CpG island methylation and promoter usage in the parathyroid hormone-related protein gene of cultured lung cells

PTHrP in differentiating human mesenchymal stem cells: transcript isoform expression, promoter methylation, and protein accumulation

Human PTHrP gene displays a complex organization with nine exons producing diverse mRNA variants due to alternative splicing at 5' and 3' ends and the existence of three different transcriptional promoters (P1, P2 and P3), two of which (P2 and P3) contain CpG islands. It is known that the expression of PTHrP isoforms may be differentially regulated in a developmental stage- and tissue-specific manner. To search for novel molecular markers of stemness/differentiation, here we have examined isoform expression in fat-derived mesenchymal stem cells both maintained in stem conditions and induced toward adipo- and osteogenesis. In addition, the expression of the splicing isoforms derived from P2 and P3 promoters was correlated to the state of methylation of the latter. Moreover, we also performed a quantitative evaluation of intracellular and secreted PTHrP protein product in undifferentiated stem cells and in parallel cultures at various differentiation stages. The data obtained indicate that from the stemness condition to that of osteo- and adipo-genic differentiated cells, the expression of isoforms becomes increasingly selective, thereby being a potential gene signature for the monitoring of cell stem or committed/differentiating state and that the switching-off of PTHrP isoform expression is mostly promoter methylation-dependent. Moreover, PTHrP intracellular retention is down-regulated in osteo-differentiating cells whereas the secretion of the protein in the extracellular medium is up-regulated with respect to stem cells, thereby suggesting that these variations of the intracellular and extracellular levels of PTHrP could potentially be enclosed in the list of the available protein signature of osteogenic differentiation.

Increased upstream methylation has no influence on the overexpression of the parathyroid hormone-related protein gene in squamous cell carcinoma of the lung

Humoral hypercalcaemia of malignancy (HHM) commonly results from the excessive production of a parathyroid hormone-related protein (PTHrP) by tumours. We have previously shown malignancy is associated with increased DNA methylation in the 5' region of the PTHrP gene. In a series of patients with lung carcinoma and relatively high serum calcium levels, 3 patients showed substantially increased PTHrP gene methylation while 5 patients showed no change in methylation status in this region. Patients showed marked tumour-specific expression of PTHrP through the P1 and P3 promoters with more general tumour and non-tumour expression through the P2 promoter. The lack of potential key regulatory CpG sites in the P1 promoter and the complete demethylation in the P2 and P3 promoters suggests methylation does not influence tumour-specific expression of PTHrP. Although demethylation may be a prerequisite for P2 and P3 expression, the overexpression of the PTHrP gene in cancer cells must be mediated through mechanisms other than DNA methylation.