a mechanism that
is now thought to involve a decrease in 24-hydroxylase mRNA stability, which is
targeted at the 3'-noncoding domain. In contrast, PTH at the level of the
promoter controls the increase in 1α-hydroxylase gene expression. Both of these
effects of PTH are mediated by cAMP in the proximal renal cell. The significance
of this reciprocal regulation of key vitamin D hydroxylases in the proximal
tubule is evidenced by the fact that 1,25(OH)2D3 synthesis efficiently meets the
demands for calcium metabolism, and newly synthesized 1,25(OH)2D3 is a potent
stimulus for 24-hydroxylase induction in all extrarenal target tissues where VDR
is present. Therefore, the decrease in renal proximal VDR transcription ensures
that 1a-hydroxylation occurs unabated. Our laboratory is working to develop
appropriate in vitro and in vivo models as tools to study renal vitamin D
metabolism and the phenomenon of PTH-mediated down-regulation of VDR, identify
and characterize the DNA elements and factors involved in transcriptional
regulation of VDR in proximal renal cells, and elucidate the molecular mechanism
of PTH-mediated down-regulation of VDR in proximal renal cells.
Our second project focuses on the bone resorptive mechanism of prosthetic
osteolysis. Aseptic loosening in association with osteolysis is a major problem
in the field of total joint arthroplasty. It has long been recognized that an
interfacial membrane develops between bone and the surface of the prosthesis in
cases of aseptic loosening. It was mostly thought of as being fibrous in nature
until 1983, when Goldring et al. demonstrated that this membrane was
histologically quite similar to the synovial lining found in rheumatoid
arthritis. On a cellular level, this membrane consists of synovium-like lining
cells, macrophages, fibroblasts, and T-lymphocytes. A large volume of work has
focused on the biochemical production/interaction of the interfacial membrane
and its relationship to the presence of osteolysis. It is generally accepted
that membrane constituents and a cascade of osteolysis inducing cytokines result
from phagocytized particles resulting from component wear. A variety of
techniques including protein extraction, immunohistochemistry, and in situ
hybridization have been employed in our laboratory to study the individual
cytokine roles in osteolysis. Commonly implicated cytokines include IL-1β, IL-6,
PGE-2, and TNF-α. Also cited have been laminins, EGF, TGF-alpha, and RANKL. We
have focused on the fibroblast and have shown them to be involved in
osteoclastic differentiation as well as RANKL production. The IL-1b protein has
also been immunolocalized to the fibroblast cell. RANKL is stimulated by
1,25(OH)2D3 and parathyroid hormone in osteoblasts to recruit preosteoclasts and
form them into bone resorbing mature osteoclasts. That fact has led us to
hypothesize that fibroblasts in interfacial membrane express RANKL to the cell
surface and convert cells of myeloid lineage to osteoclast like cells. TRAP
staining has further confirmed the presence of osteoclast like cells in the
interfacial membrane near bone tissue. Based on these observations, the
experimental focus of this project is to cleanly isolate individual cellular
components of interfacial membranes in the hip and knee for further analysis
using magnetically activated cell sorting (MACS), and secondly, use microarray
analysis in studying all the RNA expressed in fibroblasts compared with
controls. Moreover, our laboratory has developed an in vitro bone cell culture
model to assess the interactions between isolated cells from the membrane in
order to fully elucidate the mechanism of bone resorption associated with
osteolysis, and to discover new treatment interventions.
Publications
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