EXPRESSION OF HUMAN MUTS HOMOLOG-2 (HMSH2) PROTEIN IN RESTING AND PROLIFERATING CELLS

The hMSH2 protein plays an important role in the DNA mismatch repair s ystem. Since this system is involved in the correction of errors that occur during DNA replication, we studied the expression of hMSH2 prote in in resting and DNA-replicating cells, as well as through the cell c ycle in cell types with different growth characteristics. Using Wester n blot analysis, we showed that hMSH2 protein was detectable in restin g peripheral blood lymphocytes and thymocytes. However, when these cel ls were induced to proliferate, the protein level increased at least 1 2-fold. In cell-cycle dependent expression studies we chose two DNA mi smatch repair proficient cell lines (HEL and HeLa-S3), and flow cytome try was used to monitor cell-cycle progression. At every phase in the cell cycle, the steady-state level of hMSH2 was higher than in resting lymphocytes or thymocytes, and only minor variations of expression le vel were observed through the cell cycle. In particular, a two to four fold decrease in hMSH2 expression occurred at G(1) in HEL and at early S phase in HeLa-S3, but higher expression levels resumed during the r eplicative and postreplicative phases of the cell cycle. Interestingly , hMSH2 protein expression decreased fourfold when HEL cells were indu ced to differentiate along the megakaryocyte lineage, when continuous DNA replication occurs without mitosis. These results suggest that a b asal level of hMSH2 protein expression is necessary for resting and di fferentiated cells, and that increased hMSH2 protein expression is req uired when DNA replication is activated and followed by mitosis.