Seeing through the smokescreen: Proteins implicated in bone loss induced by smoking

Smoking and bone loss

Smoking causes a number of diseases like lung cancer, cardiovascular disease and emphysema, which are brought on by some of the 7000 chemicals known to be present in cigarette smoke, the latest count issued by the US National Cancer Institute. Many other diseases have been linked to smoking, such as osteoporosis, in which the mass of bones in the body is reduced, leading to a higher risk of fractures.

A number of studies have shown that bone loss is increased in smokers due to a shift in the balance between bone formation and bone resorption but the molecular mechanism behind this shift remains a mystery. One clue lies with the gene associated with the low-density lipoprotein receptor related protein (Lrp5). People with a single amino acid mutation of this gene, in which glycine-171 is replaced by valine, have high bone mass. Conversely, loss of activity of Lrp5 leads to diseases associated with osteoporosis.

This observation has led scientists in the USA to use this mutation to see if it induces any changes in the proteins that are expressed in bone marrow cells of mice. Gary Guishan Xiao and colleagues from Creighton University, Omaha, NE, compared the protein expressions of mice with three variations of the gene and also looked at the proteins in human smokers to see if the results from mice were reflected there.

Proteins promote bone loss

Wild-type mice, mice with the Lrp5 gene knocked out, and mice with the glycine-valine mutation in the Lrp5 gene were exposed to smoke for up to 12 weeks during which the bone marrow cells were collected. The proteins present were separated by two-dimensional gel electrophoresis and their abundances were measured after staining with dye.

A total of 38 proteins with different abundances between the three sets of mice were selected for identification by mass spectrometry. Many of these proteins were expressed in different quantities in the wild-type mice compared with the other two genotypes, becoming either more or less abundant. Some of these are known to be associated with osteoclasts, which are cells that break down bone tissue in the resorption process, as opposed to osteoblasts which promote bone formation.

Two proteins caught the eye in particular. They are S100A8 and S100A9, which are phosphorylated, calcium-binding proteins that became more abundant during smoking in a time-dependent manner, regardless of the mouse genotype.

Two recent studies highlighted these two proteins, which coexist in a complex with each other, as being responsible for increased bone resorption by osteoclasts. A further study had shown that S100A8 increases bone resorption in arthritis.

So, the research team examined the monocytes from the blood of smokers and non-smokers. Monocytes are a type of white blood cell produced by the bone marrow, so should reflect any smoking-induced protein changes. Sure enough, both S100A8 and S100A9 were more abundant in the heavy smokers compared with non-smokers, suggesting a role in the response to smoke.

The team continued to test the mice after smoke exposure was halted and found that the two proteins were reduced in abundance for the wild-type and the Lrp5 knock-out mice but remained high for the glycine-valine mutant mice. So bone resorption might continue in this type after the cessation of smoking. Having identified these proteins implicated in the response to smoking, the next step is to determine their functions in the bone remodelling process in order to fully understand the role that they play.