Ectopic expression of an A-type lamin does not interfere with differentiation of lamin A-negative embryonal carcinoma...

The nuclear lamina is an intermediate filament-type network underlying the inner nuclear membrane. It is believed to be important for nuclear envelope integrity and the organization of interphase chromatin. On the basis of biochemical properties and sequence criteria, vertebrate lamin proteins are classified as either A- or B-type. While B-type lamins are expressed in almost all cell types, no A-type lamins are present in early vertebrate embryos or undifferentiated embryonal carcinoma cell lines. Intriguingly, expression of A-type lamins occurs concomitant with cell differentiation and embryonic development. These findings have led to the hypothesis that A-type lamins might play a role in establishing or stabilizing cell-type specific differences in nuclear organization, which in turn might relate to the developmental potential of a cell. To test this hypothesis, we have stably expressed chicken lamin A in undifferentiated murine embryonal carcinoma (P19) cells, and examined the consequences of ectopic lamin A expression for the differentiation state and potential of these cells. Our results demonstrate that the P19 cells, although normally devoid of lamin A, properly incorporate and process chicken lamin A. Moreover, the stably transfected cell lines maintain the properties of undifferentiated cells, demonstrating that expression of lamin A does not directly induce differentiation. Conversely, when exposed to retinoic acid, an inducer of differentiation, lamin A-expressing P19 cells are able to differentiate normally. Taken together, our results suggest that unscheduled expression of A-type lamins is not sufficient to deregulate cell differentiation programs. The implications of these findings for the possible role for lamin A expression during development are discussed.