Methods to study MYC-regulated cellular senescence

Methods to Study Myc-Regulated Cellular Senescence: An Update

Cellular senescence plays a role in several physiological processes including aging, embryonic development, tissue remodeling, and wound healing and is considered one of the main barriers against tumor development. Studies of normal and tumor cells both in culture and in vivo suggest that MYC plays an important role in regulating senescence, thereby contributing to tumor development. We have previously described different common methods to measure senescence in cell cultures and in tissues. Unfortunately, there is no unique marker that unambiguously defines a senescent state, and it is therefore necessary to combine measurements of several different markers in order to assure the correct identification of senescent cells. Here we describe protocols for simultaneous detection of multiple senescence markers in situ, a quantitative fluorogenic method to measure senescence-associated β-galactosidase activity (SA-β-gal), and a new method to detect senescent cells based on the Sudan Black B (SBB) analogue GL13, which is applicable to formalin-fixed paraffin-embedded tissues. The application of these methods in various systems will hopefully shed further light on the role of MYC in regulation of senescence, and how that impacts normal physiological processes as well as diseases and in particular cancer development.

MYC synergizes with activated BRAFV600E in mouse lung tumor development by suppressing senescence

The activated RAS/RAF cascade plays a crucial role in lung cancer, but is also known to induce cellular senescence, a major barrier imposed on tumor cells early in tumorigenesis. MYC is a key factor in suppression of RAS/BRAF(V600E)-induced senescence in vitro. However, it is still unclear whether MYC has the same role during tumor development in vivo. Using a conditional, compound knock-in model of Cre-activated BRAF(V600E) and tamoxifen-regulatable MycER, we show that tamoxifen-induced activation of MYC accelerated the onset and increased the number and size of BRAF(V600E)-driven adenomas in a dose-dependent manner, resulting in reduced survival. Furthermore, MYC activation leads to reduced expression of the senescence markers p16(INK4A), p21(CIP1), and H3K9me3-containing heterochromatin foci, and an increased percentage of Ki67(+) tumor cells. This suggests that MYC already early during tumor formation suppresses a BRAF(V600E)-induced senescence-like state. Initial activation of MYC followed by tamoxifen withdrawal still resulted in an increased number of tumors and reduced survival. However, these tumors were of smaller size, showed increased expression of p16(INK4A) and p21(CIP1), and reduced number of Ki67(+) cells, indicating that MYC inactivation restores BRAF(V600E)-induced senescence. Surprisingly, MYC activation did not promote adenoma to carcinoma progression. This suggests that senescence suppression by MYC is a discrete step in tumor development important for sustained tumor growth but preceding malignant transformation and that additional oncogenic events are required for carcinoma development and metastasis. These findings contribute to our understanding of the neoplastic transformation process, with implications for future treatment strategies.