Bax and Bcl-xS are induced at the onset of apoptosis in involuting mammary epithelial cells

Abstract

Mammary gland involution is a physiological process in which the entire organ is remodeled through the process of apoptosis. Apoptosis of secretory alveolar cells is initiated at the time of weaning, followed by the collapse and disappearance of the entire lobuloalveolar compartment. While apoptotic figures were rare in mammary epithelium of lactating mice, their number increased after weaning and reached a maximum on day 3 of involution. Active cell death continued until day 5 after weaning and only little parenchyma remained on day 8, when remodeling of the gland was completed. Bax mRNA levels increased during the first day of involution independent of the presence or absence of p53. Bax protein was detected in an increasing number of cells after weaning, peaking at day 3 and decreasing thereafter. Low levels of bcl-x mRNA and protein were present during lactation, followed by a sharp increase during the first 2 days of involution. The bcl-xS splice variant of bcl-x can promote cell death, and bcl-xL has a protective function in cell culture. The ratio of bcl-xS versus bcl-xL remained stable in the virgin, pregnant and lactating gland. However, during the first 2 days of involution, bcl-xS expression increased six-fold more than bcl-xL. To further evaluate the role of Bcl-xS which was less abundant in the mammary cells than Bcl-xL, cotransfection studies were performed in cell culture. They confirmed that Bcl-xS protein can facilitate apoptosis even when Bcl-xL is present in excess. These findings point to a significant role for Bax and Bcl-xS in the regulation of apoptosis of secretory alveolar cells during involution.

Overexpression of Bcl-2 inhibits alveolar cell apoptosis during involution and accelerates c-myc-induced tumorigenesis of the mammary gland in transgenic mice

“…It is thought that their relative amounts determine whether a cell will die or survive and that homo-and heterodimerization between the family WAP-bcl-2 transgenic mice R Ja Èger et al members represents the physical basis of their apoptosis-modulatory potential (Zha et al, 1996). Upregulation of bcl-x S and Bax has been observed during mammary gland involution (Heermeier et al, 1996). During the ®rst 2 days of involution a sixfold increase in the ratio of bcl-x S to bcl-x L RNA was found; the number of Bax expressing cells peaked at day 3 of involution, coincident with a peak in the number of apoptotic cells, and some of the Bax expressing cells showed characteristic apoptotic morphology (Heermeier et al, 1996).…”

“…Upregulation of bcl-x S and Bax has been observed during mammary gland involution (Heermeier et al, 1996). During the ®rst 2 days of involution a sixfold increase in the ratio of bcl-x S to bcl-x L RNA was found; the number of Bax expressing cells peaked at day 3 of involution, coincident with a peak in the number of apoptotic cells, and some of the Bax expressing cells showed characteristic apoptotic morphology (Heermeier et al, 1996). It is not known, however, whether this upregulation is of functional signi®cance for the induction of apoptosis in alveolar cells.…”

SV40 T/t-antigen induces premature mammary gland involution by apoptosis and selects for p53 missense mutation in mammary tumors

“…In contrast to the T/t-antigen positive ME-cells the 8/ 61B cells do not exhibit a spontaneous DNA fragmentation pattern ( Figure 1d) and only 1 ± 3% of them are positive on TUNEL or propidium iodide staining (Table 1). Since the Bax mRNA level increases in cells during mammary gland involution after lactation (Heermeier et al, 1996;Merlo et al, 1997), the Bax expression rate was analysed in the 8/61A and 8/61B cells by RT ± PCR analysis but there was no signi®cant di erence between the high and low apoptotic cells (Figure 7). To determine the expression rate of the Bcl-X gene, primers were utilized for RT ± PCR ampli®cation which allow identi®cation of the two splice variants, the anti-apoptotic Bcl-xL and the apoptotic Bcl-xS form.…”

“…Bcl-2) and there is no indication that the p53-independent cmyc pathway (Sakamuro et al, 1995;Hermeking et al, 1995;Friedlander et al, 1996) is central for ME-cell apoptosis (Figure 7). Although an elevated protein kinase A (PKA) activity and higher AP-1 levels (Marti et al, 1994), as well as a transition from anti-apoptotic Bcl-xL to the apoptotic Bcl-xS alternative splice form were observed one day after lactation (Heermeier et al, 1996), no signi®cant di erence was demonstrable in this regard between the T/t antigen positive and negative ME-cells. Therefore, it remains unclear which cellular genes or gene products are the T/t-antigen target(s) that mediate the p53-independent ME-cell death, a pathway which still can be activated in the 8/ 61A and 8/61B cells by either doxorubicin or U.V.…”

Bcl-xS and Bax induce different apoptotic pathways in PC12 cells

“…However, the ®nding that Bcl-x S mRNA levels are increased in some apoptotic systems, e.g. transient forebrain global ischemia (Dixon et al, 1997), apoptosis in involuting mammary epithelial cells (Heermeier et al, 1996) and in injured carotid artery (Igase et al, 1999), suggests that Bcl-x S may participate in the apoptotic pathway in these systems.…”